Eddystone Rock, about 14 miles S.S.W. from Plymouth. The structure which
now[9] stands upon this rock was the work of Smeaton, and was completed
in 1759. The stones forming the lower courses of this tower, which is
represented in Fig. 303, half in section and half in elevation, are
dovetailed into the rock itself and into each other. The masonry is
carried up in a solid mass for about 12 ft., the stone used being
granite, which also constitutes the whole of the exterior masonry. The
four upper apartments are formed with arched roofs, the side-thrust of
which is counteracted by iron chains surrounding the tower. These
chains, which are bedded in lead, were placed in their positions while
hot, and by their contraction bound the structure together with great
force. The masonry of the tower is 68 ft. high, and this is surmounted
by the light-room, the total height from the lowest course of stonework
to the gilt ball at the top being 94 ft., or nearly half that of the
London Monument. The diameter at the base is 26 ft., and that at the top
15 ft. The light-room is of an octagonal shape, and is made of iron
framework, glazed with thick plate glass. Below this are two
store-rooms, a kitchen, and a bed-room. The Eddystone has now breasted
the storms of more than a hundred years, and it remains as firm as the
rock it is built on. Fig. 304 is a picture of this noble lighthouse,
with the British fleet passing close to it, during a furious gale on the
22nd of October, 1859, or exactly a century after the completion of the
structure. The incident of the man in the water, which occupies the
foreground, is not an imaginary one, for it is recorded that the
_Trafalgar_ stopped in the midst of the storm to pick up a man who had
fallen overboard. For eighteen hours the ships encountered the fury of
the tempest, keeping out at sea in open order throughout the night. They
wore in at dawn, came up the Channel in line of battle, steamed into
Portland, and took up their anchorage without the loss of a sail, a
spar, or a rope-yarn.
Footnote 9:
Smeaton’s tower proving unsafe, has since been taken down and
replaced, in 1882, by one from Mr. Douglass’ design.
[Illustration:
FIG. 304.—_The Eddystone in a Storm._
]
The lighthouse tower on the Bell Rock is 100 ft. high, 42 ft. in
diameter at the base, and 15 ft. at the top. The Inchcape Rock, on which
it is placed, is the scene of Southey’s ballad of “Ralph the Rover,” and
the lighthouse here is one of the most serviceable on the Scottish
coast, for the dangerous spot on which it is placed lies in the direct
track of all vessels entering the Firth of Tay from the German Ocean.
The rock is submerged at spring tides to the depth of 12 ft. The tower
bears a close resemblance in shape to that of the Eddystone: it is
circular and faced with massive blocks of granite. The lower part, to
the height of 30 ft., is solid, and the door is reached by a bronze
ladder. The building contains five apartments, and a cistern for storing
fresh water for the use of the keepers, who have sometimes to remain in
their solitary situation for six or eight weeks together, the weather
preventing the possibility of any communication with the shore.
Still loftier than the tower on the Bell Rock is that which rises in the
midst of the Skerryvore Reef, 12 miles from Tyree, a small island off
the coast of Argyleshire. This building may be taken as a typical
specimen of a detached lighthouse, and the difficulties overcome in its
construction attest the skill of the engineer, Mr. Alan Stevenson, who
has written a highly interesting account of the work. The rocks here are
of _gneiss_, an extremely hard formation, and their surfaces are worn as
smooth as glass by the action of the water. On one of a numerous series
of these small islets, where only a narrow strip of rock, a few feet
wide, remains above the surface at high water, and this divided by
rugged lumps into narrow gullies, through which the sea constantly
rushes, the lighthouse is built. The work was commenced in 1838 by the
erection of a temporary wooden barrack on piles at a little distance
from the site chosen for the foundation. In a gale during the winter the
whole of this structure was swept away in one night. Another, more
strongly secured, was built the following summer, and in this Mr.
