CONCLUDING OPERATIONS AND EXHIBITION OF THE LIGHT.
~Harbour Works.~
The shores of Tyree, as already often noticed in these pages, and as
deplored by Martin 140 years ago, in his Account of the Hebrides,
afford few places of safety fit even for boats. It had therefore been
determined, by the Commissioners, that any attempt at the construction
of a harbour should be strictly confined to the provision of a place
of shelter for the vessel which was to attend the Lighthouse. Much
attention had been bestowed on the subject, not merely by myself
during my five years’ acquaintance with Tyree, but also by Mr Thomas
Stevenson, who succeeded me in the charge of the works at Hynish, at
the time when I was appointed Engineer to the Lighthouse Board in
January 1843, after the completion of the masonry of the Tower. A small
sandy beach at Hynish, which lies embayed between rugged rocks, had
been selected as the fittest place for the pier; and all the materials
had been landed and shipped there, so that we naturally looked to it
as the site for the projected harbour, not only as presenting works
already finished, which might be made available as part of a more
extended plan, but as a place which, during an experience of some
years, had justified our anticipations as to its being less frequently
disturbed during stormy weather than most of the neighbouring creeks.
All that was contemplated in the proposed plan, was to form a small
basin in which the vessel could lie sheltered in all states of the
weather, and from which she could find an easy departure in any
condition of the sea which would permit a landing to be made on the
Rock. The Skerryvore Steamer having been sold and a small vessel of
35 tons, named the “Francis,” having been purchased at Deal, the dock
accommodation at Hynish was, for the sake of economy, laid out with
reference to the shelter of that vessel. It was calculated that a basin
100 feet in length and 50 feet wide, would afford sufficient room for
such a vessel; and as her draught of water is between 7 and 8 feet, it
was thought sufficient to provide for a depth of about 12 feet at high
water of spring-tides, which, it was expected, would render the dock
accessible during good springs at about _three quarters flood_.
The exposure of the shore at Hynish, to the effects of heavy westerly
swells, made it desirable to avoid carrying the entrance of the
basin so far seaward, as, under more favourable circumstances, would
undoubtedly have been done; and it was accordingly determined that
the landing pier should be extended to about 40 feet seaward of
low-water mark, and terminated in a round head, as shewn in Plate X.,
having a talus wall on its seaward face, composed of rough blocks,
arranged in courses regularly receding, so as to form a slope of 45° of
inclination, as shewn in Plate XI. The inner face of that pier, being
nearly vertical and guarded by fenders of timber, had served as the
quay for landing and shipping stones and other stores and it now forms
one side of the basin or dock. The other side consists of a shorter
talus wall, built about 60 feet to the westward of the first, and,
together with the crossheads projecting from each wall and containing
the gateway, completes the inclosure of the basin. In the gateway,
_booms_ are employed, as the shifting nature of the sand and the heavy
seas render _gates_ inadmissible. The space contained between those
walls was left completely dry at low water of spring-tides, and was
chiefly composed of rock, covered with a thin layer of shifting sand,
which varied in depth with the state of the wind and sea. The rocky
matter, consisting of decomposed gneiss, was excavated to the extent
of about 5000 tons, in three separate compartments, protected by
successive dams of rubble masonry, built with Pozzolano mortar, and
presenting an aggregate area of 7339 square feet. Those dams, two feet
thick, proved so water-tight, that by the aid of a small hand-pump, the
excavation and the building of the entrance heads of the booms went
regularly forward without any delay, although the men worked in the
bottom of the pit, surrounded on all sides by the sea, which, at high
water of spring-tides, rose 17 feet above them. The dams were sheltered
from the action of the swell by a temporary breakwater of heavy blocks,
which formed a convenient roadway for the transport of the materials
during the progress of the works, and which were removed at the close
of the operations.
The talus wall for protecting the seaward side of the harbour, has
about 60 feet of its foundation laid in a depth of water varying at low
spring-tides from 18 inches to 3¹⁄₂ feet. The mode of its construction,
as already stated, is shewn in Plate XI. It is surmounted by a strong
parapet of rustic masonry of Mull granite, and is altogether a most
substantial piece of work.
The idea of a tide-basin with boom-gates facing the breach of Atlantic
waves is somewhat novel and was not very hastily entertained by me at
first; but the most complete success has attended the plan. During my
occasional visits to the works in the course of the summer 1843, our
attention had been often occupied with considering the probability of
the sand shutting up the basin; and as a single tide during heavy winds
from the N.W., made great changes on the appearance of the beach, we
feared that the vessel might often be imprisoned within the boomgates
by a bank of sand heaped against them by the sea. To such an extent
did the accumulation go before the harbour was fully opened, that on
many occasions there was not water for a rowboat to pass between the
boom-heads even at the highest spring-tides. The only remedy for such
an evil, was obviously to attempt some mode of artificial scouring; and
for that purpose, it was proposed to divert several small streams which
run from Ben Hynish and the neighbouring hills through the grounds at
Hynish, into one feeder, and pen them up in a pond, so as to afford
the means of scouring the entrance to the basin from the incumbrance
of the loose sand which might choke it. Those streams were repeatedly
gauged during the summer and were found to deliver from 13 to 50 cubic
feet of water per minute, according to the state of the weather,--a
supply which seemed ample for the purpose in view. The sand at Hynish
is of a light nature and is easily acted on by currents of very feeble
power. It consists of comminuted shells and requires about 25¹⁄₂ cubic
feet to make a ton, instead of 24 feet, which is the common allowance
for silicious sand. The leave of the Duke of Argyll and also of the
Farmer at Hynish, having been obtained, various cuts were made and the
stream was diverted into a pond capable of containing about 175,000
cubic feet and provided with a waste-weir and with sluices for opening
the communication between the pond and the scouring tunnel, from which
the water flows in a stream of about 9 square feet of area, at the rate
of about 260 feet per minute for a period of 1¹⁄₄ hour. The operation
of scouring is performed at low water and is generally found quite
sufficient for the purpose of clearing a passage down to the bare rock
in a single tide. Nothing can be more satisfactory than to witness the
effect of that process in opening the entrance to a basin apparently
inaccessible; and but for such an arrangement, the dock must have
remained permanently choked with sand and sea-weed. The position of the
various works is marked in Plate X., which shews the ground at Hynish.
On the side of the dock stands a crane, which is used for various
purposes connected with the shipment and discharging of materials. It
also serves for raising the booms, by means of a double hook, which can
be attached to the chain and which embraces the pegs in the centre of
each boom, shewn in Plate XI. The two tiers of booms are firmly lashed
down by means of chains passing through ring-bolts in the manner shewn
in the section (Plate XI.), so as to prevent their rising with the
tide.
The necessity of providing somewhere in the neighbouring island of
Tyree for the proper accommodation of the vessel which was to wait
upon the Lighthouse (if it could ever have been a matter of doubt),
was abundantly demonstrated by the experience of 1843, and more
especially by that of the months of November and December. Owing to the
shortness of the day and the distance of any place of shelter from the
Skerryvore, together with the difficulty of landing on the Rock and
the extreme variableness and uncertainty of the climate, the Regent
Tender, although constantly waiting on the coast and making trials on
every occasion that seemed to offer any prospect of success, could not
effect any communication with the people who had been left on the Rock,
during a period of no less than seven _long_ weeks. The poor seamen
who were living in the Barrack passed that time most drearily, for not
only had their clothes been literally worn to rags, but they suffered
the want of many things dearer to them than clothes, and amongst others
of tobacco, the failure of the supply of which they had despondingly
recorded in chalk on the walls of their prison-house, with the date
of the occurrence! Unless, therefore, the vessel had been stationed
near the rock, the few casual and uncertain opportunities which occur
at intervals, during the short days of winter, would often have been
lost, and the future maintenance of the Light rendered excessively
precarious. The experience of subsequent years, during which the relief
of the Light-keepers has been kept up with considerable regularity,
shews that the small Harbour at Hynish forms a most important integral
part of the Establishment of Skerryvore, than which I believe no
Lighthouse on the coast is more comfortable as a residence.
~Bo Pheg Beacon.~
In connection with the Harbour at Hynish, I naturally notice the
erection of a Beacon on a rock called Bo Pheg, which is dry only at low
water of spring tides. It lies about one mile NE. from Hynish Point,
(See Plates II. and X.) right in the track of the Tender in its passage
to and from the Lighthouse. From the great difficulty of landing on
that small rock, over which the sea almost continually breaks, even
in very fine weather, the shortness of the time during which the men
could remain at work, and the want of room on its irregular surface of
about 16 yards square, the erection of a beacon proved to be a work of
great difficulty. The Beacon consisted of an open frame-work of iron,
(somewhat on the same plan as that which is described in the Appendix)
and calculated to offer little opposition to the free passage of the
waves; but before all the fixtures could be completed, the heavy storms
of the winter of 1844, acting on its unfinished base (the bats of which
were only partially secured), destroyed it piecemeal, at a season
when no landing could be effected, nor any effort made to save almost
any part of it from the sea. In the ensuing summer, a second Beacon,
consisting of a hollow cone, composed of iron plates, united together
by strong flanges and attached to the rock by _webbed flanges_ round
its base with strong bats passing through them, was fixed down to the
rock; and in order to increase its weight, the interior was filled with
concrete gravel. That structure also has since yielded to the force of
the waves, after giving slight indications of movement, an effect which
I attribute chiefly to the smallness of the base, which the narrow
limits of the rock unhappily prescribe.
~Lightkeepers’ and Seamen’s House.~
Another necessary part of the Establishment at Hynish, was the
provision of dwelling-houses for the families of the Lightkeepers and
Seamen and of Storehouses of various kinds. Those were partly built
on purpose and partly consisted of altered forms of the buildings
which had been found necessary as barracks for workmen and stores for
materials, during the progress of the works.
~Concluding works on the Rock, such as pointing, &c.~
On landing on the Rock, on the 29th of March 1843, the Resident
Engineer had the satisfaction of finding the whole building perfectly
water-tight and saw not the slightest trace of a defective joint. The
outside joints of the building were therefore carefully “ripped,” and
repointed with mortar, composed of equal parts of Halkin lime and
Pozzolano. That operation, from the difficulty of employing many men,
where suspended scaffolds were necessary and from the care with which
it requires to be executed, occupied a great deal of time.
~Interior fittings of the Tower.~
Another tedious operation was the fitting up of the interior of the
Tower with wainscot lining and forming the various stories into
apartments separate from the staircase. Much work was also expended
in providing the fire-places with proper flues, in fitting up water
tanks, coal stores, and oil tanks, and also in conveying the air-tubes
between the Lightroom and the several apartments by which the signal
bells are rung for summoning the keepers to mount guard. The keeper
on duty is, by the rules of the Service, forbidden, under penalty
of instant dismissal, to leave the Light-room, on any pretext,
until relieved by the next who mounts guard, and who is summoned by
means of a bell placed inside his cot or sleeping berth, which is
rung by means of a small piston, propelled by simply blowing into a
mouth-piece in the Light-room. The keeper in bed answers this signal by
a “_counter-blast_,” which rings another bell in the Light-room, and
informs the keeper there that his signal has been heard and will be
obeyed.
~Arrangement of the several apartments.~
The general arrangement of the Tower, may be seen in the Section Plate
VIII.; and the details of its subdivision are very similar to those
shewn in Plate XVI. of my Father’s Account of the Bell Rock Lighthouse.
The ascent to the outside door is by a ladder or trap of gun metal,
26 feet high. The first apartment on the level of the entrance door,
is chiefly appropriated to the reception of iron water-tanks, capable
of holding a supply of 1251 gallons. The next story is set aside for
coals, which are stowed in large iron boxes. The third apartment
is a workshop; the fourth is the provision store; and the fifth is
the kitchen. Above are two stories, each divided into two sleeping
apartments, for the four Light-keepers. Over them is the room for the
Visiting Officers; then follows the oil store, and lastly comes the
Lightroom, making in all twelve apartments. The nearness of the oil
store to the Lightroom is a great convenience to the Keepers, who are
thus saved the trouble of carrying the daily supply of oil to the
Lightroom, up a long flight of steps. The passage from story to story
is by oaken trap ladders, passing through hatches in each floor and
partitioned off from each apartment in order to prevent accidents and
to check cold draughts.
~Lightroom Apparatus, and first exhibition of the Light.~
The light of Skerryvore was exhibited to the mariner on the night of
the 1st February 1844, in terms of the Statutory Notice, which will
be found in the Appendix. The light is revolving, appearing in its
brightest state once in every minute of time. It is elevated 150 feet
above the sea, and is well seen as far as the curvature of the earth
permits; it is also frequently seen as a brilliant light from the high
land of Barra, a distance of 38 miles. The apparatus consists of eight
annular lenses (of the first order, in the system of AUGUSTIN FRESNEL),
of 36·22 inches focal distance, revolving round a lamp with four
concentric wicks, and producing a bright blaze when each lens passes
between the lamp and the eye of a distant observer. Above those lenses
are placed eight pyramidal lenses of 19·68 inches focal distance,
inclined at 50° with the horizon and combined with eight plane mirrors,
inclined in the opposite direction at 50° with the horizon. By this
arrangement, that part of the light from the lamp which would otherwise
escape uselessly beyond the great lenses, upwards into the sky, being
parallelized in its passage through the smaller lenses and falling on
the mirrors, is finally projected forwards in horizontal beams, so as
to aid the effect of the light. Those lenses and mirrors, however,
instead of having their axes in the same vertical plane with the axes
of the principal lenses, are inclined about 7° horizontally to the
right hand, and by that deviation produce small premonitory blazes,
which, blending with the beams of the larger lenses, tend in some
measure to lengthen the duration of the impression on the eye. So far
the apparatus of the Skerryvore Lighthouse is identical in its general
arrangements with that of the Tour de Corduan, and differs only in
the superior workmanship of the lenses and the machinery, which the
experience of more than twenty years has brought about, since FRESNEL
designed that light in 1822. Instead, however, of employing curved
mirrors, as has been done at Corduan, to collect the light which would
otherwise escape below the lenses and, at the same time, to send it to
the horizon, I determined to put in practice a plan which I had long
contemplated, of placing totally reflecting zones below the lenses,
similar in construction to the zones of the small Harbour Light
Apparatus of the fourth order, which was also invented by FRESNEL. This
was finally carried into effect, agreeably to the design of M. LEONOR
FRESNEL, his brother, with whom I had corresponded on the subject. In
the subsequent pages of this volume, I intend to make some observations
explanatory of the principles and arrangement of the various optical
instruments employed in Lighthouses; and as that will afford me a more
convenient opportunity of describing the nature and properties of the
_totally reflecting zones_, I shall forbear in this place to enter
into further details as to the construction or action of the apparatus
at the Skerryvore. It is right, however, that I should mention here
that the lenses, mirrors and zones are from the works of M. FRANÇOIS
of Paris, whose name I shall afterwards have occasion to notice; and
that the machinery was constructed to my entire satisfaction, and in a
manner worthy of his reputation as a mechanician, by Mr JOHN MILNE of
Edinburgh.
~Removal of the Barrack from the Rock.~
In such a situation as the Skerryvore, new wants were discovered every
day; and each summer brought its round of smaller works, which the
experience of the preceding winter had suggested. The Barrack was found
very useful as a place of residence for the workmen, who were engaged
in such needful works; and it was not until the summer of 1846, that it
was taken to pieces and removed from the Rock, after having kept its
place for six years. Its removal, however, was then thought advisable,
as some of its fixtures, by the continual action of the weather, had
become very loose and precarious; and, although a very small outlay
would have made it almost as stable as at first, it was considered
inexpedient to attempt to perpetuate a structure confessedly temporary
in its nature, and the sudden destruction of which by the waves, seemed
to involve some risk of injury to the Tower itself.
~Expense.~
The expense of erecting the Skerryvore Lighthouse, including the
opening of quarries and forming wharfs at the quarries in Mull and also
the Harbour in Tyree, was, as appears from the Account in the Appendix,
L.86,977 : 17 : 7.
In the course of the Summer of 1844, a marble tablet, bearing an
inscription in letters of gold, was, by order of the Commissioners,
placed over one of the windows in the Visiting Officers’ Room. With
a representation of the Tablet and the Inscription, which, after
acknowledging the hand of Almighty God in the success which attended
the work, briefly sets forth the beneficent purposes for which the
Lighthouse was erected and records the laying of the foundation-stone
by his Grace the Duke of Argyll, I willingly close this most defective
narrative of the work.
[Illustration:
IN SALUTEM OMNIUM
NORTHERN LIGHTHOUSES
A. D. O. M.
AUCTORITATE . ET . CONCILIIS
PHARORUM . SCOTIAE . COLLEGII
HAEC . STRUCTA . FUIT . PHAROS
CUJUS . DIRECTI . FLAMMA
NAUTAE
INFAMIBUS . HIS . SCOPULIS . ADHUC . MERITO . DETERRITI
OPTATUM . PORTUM . RECTIUS . ADVENIRENT.
JOANNES . DUX . DE . ARGYLL
INSULARUM . ADJACENTIUM . DOMINUS
LAPIDEM . AUSPICALEM . RITE . STATUIT
DIE . IV . MENSIS . JULII . ANNO . IV . VICT. REG.
M . D . C . C . C . X . L .
OPERUM . MAGISTRO . ALANO . STEVENSON . L.L.B.
