THE LIGHTHOUSE (_concluded_).
Lighthouses on Sand—Literally screwed down—The Light on Maplin
Sands—That of Port Fleetwood—Iron Lighthouses—The Lanterns
themselves—Eddystone long Illuminated with Tallow Candles—Coal
Fires—Revolution caused by the invention of the Argand
Burner—Improvements in Reflectors—The Electric Light at
Sea—Flashing and Revolving Lights—Coloured Lights—Their Advantages
and Disadvantages—Lanterns obscured by Moths, Bees, and Birds.
The difficulties involved in constructing a lighthouse on solid rock have
been shown, and it was at one time thought absolutely impossible to
erect—with any prospect of permanent duration—one upon storm-exposed
sands. _Nous avons changé tout cela._ It is no longer necessary to place
floating lights in places of great danger, although for other reasons they
are constantly used. One of the greatest modern triumphs of engineering is
Mitchell’s screw-mooring apparatus. To describe it fully would necessitate
several pages of technical matter. Suffice it to say that enormous
cast-iron screws, having hollow cylindrical centres, through which
wrought-iron spindles pass, are literally screwed down into the sand, or
its substratum of other soil. One of the earliest experiments was made on
the verge of the Maplin Sand, at the mouth of the Thames. Nine of the
mooring-screws were inserted into the sand 21½ feet, one in the centre,
the rest forming an octagon 42 feet in circumference, having standards or
posts which stood 5 feet above the surface of the sand. A raft of timber
was floated over the spot, and a capstan in its centre drove the screws to
the required depth. This raft was afterwards sunk, by covering it with 200
tons of rough stone. Two years were allowed to elapse, at the termination
of which time the whole mass was found firmly embedded, and then a
lighthouse, raised on a strong open framework, was erected over this
sub-structure. During these long preparations a very similar structure was
commenced and finished at Port Fleetwood, on the River Wyre, near
Lancaster.
The preparatory steps were similar to those already described. The
foundation of the lighthouse was formed of seven screw-piles, six of them
occupying the angles of a hexagon 46 feet in diameter, the seventh being
in the centre. From each screw proceeds a pile 15 feet in length, having
at the upper end another screw for securing a wooden column. These columns
are of Baltic timber, the one in the centre being 56 feet, the others 46
feet in length, firmly secured with iron hoops and coated with pitch. The
platform, upon which the house stands, is 27 feet in diameter, the house
itself being 20 feet in diameter and 9 feet high. From the summit of the
house rises a twelve-sided lantern, 10 feet in diameter and 8 feet high.
Altogether the light is elevated about 46 feet above low-water level, and
ranges over an horizon of eight miles. The light is of the dioptric
kind—bright, steady, and uniform, and when the weather is too foggy to
allow it to be seen, a bell is tolled by machinery, to give the needful
warning.
At the period when screw-pile lighthouses were being thus successfully
erected, other and most valuable suggestions were being made for the
building of bronze and cast-iron lighthouses. The great advantage of iron
over stone and other materials in those portions of the building not
actually in contact with sea-water soon became apparent. Upon a given base
a much larger internal capacity could be obtained; plates could be cast in
large surfaces and with few joints, and a system of binding adopted which
should ensure the perfect combination of every part. The comparatively
small bulk and weight also of the component parts gave great facilities
for the transport and rapid construction of such structures. The initial
cast-iron lighthouse was designed by Mr. Gordon in 1840, and was cast and
put together within three months from the date of the contract. It was
then taken to pieces and shipped for Jamaica, on which island it now
lights up Morant Point, a point of great danger. The Commissioners of the
House of Assembly had applied to Mr. Gordon to supply a suitable
lighthouse at the smallest possible cost, and in furnishing them with the
structure of cast-iron he fulfilled their wishes admirably, the expense
not exceeding one-third of the cost of a similar building in stone. This
elegant lighthouse, the outline of which resembles that of the Celtic
towers of Ireland, was exhibited to visitors while it stood complete in
the contractor’s premises. The diameter of the tower is 18 feet 6 inches
at the base, diminishing to 11 feet under the cap. The tower is formed of
nine tiers of iron plates, each tier being 10 feet high and about
three-quarters of an inch thick. At the base of the structure eleven
plates are required to form the circumference, at the top nine plates;
they are cast with a flange around their inner edges, and when put
together these flanges form the joints, which are fastened together with
nut-and-screw bolts and caulked with iron cement. The interior of the
tower, to the height of 27 feet, was to be filled up with masonry and
concrete of the weight of 300 tons; the remainder is divided into
store-rooms and berths for the attendants. The tower is finished by an
iron railing, within which rises the light-room, also of cast-iron, with
windows of plate-glass. A copper roof and a short lightning-rod complete
the whole. The Admiralty notice announced the exhibition of this light on
Morant Point November 1st, 1842, and stated that the elevation of the
light is 97 feet above the level of the sea, and that in clear weather it
is visible at a distance of twenty-one miles. The light is of the
revolving kind, consisting of fifteen Argand lamps and reflectors, five in
each side of an equilateral triangle, and so placed as to produce a
continuous light, but with periodical flashes. The tower is painted white,
and the lower portion is coated with coal-tar to preserve it from rust. It
rests on a granite base, and is also cased with granite near the
foundation, the more certainly to prevent the action of the sea-water on
the metal.