Stevenson and his men remained sometimes for fourteen days together, the
weather preventing any passage to or from the shore: here the men were
sometimes awakened from their hard-earned repose by the water pouring
over the roof, and by its rushing through the crevices, while the
erection swayed and reeled on its supports. Mr. Stevenson relates that
one night the men became so alarmed for the stability of their shelter
that some descended, and sought in cold and darkness a firmer footing on
the rocks. Two summers were occupied in cutting the foundations, and the
blasting of the rock in so narrow a space was an operation attended with
no little danger. A small harbour had to be formed at the rocks for the
vessels bringing the ready-prepared stones of the building from the
quarries, where also piers were built expressly for the shipment of the
materials. In designing his tower, the engineer preferred to oppose the
force of the waves by the weight of his structure, rather than to rely
on dovetailed or joggled-jointed stones. Measurements were made of the
force of the waves, which at Skerryvore was sometimes equivalent to a
pressure of 4,335 lbs. on the square foot; and calculations based on
these measurements showed that the mere weight of the superstructure
would amply suffice to keep the stones immovable. Nearly 59,000 cubic
feet of stone were used, or about five times the quantity contained in
the Eddystone Lighthouse, and the total cost of the building was
£87,000.
The use of iron, as a building material advantageously replacing stone,
has extended to lighthouses, and many have been constructed entirely of
cast and wrought iron, or partly of iron and partly of gun-metal, which
is not readily acted on by the sea spray. Such lighthouses are cheap,
easily and quickly erected, strong enough to bear shocks and vibrations,
and proof against fire, lightning, and earthquakes. The lighthouse on
Morant Point, Jamaica, is made of iron, cast in England; and it was
erected in a few months at a cost of one-third of that of a stone tower
of the same altitude. Its height is 105 ft., and the shaft is formed of
iron plates in segments of 10 ft. high, which are bolted together at
their flanges. At Gibbs Hill, Bermuda, is a lighthouse 130 ft. high,
constructed in the same manner.
So inefficient, inconvenient, and uncertain were the lamps or other
means of artificial illumination known up to nearly the beginning of the
present century, that nothing better could be found for the Eddystone
Lighthouse for forty years after its erection than tallow candles stuck
in a hoop—a means of illumination which would scarcely now be tolerated
even in a booth at a village fair. To M. Argand, a Frenchman, we are
indebted for the first great improvement in lamps. The admirable
invention which bears his name is, as everybody knows, an oil lamp with
a tubular wick, which occupies the annular space between two metallic
tubes, in such a manner that a current of air rises through the inner
tube, and thus reaches the interior of the flame. This current, and the
current which supplies the exterior, are increased by surrounding the
flame with a tall glass chimney; and a contraction of the chimney, just
above the flame, aids greatly in distributing the air, so as to insure
the complete combustion of the oil. In the original lamp the supply of
oil to the flame depended on the capillary attraction in the meshes of
the wick. M. Carcel applied clockwork to continuously pump up the oil
into the burner, so that, by overflowing, it was maintained at an
invariable level. This arrangement added greatly to the intensity and
steadiness of the light; and, on account of the uniformity of its flame,
the Carcel lamp has been selected as a standard to which, in France,
photometric determinations are referred.
The power of the Argand lamp, as employed in lighthouses, is greatly
increased by the plan of employing several concentric wicks instead of
one. Between these wicks there are, of course, open spaces, through
which the air obtains access to the flame, and the current of air is
made more rapid by the use of a very tall chimney. The large amount of
heat produced by the combustion of so much oil in a small space is
partly carried off by the excess of oil which is made to overflow the
burner—about four times the quantity consumed being constantly pumped up
into the burner for this purpose. Lighthouse lamps are made with two,
three, and four wicks; and the oil is forced up in the burners either by
clockwork or by the pressure of a piston loaded with a weight. The
following table gives the sizes of the burners and the illuminating
powers of the lamps:
┌──────┬──────┬────────┬─────────┐
│Order │Number│Diameter│Intensity│
│ of │ of │ in │of Light │
│Light.│Wicks.│inches. │in Carcel│
│ │ │ │ Lamps. │
├──────┼──────┼────────┼─────────┤
│ 1 │ 4 │3½ │ 23│
│ 2 │ 3 │2–15/16 │ 15│
│ 3 │ 2 │1¾ │ 5│
└──────┴──────┴────────┴─────────┘
The quantity of oil consumed in these lamps is less than that
proportional to the increase of the light:—for example, although the
four-wick lamp gives twenty-three times the light of the simple Argand,
it only consumes nineteen times the quantity of oil. The oil used in
these lamps is colza; but experiments have been made with a view of
introducing petroleum, which has the advantages, not only of being
cheaper and uncongealable by cold, but of giving a whiter and more
brilliant light. Hitherto, however, this substance has answered only
with lamps of one wick.