_Engraved by W. & A.K. Johnston_]
PART SECOND.
NOTES
ON THE
ILLUMINATION OF LIGHTHOUSES.
PART SECOND.
NOTES ON THE ILLUMINATION OF LIGHTHOUSES, WITH SHORT NOTICES OF THEIR
EARLY HISTORY.
~Early History.~
The early history of Lighthouses is very uncertain; and some ingenious
antiquaries, finding the want of authentic records, have been anxious
to supply the deficiency by conjectures based upon casual and obscure
allusions in ancient writers, and by vague hypotheses drawn from the
heathen mythology. Some writers have gone so far as to imagine, that
the Cyclopes were the keepers of lighthouses; whilst others have
actually maintained that Cyclops was intended, by a bold prosopopœia,
to represent a lighthouse itself.[24] A notion so fanciful deserves
little consideration; and accords very ill with that mythology of which
it is intended to be an exposition, as seems sufficiently plain from a
passage in the ninth Odyssey, where Homer (who flourished about 907 B.
C.), after describing the darkness of the night, informs us that the
fleet of Ulysses actually struck the shore of the Cyclopean island,
before it could be seen.[25]
[24] This spirit of etymological conjecture has converted _Cyclops_,
_Proteus_, _Cneph_, _Phanes_, _Canobus_, _Chiron_, _Tithonus_,
_Thetis_, _Amphitrite_, _Minotaurus_, _Chronus_, _Phrontis_, and
other demigods, into celebrated lighthouses, or, at all events, has
imagined that those mythological personages were worshipped under the
emblem of fire or light in buildings, which, at the same time, served
as guides to the benighted mariner. On the faith, also, of similar
obscure and finely drawn etymologies, various places, such as _Calpe_
and _Abyla_, the opposite points of Africa and Europe, at the Straits
of the Mediterranean, have been unhesitatingly recognized as the
sites of celebrated light-towers; and the Latin words _turris_ and
_columna_ have been supposed primarily to signify a lighthouse, the
first being written _Tor-is_, the _Tower of fire_, and the _Col-on_,
the _Pillar of the Sun_.
[25]
Ἔνθ’ ὄυτις τὴν νῆσον ἐσέδρακεν ὀφθαλμοῒσιν
Ὅυτ’ οὖν κύματα μακρὰ κυλινδόμενα ποτὶ χέρσον
Ἐισίδομεν πρὶν νῆας ἐϋσσέλμους ἐπικελσαι.
_Odyss._, ix., 146.
Nor does there appear any better reason for supposing, that under the
history of Tithonus, Chiron, or any other personage of antiquity,
the idea of a lighthouse was conveyed; for such suppositions,
however reconcileable they may appear with some parts of mythology,
involve obvious inconsistencies with others. It seems, indeed, most
improbable, that, in those early times, when navigation was so little
practised, the advantages of beacon lights were so generally known and
acknowledged as to render them the objects of mythological allegory.
It must not, however, be imagined, that ancient writings are entirely
destitute of allusions to the subject of Beacon Lights for the guidance
of the Mariner. The venerable poet, already noticed, in speaking of
the shield of Achilles, has beautifully described the flash of a
beacon-light in some solitary place, as seen by seamen leaving their
friends, in those lines, which contain ample proof of the existence of
such a provision for the safety of the mariner in Homer’s time:--
Ὣς δ’ ὅταν ἐκ πόντοιο σέλας ναύτῃσι φανείη
Καιομένοιο πυρὸς, τὸ δὲ καίεται ὑψόθ’ ὄρεσφι,
Σταθμῷ ἐν οἰοπόλῳ· τοὺς δ’ οὐκ ἐθέλοντας ἄελλαι
Πόντον ἐπ’ ἰχθυόεντα φίλων ἀπάνευθε φέρουσιν.
_Il._, xix., 375.
In the Holy Scriptures the word _Beacon_ occurs but once, and that in
Prophecies of Isaiah (xxx. 17.), who lived above 200 years later than
Homer; but it is obvious that the original term, which the Septuagint
translate by the word ἱστος, merely imports a flagstaff or perch and
does not at all imply the knowledge of beacon-lights among the Hebrews,
who were not a maritime people.
~Colossus of Rhodes.~
About 300 years before the Christian era, Chares, the disciple of
Lysippus, constructed the celebrated brazen statue, called the
Colossus of Rhodes. It was of such dimensions as to allow vessels to
sail into the harbour between its legs, which spanned the entrance.
There is considerable probability in the idea that this figure served
the purposes of a lighthouse; but there is no passage in any ancient
writer, where this use of the Colossus is expressly mentioned. Many
inconsistencies occur in the account of this fabric by early writers,
who, in describing the distant objects which could be seen from it,
appear to have forgotten the height which they assign to the figure.
It was partly demolished by an earthquake, about eighty years after
its completion; and so late as the year 672 of our era, the brass of
which it was composed was sold by the Saracens to a Jewish merchant of
Edessa, for a sum, it is said, equal to L.36,000.
~Pharos of Alexandria.~
Little is known with certainty regarding the Pharos of Alexandria,
which was regarded by the ancients as one of the seven wonders of the
world. It was built in the reign of Ptolemy Philadelphus, about 300
years before the Christian era; and Strabo relates that Sostratus,
a friend of the royal family, was the architect. He describes it as
built in a wonderful manner in many stories of white stone, on a rock
forming the promontory of the island Pharos (whence the Tower derived
its name), and says that the building bore the inscription--“Sostratus
of Cnidos, the son of Dexiphanes, to the Gods, the Saviours, for the
benefit of seamen.” He concludes his brief notice of it by describing
the neighbouring shores as low and encumbered with shoals and snares,
and as calling for the establishment of a lofty and bright beacon, a
sign to guide sailors arriving from the ocean into the entrance to the
haven.[26]
[26] The passage from which the above description is drawn will be
found in the Oxford edition of Strabo, 1807, page 1123. It is as
follows: Ἔστι δε και ἀυτο τὸ τῆς νησίδος ἄχρον πετρα πολυκλύστος,
ἔχουσα πῦργον θαυμαστῶς κατεσκευασμένον λευκου λιθου, πολυοροφον,
ὁμωνυμον τῇ νὴσῳ· τουτον δε ἄνέθηκε Σωστρατος Κνὶδιος φιλος τῶν
βασιλεων, της των πλωϊζομενων σωτηριας χαριν, ὥς φησιν ἥ ἐπιγραφη
Επιγραμμα, ΣΩΣΤΡΑΤΟΣ ΚΝΙΔΙΟΣ ΔΕΧΙΦΑΝΟΥΣ, ΘΕΟΙΣ ΣΩΤΗΡΣΙΝ ΥΠΕΡ ΤΩΝ
ΠΑΩΙΖΟΜΕΝΩΝ. Ἀλιμένου γαρ ὄυσης και ταπεινῆς της ἕκατέρωθεν παραλίας,
ἐχουσης δε και χοιράδας και βράχη τινὰ, ἔδει σημεὶου τινος ὑψηλου
και λαμπρου, τοις ἀπὸ του πελάγους προσπλέουσιν, ὥστ’ ἐυστοχειν της
ἐισβολης του λιμενος. Strabo’s account of the position of the island
of Pharos at once leads to the conclusion of its having formed part
of the harbour of Alexandria (as is abundantly testified by Josephus,
Pliny, and other writers), and cannot be easily reconciled with
that of Homer (fourth Odyssey, l. 354), who describes the island as
a day’s sail with a fair wind from the mainland. His words are as
follows:--
Νῆσος ἔπειτά τις ἐστὶ πολυκλύστῳ ἐνὶ πόντῳ
Αἰγύπτου προπάροιθε (Φάρον δέ ἑ κικλήσkουσι)
Τοσσον δ’ ἂνευθ’ ὄσσον τεπανημέριη γλαφυρὴ νηῦς
Ἤνυσεν, ᾗ λιγὺς οὖρος ἐπιπνέιῃσιν ὂπισθεν.
_Odyssey_, iv., l. 354.
Pliny, however, does not scruple to identify the Pharos of Homer’s
time with that of his own day. “Pharos,” says he, “quondam diei
navigatione distans ab Ægypto, nunc è turri nocturnis ignibus cursum
navium regens.” Hist. Nat., v. 31; see also Hist. Nat., ii. 87, and
xiii. 21.
The accounts which have come down to us of the dimensions of this
remarkable edifice are exceedingly various; and the statements of
the distance at which it could be seen are clearly fabulous. That
of Josephus (who likens it to the second of Herod’s three Towers at
Jerusalem, called Phasael, in honour of his brother) is the least
removed from probability; yet even he informs us, that the fire which
burnt on the top to enable seamen to anchor in sight of it, before
coming near the shore, and so to avoid the difficulty of the navigation
by night, was visible at a distance equal to about thirty-four English
miles. Such a range for a lighthouse on the low shores of Egypt, would
require a tower about 550 feet in height![27] Ammianus Marcellinus[28]
and Pliny[29] are both very circumstantial in their notices of the
Pharos as a beacon-light to guide seamen in approaching the coast of
Egypt and port of Alexandria. The latter adds the interesting fact,
that the cost of the Tower was reckoned at a sum equal to about
L.390,000 of our money;[30] and both of them agree in stating that a
light was shewn from it at night. Ammianus Marcellinus differs from all
the other writers, in attributing the erection of the Tower to Queen
Cleopatra. Pliny mentions in passing, that there were also lighthouses
at Ostia and Ravenna.
[27] Bell. Judaic. iv., cap. 10, sec. 5. (Havercamp’s Josephus,
tom. ii., p. 309. Amsterdam, 1726.) ἐν δεξιᾳ δε ἡ προσαγορευομενη
Φαρος νησις προκειται, πῦργον ανἐχουσα μεγιστον, εκπυρσευοντα
τοις καταπλὲουσιν, ἐπι τριακοσιους σταδιους, ὤς ἐν νυκτὶ πὂρρωθεν
ὁρμιζοιντο προς την δυσχέρειαν του καταπλοῦ. And again, in the sixth
Book of the same History (v. 4, sec. 3, tom. ii., p. 330), he says,
και τὸ μεν σχημα παρεῴκει τῳ κατα τὴν Φαρον ἐκπυρσεὐοντι τοις επ’
Αλεξανδρείας πλέουσι. The height of the Tower in the text proceeds on
the idea of the observer’s eye being ten feet above the sea.
[28] Ammianus Marcellinus, l. xxii., c. 16. (Leipsic 1807, tom.
i., p. 306.) Hoc littus cum fallacibus et insidiosis accessibus
affligeret antehac navigantes discriminibus plurimis, excogitavit
in portu Cleopatra turrim excelsam, quae Pharos a loco ipso
cognominatur, praelucendi navibus nocturna suggerens ministeria; cum,
quondam ex Parthenio pelago venientes aut Libyco, per pandas oras
et patulas, montium nullas speculas vel collium signa cernentes,
harenarum inlisae glutinosae mollitiae frangerentur.
[29] Plinii Hist. Nat., xxxvi. 18. (Paris, 1723, p. 739.)
Magnificatur et alia turris a rege facta in insula Pharo portum
obtinente Alexandriae, quam constitisse octingentis talentis tradunt;
magno animo (nequid omittamus) Ptolemai regis, quod in ea permiserit
Sostrati Guidii Architecti structuræ ipsius nomen inscribi. Usus
ejus, nocturno navium cursu ignes ostendere ad praenuncianda vada
portûsque introïtum: quales, jam compluribus locis flagrant, ut
Ostiae ac Ravennae. Periculum in continuatione ignium, ne sidus
existimetur, quoniam è longinquo similis flammarum aspectus est.
[30] Supposing, as is most probable, that Pliny means the _Egyptian_
talent; the _Attic_ talent was about one-half the value of the other.
If the reports of some writers are to be believed, this Tower must
have far exceeded in size the great Pyramid itself; but the fact that
a building of comparatively so late a date should have so completely
disappeared, whilst the Pyramid remains almost unchanged, is a
sufficient reason for rejecting, as erroneous, the dimensions which
have been assigned by most writers to the Pharos of Alexandria. Some
have pretended that large mirrors were employed to direct the rays of
the beacon-light on its top, in the most advantageous direction; but,
in so far as I know, there is no definite evidence in favour of this
supposition. Others, with greater probability, have imagined that this
celebrated beacon was known to mariners, simply by the uncertain and
rude light afforded by a common fire. In speaking of the Pharos, the
poet Lucian, on most occasions sufficiently fond of the marvellous,
takes no notice of the gigantic mirrors which it is said to have
contained. He thus speaks of this celebrated lighthouse as having
indicated to Julius Cæsar his approach to the Pharos of Egypt on the
seventh night after he sailed from Troy:
Septima nox, Zephyro nunquam laxante rudentes,
Ostendit Phariis Ægyptia littora _flammis_.
Sed prius orta dies nocturna _lampada_ texit,
Quam tutas intraret aquas.
_Pharsal._, ix., 1004.
It is true that, by using the word “_lampada_,” which can only with
propriety be applied to a more perfect mode of illumination than an
open fire, he appears to indicate that the “_flammis_” of which he
speaks, were not so produced. The word _lampada_ may however, be used
metaphorically; and _flammis_ would, in this case, not improperly
describe the irregular appearance of a common fire.
Perhaps, also, the opinion that some kind of lamp was used in the
Pharos, may seem to receive countenance from the remarkable words of
Pliny, in the passage above cited--“Periculum in continuatione ignium,
ne sidus existimetur, quoniam è longinquo similis flammarum aspectus
est.” The fear he expresses lest the light viewed from a distance
should be mistaken for a star, could hardly be applicable to the
diffuse, oscillating, lambent light derived from an open fire, and
certainly gives some reason for imagining that, even at that remote
time, the art of illuminating lighthouses was better understood than in
the early part of the present century.
Before leaving the subject of the Pharos of Alexandria, I wish to
vindicate the memory of its architect Sostratus from the calumny of
Lucian, who, in his Treatise on the art of writing history, with his
usual acrimony, accuses the builder of the Pharos of a fraud, in
cutting his own name on the solid walls of the Tower, and covering the
inscription with plaster, on which he carved the name of his royal
master Ptolemy.[31] Against this assertion I would oppose the testimony
of Strabo, who calls Sostratus the “friend of the Kings” (see the
quotation at the foot of page 183), and the direct evidence of Pliny,
who, in the passage above cited, expressly states, as a proof of
Ptolemy’s magnanimity, his giving the architect liberty to inscribe
his own name on the Tower. The only other notices of the Pharos
which I have been able to find in ancient writers are from Cæsar’s
Commentaries, Valerius Flaccus, and Pomponius Mela.[32] At Alexandria,
there is a modern lighthouse called the Pharos, which is maintained by
the Pacha of Egypt.
[31] Lucian, in his Treatise (Amsterdam, 1743, vol. ii., p. 68.)
Πως δει ἱστοριαν συγγραφειν, thus details the merit and fraud of
Sostratus. Ορᾷς τον Κνίδιον ἐκεῖνον ἀρχιτεκτονα, οἶον ἐποίησεν;
οἰκοδομὴσας γαρ τον ἐπὶ τῇ Φαρῳ πῦργον, μεγιστον και καλλιστον ἔργων
ἀπαντων, ὡς πυρσεύοιτο ἀπ’ αὐτου τοις ναυτιλλομενοις, ἐπὶ πολὺ της
θαλασσης, και μὴ καταφέροιντο εἰς την Παραιτονίαν, παγχάλεπον,
ὣς φασιν, οὖσαν και ἄφυκτον, εἴ τις ἐμπεσοι εἴς τα, ἕρματα.
Οἰκοδομήσας οὐν το ἔργον, ἔνδοθεν μεν, κατα των λιθων, το αὐτου
ὄνομα ἐπέγραψεν. Ἐπιχρίσας δὲ τιτάνῳ και ἐπικαλύψας, ἐπέγραψε το
ὀνομα του τότε βασιλέυοντος, εἰδὼς, ὃσπερ και ἐγενετο, πάνυ ὀλιγου
χρονου, συνεκπεσουμενα μεν τῳ χρισματι τα γραμματα, εκφανησόμενον δε
ΣΩΣΤΡΑΤΟΣ ΔΕΧΙΦΑΝΟΥΣ Κνιδιος. θεοις σωτηρσιν ὑπερ των πλωΐζομεων,
κ.τ.λ.
[32] Cæsar de Bell. Civil., iii. 98 (Lond. 1712, p. 355). Pharus est
in insula turris, magna altitudine, mirificis operibus extructa, quae
nomen ab insula accepit.
Valerius Flaccus very distinctly sets forth the great advantage
of lighthouses to the seaman, and especially speaks of those at
Alexandria and Ostia in these lines--
Non ita Tyrrhenus stupet Ioniusve magister,
Qui portus, Tyberine, tuos, claramque serenâ
Arce Pharon præceps subiit:
_Argonaut_, vii., v. 84.
Pompon. Mela, ii. cap. 7.
