_Jointing of Stones in Rock Towers._--Various methods of jointing the
stones in rock towers are shown in figs. 6 and 22. The great distinction
between the towers built by successive engineers to the Trinity House
and other rock lighthouses is that, in the former the stones of each
course are dovetailed together both laterally and vertically and are not
connected by metal or wooden pins and wedges and dowled as in most other
cases. This dovetail method was first adopted at the Hanois Rock at the
suggestion of Nicholas Douglass. On the upper face, one side and at one
end of each block is a dovetailed projection. On the under face and the
other side and end, corresponding dovetailed recesses are formed with
just sufficient clearance for the raised bands to enter in setting (fig.
23). The cement mortar in the joint formed between the faces so locks
the dovetails that the stones cannot be separated without breaking (fig.
24).
TABLE I.--_Comparative Cost of Exposed Rock Towers_.
+----------------------------------------------------------+--------------+--------+-----------+
| | | | Cost per |
| Name of Structure. | Total Cost. |Cub. ft.|cub. ft. of|
| | | | Masonry. |
+----------------------------------------------------------+--------------+--------+-----------+
| Eddystone, Smeaton (1759) |£40,000 0 0 | 13,343 | £2 9 11½ |
| Bell Rock, Firth of Forth (1811) | 55,619 12 1 | 28,530 | 1 19 0 |
| Skerryvore, west coast of Scotland (1844) | 72,200 11 6 | 58,580 | 1 4 7¾ |
| Bishop Rock, first granite tower (1858) | 34,559 18 9 | 35,209 | 0 19 7½ |
| Smalls, Bristol Channel (1861) | 50,124 11 8 | 46,386 | 1 1 7¼ |
| Hanois, Alderney (1862) | 25,296 0 0 | 24,542 | 1 0 7¼ |
| Wolf Rock, Land's End (1869) | 62,726 0 0 | 59,070 | 1 1 3 |
| Dhu Heartach, west coast of Scotland (1872) | 72,584 9 7 | 42,050 | 1 14 6 |
| Longships, Land's End (1872) | 43,869 8 11 | 47,610 | 0 18 5 |
| Eddystone, Douglass (1882) | 59,255 0 0 | 65,198 | 0 18 2 |
| Bishop Rock, strengthening and part reconstruction (1887)| 64,889 0 0 | 45,080 | 1 8 9 |
| Great Basses, Ceylon (1873) | 63,560 0 0 | 47,819 | 1 6 7 |
| Minot's Ledge, Boston, Mass. (1860) | 62,500 0 0 | 36,322 | 1 17 2 |
| Spectacle Reef, Lake Huron (1874) | 78,125 0 0 | 42,742 | 1 16 2 |
| Ar'men, France (1881) | 37,692 0 0 | 32,400 | 1 3 3 |
| Fastnet, Ireland (1904) | 79,000 0 0 | 62,600 | 1 5 5½ |
+----------------------------------------------------------+--------------+--------+-----------+
_Effect of Waves._--The wave stroke to which rock lighthouse towers are
exposed is often considerable. At the Dhu Heartach, during the erection
of the tower, 14 joggled stones, each of 2 tons weight, were washed away
after having been set in cement at a height of 37 ft. above high water,
and similar damage was done during the construction of the Bell Rock
tower. The effect of waves on the Bishop Rock and Eddystone towers has
been noted above.
_Land Structures for Lighthouses._--The erection of lighthouse towers
and other buildings on land presents no difficulties of construction,
and such buildings are of ordinary architectural character. It will
therefore be unnecessary to refer to them in detail. Attention is
directed to the Phare d'Eckmühl at Penmarc'h (Finistère), completed in
1897. The cost of this magnificent structure, 207 ft. in height from the
ground, was largely defrayed by a bequest of £12,000 left by the marquis
de Blocqueville. It is constructed entirely of granite, and is octagonal
in plan. The total cost of the tower and other lighthouse buildings
amounted to £16,000.
[Illustration: FIG. 19.--Maplin Pile Lighthouse.]
The tower at Île Vierge (Finistère), completed in 1902, has an elevation
of 247 ft. from the ground level to the focal plane, and is probably the
highest structure of its kind in the world.