Coal-gas has been applied to the illumination of lighthouses, and as it
gives a light of great brilliancy and steadiness, when consumed in
proper burners, it has certain advantages over oil lamps, which have
caused it to be employed in situations where a supply can be readily
obtained. The light produced by lime, ignited by the combustion of
coal-gas or hydrogen mixed with oxygen, has also been suggested; but
this plan is not without risk of interruptions and of dangerous
accidents, and it has been considered inadvisable to entrust the
apparatus to the persons who commonly take charge of lighthouses.
The electric light has been very successfully applied in certain
lighthouses, since the mode of producing steady currents by
magneto-electric[10] machines has come into use. The lighthouses at the
South Foreland have been thus illuminated by a machine constructed by
Mr. Holmes in 1862. A very powerful electric light is exhibited from the
lighthouse on Cape Grisnez; and the adoption of this source of light has
been extending, as it is far more intense than any other artificial
light, and can be sent in more concentrated beams across the sea, on
account of its being emitted from a space which is practically a point.
These circumstances cause the beams of the electric light to possess
greater power of penetrating the atmosphere than those from any other
source.
Footnote 10:
Now superseded by the dynamo-electric machines.
But it is perhaps in the optical apparatus of lighthouses that the
greatest improvements and most admirable inventions are to be found.
When only the blaze of an open fire furnished the guide to the mariner,
the means resorted to in order to throw across the sea the light which
issued from the flames upwards or landwards, appear to have been of the
rudest kind, even where such attempts were made at all. The inverted
cone on the Tour de Cordouan has been already mentioned, and we read of
cases in which screens of sheet brass were placed on the landward side,
to throw back the light seaward.
Here it may be proper to examine the conditions which determine how the
light can be made most available for the guidance of the mariner.
Everybody knows that the light from a luminous body spreads out from it
in all directions equally. Thus, if we simply place an electric light on
a tower such as that on the Bell Rock, but few of the luminous rays can
benefit the mariner: namely, those which fall upon the sea or are
directed to the horizon. A much larger portion of the light will stream
upwards and be lost in space; another part will descend towards the base
of the tower, and be equally wasted. Again, if the situation of our
lighthouse were on the shore of the mainland, all the light which passes
landwards, whether horizontally or not, would be entirely lost for our
purpose. Even if, in the case of an isolated lighthouse, we can send out
all the light in a nearly level zone over the sea to the horizon, the
intensity of the illumination will diminish, on account of the widening
space, as the distance increases. The question, therefore, arises
whether it is possible to send the whole of the light in one unbroken
beam, not liable to this kind of enfeeblement, so that the only loss it
can experience may be absorption by the imperfectly transparent
atmosphere.
There are two means of gathering up all the otherwise useless beams, and
sending them in such a direction as to reach the eye of the distant
mariner. The one is by reflection from mirrors, and the other by
refraction through lenses. The apparatus employed in the first process
is termed _catoptric_, and in the latter _dioptric_.
When a luminous point is placed at the focus of a parabolic mirror, all
the rays which fall upon the mirror are reflected by it in a direction
parallel to its axis, so that they form a cylindrical beam. This is the
method which was adopted in the first improvements effected in
lighthouses. The parabolic reflector was first used at the Tour de
Cordouan in 1780, and soon afterwards metallic reflectors became the
ordinary appliances of lighthouses, and they are still largely used.
Such reflectors are made of sheet copper, thickly plated with silver,
about 6 oz. of this metal being applied to 16 oz. of copper. They are
formed by carefully beating a circular sheet of the plated copper into a
concave shape, which is finally brought to the exact curve by the aid of
gauges, and is then turned and polished. The largest of these reflectors
have a diameter of 2 ft. at the _mouth_, as it is termed, for the
reflector comes forward in advance of the lamp, the chimney and burner
passing through openings in the metal. The flame of the lamp occupies
such a position that its brightest part is in the focus of the mirror;
but since the focus is a point merely, whereas the flame has a certain
magnitude, it follows that the want of coincidence of the other luminous
points with the focus produces a certain divergence in the reflected
rays, so that the beam is not accurately cylindrical. This, however, is
far from being a disadvantage practically, for it has the effect of
widening a little the strip of sea illuminated by the beams. But all
that portion of the light which escapes from the mouth of the mirror
without being reflected is radiated in the ordinary manner, and is
practically lost. We shall presently see how even this light may be
gathered up and brought into the main beam.