~Coruña Tower~
Mr Moore, in his _History of Ireland_, vol. i., p. 16, speaks of
the Tower of Coruña, which he says is mentioned in the traditionary
history of that country, as a lighthouse erected for the use of the
Irish in their frequent early intercourse with Spain. In confirmation
of this opinion, he cites a somewhat obscure passage from Æthicus,
the cosmographer. This in all probability is the tower which Humboldt
mentions in his Narrative under the name of the _Iron Tower_, which was
built as a lighthouse by Caius Saevius Lupus, an architect of the city
of Aqua Flavia, the modern Chaves.[33] A Lighthouse has lately been
established on this headland, for which Dioptric apparatus was supplied
from the workshop of M. LETOURNEAU of Paris.
[33] “The traditionary history,” says Mr Moore, “of the latter
country (Ireland) gives an account of an ancient Pharos or lighthouse
erected in the neighbourhood of the port now called Coruña, for the
use of navigators on their passage between that coast and Ireland.
There is a remarkable coincidence between this tradition and an
account given by Æthicus, the cosmographer, of a lofty Pharos or
lighthouse standing formerly on the sea-coast of Gallicia, and
serving as a beacon in the direction of Britain. _Secundus Angulus
intendit ubi Brigantia civitas sita est Galliciae, et altissimum
Pharum et inter pauca memorandi operis ad speculum Britanniae._
Whether the translation I have given of the last three words of this
passage convey their real meaning, I know not; but they have been
hitherto pronounced unintelligible. The passage is thus noticed
by Casaubon, in a note on Strabo, lib. iii. ‘Æthicus in Hispaniae
descriptione altissimi cujusdam Fari meminit.’” The passage in Strabo
above referred to is on page 179 of the first volume of the Oxford
folio edition of 1807, where the geographer speaks of Cape Νεριον,
which Casaubon distinctly identifies with the _Cabo de Finisterra_ of
modern seamen.
~Lighthouse at the mouth of the Guadalquivir.~
There is also a record in Strabo of a magnificent lighthouse of stone
at Capio, or Apio, near the Harbour of Menestheus (the modern Mesa
Asta, or Puerto de Sta. Maria), which he describes as built on a rock
nearly surrounded by the sea, as a guide for the shallows at the mouth
of the Guadalquivir, in terms almost identical with those used by him
in speaking of the Pharos of Alexandria. I am not aware of any other
notice of this great work, for such it seems to have been, to have
deserved the praises of Strabo.[34]
[34] The words of Strabo are (Oxon. 1807, p. 184), Και ὁ του Καπὶωνος
(vel Ἀπιωνος) πὺργος ἵδρυται ἐπι πέτρας ἀμφκλυστου, θαυμασιως
κατεσκευασμένος, ὥσπερ ό Φαρος τῆς των πλωϊζομενων σωτηριας χαριν, ῃ
τε γαρ ἐκβαλλομενη χους ὕπο του ποταμου βραχεα ποιει και χοιραδωδης
ἐστὶν ὁ πρὸ ἀυτου τοπος ὥστε δει σημειου τινος ἐπιφανοῦς.
~Ancient Phari in Britain.~
In Camden’s Britannia, a passing notice is taken of the ruins called
_Cæsar’s Altar_, at Dover, and of the _Tour d’Ordre_, at Boulogne, on
the opposite coast; both of which are conjectured to have been ancient
lighthouses. Pennant describes the remains of a Roman Pharos near
Holywell, but cites no authorities for his opinion as to its use. There
were likewise remains of a similar structure at Flamborough-head. A
very meagre and unintelligible account is also given of a lighthouse
at St Edmund’s Chapel, on the coast of Norfolk, in Gough’s additions
to Camden, by which it might seem that the lighthouse was erected in
1272.[35]
[35] Gough’s Camden’s Britannia, vol. i., 318, and vol. ii., p.
198; Batcheller, in his Dover Guide (1845, p. 111), says, that the
Dover Pharos was built “during the lieutenancy of Aulus Plautius
and Ostorius Scapula, the latter of whom left Britain, A. D.
53.”--Pennant’s History of Whiteford and Holywell, p. 112.
Such seems to be the sum of our knowledge of the ancient history of
lighthouses, which, it must be admitted, is neither accurate nor
extensive. Our information regarding modern lighthouses is of course
more minute in its details and more worthy of credit. The greater part
of it is drawn from authentic sources; and much of what is afterwards
stated is the result of my own observation, during my visits to the
most important lighthouses of Europe.
~Tour de Corduan.~
The first lighthouse of modern days that merits attention, is the
_Tour de Corduan_, which, in point of architectural grandeur, is
unquestionably the noblest edifice of the kind in the world. It is
situated on an extensive reef at the mouth of the river Garonne, and
serves as a guide to the shipping of Bordeaux and the Languedoc Canal,
and indeed of all that part of the Bay of Biscay. It was founded in
the year 1584, but was not completed till 1610, under Henri IV. It
is minutely described in Belidor’s _Architecture Hydraulique_. The
building is 197 feet in height, and consists of a pile of masonry,
forming successive galleries, enriched with pilasters and friezes,
and rising above each other with gradually diminished diameters.
Those galleries are surmounted by a conical tower, which terminates
in the lantern. Round the base is a wall of circumvallation, 134
feet in diameter, in which the light-keepers’ apartments are formed,
somewhat in the style of casemates. This wall is an outwork of
defence, and receives the chief shock of the waves. The tower itself
contains a chapel, and various apartments; and the ascent is by a
spacious staircase. The first light exhibited in the Tour de Corduan
was obtained by burning billets of oak-wood, in a chauffer at the
top of the tower; and the use of coal instead of wood, was the first
improvement which the light received. A rude reflector, in the form of
an inverted cone, was afterwards added, to prevent the loss of light
which escaped upwards. About the year 1780, M. LENOIR was employed to
substitute paraboloïdal reflectors and lamps; and in 1822, the light
received its last improvement, by the introduction of the dioptric
instruments of AUGUSTIN FRESNEL, the celebrated French Academician.
~Eddystone.~
The history of the famous Lighthouse on the Eddystone Rocks is well
known to the general reader, from the narrative of SMEATON the
Engineer. Those Rocks are 9¹⁄₂ miles from the Ram-Head, on the coast
of Cornwall; and from the small extent of the surface of the chief
Rock and its exposed situation, the construction of the Lighthouse was
a work of very great difficulty. The first erection was of timber,
designed by Mr WINSTANLEY; and was commenced in 1696. The light was
exhibited in November 1698. It was soon found, however, that the
sea rose upon that tower to a much greater height than had been
anticipated; so much so, it is said, as to “_bury under the water_”
the lantern, which was sixty feet above the Rock; and the Engineer
was therefore afterwards under the necessity of enlarging the Tower,
and carrying it to the height of 120 feet. In November 1703, some
considerable repairs were required, and Mr WINSTANLEY, accompanied by
his workmen, went to the Lighthouse to attend to their execution; but
the storm of the 26th of that month, carried away the whole erection,
when the Engineer and all his assistants unhappily perished!
The want of a light on the Eddystone, soon led to a fatal accident;
for not long after the destruction of Mr WINSTANLEY’S lighthouse,
the Winchilsea man-of-war was wrecked on the Eddystone Rocks, and
most of her crew were lost. Three years, however, elapsed, after this
melancholy proof of the necessity for a light, before the Trinity-House
of London could obtain a new Act of Parliament, to extend their powers;
and it was not till the month of July 1706, that the construction of
a new lighthouse was begun under the direction of Mr JOHN RUDYERD of
London. On the 28th of July 1708, the new light was first shewn, and it
continued to be regularly exhibited till the year 1755, when the whole
fabric was destroyed by accidental fire, after it had stood forty-seven
years. But for this circumstance, it is impossible to tell how long the
lighthouse might, with occasional repair, have lasted, as Mr RUDYERD
seems to have executed his task with much judgment, carefully rejecting
all architectural decoration, as unsuitable for such a situation, and
directing his attention to the formation of a tower which should offer
the least resistance to the waves. The height of the tower, which was
of a conical form and constructed of timber, was 92 feet, including the
lantern; and the diameter at the base, which was a little above the
level of high water, was 23 feet.
The advantages of a light on the Eddystone having been so long known
and acknowledged by seamen, no time was permitted to elapse before
active measures were taken for its restoration; and SMEATON, to whom
application was made for advice on the subject, recommended the
exclusive use of stone as the material, which, both from its weight
and other qualities, he considered most suitable for the situation.
On the 5th of April 1756, SMEATON first landed on the Rock and made
arrangements for erecting a Lighthouse of stone and preparing the
foundations, by cutting the surface of the rock into regular horizontal
benches, into which the stones were carefully dovetailed or notched.
The first stone was laid on 12th June 1757 and the last on the 24th
of August 1759. The Tower measures 68 feet in height and 26 feet in
diameter at the level of the first entire course; and the diameter
under the cornice is 15 feet. The first 12 feet of the Tower form
a solid mass of masonry; and the stones of which it is composed
are united by means of stone joggles, dovetailed joints, and oaken
treenails. It is remarkable that SMEATON should have adopted an arched
form for the floors of his building, instead of employing the floors
as tie-walls formed of dovetailed stones. To counteract the injurious
tendency of the outward thrust of those arched floors, he had recourse
to the ingenious expedient of laying, in circular trenches or grooves
cut in the stones which form the outside casing, tie-belts of chain,
which were heated before being set in the grooves by means of an
application of hot lead and became tight in cooling, after they were
fixed in the wall. The light was exhibited on the 16th October 1759;
but such was the state of lighthouse apparatus in Britain at that
period, that a feeble light from tallow candles was all that decorated
this noble structure. In 1807, when the property of this lighthouse
again came into the hands of the Trinity-House, at the expiry of a long
lease, Argand burners, and paraboloïdal reflectors of silvered copper,
were substituted for the chandelier of candles.
~Bell Rock.~
The dangerous reef called the Inch Cape, or Bell Rock, so long a
terror to mariners, was well known to the earliest navigators of
Scotland. Its dangers were so generally acknowledged, that the Abbots
of Aberbrothwick, from which the Rock is distant about twelve miles,
caused a float to be fixed upon the Rock with a bell attached to it,
which being swung by the motion of the waves, served by its tolling to
warn the mariner of his approach to the reef. From this circumstance,
which formed the groundwork of SOUTHEY’S striking ballad of Sir Ralph
the Rover, the Rock is said to have derived its name. Amongst the many
losses which occurred on the Bell Rock in modern times, one of the
most remarkable is that of the _York_, _seventy-four_, with all her
crew, part of the wreck having been afterwards found on the Rock and
part having come ashore on the neighbouring coast. During the survey
of the Rock also, several instances were discovered of the extent
of loss which this reef had occasioned; and many articles of ships’
furnishings were picked up on it, as well as various coins, a bayonet,
a silver shoe-buckle, and many other small objects. Impressed with the
great importance of some guide for the Bell Rock, Captain Brodie, R.N.,
set a small subscription on foot and erected a beacon of spars on the
Rock, which, however, was soon destroyed by the sea. He afterwards
constructed a second beacon, which soon shared the same fate. It was
not, therefore, until 1802, when the Commissioners of Northern Lights
brought a bill into Parliament for power to erect a lighthouse on it,
that any efficient measures were contemplated for the protection of
seamen from this Rock, which, being covered at every spring-tide to the
depth of from twelve to sixteen feet, and lying right in the fairway
to the Friths of Forth and Tay, had been the occasion of much loss
both of property and life. In 1806, the bill passed into a law; and
various ingenious plans were suggested for overcoming the difficulties
which were apprehended, in erecting a lighthouse on a rock twelve miles
from land, and covered to the depth of twelve feet by the tide. But
the suggestion of Mr ROBERT STEVENSON, the Engineer to the Lighthouse
Board, after being submitted to the late Mr RENNIE, was at length
adopted; and it was determined to construct a tower of masonry, on the
principle of the Eddystone. On the 17th of August 1807, Mr STEVENSON
accordingly landed with his workmen and commenced the work by preparing
the Rock to receive the supports of a temporary pyramid of timber,
on which a barrack-house for the reception of the workmen (similar
to that which has already been described in a preceding part of this
volume) was to be placed; and during this operation, much hazard was
often incurred in transporting the men from the Rock, which was only
dry for a few hours at spring-tides, to the vessel which lay moored off
it. The lowest floor of this temporary erection, in which the mortar
for the building was prepared, was often broken up and removed by the
force of the sea. The foundation for the tower having been excavated,
the first stone was laid on the 10th July 1808, at the depth of sixteen
feet below the high water of spring-tides; and at the end of the second
season, the building was five feet six inches above the lowest part of
the foundation. The third season’s operations terminated by finishing
the solid part of the structure, which is thirty feet in height; and
the whole of the masonry was completed in October 1810. The light was
first exhibited to the public on the night of the 1st of February 1811.
The difficulties and hazards of this work were chiefly caused by the
short time during which the Rock was accessible between the ebbing and
flowing tides; and amongst the many eventful incidents which render
the history of this work interesting, was the narrow escape which the
Engineer and thirty-one persons made from being drowned, by the rising
of the tide upon the Rock, before a boat came to their assistance, at
a time when the attending vessel had broken adrift. This circumstance
occurred before the Barrack-house was erected, and is narrated by Mr
STEVENSON, in his Account of the work, published at the expense of
the Lighthouse Board in 1824, to which I would refer for more minute
information on the subject of this work and the other lighthouses on
the coast of Scotland.
The Bell Rock Tower is 100 feet in height, 42 feet in diameter at the
base, and 15 at the top. The door is 30 feet from the base and the
ascent is by a massive copper ladder. The apartments, including the
light-room, are six in number. The light is a _revolving red and white
light_; and is produced by the revolution of a frame containing sixteen
Argand lamps, placed in the foci of paraboloïdal mirrors, arranged on
a quadrangular frame, whose alternate faces have shades of red glass
placed before the reflectors, so that a red and white light is shewn
successively. The machinery, which causes the revolution of the frame
containing the lamps, is also applied to tolling two large bells,
to give warning to the mariner of his approach to the Rock in foggy
weather. The erection of the Bell Rock Lighthouse cost L.61,331 : 9 : 2.
~Carlingford.~
The most remarkable Lighthouse on the coast of Ireland is that of
Carlingford, near Cranfield Point, at the entrance of Carlingford
Lough. It was built according to the design of Mr GEORGE HALPIN, the
Inspector of the Irish Lights; and was a work of an arduous nature,
being founded 12 feet below the level of high-water, on the Hawlbowling
Rock, which lies about two miles off Cranfield Point. The figure of the
Tower is that of a frustum of a cone, 111 feet in height, and 48 in
diameter at the base. The light, which is fixed, is from oil burned in
Argand lamps, placed in the foci of paraboloïdal mirrors. It was first
exhibited on the night of December 20, 1830.
~Iron Lighthouses.~
There are various other Lighthouses, which, in themselves, are
sufficiently deserving of a separate notice, were it not that they
have more or less something in common with those already described,
which are unquestionably the most remarkable edifices of the kind.
The first design for an Iron Lighthouse, is that by my Father for the
Bell Rock, in the year 1800. The invention of Mr MITCHELL of Belfast,
for applying the principle of the screw to the erection of Lighthouses
on soft foundations, deserves a longer notice than is consistent with
the nature of these notes. It must therefore be sufficient to say,
that the principal Lighthouses on this plan (those of the Maplin,
Fleetwood, and Belfast Lough) consist of piles or of hollow pillars of
cast-iron, grouped together in the form of a truncated pyramid, and
closely resembling, in the general arrangement of their parts, the
Beacon shewn in Plate XXX., and that erected on the Carr Rock in 1821.
The lower end of each pillar is furnished with a flat screw or worm
and a sharp point, which is screwed into the sand, clay, or gravel,
or other soft subsoil. Mr ALEXANDER GORDON of London also fitted up
a Lighthouse, composed of cast-iron plates, which was erected at
Morant, in the West Indies, a style of building in itself by no means
eligible, and which seems suitable only where stone cannot be easily
obtained, or conveniently applied. Both those plans (except in so far
as the screw is concerned, which is indeed the distinguishing feature
of Mr MITCHELL’S ingenious plan) are to be found in one of my Father’s
designs for the Bell Rock Lighthouse (see his Account, at Plate VII.,
figs. 2, 3, 4, and 5, and pp. 499, 500). Dr POTTS has also invented a
method of driving piles by means of atmospheric pressure, which has
been used at the South Galliper Beacon, on the Goodwin Sands.
~Early modes of Illumination.~
Having thus hastily described the most interesting and celebrated
Lighthouses, I proceed to the proper object of these Notes, which are
chiefly intended to make known the various methods now in use for the
illumination of Lighthouses. There can be little doubt, that down
to a very late period, the only mode of illumination adopted in the
Lighthouses, even of the most civilized nations of Europe, was the
combustion of wood or coal in _chauffers_, on the tops of high towers
or hills. It consists with the personal knowledge of many persons now
living, that the Isle of May Light, in the Frith of Forth, previous to
its being assumed by the Commissioners of the Northern Lights in 1786,
was of that kind; and, even in England, the art of illumination had
made so little progress, that the magnificent Tower of the Eddystone,
for about forty years after it came from the hands of SMEATON, could
boast of no better Light than that derived from a few miserable
tallow candles. Such methods were most imperfect, not only in point of
efficiency and power, but also as respects the distinction of one light
from another, an object which, on a difficult and rugged coast, may be
considered as of almost equal importance with the distance at which the
Light can be seen.