The brick tower, constructed at Spurn Point, at the entrance to the
Humber and completed in 1895, replaced an earlier structure erected by
Smeaton at the end of the 18th century. The existing tower is
constructed on a foundation consisting of concrete cylinders sunk in the
shingle beach. The focal plane of the light is elevated 120 ft. above
high water.
Besides being built of stone or brick, land towers are frequently
constructed of cast iron plates or open steel-work with a view to
economy. Fine examples of the former are to be found in many British
colonies and elsewhere, that on Dassen Island (Cape of Good Hope), 105
ft. in height to the focal plane, being typical (fig. 25). Many openwork
structures up to 200 ft. in height have been built. Recent examples are
the towers erected at Cape San Thomé (Brazil) in 1882, 148 ft. in height
(fig. 26), Mocha (Red Sea) in 1903, 180 ft. and Sanganeb Reef (Red Sea)
1906, 165 ft. in height to the focal plane.
[Illustration: FIG. 20.--American Shoal Lighthouse, Florida.]
3. OPTICAL APPARATUS.--Optical apparatus in lighthouses is required for
one or other of three distinct purposes: (1) the concentration of the
rays derived from the light source into a belt of light distributed
evenly around the horizon, condensation in the vertical plane only being
employed; (2) the concentration of the rays both vertically and
horizontally into a pencil or cone of small angle directed towards the
horizon and caused to revolve about the light source as a centre, thus
producing a flashing light; and (3) the condensation of the light in the
vertical plane and also in the horizontal plane in such a manner as to
concentrate the rays over a limited azimuth only.
Apparatus falling under the first category produce a fixed light, and
further distinction can be provided in this class by mechanical means of
occultation, resulting in the production of an occulting or intermittent
light. Apparatus included in the second class are usually employed to
produce flashing lights, but sometimes the dual condensation is taken
advantage of to produce a fixed pencil of rays thrown towards the
horizon for the purpose of marking an isolated danger or the limits of a
narrow channel. Such lights are best described by the French term _feux
de direction_. Catoptric apparatus, by which dual condensation is
produced, are moreover sometimes used for fixed lights, the light
pencils overlapping each other in azimuth. Apparatus of the third class
are employed for sector lights or those throwing a beam of light over a
wider azimuth than can be conveniently covered by an apparatus of the
second class, and for reinforcing the beam of light emergent from a
fixed apparatus in any required direction.
The above classification of apparatus depends on the resultant effect of
the optical elements. Another classification divides the instruments
themselves into three classes: (a) catoptric, (b) dioptric and (c)
catadioptric.
_Catoptric_ apparatus are those by which the light rays are reflected
only from the faces of incidence, such as silvered mirrors of plane,
spherical, parabolic or other profile. _Dioptric_ elements are those in
which the light rays pass through the optical glass, suffering
refraction at the incident and emergent faces (fig. 27). _Catadioptric_
elements are combined of the two foregoing and consist of optical prisms
in which the light rays suffer refraction at the incident face, total
internal reflexion at a second face and again refraction on emergence at
the third face (fig. 28).
The object of these several forms of optical apparatus is not only to
produce characteristics or distinctions in lights to enable them to be
readily recognized by mariners, but to utilize the light rays in
directions above and below the horizontal plane, and also, in the case
of revolving or flashing lights, in azimuths not requiring to be
illuminated for strengthening the beam in the direction of the mariner.
It will be seen that the effective condensation in flashing lights is
very much greater than in fixed belts, thus enabling higher intensities
to be obtained by the use of flashing lights than with fixed apparatus.
[Illustration: FIG. 21.--Rothersand Lighthouse.]
_Catoptric System._--Parabolic reflectors, consisting of small facets
of silvered glass set in plaster of Paris, were first used about the
year 1763 in some of the Mersey lights by Mr Hutchinson, then dock
master at Liverpool (fig. 29). Spherical metallic reflectors were
introduced in France in 1781, followed by parabolic reflectors on
silvered copper in 1790 in England and France, and in Scotland in
1803. The earlier lights were of fixed type, a number of reflectors
being arranged on a frame or stand in such a manner that the pencils
of emergent rays overlapped and thus illuminated the whole horizon
continuously. In 1783 the first revolving light was erected at
Marstrand in Sweden. Similar apparatus were installed at Cordouan
(1790), Flamborough Head (1806) and at the Bell Rock (1811). To
produce a revolving or flashing light the reflectors were fixed on a
revolving carriage having several faces. Three or more reflectors in a
face were set with their axes parallel.