Let us suppose a number of such reflectors, each with its own lamp,
placed in a horizontal circle, so as to throw their beams towards
different points of the compass. If eight lamps were so placed, eight
beams of light would stream out across the water, like eight spokes of a
wheel; eight sectors would, however, be left unilluminated, and for
ships in these spaces the lighthouse would be virtually non-existent:
its rays could only reach vessels within the eight narrow strips
traversed by the beams. If we double the number of reflectors in the
circle, or if we arrange another series of eight in a circle above or
below the others, so that a lamp in the second circle coincides
vertically with an interval in the first, the effect will be that we
shall have sixteen beams, and sixteen dark sectors, instead of eight;
that is, only a very small part of the expanse of water will receive the
benefit of the light. It must be remembered that the breadth of the
cylindrical beam would not be greater than the diameter of the mirrors,
and that the space illuminated by it has the same breadth at all
distances; or rather, that this is nearly the case, for the light does
not all issue precisely from the focus of the mirror. Thus, even if we
use a very great number of mirrors, we shall succeed in illuminating but
an extremely small proportion of the sea horizon. This evil is met by
giving a horizontal rotatory motion to the reflectors, causing the beams
to sweep over the whole expanse of the waters; and thus from every ship
the light will be visible for an instant. The rotation is produced by
clockwork, duly regulated, so that an uniform motion is obtained. The
regular appearances and eclipses of the light prevent the mariner from
mistaking for a lighthouse a bright star near the horizon or an
accidental fire on the coast; and, further, it being necessary that the
lighthouses along any particular coast should be readily distinguishable
from each other, it becomes easy, by assigning to each lighthouse a
different period of revolution, to individualize them, so that the
mariner shall be in no danger of confounding one with another.
But when the lighthouses on a certain extent of coast are numerous, this
mode of distinguishing them becomes inconvenient, as mistakes might
easily be made in small differences of time; and it would be inexpedient
to keep long intervals of darkness. Hence other methods have been
resorted to in addition—such as red lights, or lights alternately red
and white. The following are the distinctions made use of among the
Scottish lighthouses, including the double lighthouses, which give a
leading line to the navigator:
1. Fixed lights.
2. Revolving lights.
3. Revolving, with red and white beams alternately.
4. Revolving, with alternately two white beams and one red.
5. Revolving, with alternately two red beams and one white.
6. Flashing, in which the light increases and decreases at regular
intervals.
7. Intermittent, in which, by means of a revolving screen, the light
is abruptly cut off and exhibited.
8. Double fixed lights.
9. Double revolving lights, which appear and disappear at the same
instant.
The efficiency of reflectors depends on the state of polish of the
surface, and even with the most brilliant polish there is a very large
loss of light: in the ordinary condition of lighthouse reflectors, it is
found that one-half of the light is lost at the surface of the mirrors.
An attempt was made in England, about the beginning of the present
century, to substitute glass lenses for mirrors. But it was found that,
in spite of the loss occurring in reflection, the mirrors produced a
more intense beam. No doubt the person who made the attempt did not
observe the true conditions of the problem. It was Fresnel, the
illustrious Frenchman, whose name has already been mentioned in these
pages, who successfully solved the problem. He saw that it would be
necessary to give the lenses a short focal length, and at the same time
to have their diameters very great. The dimensions required by these
conditions far exceeded any that could be given to lenses formed in the
ordinary manner; and even if they could be so formed, the great
thickness of glass which would be necessary would diminish the
transparency, and unduly increase the weight of the apparatus to the
detriment of the revolving apparatus. An idea now occurred to Fresnel’s
mind, which, although similar to previous projects, he conceived
independently, and was undoubtedly the first to carry out. This was the
idea of the _lentille à échelons_, or “lens in steps.” The construction
of this will be understood from Fig. 305, where _a b_ is a section of a
lens in steps, and the dotted line, _c_, shows the thickness an ordinary
lens of the diameter _a b_ would have. Fresnel kept only the marginal
part of such a lens; and inside of the ring formed by this, he fitted
the margin of a second large lens having the same focal distance; inside
of this another ring, and so on; and in the centre a large lens of
moderate thickness. He also placed above and below the lens the
concentric prisms, _e e´_ and _f f´_, which, by refraction and total
reflections (see page 399), send the rays parallel to the axis of the
lens. Fresnel also contrived methods of economically grinding such
lenses and prisms with precision.