~Flame.~
Solid substances which remain so throughout their combustion, are
only luminous at their own surface, and exhibit phenomena, such as
the dull red heat of iron, or of most kinds of pit-coal, and are
therefore more suited for the purpose of producing heat than light.
But by using substances which are formed into inflammable vapours,
at a temperature below that which is required for the ignition of
the substances themselves, gas is obtained and _flame_ is produced.
Much light is thus evolved at a comparatively low temperature. The
gas necessarily rises _above_ the combustible substance from which it
is evolved, owing to its being formed at a temperature considerably
higher than that of the surrounding air, than which it is necessarily
rarer. Of this description are the flames obtained by the burning of
the various oils, which are generally employed in the illumination of
lighthouses. In the combustion of oil, wicks of some fibrous substance,
such as cotton, are used, into which the oil ascends by capillary
action, and being supplied in very thin films, is easily volatilized
into vapour or gas by the heat of the burning wick. The gas of pit-coal
has been occasionally used in lighthouses; it is conveyed in tubes to
the burners, in the same manner as when employed for domestic purposes.
There are certain advantages, more especially in dioptric lights,
where there is only one large central flame, which would render the
use of gas desirable. The form of the flame, which is an object of
considerable importance, would thus be rendered less variable, and
could be more easily regulated, and the inconvenience of the clock-work
of the lamp would be wholly avoided. But it is obvious, that gas is
by no means suitable for the majority of lighthouses, their distant
situation and generally difficult access rendering the transport
of large quantities of coal expensive and uncertain; whilst in many
of them there is no means of erecting the apparatus necessary for
manufacturing gas. There are other considerations which must induce us
to pause before adopting gas as the fuel of lighthouses; for, however
much the risk of accident may be diminished in the present day, it
still forms a question, which ought not to be hastily decided, how
far we should be justified in running even the most remote risk of
explosion in establishments such as lighthouses, whose sudden failure
might involve consequences of the most fatal description, and whose
situation is often such, that their re-establishment must be a work of
great expense and time. Gas is, besides, far from being suitable in
catoptric lights, to which, in many cases (especially when the frame is
moveable, as in revolving lights), it could not be easily applied. The
oil most generally employed in the Lighthouses of England is the sperm
oil of commerce, which is obtained from the South Sea whale (_Physeter
macrocephalus_). In France, the colza oil, which is expressed from the
seed of a species of wild cabbage (_Brassica oleracea colza_), and the
olive oil are chiefly used; and a species of the former has lately
been successfully introduced into the Lighthouses of Great Britain.
Of all these oils, the purified sperm oil has hitherto been generally
considered the most advantageous for lighthouse purposes; but there is
every reason for anticipating that the late adoption of the colza oil
in many of the British Lights, on the suggestion of Mr JOSEPH HUME,
M.P., while chairman of a select committee of the House of Commons
on Lighthouses, will lead to an important saving, as its combustion
produces an equal quantity of light at somewhat more than one-half of
the expense for spermaceti oil. Careful trials have been made of this
oil; and on the 10th of March 1847, I was enabled to report the results
to the Commissioners of Northern Lighthouses in the following terms:
“1. The colza oil possesses the advantage of remaining fluid at
temperatures which thicken the spermaceti oil so that it requires the
application of the frost lamp.
“2. It appears, from pretty careful photometrical measurements of
various kinds, that the light derived from the colza oil is, in point
of intensity, a little superior to that derived from the spermaceti
oil, being in the ratio of 1·056 to 1.
“3. The colza oil burns both in the Fresnel lamp and the single Argand
burner with a thick wick during seventeen hours without requiring
any coaling of the wick or any adjustment of the damper; and the
flame seems to be more steady and free from flickering than that from
spermaceti oil.
“4. There seems (most probably owing to the greater steadiness of the
flame) to be less breakage of glass chimneys with the colza than with
the spermaceti oil.
“5. The consumption of oil, in so far as that can be ascertained
during so short a period of trial, seems in the Fresnel lamp to be 121
for colza, and 114 for spermaceti; while in the common Argand, the
consumption appears to be 910 for colza, and 902 for spermaceti.
“6. If we assume the means of these numbers, 515 for colza, and 508 for
spermaceti, as representing the relative expenditure of these oils, and
if the price of colza be 3s. 9d., while that of spermaceti is 6s. 9d.
per imperial gallon, we shall have a saving in the ratio of 1 to 1·755,
which, at the present rate of supply for the Northern Lights, would
give a saving of about L.3266 per annum.
“Of these conclusions, the three last may be considered as more or less
conjectural, being founded on data derived from too short a trial;
but the striking agreement of the results obtained at the six lights
in which the experiments were made, tends in some measure to supply
the place of a longer period of trial; and I have no hesitation,
therefore, in recommending the Board at once to introduce the use of
the colza oil into all the dioptric lights, except that of Skerryvore,
where some special reasons induce me to defer the change for another
season. In the catoptric lights, the only reason for not making an
equally extensive trial is the necessity for renewing all the burners,
which require to be so constructed as to receive thick wicks of brown
cotton; and it may perhaps be considered prudent to proceed with
some caution in changing the apparatus, so as to suit it for burning
a patent oil, the circumstances attending the regular and extensive
supply of which are not yet fully known. I may remark, that I have
burnt the colza oil in the solar lamp alluded to in my last report; but
I disapprove of it as tending to elongate the flame vertically, and
thus to decrease its horizontal volume. The elongated form of flame
increases the divergence vertically where the light is lost, and so far
circumscribes its horizontal range where it is most required. I have
therefore substituted the thick wick burner for the solar lamp, whereby
an equally complete combustion is obtained, and the proper form of the
flame is at the same time preserved.”[36]
[36] Since the above report was written, the price of colza oil has
risen; and other circumstances have occurred to justify the caution
as to the universal adoption of that oil.
* * * * *
~Drummond and Voltaic Lights.~
The application of the Drummond and Voltaic lights[37] to lighthouse
purposes is, owing to their prodigious intensity, a very desirable
consummation; but it is surrounded by so many practical difficulties
that, in the present state of our knowledge, it may safely be
pronounced unattainable. The uncertainty which attends the exhibition
of both these lights, is of itself a sufficient reason for coming to
this conclusion. But other reasons unhappily are not wanting. The
smallness of the flame renders them wholly inapplicable to dioptric
instruments, which require a great body of flame in order to produce a
degree of divergence sufficient to render the duration of the flash in
revolving lights long enough to answer the purpose of the mariner. M.
FRESNEL made some experiments on the application of the Drummond light
to dioptric instruments, which completely demonstrate their unfitness
for this combination. He found that the light obtained by placing
it in the focus of a great annular lens was much more intense than
that produced by the great lamp and lens; but the divergence did not
exceed 30′; so that, in a revolution like that of the Corduan Light,
the flashes would last only 1¹⁄₃ second, and would not, therefore, be
seen in such a manner as to suit the practical purposes of a revolving
light. The great cylindric refractor used in fixed lights of the first
order, was also tried with the Drummond light in its focus; but it
gave coloured spectra at the top and bottom, and only a small bar of
white light was transmitted from the centre of the instrument. The
same deficiency of divergence completely unfits the combination of the
Drummond light with the reflector for the purposes of a fixed light,
and even if this cause did not operate against its application in
revolving lights on the catoptric plan, the supply of the gases, which
is attended with almost insurmountable difficulties, would, in any
case, render the maintenance of the light precarious and uncertain in
the last degree.
[37] The Drummond light is produced by the ignition or combustion of
a ball of lime (³⁄₈ inch diameter) in the united flames of hydrogen
and oxygen gases, and is equal to about 264 flames of an ordinary
Argand Lamp with the best Spermaceti oil. It derives its name from
the late LIEUT. DRUMMOND, R. E., who first applied it in the focus of
a paraboloïd for geodetical purposes, and afterwards proposed it for
Lighthouses. (See his Account of the Light in the Phil. Trans. for
1826, p. 324, and for 1830, p. 383.) The Voltaic light is obtained
by passing a stream of Voltaic electricity from a powerful battery
between two _charcoal points_, the distance between which requires
great nicety of adjustment, and is the chief circumstance which
influences the stability and the permanency of the light. The Voltaic
light greatly exceeds the Drummond light in intensity, as ascertained
by actual comparison of their effects; but the ratio of their power
has not been accurately determined. It was first exhibited in the
focus of a reflector by Mr JAMES GARDNER, formerly engaged in the
Ordnance Survey of Great Britain.
~Mr Gurney’s Lamp.~
In 1835, Mr GURNEY proposed the combination of a current of oxygen with
the flame of oil, in order to obtain a powerful light of sufficient
size to produce the divergence required for the illumination of
lighthouses. The Trinity-House of London entertained the proposal,
and made some experiments on this important subject; but the plan was
finally rejected as disadvantageous in practice.
~Argand Burners.~
Until the invention by ARGAND (about the year 1784), of the lamp with a
double current of air, the art of illumination seems to have received
no improvement, and to have occupied very little attention from the
time of CARDAN, or at all events of Dr HOOK, who, about the year 1677,
in a monograph entitled “Lampas,” made some important observations on
the constitution of _flame_, so as to make one wonder that he should
have stopped short of the discoveries of later inventors. Before
ARGAND’S time, every wick consisted of a solid cord, whose flame was
fed only by the current of air on its outside; and the consequence of
this arrangement is, that the stream of vapour or smoke, especially
from the centre of thick wicks, escapes unburnt, because, before it
reaches the height at which the combustion of the central stream
can take place, its temperature has become too low to admit of its
ignition.[38] The chief improvements which had been made, consisted
in varying the level of the oil in the cistern, or in attempts to
render that level constant, by mechanical means, and in lessening the
thickness of the wick, by spreading its substance into a flat form,
thus reducing the stream of gas which escapes from the centre of a
thick cylindric wick without being burnt, and thereby causing a more
complete combustion, and producing less smoke and a whiter flame. To
ARGAND belongs the great merit of having first formed the wick into a
hollow cylinder, thus supplying the flame with two currents of air, one
of which, as in the case of the solid wick, envelopes the flame, and
the other, passing through the centre of the wick, is enveloped by the
flame itself. He also added a chimney, which served to defend the flame
from irregular draughts of air, and to regulate the proportion between
the velocities of the currents of air and the stream of gas. This was
indeed a most important step in the art of illumination, and causes
the great difference between the incomplete combustion, which, owing
chiefly, as we have seen, to a defect in the supply of air, always
takes place with a solid wick (from which much unburnt gas escapes in
the form of smoke), and that more perfect combustion in which passage
is given for a free current of air through the centre of the wick. The
invention of ARGAND came nearly perfect from his hands; and but a few
slight modifications of his original arrangement have been introduced.
The Argand burner consists of two concentric tubes or cylinders,
separated by a small annular space, which is shut at the bottom, and
communicates by a pipe with the oil fountain, whose level ought to
be a little _below_ the level of the upper edge of the cylinders. In
this annular space, partly filled with oil from the fountain, stands
a cylindric wick of cotton, loosely wove, into which the oil rises
freely by capillary action. The wick has its lower edge fixed to a
metallic ferule or ring, called a wickholder, which (by means of a
peculiar arrangement, to be afterwards described) gives the power of
raising or depressing the wick to any convenient level with regard to
the burner. A cylinder of glass, of greater diameter than the burner,
rests on a gallery or ring which hangs from the burner and surrounds
it. This glass cylinder, or chimney as it is generally called, should
stand vertically with its axis coincident with that of the burner
itself. The effect of this arrangement is obvious, and has already
in part been indicated. The flame is thus necessarily bounded on all
sides by two conical concentric surfaces, one external and concave, and
the other internal and convex, both of which receive a free current
of air. The flame is therefore very thin in every direction; and, as
a consequence of the mutual radiation of its different parts on each
other, it is throughout its entire surface of more equal temperature
than can ever be attained in the thick solid wick or the narrow flat
one. The glass cylinder also increases the force of the two currents
which pass outside and inside of the flame; and the union of so many
favourable circumstances produces a greater amount of pure light than
has yet been obtained by any other method. The contraction of the glass
chimney (known by the technical name of the _shoulder_) at a point a
little above the level of the wick, tends to direct the current of air
inwards on the flame, thereby causing a more perfect combustion and the
evolution of more light.
[38] That the form of a flame is necessarily conoidal, and that
its height is determined by the relation subsisting between its
diameter and the continually varying velocities of the currents
of gas and air, may be easily shewn; and the combustion of each
annular film of the stream of gas from the wick can take place only
at a level determined by, and continually varying with, the ratio
of the velocities of the streams of gas and air. I am unwilling to
offer this explanation in my own words, when those of M. Peclet,
in his excellent work, Traité de l’Eclairage, are at hand,--“Let
us conceive,” says he, “a very thin film or layer of inflammable
gas placed horizontally, and which rises into the air parallel to
itself, with a uniform motion. We shall suppose that it cannot be
burnt, except at its circumference, and that the top and bottom of
the film are, by some means, preserved from combustion (they are so
preserved in ordinary flames, by the films which precede and follow
them). If the circumference is at a high enough temperature it will
burn; at each instant the film or layer of air, which has assisted
the combustion and also the products of that combustion, being very
hot, will rise very rapidly, and will make room for other layers or
films of air, which will rise in their turn; and as the diameter
of the film of gas is continually diminishing, it is obvious that
its combustion will offer the appearance of a series of circles
continually growing smaller, and terminating at length in a point.
If we trace in thought the series of circles which the combustion
has successively developed, we shall form a cone whose length will
depend on the ratio of the velocities of the films of gas and of
air which escape after combustion. If, for example, the velocity
of the current of air were very great, compared to the velocity of
the cylinder of gas, the entire combustion would take place, while
the film of gas passes over a very small space; and the cone formed
by the succession of luminous circles would, consequently, be very
short. If, on the contrary, there were but a very small difference
between these velocities, the luminous circles would only appear
at considerable intervals from each other; for the air which had
served for combustion, being unable to feed it longer, the surface
of the cylinder could not become luminous until the difference of
velocity had freed it from the air which had served for the preceding
combustion. If, then, we imagine a set of similar films succeeding
each other, each of them would give rise to the same series of
coloured rings; and as there would be a film in each section of the
cone in a state of combustion at the same instant of time, the cone
would, of course, appear luminous throughout its height.”--Peclet,
_Traité de l’Eclairage_, p. 51.
Great as the improvement of ARGAND undoubtedly was, the value of the
lamp alone as a means for the illumination of lighthouses must be
regarded as comparatively small. The primary object of a lighthouse
is to give early notice to the mariner of his approach to the coast,
and it is therefore necessary that the light be of such a kind that it
may be seen at a great distance. Every one is practically acquainted
with the fact that the rays proceed in all directions from a luminous
body in straight lines; and if we could obtain a ball equally luminous
in every part of its surface, it would give an equal share of light
to every part of the inner surface of a hollow sphere, whose centre
coincided with the centre of the ball. Again, if an opaque body were
placed between the luminous ball and the hollow sphere, the part
opposite that body would be deprived of the light by the interception
of the rays, and no light would emerge from a hole bored in that
part of the surface of the hollow sphere. The bearing of these facts
is obvious; and no one can fail to perceive that in the case of a
lighthouse illuminated by a single unassisted burner, a seaman could
only receive the benefit of that small portion of light which emerges
from the lamp in a line joining his eye and the centre of the flame.
The other rays would be occupied partly in making the light visible
in other parts of the horizon, and but a very small portion of them
would be usefully employed for that purpose, while all the rest would
be lost by escaping upwards into the sky, or downwards below the plane
in which seamen can see a lighthouse. This state of matters would be
little improved by increasing the number of burners, as the effective
part of the light would only be augmented by the addition of an equally
trifling portion of light from each burner. The small pencils of rays
thus meeting at the eye of a distant observer, would form a very minute
fraction of the whole quantity of light uselessly escaping above and
below the horizon, and also at the back of each flame; and the wasteful
expenditure of light would be enormous. By such a method no practically
efficient sea-light could ever be obtained.
CATOPTRIC[39] SYSTEM OF LIGHTS.
[39] From the Greek κατοπτρον, a _mirror_; a compound of κατα,
_opposite to_, and ὂπτομαι, _I see_.
For those defects a simple remedy is found in the well known power
possessed by most bodies, of _reflecting_ or throwing back from them
the light which falls upon them. This property is not possessed by all
reflecting bodies in an equal degree, some absorbing more and some
less of the incident light. Perhaps the earliest attempts to apply
this property as a corrective for the direction of the rays from a
Lighthouse, would be confined to placing plane mirrors behind each
lamp; yet this would prove but a partial remedy, as it would still
leave the greater part of the light to stray above and below the proper
direction. Hollow mirrors of a spherical form might next be tried;
and if properly placed with reference to the flame, would constitute
a very great improvement in lighthouse illumination. But those steps
in the march of improvement are more imaginary than real; and I am
not aware of any well authenticated records of such gradual attempts
having preceded the adoption of the right mode of applying reflection
as a means of rectifying the direction of the rays emerging from a
lighthouse. There is, on the contrary, distinct evidence that the
impulse given by ARGAND’S invention, led to an immediate adoption of
the most perfect form of reflecting instruments.
~Application of Paraboloïdal Mirrors into Lighthouses.~
The name of the inventor of paraboloïdal mirrors and the date of their
first application to Lighthouses, have not been accurately ascertained.