A type of parabolic reflector now in use is shown in fig. 30. The
sizes in general use vary from 21 in. to 24 in. diameter. These
instruments are still largely used for light-vessel illumination, and
a few important land lights are at the present time of catoptric type,
including those at St Agnes (Scilly Islands), Cromer and St Anthony
(Falmouth).
_Dioptric System._--The first adaptation of dioptric lenses to
lighthouses is probably due to T. Rogers, who used lenses at one of
the Portland lighthouses between 1786 and 1790. Subsequently lenses by
the same maker were used at Howth, Waterford and the North Foreland.
Count Buffon had in 1748 proposed to grind out of a solid piece of
glass a lens in steps or concentric zones in order to reduce the
thickness to a minimum (fig. 31). Condorcet in 1773 and Sir D.
Brewster in 1811 designed built-up lenses consisting of stepped
annular rings. Neither of these proposals, however, was intended to
apply to lighthouse purposes. In 1822 Augustin Fresnel constructed a
built-up annular lens in which the centres of curvature of the
different rings receded from the axis according to their distances
from the centre, so as practically to eliminate spherical aberration;
the only spherical surface being the small central part or "bull's
eye" (fig. 32). These lenses were intended for revolving lights only.
Fresnel next produced his cylindric refractor or lens belt, consisting
of a zone of glass generated by the revolution round a vertical axis
of a medial section of the annular lens (fig. 33). The lens belt
condensed and parallelized the light rays in the vertical plane only,
while the annular lens does so in every plane. The first revolving
light constructed from Fresnel's designs was erected at the Cordouan
lighthouse in 1823. It consisted of 8 panels of annular lenses placed
round the lamp at a focal distance of 920 mm. To utilize the light,
which would otherwise escape above the lenses, Fresnel introduced a
series of 8 plain silvered mirrors, on which the light was thrown by a
system of lenses. At a subsequent period mirrors were also placed in
the lower part of the optic. The apparatus was revolved by clockwork.
This optic embodied the first combination of dioptric and catoptric
elements in one design (fig. 34). In the following year Fresnel
designed a dioptric lens with catoptric mirrors for fixed light, which
was the first of its kind installed in a lighthouse. It was erected at
the Chassiron lighthouse in 1827 (fig. 35). This combination is
geometrically perfect, but not so practically on account of the great
loss of light entailed by metallic reflection which is at least 25%
greater than the system described under. Before his death in 1827
Fresnel devised his totally reflecting or catadioptric prisms to take
the place of the silvered reflectors previously used above and below
the lens elements (fig. 28). The ray Fi falling on the prismoidal ring
ABC is refracted in the direction i r and meeting the face AB at an
angle of incidence greater than the critical, is totally reflected in
the direction r e emerging after second refraction in a horizontal
direction. Fresnel devised these prisms for use in fixed light
apparatus, but the principle was, at a later date, also applied to
flashing lights, in the first instance by T. Stevenson. Both the
dioptric lens and catadioptric prism invented by Fresnel are still in
general use, the mathematical calculations of the great French
designer still forming the basis upon which lighthouse opticians work.
[Illustration: FIG. 22.--Courses of various Lighthouse Towers.]
[Illustration: FIG. 23.--Perspective drawing of Dovetailed Stone (Wolf
Rock).]
[Illustration: FIG. 24.--Section of Dovetail.]
[Illustration: FIG. 25.--Dassen Island Lighthouse (cast iron).]
[Illustration: FIG. 26.--Cape San Thomé Lighthouse.]
[Illustration: FIG. 27.--Dioptric Prism.]
Fresnel also designed a form of fixed and flashing light in which the
distinction of a fixed light, varied by flashes, was produced by
placing panels of straight refracting prisms in a vertical position on
a revolving carriage outside the fixed light apparatus. The revolution
of the upright prisms periodically increased the power of the beam, by
condensation of the rays emergent from the fixed apparatus, in the
horizontal plane.