[Illustration:
FIG. 305.—_Revolving Light Apparatus._
]
Fresnel saw that it would be useless to apply lenses in lighthouse
illumination unless the intensity of the light given out by the
single-wick Argand lamps then in use could be considerably increased,
without much enlarging the flame. Accordingly he devoted himself, in
conjunction with his friend Arago, to this preliminary consideration.
Their studies and experiments led them to the construction of the lamp
with several concentric wicks—by which a brilliancy of light is
obtainable twenty-five times greater than that of the single-wick
Argand. The light which the improved lamp, when combined with Fresnel’s
lenses, could send to the horizon, was equivalent to that which would be
given by the united beams of 4,000 Argand lamps without optical
apparatus; and it was eight times greater than any which could be
produced by the reflectors then in use. The first apparatus constructed
on Fresnel’s plan was placed on the Tour de Cordouan in July, 1823.
France led the van in the erection of the most perfect lighthouses in
the world, and it was not until 1835 that, by the strenuous advocacy of
Mr. Alan Stevenson, a dioptric apparatus was employed in a British
lighthouse; but at the present time Fresnel’s principle has been adopted
in the majority of British lighthouses. Fig. 305 is a part elevation,
with the section, of a catadioptric apparatus of the first class. In
plan it is a regular octagon, and it sends out eight beams, which are
directed to the horizon, and made to sweep over the sea by its regular
rotation, produced by clockwork contained in the case, A. The whole
frame is very accurately balanced, and turns on its bearings, and the
rollers, _h_, _h_, with great smoothness and steadiness. The moving
power is given by the descent of a weight attached to a chain or cord,
which is wound round a barrel. One train of wheels is connected with
apparatus for regulating the speed, and to this an indicator is attached
which registers the number of revolutions made in an hour. There is also
a contrivance of some kind for maintaining the motion while the weight
is being wound up. The reader will observe that all the light of the
lamp, L, is utilized, except that which is directed towards the base and
top of the apparatus—a quantity less than one-fifth of the whole. About
45 per cent. of the light emitted by the lamp falls on the refracting
lenses; 22½ on the upper reflecting prisms; and 13½ on the lower
reflecting prisms. The brightest part of the flame is placed so that the
beams from it are directed towards the sea horizon, and the space
between the horizon and the neighbourhood of the lighthouse receives
ample light from the other parts of the flame. Thus a ship, or any part
of the sea within the range of the lighthouse, will see the light
appearing at regular intervals, as one after another of the eight beams
passes across it, the intervals being one eighth of the time in which
the apparatus completes its revolution. The zones of totally reflecting
prisms, shown at _e e´_, _f f´_, Fig. 305, were not adopted in British
lighthouses until 1844, when the Skerryvore light was exhibited with the
complete apparatus represented in the drawing.
The optical apparatus for lighthouses is constructed of certain sizes,
adapted to the different situations in which it is to be used. The
apparatus we have just described is made in six forms, according to the
_order_ of light required. The first three orders are for sea lights,
the rest for harbour lights; and the following are the dimensions of the
apparatus for each order of revolving or fixed lights:
┌─────────────┬─────────────┬─────────────┬───────────────────────────┐
│ Order. │ Height in │ Internal │ Number of Reflecting │
│ │ Inches. │ Diameter in │ Prisms. │
│ │ │ Inches. │ │
├─────────────┼─────────────┼─────────────┼─────────────┬─────────────┤
│ │ │ │ In Upper │ In Lower │
│ │ │ │ Zone. │ Zone. │
├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤
│ 1 │ 106½│ 72½│ 18 │ 8 │
│ 2 │ 83½│ 55 │ 16 │ 4 │
│ 3 │ 61½│ 39½│ 11 │ 4 │
│ 4 │ 29 │ 19¾│ 5 │ 4 │
│ 5 │ 21¾│ 14¾│ 5 │ 4 │
│ 6 │ 17½│ 12 │ 5 │ 4 │
└─────────────┴─────────────┴─────────────┴─────────────┴─────────────┘
When a revolving apparatus of the above description is erected on shore,
a reflector of suitable shape and dimensions is placed on the landward
side of the lamp, so as to throw its rays back upon itself and towards
the lenses which are directed seaward.