The earliest notice which I have been able to find, is that by Mr
WILLIAM HUTCHINSON, the pious and intelligent author of a quarto volume
on “Practical Seamanship” (published at Liverpool in 1791), who notices
(at p. 93) the erection of the four lights at Bidstone and Hoylake, in
the year 1763, and describes large parabolic moulds, fashioned of wood
and lined with mirror-glass, and smaller ones of polished tin-plate, as
in use in those Lighthouses. Mr HUTCHINSON seems to have understood the
nature, properties, and defects of the instruments which he describes,
and has shewn a good acquaintance with many of the most important
circumstances to be attended to in the illumination of Lighthouses.
Many claims to inventions rest on more slender grounds than might be
found in Mr HUTCHINSON’S book for concluding him to have first invented
the paraboloïdal mirror and applied it to use in a Lighthouse;[40]
but, in the absence of any statement as to the date when the mirrors
were really adopted, the merit of the improvement must, in justice, be
awarded to others.
[40] Mr HUTCHINSON seems also (“Practical Seamanship,” p. 198) to
have tried speculum metal as a material for Lighthouse reflectors.
M. TEULERE, a member of the Royal Corps of Engineers of Bridges and
Roads in France, is, by some, considered the first who hinted at the
advantages of paraboloïdal reflectors; and he is said, in a memoir
dated the 26th June 1783, to have proposed their combination with
Argand lamps, ranged on a revolving frame, for the Corduan Lighthouse.
Whatever foundation there may be for the claim of M. TEULERE, certain
it is that this plan was actually carried into effect at Corduan, under
the directions of the CHEVALIER BORDA; and to him is generally awarded
the merit of having conceived the idea of applying paraboloïdal mirrors
to lighthouses. These were most important steps in the improvement
of lighthouses, as not only the power of the lights was thus greatly
increased, but the introduction of a revolving frame proved a valuable
source of differences in the appearance of lights, and, in this way,
has since been the means of greatly extending their utility. The exact
date of the change on the light of the Corduan is not known; but as it
was made by LENOIR, the same young artist to whom BORDA, about the year
1780, entrusted the construction of his reflecting circle, it has been
conjectured by some that the improvement of the light was made about
the same time. The reflectors were formed of sheet-copper, plated with
silver, and had a double ordinate of 31 French inches. It was not long
before these improvements were adopted in England, by the Trinity-House
of London, who sent a deputation to France to inquire into their
nature. In Scotland, one of the first acts of the Northern Lights Board
in 1786, was to substitute reflectors in the room of the coal-light
then in use at the Isle of May in the Frith of Forth, which, along with
the light on the Cumbrae Isle in the Frith of Clyde, had, till that
period, been the only beacons on the Scotch coast. The first reflectors
employed in Scotland were formed of _facets_ of mirror glass, placed in
hollow paraboloïdal moulds of plaster, according to the designs of the
late Mr THOMAS SMITH, the Engineer of the Board, who (as appears from
the article _Reflector_, in the Supplement to the third edition of the
Encyclopædia Britannica) was not aware of what had been done in France,
and had himself conceived the idea of this combination. The same system
was also adopted in Ireland; and in time, variously modified, it became
general wherever lighthouses are known.
~Reflection.~
To enable us to enter on the subject of the proper forms of reflectors,
we must glance very briefly at the _laws of reflection_. Those laws
are two in number. _1st_, The ray which falls on a reflecting surface,
called the _incident_ ray, and the ray which leaves the reflector,
called the _reflected_ ray, are always in one _plane_, which plane is
perpendicular to the _reflecting surface_. _2d_, The angle which the
_reflected_ ray makes with the reflector is always equal to the angle
which the _incident_ ray makes with it, or, in other words, the angle
of _incidence_ is equal to the angle of _reflection_.[41]
[41] This will be more readily understood by referring to the
accompanying figure (No. 22), in which CDEF is the reflecting
surface; GHOKI the plane of reflection perpendicular to that surface;
BO a line perpendicular or _normal_ to the surface CDEF; and AO the
incident ray. Then if in the plane GHOKI, the angle BOI be made equal
to AOB, OA′ is the reflected ray; BOG is then the angle of incidence;
and BOI the angle of reflection. GOH and IOK, which are the
complements of those angles, are, indeed, more strictly speaking, the
angles of incidence and reflection; but in cases where the reflecting
surface is curved, it is more convenient to refer the angles to the
normal BO.
[Illustration: Fig. 22.]
It would lead to prolixity altogether superfluous in this place,
to explain, in a rigorous manner, the effects produced by various
reflecting surfaces on the direction of the rays incident on them; as
any one who comprehends the laws of reflection just enumerated, may
easily satisfy himself of the following truths: _1st_, That a plane
mirror makes no change on the divergence of the rays, but merely causes
them to emerge from its surface in the same direction as if they had
come from a point as much behind the mirror as the luminous body lies
in front of it. _2d_, A convex reflecting surface increases divergence,
and disperses the rays in the same manner as if they had come directly
from a point behind it, whose distance from the mirror increases with
the distance of the luminous body from its surface, and diminishes
with the degree of convexity of the mirror. _3d_, A concave surface
diminishes the divergence of the rays incident upon it from a point
between the surface and its centre of curvature; the distance of the
point in which the reflected rays converge diminishing as the distance
of the radiant point or the concavity of the mirror is increased. It is
obvious, therefore, that concave mirrors are those which are required
to produce a correction of the path of the rays, so as to apply them to
most advantage in a lighthouse, the object to be attained being that
of throwing the greatest amount of light towards given points in the
horizon, and collecting the divergent rays, which, as we have already
seen, are scattered above and below it.
To simplify our view of this matter, I shall, in the first place,
suppose that the object to be attained is to throw the whole rays of a
single lamp, with an infinitely small flame, to a given mathematical
point at a moderate distance; and, as this is a case which can hardly
occur in the practice of Lighthouse illumination, I content myself
with observing that this object may be attained _approximately_ by
placing the lamp in front of a spherical mirror at any distance
greater than half the radius of the curve surface, or _accurately_
by placing it in one focus of an elliptical mirror; in all those
cases the rays would meet in the opposite, or, as they are termed,
_conjugate foci_. Let us next suppose that our object is to illuminate,
by means of a mathematical point of light, a small circular space on
the horizon equal in diameter to the mirror employed; this object will
be rigorously attained only by placing the light in the focus of a
paraboloïdal reflector. The same object may be approximately attained
by placing the light in a spherical mirror, at a point _half-way_
between the centre of curvature and the surface of the mirror, provided
the surface of the mirror shall subtend only a small angle at the
centre of curvature. The paraboloïdal mirror, on the contrary, has the
property of converging to the focus parallel rays falling upon every
point of its surface, however extended it may be.
~Paraboloïdal Mirrors.~
Any one practically acquainted with this subject, must at once
perceive that the paraboloïdal mirror completely fulfils one great
object required in a lighthouse; and to render this more obvious to
the general reader, I shall, for the present, confine my remarks to
the case of those lighthouses which exhibit to the mariner in every
part of the horizon, pencils of light at certain intervals of time,
separated by periods of darkness, reserving the consideration of Lights
which are continually in sight all round the horizon or over a given
portion of it, for a subsequent part of these Notes. In doing this,
I am aware that I may appear to be departing from the strict order
of investigation, by suddenly introducing the idea of motion; but a
little consideration will, I think, satisfy the reader that this is,
in reality, the more convenient mode of treating the subject. Let us
suppose, then, that our object is to give occasional flashes of light,
separated by intervals of darkness, to seamen in various azimuths and
at various distances from a lighthouse. It is obvious that this may
be most efficiently done by causing concave mirrors, which collect
the rays from lamps placed in them and thereby increase the light in
front of the mirror, to revolve round a vertical axis with a velocity
suited to produce the required number of flashes in a given time. The
paraboloïdal mirror is best adapted for producing this effect, for
the following reasons: _1st_, Because it alone produces a rigorous
parallelism of all rays proceeding from its focus, and falling upon
any point of its surface, however distant the point of reflection
from that focus, or however far _in front_ of it. _2d_, Because it
therefore embraces in its action the greatest number of the whole
rays coming from the focus, and, _cæteris paribus_, will produce the
strongest light. _3d_, Because the _theoretical_ object to be attained
is to make those flashes equally powerful at any distance, an effect
which would be rigorously fulfilled by placing an infinitely small
flame in a perfect paraboloïdal mirror. And, _4th_, Because, although
absolute equality of luminousness at any distance is not attainable,
and, in practice, is inconsistent with other conditions required in a
useful light, we still, by using the parabolic mirror, make the nearest
approach to parallelism of the reflected rays, and consequently obtain
the strongest light which is consistent with a due regard to a certain
duration of the flash on the eye of a distant observer, which is
measured by the angle of the luminous cone projected to the horizon.
Having thus so far anticipated what some might think would more
naturally have occurred in a subsequent part of these Notes, I return
to a more detailed consideration of the parabola itself, and its
product, the paraboloïdal mirror. I content myself, however, with
describing the parabola, by that property which peculiarly adapts it
to the purposes of a lighthouse. The parabola, then, is a curve of the
second order, obtained by cutting a cone in a plane parallel to one
side, which possesses this remarkable property, _that a line drawn from
the focus to any point in the curve, makes, with a tangent at that
point, an angle equal to that which a line parallel to the axis of the
curve makes with that tangent_.[42]
[42] See third corollary to Proposition III. of Wallace’s Conic
Sections, which shews that a tangent to the parabola makes equal
angles with the diameter which passes through the point of contact
and a straight line drawn from that point to the focus. The curve
may be traced in two different ways, both dependent on the property,
_that the distance of any point in the parabola from the focus is
equal to its distance from the directrix_.
To draw the curve mechanically (fig. 23), let F be the focus, MF
the focal distance (chosen at pleasure according to rules which I
shall afterwards notice), KMX is the axis, and AB the directrix (the
dotted line _f_ F _e_, bounded by the curve at either end, would then
be the _parameter_ or _latus rectum_). Place the edge of the straight
ruler AKHB along the directrix; and let LHB be a square ruler which
may slide along the fixed ruler AKHB, so that the edge HL may be
constantly perpendicular to AB, or parallel to MX, the axis; let LDF
be a string equal in length to HL, and having one end fixed in F, and
the other at L, a point in the sliding square. Then if the string be
stretched by a pencil D, so as to keep the part DL close to the edge
of the square, and if at the same time the square be gently pushed
along the line AB, the point D will be forced to move along the
edge LH of the square, and will trace out a curve which will be the
required parabola. This is obvious from the consideration, that the
string LDF being equal in length to LH, and LD being common to both,
the remainder DF must be equal to the remainder DH, so that the point
which traces the curve being equidistant from the directrix and the
focus must, in terms of the above definition, describe a parabola.
[Illustration: Fig. 23.]
In the second place, the same property, as already stated, furnishes
us with the means of tracing the curve by finding successive points
therein. Draw a line _a b_ perpendicular to the axis OX, and the
position in this line, of a point _p_ through which the curve passes,
is easily found thus: Describe from F the focus as a centre with a
radius equal to the perpendicular distance O _d_ of the line _a b_
from the directrix AB, a circle cutting the line _a b_ in two points
_p_ and _p′_; then both these points are in the curve. By repeating
the same process, any number of points in the curve may be obtained.
[Illustration: Fig. 24.]
Lastly, from the equation to the curve, the length _y_ of any
ordinate may be computed, in terms of _m_ its principal focal
distance, and x its abscissa, by the simple expression,--
_y_ = √(4 _m_ _x_).
It is easy to see, that if this curve revolve about its axis, it will
generate a parabolic conoid, which we may conceive to be concave or
convex, as we please. If the surface be concave, we obtain the mirror
of which we are in search; for every principal section, or that passing
through the axis of such a mirror, will necessarily possess the same
properties as that of the plane curve, and will each have a focus
meeting in one and the same point; the union of all these sections will
therefore form a mirror capable of reflecting, in a direction parallel
to the axis and to each other, all the rays of light which fall on its
surface.
~Divergence of Paraboloïdal Mirrors.~
We have already seen that a perfect paraboloïdal mirror, with a
point of light infinitely small placed in the focus, would project
a beam equally intense at any distance, every transverse section of
which would be of the same superficial extent. In practice, these
conditions can never be rigorously fulfilled. No perfect instrument
can come from the hands of man, and every mirror must of necessity
possess many defects. To obtain a true mathematical point of light
is also impossible; and for the purposes of a lighthouse, it would
be completely useless, as will appear from the following simple
considerations. Let us suppose that a true paraboloïdal mirror, having
a double ordinate or space of two feet, and illuminated by a point
of light, projects a truly cylindric beam of light to the horizon,
and that it revolves horizontally round a vertical axis, with such
a velocity as to cause the beam to pass over the eye of an observer
stationed at the distance of 100 feet in one second of time, and we
shall find that another observer, at a distance of 15 miles from the
mirror, would not see the light at all, although of equal size, because
its velocity at that distance would be so great as only to be present
to his eye for ¹⁄₇₉₂d of a second, a space of time far too short to
make a perceptible impression on the eye of a distant observer. This
is no mere hypothesis unsupported by facts; for I shall have occasion,
in another part of these Notes, to describe certain experiments, by
which it was ascertained that a beam of light emerging from a lens, and
passing over the eye of an observer at 14 miles distance, in a space of
time equal to ¹⁄₁₆₆th of a second, became altogether invisible at that
distance.
For this evil, happily a very simple and efficient remedy may be
found in what may be said to constitute a _theoretical_ defect in
the combination of the Argand burner with the reflector. The burner,
instead of being a mathematical point, has generally a diameter of
about one inch, and a ray proceeding from the edge of the flame to
any point on the surface of the mirror, makes with the line joining
that point and the principal focus an angle which, being repeated by
reflection, gives the effective divergence of _each_ side of the mirror
at that point.[43]
[43] This is easily understood by reference to the accompanying
figure (No. 25.), in which AOB is a central section of a paraboloïdal
mirror.
[Illustration: Fig. 25.]
PF = distance from the focus F to a point in the curve P, and PG a
tangent drawn from P to the surface of the flame at G;
FG = radius of the wick or flame;
and GPF = G′PF′ = divergence of one side of mirror, and consequently
2 GPF = the whole effective divergence of the mirror at that cross
section.
GF
Now sin GPF = --
PF
or the sine of the divergence from each point
Radius of flame.
= -------------------------------------------
Distance from focus to point of reflection.
[Illustration: Fig. 26.]
It is obvious that this quantity which varies _inversely_ with the
distance of the reflecting surface from the focus, is greatest
at the vertex of the curve, and least at the sides or edges of
the paraboloïd. The most useful part of the light, or that which
conduces to the strongest part of the flash in a revolving light,
is that which is derived from the cone of rays which is bounded by
the limits of this _minimum_ divergence; for the faint light which
first reaches the eye of a distant observer, in the revolution of a
reflector, is not that which is reflected by the sides or edges, as
might at first be supposed, but proceeds from the centre. The light,
in fact, gradually increases in power in proportion as additional
rays of reflected light are brought to bear on the observer’s eye,
until, last of all, the extreme edge of the mirror adds its effect.
The light continues in its best state until the opposite limit of
minimum divergence has been reached, when it begins gradually to
decline, receding from the margin of the mirror towards the centre,
and, having at length reached the limit of its maximum divergence,
it finally disappears at the centre. The increase and decline of the
power of a mirror in the course of its movement round the circle of
the lantern, as seen by a distant observer, will, therefore, in all
its different states, be measured by the areas of a series of circles
described from its focus, with radii equal to the distance of the
focus from the point of the mirror which reflects to the observer’s
eye the extreme ray which can reach him in any given position of
the mirror. This will be more easily understood by referring to the
accompanying diagram, Fig. 26, in which _e a e′_ is the principal
section of a paraboloïdal mirror, F its focus, αFA its axis, and FK
the radius of the flame. If the reflector revolve round a vertical
axis at O, an observer placed in front of it (at a distance so great
that the subtense of the mirror’s width would be small enough to
allow us safely to consider the lines drawn from _e_ and _e′_ to
his eye as parallel), would receive the first ray of light in the
direction _a_ D, as reflected at _a_, from a single point on the
edge of the flame (where a tangent to the flame would pass through
_a_); and conversely he would lose the last ray at D′, as reflected
at _a_, from a single point on the opposite margin of the flame; and
hence, as above, the greatest divergence is measured by the angle
which the flame subtends at the vertex a of the mirror, being the
sum of the angles _α_ and _α′_. We shall next suppose the mirror to
move a little, so that the observer may receive at G a ray of light
from some other point in the flame which is reflected at _b_; while
another ray from an opposite point reflected at _b′_ would be seen
in the parallel direction _b′_ G′, thus indicating the boundary of
a circular portion of the mirror _b a b′_, the whole of which would
reflect light to the distant observer’s eye. Again, let us suppose
a ray to come from another part of the flame, and be reflected at
the mirror’s edge _e_ into the direction _e_ H, and another from
the opposite side of the flame to be reflected at its opposite edge
_e′_, into the direction _e′_ G″, and we obtain the full effect of
the whole reflecting surface, which will continue unabated until
the mirror in the course of its revolution shall reflect at _e′_ to
the observer’s eye, a ray from a point in the margin of the flame
(through which a tangent drawn from _e′_ to the flame would pass)
in such a direction, that the angle which it makes with the axis of
the mirror is equal to that subtended by the radius of the flame at
the distance F _e_ or F _e′_. After this the light would recede from
the edges of the mirror in the same gradual manner, until it should
vanish in the direction _a_ D′, which is the opposite limit of the
extreme divergence of the instrument. In the above explanation, I
have confined myself simply to the effects of the outer ring of the
flame, which is the source of divergence; but I need not remind the
reader that every portion of the flame radiates light, which, being
reflected, conduces to the effect. Some rays also are passing from
the opposite sides of the flame through the true focus, so as to be
normally reflected in lines parallel to its axis. The solid lines
in the diagram shew the theoretical reflection of rays proceeding
from F to _b_, _b′_, _e_, _e′_, where they are diverted into the
directions _b_ B, _b′_ B′, _e_ E, and _e′_ E′; and by contrast with
the dotted lines, serve to render more perceptible the path of the
divergent rays which come from the edge of the flame. The Greek
letters indicate the angles of divergence, and point out their
relations to each other on either side of the mirror. The arcs of
greatest and least divergence are marked in the diagram. This subject
will be found treated less directly, but, certainly, more concisely
and neatly, by Mr W. H. BARLOW, in a paper on the Illumination of
Lighthouses in the London Transactions for 1837, p. 218.