The lens segments in Fresnel's early apparatus were of polygonal form
instead of cylindrical, but subsequently manufacturers succeeded in
grinding glass in cylindrical rings of the form now used. The first
apparatus of this description was made by Messrs Cookson of Newcastle
in 1836 at the suggestion of Alan Stevenson and erected at Inchkeith.
[Illustration: FIG. 28.--Catadioptric or Reflecting Prism.]
In 1825 the French Commission des Phares decided upon the exclusive
use of lenticular apparatus in its service. The Scottish Lighthouse
Board followed with the Inchkeith revolving apparatus in 1835 and the
Isle of May fixed optic in 1836. In the latter instrument Alan
Stevenson introduced helical frames for holding the glass prisms in
place, thus avoiding complete obstruction of the light rays in any
azimuth. The first dioptric light erected by the Trinity House was
that formerly at Start Point in Devonshire, constructed in 1836.
Catadioptric or reflecting prisms for revolving lights were not used
until 1850, when Alan Stevenson designed them for the North Ronaldshay
lighthouse.
_Dioptric Mirror._--The next important improvement in lighthouse
optical work was the invention of the dioptric spherical mirror by Mr
(afterwards Sir) J. T. Chance in 1862. The zones or prisms are
generated round a vertical axis and divided into segments. This form
of mirror is still in general use (figs. 36 and 37).
[Illustration: FIG. 29.--Early Reflector and Lamp (1763).]
_Azimuthal Condensing Prisms._--Previous to 1850 all apparatus were
designed to emit light of equal power in every azimuth either
constantly or periodically. The only exception was where a light was
situated on a stretch of coast where a mirror could be placed behind
the flame to utilize the rays, which would otherwise pass landward,
and reflect them back, passing through the flame and lens in a seaward
direction. In order to increase the intensity of lights in certain
azimuths T. Stevenson devised his azimuthal condensing prisms which,
in various forms and methods of application, have been largely used
for the purpose of strengthening the light rays in required directions
as, for instance, where coloured sectors are provided. Applications of
this system will be referred to subsequently.
[Illustration: FIG. 30.--Modern Parabolic Reflector.]
_Optical Glass for Lighthouses._--In the early days of lens lights the
only glass used for the prisms was made in France at the St Gobain and
Premontré works, which have long been celebrated for the high quality
of optical glass produced. The early dioptric lights erected in the
United Kingdom, some 13 in all, were made by Messrs Cookson of South
Shields, who were instructed by Léonor Fresnel, the brother of
Augustin. At first they tried to mould the lens and then to grind it
out of one thick sheet of glass. The successors of the Cookson firm
abandoned the manufacture of lenses in 1845, and the firm of
Letourneau & Lepaute of Paris again became the monopolists. In 1850
Messrs Chance Bros. & Co. of Birmingham began the manufacture of
optical glass, assisted by M. Tabouret, a French expert who had been a
colleague of Augustin Fresnel himself. The first light made by the
firm was shown at the Great Exhibition of 1851, since when numerous
dioptric apparatus have been constructed by Messrs Chance, who are, at
this time, the only manufacturers of lighthouse glass in the United
Kingdom. Most of the glass used for apparatus constructed in France is
manufactured at St Gobain. Some of the glass used by German
constructors is made at Rathenow in Prussia and Goslar in the Harz.
The glass generally employed for lighthouse optics has for its
refractive index a mean value of µ = 1.51, the corresponding critical
angle being 41° 30'. Messrs Chance have used dense flint glass for the
upper and lower refracting rings of high angle lenses and for dioptric
mirrors in certain cases. This glass has a value of µ = l.62 with
critical angle 38° 5'.
[Illustration: FIG. 31. Buffon's Lens.]
[Illustration: FIG. 32. Fresnel's Annular Lens.]
[Illustration: FIG. 33. Fresnel's Lens Belt.]
[Illustration: FIG. 34.--Fresnel's Revolving Apparatus at Cordouan
Lighthouse.]