It is still more obvious that a perfect paraboloïdal figure, and a
luminous _point_ mathematically true, would render the illumination
of the whole horizon by means of a fixed light _impossible_; and it
is only from the divergence caused by the size of the flame which
is substituted for the _point_, that we are enabled to render even
revolving lights practically useful. But for this aberration, the
slowest revolution in a revolving light would be inconsistent with a
continued observable series, such as the practical seamen could follow,
and would, as we have seen, render the flashes of a revolving light too
transient for any useful purpose; whilst fixed lights, being visible
in the azimuths only in which the mirrors are placed, would, over the
greater part of the distant horizon, be altogether invisible. The size
of the flame, therefore, which is placed in the focus of a paraboloïdal
mirror, when taken in connexion with the form of the mirror itself,
leads to those important modifications in the paths of the rays and the
form of the resultant beam of light, which have rendered the catoptric
system of lights so great a benefit to the benighted seamen.
In order to obtain a mirror capable of producing a given divergence of
the reflected beam, therefore, we must proportion its focal distance to
the diameter of the flame in such a manner, that the sine of _one-half_
of the whole effective divergence of the mirror, may be equal to the
_quotient of the radius of the flame, divided by the distance of a
given point on the surface of the mirror from the focus_. The best
proportions for paraboloïdal mirrors depend on the objects which they
are meant to attain. Those which are intended to give great divergence
to the resultant beams, as in fixed lights, capable of illuminating
the whole horizon at one time, should have a short focal distance;
while those mirrors which are designed to produce a nearer approach to
parallelism (as in the case of revolving lights which illuminate but a
few degrees of the horizon at any one instant of time), will have the
opposite form. Those two objects may, no doubt, be attained with the
same mirror, by increasing or diminishing the size of the burner; but
that is by no means desirable, as any change on the size of a burner,
which is found to be the best in other respects, must be considered as
to some extent disadvantageous.
What I have stated above as to the use of mirrors with a short focal
distance for lights of great divergence, proceeds on the assumption,
that the penumbral portion of the light on each side of the strongest
beam (which is confined within the limits of the least divergence, due
to that portion of the mirror where the focal distance is the greatest)
is to be pressed into service in the illumination of the horizon;
and it is the chief inconvenience which attends the application of
paraboloïdal mirrors to fixed lights, that because it is impracticable
to apply a number of mirrors sufficient to light the whole horizon with
an equally strong light, spaces occur on either side of each reflector
in which the mariner has a light sensibly inferior to that which
illuminates the sector near the axis of each mirror. This will be best
explained by stating the numerical results of the computations of the
divergence of the mirrors used in the Northern Lights for this purpose,
both at the vertex and the sides. In a mirror whose focal distance is
4 inches, and its greatest double ordinate 21 inches, illuminated by
a flame 1 inch in diameter, we find by computation, that the greatest
divergence is 14° 22′, and that the strongest arc of light is only 5°
16′; a difference so great, that while the one may admit of the horizon
being imperfectly illuminated by means of 26 reflectors, the superior
light which would result from confining the duty of each instrument
within the range of its best effect, could only be obtained by the
use of 68 reflectors, and the expenditure of a proportionately great
quantity of oil, not to speak of the great practical difficulty which
would attend the arrangement of so many lamps in a lantern of moderate
size. In revolving lights, the mirrors are not, as in fixed lights,
inconveniently taxed for horizontal divergence, because each portion of
the divergent beam visits successively each point of the horizon. In
this view of the merits of _fixed_ and _revolving_ lights, I should be
disposed to recommend, in any new organisation of lights with parabolic
reflectors, the adoption, in fixed lights, of reflectors with a short
focal distance and small span, so as to admit of many being ranged
around the frame; while in revolving lights, it would be my aim to
approach the largest size of reflector that could be made, so as if
possible to illuminate each face of the revolving frame by means of a
large lamp in a single mirror, with a great focal distance, thereby
diminishing the difference between the divergence of the powerful cone
of rays reflected from the more distant parts of the mirror and that of
the feebler and more diffuse light from its apex.
~Effect of Paraboloïdal mirrors.~
The maximum luminous effect of the reflectors ordinarily employed in
fixed lights, as determined by observation, is generally equal to
about 350 times the effect of the unassisted flame which is placed in
the focus; while for those employed in revolving lights, which are of
larger size, it is valued at 450. This estimate, however, is strictly
applicable only at the distances at which the observations have been
made, as the proportional value of the reflected beam must necessarily
vary with the distance of the observer, agreeably to some law dependent
upon the unequal distribution of the light in the illuminous cone
which proceeds from it. The effect also varies very much in particular
instruments. The ordinary burners used in lighthouses are one inch
in diameter, and the focal distance generally adopted is 4 inches,
so that the extreme divergence of the mirror in the horizontal plane
may be estimated at about 14° 22′; while the divergence of the most
luminous cone is 5° 16′ for the small reflectors, and 4° 25′ for the
larger size. In arranging reflectors on the frame of a fixed light,
however, it is advisable to calculate upon a less amount of effective
divergence, for beyond 11° the light is very feeble; but the difficulty
of placing many mirrors on one frame, and the great expense of oil
required for so many lamps, have generally led to the adoption of the
first valuation of the _effective_ divergence.
~Power of Paraboloïdal Mirrors.~
The measure of the illuminating power of a paraboloïdal mirror may be
estimated as the _quotient of the_ SURFACE _of the circle which cuts it
in the plane of its greatest double ordinate, divided by the surface of
the largest vertical section of the flame, and diminished by the loss
of light in the process of reflection_. This estimate will be found
near enough for all practical purposes; but it is obviously inaccurate,
inasmuch as it overlooks the circumstance of the focal distance of each
portion of the mirror being different, and the consequent increase in
the length of the various trajectories at each point of the surface
as you recede from the axis; and the only correct rule, therefore,
is, to find an imaginary focal distance which must be the radius of a
spherical segment which shall answer the double condition of having
its surface equal to that of the greatest cross section of the mirror,
and of including, at the same time, a number of degrees equal to those
which are brought under the influence of the reflecting action of the
paraboloïd. This subject, however, as I have already hinted, is not
of great practical importance; and I shall not therefore dilate on it
farther, but content myself with saying, that such a line will be found
to be a _mean proportional_ between the _greatest_ and _least_ focal
distances of the mirror.[44] The large mirrors used in the Northern
Lights have about ¹²⁄₁₇ths of the whole light of the lamp incident on
their surface; the rest escapes in the comparatively useless state of
naturally radiating light.
[44] This subject is treated in detail in M. BARLOW’S Paper already
noticed. (London Transactions for 1837, p. 212.)
~Manufacture of Reflectors.~
The reflectors used in the best lighthouses, are made of sheet-copper
plated in the proportion of six ounces of silver to sixteen ounces
of copper. They are moulded to the paraboloïdal form, by a delicate
and laborious process of beating with mallets and hammers of various
forms and materials, and are frequently tested during the operation
by the application of a mould carefully formed. After being brought
to the curve, they are stiffened round the edge by means of a strong
bizzle, and a strap of brass which is attached to it for the purpose of
preventing any accidental alteration of the figure of the reflector.
Polishing powders are then applied, and the instrument receives its
last finish. The details of this manufacture are given in the Appendix.
~Testing of Mirrors.~
Two gauges of brass are employed to test the form of the reflector.
One is for the back, and is used by the workmen during the process of
hammering, and the other is applied to the concave face as a test,
while the mirror is receiving its final polish. It is then tested, by
trying a burner in the focus, and measuring the intensity of the light
at various points of the reflected conical beam. Another test may also
be applied successively to various points in the surface, by masking
the rest of the mirror; but as it proceeds upon the assumption that the
surface of the reflector is perfect, and that we can measure accurately
the distance from a radiant coincident with the focus to the point of
the mirror to be tried, it is in practice almost useless. For such a
trial we must place a screen in the line of the axis of the mirror
at some given distance from it, and ascertain whether the image of a
very small object placed in the conjugate focus, which is due to the
distance of the screen in front of the focus, be reflected to any point
considerably distant from the centre of the screen through which the
prolongation of the axis of the mirror should pass. We thus obtain a
measure of the error of the instrument. For this purpose, we must find
the position of the conjugate focus, which corresponds to the distance
of the screen. If _b_ be the distance to which the object should be
removed outwards from the principal focus of the mirror, _d_ the
distance from the focus to the screen, and _r_ the distance from the
focus to that point of the mirror which is to be tested, we shall have
_r_²
_b_ = ----
_d_
as the distance to which the object must be removed outwards from the
true focus on the line of the axis.[45]
[45] The truth of this equation may be easily ascertained as follows
(See fig. 27):--Let AP be the mirror, F its principal focus, and PH
the line of reflection of the ray FP; then an object at I will be
reflected at P to the conjugate focus O, where the screen is supposed
to be placed. But by construction, FPI = HPO = POF, and the angle at
F being common, the triangle FPI is similar to FPO, and hence FO ∶
PF ∷ PF ∶ FI, and
PF²
FI = ---;
FO
and substituting the letters in the text, we get _d_ ∶ _r_ ∷ _r_ ∶
_b_, and
_r_²
_b_ = ----.
_d_
[Illustration: Fig. 27.]
[Illustration: Fig. 28.]
[Illustration: Fig. 29.]
[Illustration: Fig. 30.]
[Illustration: Fig. 31.]
~Argand Lamps used in Reflectors.~
The flame generally used in reflectors, is from an Argand
fountain-lamp, whose wick is an inch in diameter. Much care is bestowed
upon the manufacture of the lamps for the Northern Lighthouses, which
sometimes have their burners tipped with silver to prevent wasting
by the great heat which is evolved. The burners are also fitted with
a sliding apparatus, accurately formed, by which they may be removed
from the interior of the mirror at the time of cleaning them, and
returned exactly to the same place, and locked by means of a key. This
arrangement, which is shewn in figs. 28, 29, and 30, is very important,
as it insures the burner always being in the focus, and does not
require that the reflector be lifted out of its place every time it
is cleaned; so that, when once carefully set and screwed down to the
frame, it is never altered. In these figs. _a a a_ represents one of
the reflectors, _b_ is the burner, and _c_ a cylindric fountain, which
contains 24 ounces of oil. The oil-pipe, the fountain _c_ for supplying
oil, and the burner _b_, are connected with the rectangular frame _d_,
which is moveable in a vertical direction upon the guide-rods _e_ and
_f_, by which it can be let down, so that the burner may be lowered
out of the reflector, by simply turning the handle _g_ (as will be
more fully understood by examining figs. 28 and 29), which has the
effect of forcing a _thread_ (like that of a screw) on the outside
of the guide into a groove in the frame, or withdrawing it, and thus
allows it to slide down or locks it at pleasure. An aperture of an
elliptical form, measuring about two inches by three, is cut in the
upper and lower part of the reflector, the lower serving for the free
egress and ingress of the burner, and the upper, to which the copper
tube _h_ is attached, serving for ventilation; _i_ shews a cross
section and a back view of the main bar of the chandelier or frame on
which the reflectors are ranged, each being made to rest on knobs of
brass, one of which, as seen at _k k_, is soldered to the brass band
_l_, that clasps the exterior of the reflector. Fig. 28 is a section
of the reflector _a a_, shewing the position of the burner _b_, with
the glass chimney _b′_, and oil-cup _l_, which receives any oil that
may drop from the lamp. Fig. 30 shews the apparatus for moving the
lamp up and down, so as to remove it from the reflector at the time of
cleaning it. In the diagram (fig. 30) the fountain _c_ is moved partly
down; _d d_ shews the rectangular frame on which the burner is mounted,
_e e_ the elongated socket-guides through which the guide-rods slide,
and _f_ the guide-rod, connected with the perforated sockets on which
the _checking-handle g_ slides. The oil-cup _l_ (covered with a lid
and wick-holder, as shewn in fig. 31) also serves as a _frost-lamp_
during the long nights of winter, when the oil is apt to turn thick.
It is attached to the lower part of the oil-tube by the arm _h_; and
is lighted about an hour before sunset, so as to prepare the reflector
lamp for lighting at the proper time. The communication between the
burner and the fountain is easily opened or shut in the burners used
in the Scotch Lighthouses, by simply giving the fountain a turn of one
quadrant of the horizon round its own vertical axis by means of the
round knob at its top, and thereby moving a simple slide-valve, which
shuts off the communication between the fountain-tube and lamp-tube.
By this mode, the oil is cut off about fifteen minutes before
extinguishing the lights, so that when that is done, the burner is
quite free of oil.
[Illustration: Fig. 32.]
~Arrangements for Raising or lowering the Argand Wick.~
It would needlessly occupy much time and space to describe the various
means (many of them sufficiently clumsy) which have been employed,
and in many places are still in use, for raising and depressing the
wick; it will be enough to say, that they all involve some application
of the rack and pinion. I shall, therefore, only describe the method
(invented, it is believed, by M. VERZY) which is adopted in all the
Lighthouses in the district of the Commissioners of Northern Lights.
The arrangement is as follows (see figs. 32, 33, 34, 35, 36):--The
inner tube _t_ of the burner is enclosed by a strong tube _s_, which
fits to it tightly, so as not to be easily moved. This strong tube has
a spiral groove cut on its outer or convex surface. The wick-holder has
two small pegs projecting from it, the one on the inside (not seen),
and the other on the outside at _a_ (fig. 33). That on the inside
works in the spiral groove of the tube S (figs. 32 and 33), already
described as embracing the inner tube _t_; and all that is required for
raising the wick is to make the wick-holder turn round on its vertical
axis. This is effected by means of the small external peg _a_ of the
wick-holder (fig. 33), which moves in a vertical slit _a_ (figs. 32 and
34), cut in a tube standing in the burner, and concentric with it, and
which also moves freely round its axis. Small knobs _n_ _n_ (figs. 32,
34, and 36), at the top of this tube, fit into a notch in the upper
ring of the gallery, which supports the glass chimney. By turning this
gallery _g_ (see figs. 33 and 36), therefore, motion is given to the
tube, with its knobs _n_ _n_, whose vertical slit _a_ (while it holds
the external peg of the wick-holder, and also turns it round along with
it) permits that peg _a_ to slide upwards or downwards, and thus the
wick-holder rises or falls, according as its own internal peg moves up
or down the spiral groove in the tube S. In fig. 32, C shews the glass
chimney resting on the gallery _g g_.
[Illustration: Fig. 33.]
[Illustration: Fig. 34.]
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
~Flowing of the Lamp.~
An important point in the economy of the Argand lamp, is the level at
which the outlet for the oil, in its passage from the fountain to the
burner, should be cut. The cutting of this hole (generally called
the _flow-hole_) in the pipe is termed the _flowing_ of the lamp, and
is commonly done by successive trials, until the oil stands at the
proper level of the burner, before the wick is put in. A more ready and
accurate method of accomplishing this object and at once determining
the level at which the _flow-hole_ should be cut, was introduced by Mr
JAMES MURDOCH, the Foreman of Lightroom Repairs to the Scotch Board,
and is generally employed in the Northern Lighthouses. Its nature will
be readily understood by a reference to the accompanying diagram, No.
37:
[Illustration: Fig. 37.]
The _hatched_ surface represents a metallic ruler, with a spirit-level
at L; C is the cup in which the bottom of the fountain _f_ (shewn in
dotted lines) rests. When the fountain is removed, and the ruler rests
on the edge of the cup C, the screw at A is used to adjust the level at
L; and a gauge GG is allowed to fall until a notch in it at _x′_ rests
on the outer tube of the burner F; the pinching-screw B retains this
ruler in its place, and the point _x′_ indicates the level at which
the oil should stand in the burner. The level line _x′ x_ indicates
the level on which the top of the _flow-hole_ H should be cut in the
fountain-tube, which is shewn in dotted lines within the outer tube,
or _body_ of the lamp. In other words, _y′ x′_ measures the level at
which the oil should stand in the burner _below_ the lower edge of the
metallic ruler, while the corresponding line _y x_, at the opposite
end, shews the level of the top of the _flow-hole_ H, below the edge
of the cup C. The gauge GG applied to that point of the fountain which
coincides with the edge of the cup (so that _y′_ coincides with _y_)
measures the length _yx_ = _y′ x′_; and a _set-square_ applied at _x_
gives the position of H on the fountain-tube. The round dot at _a_
shews the position of the air-hole in the body of the lamp, which
establishes a connection between the external air and the surface of
the oil. The rods SS′ shew the sliding gear (described as _d_ and _f_,
page 221), and are only introduced to identify this diagram with those
of the fountain and burner which have preceded it.
The most advantageous level of the flow-hole depends on many
circumstances too obscure and complicated to admit of any systematic
elucidation; and it is enough for all practical purposes, to know that
the capillary powers of the wick, and the greater or less viscidity of
the oil, are the chief circumstances which determine that level. Actual
experience is the only sure guide to the best practice in this respect;
and I therefore content myself with stating, that it is generally found
that the sperm oil should stand in the empty burner at about ³⁄₈ inch
below its top. For colza oil ²⁄₈ inch is sufficient. In summer, owing
to the oil being more fluid, there is sometimes a tendency to overflow
the burner; but any inconvenience arising from it is avoided by the
plan adopted in the Northern Lights, of shutting off the oil (by means
of the apparatus already alluded to on p. 222) about fifteen minutes
before extinguishing the lights in the morning.
[Illustration: Fig. 38.]
The arrangement for cutting off the oil is very simple, as will be
seen from the annexed diagram (fig. 38), in which F is the fountain,
T the oil-tube leading to the burner, and V the _flow-hole_, with its
sliding valve. By turning the handle H one quadrant of the circle, the
whole fountain F and tube T turn round their vertical axis, while the
valve V, which rests in a notch in the cup of the lamp, remains still,
and sliding over T, opens the _flow-hole_. S is the screw-plug which
retains the oil in the fountain, and which is unscrewed and removed
when the fountain is to be filled.
~Placing the Lamp in the Focus.~
[Illustration: Fig. 39.]
[Illustration: Fig. 40.]
In the reflecting apparatus of the Northern Lighthouses, the focal
position of the lamp is not, as we have already seen, liable to
derangement, by the removal of the burner for the purpose of cleaning,
as the sliding gear described at p. 221 insures the return of the lamp
to its true place. The burner is originally set by means of a gauge,
which touches four points of the mirror’s surface (one of them being
its vertex, and the other three in the vertical plane of its greatest
double ordinate). This gauge being provided with a short tube or collar
properly placed for the purpose of receiving the burner, at once
verifies its true position, both vertical and horizontal. The diagrams
39 and 40 shew the nature of the apparatus for adjusting the burners,
the one being a plan and the other a section. The four points which
touch the curve are one _g_ at the vertex, two in the same horizontal
plane with the focus, and near the edge of the mirror at PP, and the
fourth, also near the edge, and in the same vertical plane with the
focus. F is the focus. The horizontal arms are graduated, and fitted
with sliding pieces and clamping screws at R, so as to admit of being
varied with the width of the mirror; but each gauge applies only to
curves of the same focal distance; the distance F _g_ being fixed. The
gauge, when applied to the mirror, is properly secured by the screws
at R, R, and R′; and the burner which is attached to the oil-tube in a
temporary manner at A, is raised into the interior of the mirror. If
the tube of the burner ascends into the circular tube at F until (when
fixed by the checking handle already noticed at p. 221) its upper edge
just touches a narrow projection inside the tube F (so placed that the
rim of the burner should just touch it when it is on the level required
for putting the brightest part of the flame in the focus), then the
burner is in the proper position; but if, on the one hand, the axis of
the burner stands beyond F, at some point between it and N (which lies
in the plane of the mirror’s edge), the bent tube O from the fountain
must be shortened at A; and if it rise too high, that tube must be bent
down (and _vice versa_), until, by successive trials, it shall exactly
fit into the tube F, and stand at the proper level. A skilful workman
soon comes to guess those quantities very accurately; and, almost at
the first trial, curtails the tube to the proper length, and bends it
to the suitable level. All that is needful is to proceed cautiously, so
as not to cut the tube too short, for this leads to some trouble.
~Distinctions of Catoptric Lights.~
The great advantage derived by seamen from the establishment of lights
on a coast, soon makes the calls for additional lights so frequent,
that their very number itself produces a new evil, in the difficulty
of distinguishing the lights from each other. As the object of a light
is to make known to the benighted mariner the land he has made, with
as much certainty as the sight of a hill or tower would shew him his
position during the day, it becomes an object of the first importance
to impress upon each light a distinctive character, which shall
effectually prevent the possibility of its being mistaken for any
other.
Catoptric lights are susceptible of nine separate distinctions, which
are called _fixed_, _revolving white_, _revolving red and white_,
_revolving red with two whites_, _revolving white with two reds_,
_flashing_, _intermittent_, _double fixed lights_, and _double
revolving white lights_. The first exhibits a steady and uniform
appearance, which is not subject to any change; and the reflectors
used for it (as already noticed) are of smaller dimensions than those
employed in revolving lights. This is necessary, in order to permit
them to be ranged round the circular frame, with their axes inclined at
such an angle, as shall enable them to illuminate every point of the
horizon. The revolving light is produced by the revolution of a frame
with three or four sides, having reflectors of a larger size grouped
on each side, with their axes parallel; and as the revolution exhibits
once in two minutes, or once in a minute, as may be required, a light
gradually increasing to _full strength_, and in the same gradual manner
decreasing to total darkness, its appearance is extremely well marked.
The succession of _red_ and _white_ lights is caused by the revolution
of a frame whose different sides present red and white lights; and
these, as already mentioned, afford three separate distinctions,
namely, alternate red and white; the succession of two white lights
after one red, and the succession of two red lights after one white
light. The _flashing_ light is produced in the same manner as the
_revolving_ light; but owing to a different construction of the frame,
the reflectors on each of eight sides are arranged with their rims or
faces in one vertical plane, and their axes in a line inclined to the
perpendicular, a disposition of the mirrors which, together with the
greater quickness of the revolution, which shews a flash once in five
seconds of time, produces a very striking effect, totally different
from that of a revolving light, and presenting the appearance of
the flash alternately rising and sinking. The brightest and darkest
periods being but momentary, this light is farther characterised by a
rapid succession of bright flashes, from which it gets its name. The
_intermittent_ light is distinguished by bursting suddenly into view
and continuing steady for a short time, after which it is suddenly
eclipsed for half a minute. Its striking appearance is produced by the
perpendicular motion of circular shades in front of the reflectors, by
which the light is alternately hid and displayed. This distinction,
as well as that called the _flashing light_, is peculiar to the
Scotch coast, having been first introduced by the late Engineer of
the Northern Lights Board. The double lights (which are seldom used
except where there is a necessity for a _leading_ line, as a guide for
taking some channel or avoiding some danger) are generally exhibited
from two Towers, one of which is higher than the other. At the Calf
of Man, a striking variety has been introduced into the character of
leading lights, by substituting, for two _fixed_ lights, two lights
which revolve in the same periods, and exhibit their flashes at the
same instant; and these lights are, of course, susceptible of the other
variety enumerated above, that of two revolving red and white lights,
or flashing lights, coming into view at equal intervals of time. The
utility of all these distinctions is to be valued with reference to
their property of at once striking the eye of an observer and being
instantaneously obvious to strangers.
The introduction of colour, as a source of distinction, is necessary,
in order to obtain a sufficient number of distinctions; but it is in
itself an evil of no small magnitude; as the effect is produced by
interposing coloured media between the burner and the observer’s eye,
and much light is thus lost by the absorption of those rays, which are
held back in order to cause the appearance which is desired. Trial
has been made of various colours; but red, blue, and green alone have
been found useful, and the two latter only at distances so short as to
render them altogether unfit for sea-lights. Owing to the depth of tint
which is required to produce a marked effect, the red shades generally
used absorb from ⁴⁄₇ths to ⁵⁄₆ths of the whole light, an enormous loss,
and sufficient to discourage the adoption of that mode of distinction
in every situation where it can possibly be avoided. The red glass
used in France absorbs only ⁴⁄₇ths of the light; but its colour
produces, as might be expected, a much less marked distinction to the
seaman’s eye. In the Lighthouses of Scotland, a simple and convenient
arrangement exists for colouring the lights, which consists in using
chimneys of red glass, instead of placing large discs in front of the
reflectors.
~Arrangement of Reflectors on the Frame.~
[Illustration: Fig. 41.]
After what has been already said on the subject of divergence, it
will at once be seen, that in revolving lights the reflectors are
placed with their axes parallel to each other, so as to concentrate
their power in one direction; whilst in fixed lights it is necessary,
in order to approach as near as possible to an equal distribution of
the light over the horizon, to place the reflectors, with their axes
inclined to each other, at an angle somewhat less than that of the
divergence of the reflected cone. For this purpose, a brass gauge (see
fig. 41), composed of two long arms, AM, AM, somewhat in the form of a
pair of common dividers, connected by a means of a graduated limb A,
is employed. The arms having been first placed at the angle, which is
supplemental to that of the inclination of the axes of the two adjacent
mirrors at O, are made to span the faces of the reflectors, one of
which is moved about till its edges are in close contact with the flat
surface of one of the arms of the gauge.
[Illustration: Fig. 42.]
[Illustration: Fig. 43.]
Figs. 42 and 43 shew an elevation and plan of a revolving apparatus on
the catoptric principle. In these figures, _n n_ shews the reflector
frame or chandelier; _o o_, the reflectors with their oil-fountains
_p_ _p_. The whole is attached to the revolving axis or shaft _q_.
The copper tubes _r_ _r_ convey the smoke from the lamps; _s_ _s_ are
cross bars which support the shaft at _t t_; _u u_ is a copper pan for
receiving any moisture which may accidentally enter at the central
ventilator in the roof of the light-room; _l_ is a cast-iron bracket,
supporting the cup in which the pivot of the shaft turns; _m_ _m_ are
bevelled wheels, which convey motion from the machine to the shaft. The
machinery does not require any particular notice, being that of common
clock-work, moved by the descent of a weight.
[Illustration: Fig. 44.]
Fig. 44 shews a plan of one tier of reflectors arranged in the manner
employed in a fixed catoptric light; _n_ _n_ shews the chandelier,
_q_ the fixed shaft in the centre, which supports the whole, _o_ _o_
the reflectors, and _p_ _p_ the fountains of their lamps. In this
figure (in order to prevent confusion) only one tier of reflectors is
shewn; the other tiers are so arranged, that their axes divide into
equal angles the arcs intercepted between the axes of the adjoining
reflectors on the first tier, thereby producing the nearest approach
to an equal distribution of the light, which is attainable by this
arrangement.
In lighthouses of moderate height, the proper position for the
reflector itself is perfect horizontality of its axis, which may be
ascertained with sufficient accuracy, by trying with a plummet, whether
the lips of the instrument, which we may conclude to be at right
angles to the plane of its axis, be truly vertical. In lightrooms very
much elevated above the sea, however, the dip of the horizon becomes
notable; and a slight inclination forwards should be given to the face
of the reflectors, so that their axes produced may be tangents to the
earth at the visible horizon of the light-room. This, however, must not
be permitted to interfere with the perfect horizontality of the top of
the burner, which is indispensable to its proper burning.
~Bordier Marcet’s Reflectors.~
~Fanal Sidéral.~
Various forms of the parabolic mirror were invented by M. BORDIER
MARCET, the pupil and successor of ARGAND, who has laboured with
much enthusiasm in perfecting catoptric instruments, more especially
with a view to their application in the illumination of lighthouses
and the streets of towns. Amongst many other ingenious combinations,
he has invented and constructed an apparatus which is much used in
harbour-lights on the French coast, where it is known by the fanciful
name of _Fanal[46] sidéral_. The object is to fulfil, as economically
as possible, the conditions required in a fixed light, by illuminating,
with perfect equality, every part of the horizon, by means of a single
burner; and M. BORDIER MARCET has in his work-shop an instrument of
this kind, eight feet in diameter, which he constructed on speculation.
The apparatus used in harbour-lights, on the French coast, is of much
smaller dimensions, and does not exceed fifteen inches in diameter.
A perfect idea of the construction and effect of this instrument may
be formed, by conceiving a parabola to revolve about its parameter as
a vertical axis, so that its upper and lower limbs would become the
generating lines of two surfaces possessing the property of reflecting,
in lines parallel to the axis of the parabola, all the rays incident
upon them, from a light placed in the point where the parameter and
axis of the generating parabola intersect each other. This point
being the focus of each parabolic section of this apparatus, light
is equally dispersed in every point of the horizon, when the axis of
the parabolic section is in a plane perpendicular to a vertical line.
But however perfectly this apparatus may attain its important object,
it necessarily produces a feeble effect; because as its action is
entirely confined to the vertical direction, the light distributed by
it decreases directly as the distance of the observer. This beautiful
little instrument is shewn at fig. 45, in which _b_ shews the burner,
_p p_ the upper reflecting surface, and _p′ p′_ the lower reflecting
surface, both generated in the manner above described by the revolution
of a parabola about its parameter _x b_; F is the focus of the
generating parabola; and _l_ _l_ are small pillars, which connect the
two reflecting plates, and give strength to the apparatus.
[46] Fanal, from φανεν, a lantern.
[Illustration: Fig. 45.]
~Fanal à double effet.~
M. BORDIER MARCET has also prepared an ingenious modification of the
paraboloïdal mirror, which he has described under the name of _fanal
à double effet_; and the object of which is to obtain a convenient
degree of divergence from parabolic mirrors, by the use of two flames
and two reflecting surfaces, each of which is acted upon by its own
flame, and also by that of the other. This modification consists in
the union of two portions of hollow paraboloïdal mirrors, generated by
the revolution of two parabolas about a common horizontal axis, and
illuminated by two lamps placed in the focus of each. The first surface
is generated by the revolution on its axis of a segment of a paraboloid
intercepted between the parameter and some double ordinate greater
than it, and may, from its form, be called the ribbon-shaped mirror.
The second surface is that of a parabolic conoid, which is cut off by
a vertical plane passing through a double ordinate, which is equal to
the parameter of the parabolic ribbon, which is placed in front of it.
The elements of the curve which forms the conoïdal mirror, must be so
chosen as to have its focus at a convenient distance in _front_ of
that of the ribbon-shaped mirror, so as to admit of placing the two
lamps separate from each other, as well as to produce the necessary
degree of divergence, which is to be obtained by the action of these
mirrors respectively on the flame placed in the focus of the other.
These two mirrors are joined together in the line of the parametric
section of the ribbon, which coincides with the lips of the conoid at
some double ordinate _behind_ its parameter. Each mirror produces, by
means of the lamp placed in its focus, an approach to parallelism of
the reflected rays, which M. BORDIER MARCET has not inaptly termed
the _principal effect_; whilst the action of each surface on the lamp
which is placed in the focus of the other, causes what the inventor
calls the _secondary_ or _lateral effect_. Their secondary action may
be described thus: The lamp, which is in the focus of the ribbon, is
much nearer the vertex of the conoid than its own focus; so that its
rays making, with normals to the surface of the conoid, angles greater
than those which are formed by the rays proceeding from its focus, are
of necessity reflected in lines diverging from the axis of the mirror.
Those, on the contrary, which proceed from the focus of the conoid,
meet the ribbon-shaped surface, so as to make angles with its normals
more acute than those which the rays from its own focus could do, and
which are, therefore, reflected in lines converging to the axis of the
mirror. Those reflected rays must therefore cut the axis, and diverge
from it on the other side. This apparatus has been used at La Hève
and some other lights on the French coast; but it is impossible not
to perceive the great loss of light which results from the use of two
flames in one mirror; and it must not be forgotten, that the divergence
which is obtained by means of it is not confined to the horizontal
direction in which only it is wanted; but that the light is at the same
time scattered in every direction round the edge of the mirror.
Arrangements of a similar kind were proposed and executed for the
same purpose of uniting greater divergence with considerable power in
the central parts of the resultant beam, by ARGAND himself, in 1806,
and also in 1808, by M. HAUDRY, _Ingénieur des Ponts et Chaussées_.
ARGAND proposed the union of a paraboloid, and an ellipsoid having
their foci coincident in one point, which being the posterior focus
of the latter curve, was illuminated by the rays reflected to it by
means of the ellipsoïdal surface from the lamp placed in the anterior
focus. From the _optical focus_ thus obtained, some rays would fall
on the paraboloïdal surface and produce, by reflection, a cylinder of
parallel rays, while the rest would diverge from the axis, and form a
zone of spreading rays. M. HAUDRY’S plan consisted of a combination of
a conical with a paraboloïdal mirror, so placed, that the rays from the
front part of the hollow cone might be nearly parallel to those sent
out by the paraboloid; while the rays from its base diverging from the
axis might produce a ring of divergent rays, similar to that obtained
from the ellipsoid of ARGAND’S apparatus.
It would occupy much time to exhibit all the disadvantages of the
arrangements in the _fanal à double effet_ of M. BORDIER MARCET, and
also in those of ARGAND and HAUDRY; and I shall therefore dismiss the
subject by observing, that the loss of light due to the position of
the flame in the apparatus of ARGAND, is so great as to induce one
to wonder that such combinations should ever have been attempted.