_Occulting Lights._--During the last 25 years of the 19th century the
disadvantages of fixed lights became more and more apparent. At the
present day the practice of installing such, except occasionally in
the case of the smaller and less important of harbour or river lights,
has practically ceased. The necessity for providing a distinctive
characteristic for every light when possible has led to the conversion
of many of the fixed-light apparatus of earlier years into occulting
lights, and often to their supersession by more modern and powerful
flashing apparatus. An occulting apparatus in general use consists of
a cylindrical screen, fitting over the burner, rapidly lowered and
raised by means of a cam-wheel at stated intervals. The cam-wheel is
actuated by means of a weight or spring clock. Varying characteristics
may be procured by means of such a contrivance--single, double, triple
or other systems of occultation. The eclipses or periods of darkness
bear much the same relation to the times of illumination as do the
flashes to the eclipses in a revolving or flashing light. In the case
of a first-order fixed light the cost of conversion to an occulting
characteristic does not exceed £250 to £300. With apparatus
illuminated by gas the occultations may be produced by successively
raising and lowering the gas at stated intervals. Another form of
occulting mechanism employed consists of a series of vertical screens
mounted on a carriage and revolving round the burner. The carriage is
rotated on rollers or ball bearings or carried upon a small mercury
float. The usual driving mechanism employed is a spring clock. "Otter"
screens are used in cases when it is desired to produce different
periods of occultations in two or more positions in azimuth in order
to differentiate sectors marking shoals, &c. The screens are of sheet
metal blacked and arranged vertically, some what in the manner of the
laths of a venetian blind, and operated by mechanical means.
_Leading Lights._--In the case of lights designed to act as a lead
through a narrow channel or as direction lights, it is undesirable to
employ a flashing apparatus. Fixed-light optics are employed to meet
such cases, and are generally fitted with occulting mechanism. A
typical apparatus of this description is that at Gage Roads,
Fremantle, West Australia (fig. 38). The occulting bright light covers
the fairway, and is flanked by sectors of occulting red and green
light marking dangers and intensified by vertical condensing prisms. A
good example of a holophotal direction light was exhibited at the 1900
Paris Exhibition, and afterwards erected at Suzac lighthouse (France).
The light consists of an annular lens 500 mm. focal distance, of 180°
horizontal angle and 157° vertical, with a mirror of 180° at the back.
The lens throws a red beam of about 4½° amplitude in azimuth, and
50,000 candle-power over a narrow channel. The illuminant is an
incandescent petroleum vapour burner. Holophotal direction lenses of
this type can only be applied where the sector to be marked is of
comparatively small angle. Silvered metallic mirrors of parabolic form
are also used for the purpose. The use of single direction lights
frequently renders the construction of separate towers for leading
lights unnecessary.
If two distinct lights are employed to indicate the line of navigation
through a channel or between dangers they must be sufficiently far
apart to afford a good lead, the front or seaward light being situated
at a lower elevation than the rear or landward one.
_Coloured Lights._--Colour is used as seldom as possible as a
distinction, entailing as it does a considerable reduction in the
power of the light. It is necessary in some instances for
differentiating sectors over dangers and for harbour lighting
purposes. The use of coloured lights as alternating flashes for
lighthouse lights is not to be commended, on account of the unequal
absorption of the coloured and bright rays by the atmosphere. When
such distinction has been employed, as in the Wolf Rock apparatus, the
red and white beams can be approximately equalized in initial
intensity by constructing the lens and prism panels for the red light
of larger angle than those for the white beams. Owing to the
absorption by the red colouring, the power of a red beam is only 40%
of the intensity of the corresponding white light. The corresponding
intensity of green light is 25%. When red or green sectors are
employed they should invariably be reinforced by mirrors, azimuthal
condensing prisms, or other means to raise the coloured beam to
approximately the same intensity as the white light. With the
introduction of group-flashing characteristics the necessity for using
colour as a means of distinction disappeared.
[Illustration: FIG. 35.--Fixed Apparatus at Chassiron Lighthouse
(1827).]
[Illustration: FIG. 36.--Vertical Section. Prism of Dioptric Spherical
Mirror.]
_High-Angle Vertical Lenses._--Messrs Chance of Birmingham have
manufactured lenses having 97° of vertical amplitude, but this result
was only attained by using dense flint glass of high refractive index
for the upper and lower elements. It is doubtful, however, whether the
use of refracting elements for a greater angle than 80° vertically is
attended by any material corresponding advantage.
[Illustration: FIG. 37.--Chance's Dioptric Spherical Mirror.]
_Group Flashing Lights._--One of the most useful distinctions consists
in the grouping of two or more flashes separated by short intervals of
darkness, the group being succeeded by a longer eclipse. Thus two,
three or more flashes of, say, half second duration or less follow
each other at intervals of about 2 seconds and are succeeded by an
eclipse of, say, 10 seconds, the sequence being completed in a period
of, say, 15 seconds. In 1874 Dr John Hopkinson introduced the very
valuable improvement of dividing the lenses of a dioptric revolving
light with the panels of reflecting prisms above and below them,
setting them at an angle to produce the group-flashing characteristic.
The first apparatus of this type constructed were those now in use at
Tampico, Mexico and the Little Basses lighthouse, Ceylon (double
flashing). The Casquets apparatus (triple flashing) was installed in
1877. A group-flashing catoptric light had, however, been exhibited
from the "Royal Sovereign" light-vessel in 1875. A sectional plan of
the quadruple-flashing first order apparatus at Pendeen in Cornwall
is shown in fig. 39; and fig. 55 (Plate 1.) illustrates a double
flashing first order light at Pachena Point in British Columbia.
Hopkinson's system has been very extensively used, most of the
group-flashing lights shown in the accompanying tables, being designed
upon the general lines he introduced. A modification of the system
consists in grouping two or more lenses together separated by equal
angles, and filling the remaining angle in azimuth by a reinforcing
mirror or screen. A group-flashing distinction was proposed for gas
lights by J. R. Wigham of Dublin, who obtained it in the case of a
revolving apparatus by alternately raising and lowering the flame. The
first apparatus in which this method was employed was erected at
Galley Head, Co. Cork (1878). At this lighthouse 4 of Wigham's large
gas burners with four tiers of first-order revolving lenses, eight in
each tier, were adopted. By successive lowering and raising of the gas
flame at the focus of each tier of lenses he produced the
group-flashing distinction. The light showed, instead of one prolonged
flash at intervals of one minute, as would be produced by the
apparatus in the absence of a gas occulter, a group of short flashes
varying in number between six and seven. The uncertainty, however, in
the number of flashes contained in each group is found to be an
objection to the arrangement. This device was adopted at other
gas-illuminated stations in Ireland at subsequent dates. The
quadriform apparatus and gas installation at Galley Head were
superseded in 1907 by a first order bi-form apparatus with
incandescent oil vapour burner showing five flashes every 20 seconds.
[Illustration: FIG. 38.--Gage Roads Direction Light.]
[Illustration: FIG. 39.--Pendeen Apparatus. Plan at Focal Plane.]
_Flashing Lights indicating Numbers._--Captain F. A. Mahan, late
engineer secretary to the United States Lighthouse Board, devised for
that service a system of flashing lights to indicate certain numbers.
The apparatus installed at Minot's Ledge lighthouse near Boston
Harbour, Massachusetts, has a flash indicating the number 143, thus: -
---- ---, the dashes indicating short flashes. Each group is separated
by a longer period of darkness than that between successive members of
a group. The flashes in a group indicating a figure are about 1½
seconds apart, the groups being 3 seconds apart, an interval of 16
seconds' darkness occurring between each repetition. Thus the number
is repeated every half minute. Two examples of this system were
exhibited by the United States Lighthouse Board at the Chicago
Exhibition in 1893, viz. the second-order apparatus just mentioned and
a similar light of the first order for Cape Charles on the Virginian
coast. The lenses are arranged in a somewhat similar manner to an
ordinary group-flashing light, the groups of lenses being placed on
one side of the optic, while the other is provided with a catadioptric
mirror. This system of numerical flashing for lighthouses has been
frequently proposed in various forms, notably by Lord Kelvin. The
installation of the lights described is, however, the first practical
application of the system to large and important coast lights. The
great cost involved in the alteration of the lights of any country to
comply with the requirements of a numerical system is one of the
objections to its general adoption.
[Illustration: PLATE I.
FIG. 54.--FASTNET LIGHTHOUSE--FIRST ORDER SINGLE-FLASHING BIFORM
APPARATUS.
FIG. 55.--PACHENA POINT LIGHTHOUSE, B.C.--FIRST ORDER DOUBLE-FLASHING
APPARATUS.]
[Illustration: PLATE II.