There can be no doubt, that the most efficient mode of obtaining due
divergence from mirrors, is to adopt the paraboloid, with a short focal
distance, which has the double advantage of increasing the divergence
which is due inversely to the focal distance, and, at the same time,
subjecting to the action of the mirror a larger portion of the luminous
sphere proceeding from the flame.
[Illustration: Fig. 46.]
~Fanal à double face.~
Lastly, I shall notice M. BORDIER MARCET’S _fanal à double face_,
which consists of two paraboloïdal mirrors, truncated in the vertical
plane of the parameter, and united together back to back, so as to
be illuminated by the same lamp placed in their common focus. To
save the light which would otherwise escape the catoptric action, he
adds a parabolic conoid of greater focal distance, and so placed,
that while its focus may coincide with the common focus of the other
mirrors, its size may be so restricted, that it shall not interfere
with the effect of the truncated mirror opposite which it is placed.
The obvious consequence of such an arrangement is, that the rays (see
fig. 46) produced from a lamp in the common focus of the three mirrors,
will produce in opposite directions a luminous ring from each of the
truncated mirrors AC, BC, and A′C′, B′C′, while the central or conoïdal
mirror MN will fill the interior of one of those luminous rings with
a cone of rays, whose intensity will be in the inverse ratio of MN²
to _a_ _b_² (or FM² to F _a_²), which latter surface represents the
whole amount of naturally divergent rays, which strike on _a_ _b_, and
which are spread over MN. Two sets of reflectors of this form facing
in opposite directions (each set arranged in one plane, and fixed on
a frame which could be made to revolve round a vertical axis), would
thus present their brightest effect after considerable intervals of
darkness; but, by arranging them with their axes slightly inclined,
they were made to prolong the light periods and curtail the dark ones.
M. BORDIER MARCET speaks of this apparatus with all the satisfaction
generally felt by inventors; but it is no difficult matter to identify
its effect with that of the common paraboloïdal mirrors. It is obvious,
that all the rays which fall from a true focal point on the three
reflectors AC, BC, A′C′, B′C′, and MN, are merely those which would
fall on a single reflector, whose double ordinate and the portion of
the abscissa between that ordinate and the focus, are equal to those
of the first reflector of the compound system, so that the quantity of
light reflected by the three reflectors is neither more nor less than
that which would be projected by one. All the difference that can exist
is, that in the case of a flame which has a notable size, the surface
MN being farther distant than _a_ _b_, would produce less aberration
and, consequently, a very slight increase of intensity in the small
portion of the reflected beam of parallel rays due to that part of the
compound mirror. We cannot, therefore, sensibly err in rejecting any
advantage to be derived from this arrangement as insignificant.[47]
[47] See Peclet′s Traité de l′Eclairage, p. 302, from which fig. 46
is copied.
~Mr Barlow′s Spherical Mirrors.~
Spherical mirrors have been employed in Lighthouses chiefly when they
can be introduced to aid the effect of refracting apparatus: and it
will not be necessary to say much of them in this place. I must,
however, notice an ingenious proposal of Mr W. H. BARLOW,[48] who
suggests placing in front of the flame a small spherical reflector,
whose centre is coincident with the focus of a paraboloïd, and
whose subtense is the parameter of the generating curve. The small
mirror, being somewhat less than a hemisphere, would cause the light
falling upon it to be returned through the focus so as to reach the
paraboloïdal surface and to be finally reflected from that portion of
it which is embraced between the limits of its extreme divergence. If
there were no loss of light at the surface of the small mirror, its
effect would be to increase the power of the beam of parallel rays
by an amount equal to the sum of the rays incident on the spherical
surface, but at the same time to diminish it by intercepting a portion
of the light reflected from the paraboloïd. I am not aware that such
a combination has been tried, as it applies most advantageously to
reflectors whose span does not exceed the parameter of the generating
curve, a form rarely adopted in lighthouses; but it might also be
adapted to reflectors which intercept a larger portion of light, by
making the spherical reflector some segment less than the hemisphere.
[48] In an excellent paper above noticed, on the Illumination of
Lighthouses, in the London Transactions, for 1837.
~Captain Smith’s Mirrors in the form of a parabolic spindle.~
CAPTAIN SMITH of the Madras Engineers, has described in the
“Professional papers of the ‘Corps of Royal Engineers,’[49] a new
system of fixed lights,” which consists in placing a flat wick in
the focus of one-half of a hollow parabolic spindle generated by the
rotation of a parabola about its parameter as a vertical axis. The
action of the instrument is obvious, for each vertical section being
parabolic, effects a change only in the _vertical_ divergence of the
rays incident on it from the focus, and suffers their horizontal
direction to remain unaltered; thus each vertical plate of reflected
rays passes through the parameter of the curve and illuminates the
opposite point of the horizon by means of a narrow strip or line of
light. Two hollow spindles of that form, each lighting 180° and facing
opposite azimuths, would, therefore, be sufficient to illuminate the
whole horizon. The author of the paper, however, appears to contemplate
the employment of a series of those mirrors ranged one above another
and _breaking joint_ vertically, somewhat in the manner already
described in speaking of the arrangement of the paraboloïdal mirrors
used in fixed lights. The advantages of this mode of illumination are
much overrated by CAPTAIN SMITH, who seems to magnify beyond its real
importance the risk attending the use, in the dioptric apparatus,
of a single lamp, whose sudden extinction would deprive at once the
whole horizon of the benefit of the light; while, on the contrary, he
reckons the security obtained by his arrangement as an advantage of the
highest value. In certain situations, where no regular establishment of
trained light-keepers is maintained, that security may be an object of
more importance and may warrant a greater sacrifice, than is necessary
in Great Britain; but I have no hesitation in saying, that I know of
no situation in which the plan proposed by CAPTAIN SMITH could bear
comparison with the mode of illumination for fixed lights by means of
the catadioptric instruments of FRESNEL.
[49] Vol. v., p. 56.
DIOPTRIC[50] SYSTEM OF LIGHTS.
[50] Most probably directly derived from the Greek διόπτρον, an
optical instrument with holes for looking through, whose name is a
compound of διὰ, through, and ὄπτομαι, _I see_.
One of the earliest notices of the application of lenses to lighthouses
is that recorded by SMEATON in his Narrative of the Eddystone
Lighthouse, where he mentions a London optician, who, in 1759, proposed
grinding the glass of the lantern to a radius of seven feet six
inches; but the description is too vague to admit of even a conjecture
regarding the proposed arrangement of the apparatus. About the middle
of the last century, however, lenses were actually tried in several
lighthouses in the south of England, and in particular at the South
Foreland in the year 1752; but their imperfect figure and the quantity
of light absorbed by the glass, which was of impure quality and of
considerable thickness, rendered their effect so much inferior to
that of the parabolic reflectors then in use, that after trying some
strange combinations of lenses and reflectors, the former were finally
abandoned. Lenses were also tried at the lights of Portland, Hill
of Howth, and Waterford, by Mr Thomas Rogers, a glass manufacturer
in London; who possessed, it is said, the art of blowing mirrors of
glass, “and by a new method silvered over the convex side without
quicksilver.”[51]
[51] Hutchinson’s Practical Seamanship, p. 200. See also the notice
of the spherical mirrors made by Messrs François and Letourneau of
Paris in a subsequent part of this volume.
The object to be attained by the use of lenses in a Lighthouse is, of
course, identical with that which is answered by employing reflectors;
and both instruments effect the same end by different means, collecting
the rays which diverge from a point called the _focus_, and projecting
them forward in a beam, whose axis coincides with the produced axis of
the instrument. We have already seen that, in the case of _reflection_,
this result is produced by the light being _thrown back_ from a surface
so formed as to make all the rays to proceed in one and the same
required direction. In the case of _refraction_, on the other hand, the
rays pass through the refracting medium, and are _bent_ or _refracted_
from their natural course into that which is desired.
The celebrated BUFFON, to prevent the great absorption of light by
the thickness of the material, which would necessarily result from
giving to a lens of great dimensions a figure continuously spherical,
proposed to grind out of a solid piece of glass, a lens in steps or
concentric zones. This suggestion of BUFFON regarding the construction
of large burning glasses, was first executed, with tolerable success,
about the year 1780, by the Abbé ROCHON; but such are the difficulties
attending the process of working a solid piece of glass into the
necessary form, that it is believed the only other instrument ever
constructed in this manner, is that which was made by Messrs COOKSON of
Newcastle-upon-Tyne, for the Commissioners of Northern Lighthouses.
The merit of having first suggested the building of lenses in separate
pieces, seems to be due to CONDORCET, who, in his _Eloge de Buffon_,
published so far back as 1773, enumerates the advantages to be derived
from this method. Sir DAVID BREWSTER also described this mode of
building lenses in 1811, in the _Edinburgh Encyclopædia_; and in
1822, the late eminent FRESNEL, unacquainted with the suggestions of
CONDORCET or the description by Sir DAVID BREWSTER, explained, with
many ingenious and interesting details, the same mode of constructing
those instruments. To FRESNEL belongs the additional merit of having
first followed up his invention, by the construction of a lens and, in
conjunction with MM. ARAGO and MATHIEU, of placing a powerful lamp in
its focus, and indeed of finally applying it to the practical purposes
of a Lighthouse.
The great advantages which attend the mode of construction proposed by
CONDORCET are,--the ease of execution, by which a more perfect figure
may be given to each zone and spherical aberration in a great measure
corrected, and the power of forming a lens of larger dimensions than
could easily be made from a solid piece. Both BUFFON and CONDORCET,
however, chiefly speak of reducing the thickness of the material, and
do not seem to have thought of determining the radius and centre of
the curvature of the generating arcs of each zone, having contented
themselves with simply depressing the spherical surface in separate
portions. FRESNEL, on the other hand, determined those centres,
which constantly recede from the vertex of the lens in proportion as
the zones to which they refer are removed from its centre; and the
surfaces of the zones of the annular lens, consequently, are not parts
of concentric spheres, as in BUFFON’S lens. It deserves notice, that
the first lenses constructed for FRESNEL by M. SOLEIL had their zones
polygonal, so that the surfaces were not annular, a form which FRESNEL
considered less accommodated to the ordinary resources of the optician.
He also, with his habitual penetration, preferred the plano-convex
to the double-convex form, as more easily executed.[52] After mature
consideration, he finally adopted crown glass, which, notwithstanding
its greenish colour, he preferred to flint glass, as being more free
from _striæ_. All his calculations were made in reference to an index
of refraction of 1·51, which he had verified by repeated experiments,
conducted with that patience and accuracy for which, amidst his higher
qualities, he was so remarkably distinguished.[53] The instruments have
received the name of _annular_ lenses, from the figure of the surface
of the zones.
[52] The plano-convex lens, with its curved side towards the parallel
rays, is also a form producing small spherical aberration, a
circumstance which may also have influenced his choice.
[53] My friend, Mr WILLIAM SWAN, carefully examined, by his new
and ingenious method, described in the Edinburgh New Philosophical
Journal, January 1844, several specimens of the St Gobain glass
(which is now used in the manufacture of the lenses), and found its
refractive index to be 1·51793, the _difference_ between the greatest
and least values being only 0·00109.
~Refraction.~
A ray of light, in passing _obliquely_ from one transparent body
into another of different density, experiences at the point of the
intersection of the common surface of the two planes, a sudden change
of direction, to which the name of _refraction_ has naturally been
given, in connection with the most familiar instance of the phenomenon,
which is exhibited by a straight ruler with one half plunged into a
basin of water while the other remains in the air. The ruler no longer
appears straight, but seems to be _bent_ or _broken_ at the point where
it enters the water. It may not be out of place to call attention to
the laws which regulate the change of direction in the incident light,
which are _three_ in number.
1. Incidence and refraction, in uncrystallized media of homogeneous
structure such as glass, always occur in a plane perpendicular to that
of the refracting surface.
2. In the same substances, the angle formed with the perpendicular by
the ray at its entering the surface of the second medium, has to the
angle which it makes with the normal after it has entered the surface,
such a relation, that their sines have a fixed ratio, which is called
the _refractive index_. When a ray falls normally on the surface of any
substance, it suffers no refraction.
3. The effect of passing from a rare to a dense medium, as from air
into water or glass, is to make the angle of _refraction_ less than the
angle of _incidence_; and those angles are measured with reference
to a normal to the plane which separates the media at the point of
incidence. The converse phenomenon, of course, takes place in the
passage from a dense to a rare medium, in which case the angle of
_incidence_ is less than the angle of _refraction_. To this rule there
are a few exceptions; for there are certain combustible bodies, such as
diamond, whose refractive powers are much greater than other substances
of equal density.
[Illustration: Fig. 47.]
The diagram (fig. 47) will serve to render those laws more
intelligible. Let a ray of light _a_ O meet a surface of water _n m_
at O, it will be immediately bent into the direction O _a′_; and if,
from the centre O, we describe any circle, and draw a line _b_ O _b′_,
perpendicular to _nm_; then _ab_ and _a′ b′_, perpendiculars drawn to
the normal _bb′_, from the points _a_ and _a′_ where the circle cuts
the incident and refracted rays, will be the sines of the angle of
incidence _b_ O _a_, and of the angle of refraction _b′_ O _a′_, and
the ratio of those sines to each other, or
_b a_
-------
_b′ a′_
will be the relative index of _refraction_ for the two media.
4. It may perhaps be added, for convenience, as a _fourth_ law,
deducible from the others, that since rays passing from a dense into a
rare medium, have their angle of refraction greater than the angle of
incidence, there must be some angle of incidence whose corresponding
angle of refraction is a right angle; beyond which no refraction can
take place, because there is no angle whose sine can be greater than
the radius. In such circumstances, _total reflection_ ensues. For
common glass, whose index of refraction is 1·5, we have (in the case of
emergent rays) sine of
sine of refraction
incidence = ------------------;
1·5
but, as no sine can exceed radius or unity, the angle of incidence must
be limited to 41° 49′; beyond which total reflection will take place,
and the light will return _inwards_ into the glass, being _reflected_
at its surface.
Thus, if a ray proceed from a point O (fig. 48), within a piece of
glass, to a point C, at its surface A B; and if O C _b_, its incidence,
be less than 41° 49′, it will be _refracted_ in some direction C _f_;
but if this angle be greater than 41° 49′, as O C′ _b′_, the ray will
be _reflected_ back into the glass in the direction C′ O′.
[Illustration: Fig. 48.]
The material hitherto employed in the construction of lighthouse
apparatus is crown glass, which, although it possesses a lower
refractive power than flint glass and has, besides, a slightly
greenish tinge, offers the great practical advantages of being more
easily obtained of homogeneous quality; and, being less subject to
deterioration from atmospheric influences, it is peculiarly suitable
for use in the exposed situations generally occupied by Lighthouses.
The refractive index of crown glass, as already noticed, is about 1·5.
Any one may easily satisfy himself by a careful protraction of the
angles of _incidence_ and _refraction_, in the manner above described,
as to the truth of the following general propositions resulting from
those laws:--
1. A ray of light passing through a plate of some diaphanous
substance such as glass, with parallel surfaces, suffers no change
of _direction_, but emerges in a line parallel to its original path,
merely suffering a _displacement_, depending on the obliquity of the
incident ray, and the refractive power and thickness of the plate. The
effect of this displacement is merely to give the ray an apparent point
of origin different from the true one. This will be easily understood
by the diagram (fig. 49), in which _a b_ is a normal to the plate,
whose surfaces _x x_ and _x′ x′_ are parallel, _r r r r_ shews the path
of the ray, _r r_ the displacement, and _r′_ the apparent point of
origin resulting from its altered _direction_.
[Illustration: Fig. 49.]
2. When a ray passes through a triangular prism _a b c_, the
inclination of the faces _a c_ and _c b_ causes the emergent ray _r′_
to be bent towards _a b_, the base of the prism, in a measure depending
on the inclination of the sides of the prism and the obliquity of the
incident ray to the first surface.
[Illustration: Fig. 50.]
3. When parallel rays fall on a concave lens, they will, at their
emergence, be divergent. The section of the diaphanous body _a b c d_
may be regarded as composed of innumerable frusta of prisms, having
their apices directed towards the centre line _x r_; and the rays which
pass through the centre, being normal to the surface, will be unchanged
in their direction, while all the others will (as shewn in the figure)
suffer a change of direction, increasing with their distance from the
centre, owing to the increasing inclination of the surfaces of the lens
as they recede from its axis.
[Illustration: Fig. 51.]
4. Lastly, when divergent rays fall on a convex lens _a b_, from a
point _f_, called the principal focus, they are made parallel at their
emergence; while, _conversely_, parallel rays which fall on the lens
are united in that point.[54] This effect, which is the opposite of
that caused by the concave lens, may be explained in a similar manner,
by conceiving the section _a b_ of the convex lens to be composed of
innumerable frusta of prisms, arranged with their _bases_ towards the
centre of the lens.
[54] It is, of course, to be understood that only rays incident near
the axis of the lens are refracted accurately to a focus.
[Illustration: Fig. 52.]
Now, it is obvious, that we can derive no assistance, in economising
the rays of a lamp for Lighthouse purposes, from concave lenses, whose
property is to increase the dispersion of the rays incident on them.
With concave lenses, therefore, we have no concern; and we shall
confine ourselves to the consideration of the convex or converging
lenses.