While the engineer had attained some of his greatest triumphs in the
construction of lighthouses, the optician had not once directed his
attention to the invention of a brilliant light, worthy to be placed upon
the structure which proudly rose high above the fierce waves with the
strength and solidity of a rock. During a period of forty years after the
completion of the Eddystone tower by Smeaton, the lantern was illuminated
by tallow candles stuck in hoops, just as a stand or booth is lighted at a
country fair, and so lately as the year 1811 it was lighted with
twenty-four wax candles. In 1812 the Lizard Light was maintained with coal
fires; and in 1816, when the Isle of May Light, in the Firth of Forth, was
taken possession of by the Commissioners of the Northern Lighthouses, a
coal fire was exhibited in a _chauffer_—a description of light which had
been exhibited for 181 years. In 1801 the light at Harwich, in addition to
the coal fire, had a _flat_ plate of rough brass on the landward side, to
serve as a reflector. Such methods of lighting were of course very
deficient in power, and did not enable the mariner to distinguish one
light from another—a point which is often of as much importance as the
brilliancy of the light itself. Prior to the invention of the Argand lamp
(about 1784) the production of a strong and brilliant light from a single
source was scarcely possible, and even such a lamp, by its unassisted
powers, would not be of very great value in giving early notice to the
mariner of his approach to the coast, which ought to be the primary object
of a lighthouse. As the rays of a luminous body proceed in all directions
in straight lines, it is obvious that in the case of a single lamp the
mariner would derive benefit only from that small portion of light which
proceeded from the centre of the flame to his eye. The other rays would
proceed to other parts of the horizon, or escape upwards to the sky, or
downwards to the earth, and thus be of no value to him. By increasing the
number of burners a small portion of light from each burner would slightly
increase the effective action, but by far the greater portion of the light
produced would escape uselessly above and below the horizon and also at
the back of each flame. Next, these defects were remedied, and the
efficiency of the light greatly increased, by placing behind each lamp a
reflector of such a form as to collect the rays that would otherwise be
lost, and throw them forward to the horizon. The adoption of such a method
has led to what is called the catoptric system of lights.
[Illustration: HOLOPHOTAL REVOLVING LIGHT. (FIRST ORDER) FLOATING LIGHT
LANTERN. HOLOPHOTAL REVOLVING LIGHT. (FOURTH ORDER.)
REVOLVING LIGHT APPARATUS.
(_From Drawings supplied by Messrs. W. Wilkins & Co._)]
Alan Stevenson states that the earliest notice he has been able to find of
the application of paraboloidal mirrors to lighthouses is in a work on
“Practical Seamanship” (Liverpool, 1791), by Mr. William Hutchinson, who
notices the erection of the four lights at Bidstone and Hoylake for the
entrance of the Mersey, in 1763, and describes large paraboloidal moulds
of wood lined with mirror glass and smaller ones of polished tin-plate, as
in use in those lighthouses. In France M. Téulère, a Member of the Royal
Corps of Engineers of Bridges and Roads, is regarded as the inventor of
the catoptric system of lights. In a memoir dated 26th June, 1783, he is
said to have proposed for the Cordouan Lighthouse a combination of
paraboloidal reflectors with Argand lamps, arranged on a revolving frame,
a plan which was actually carried into execution, under the direction of
the Chevalier Borda.(60) The plan was so successful that it was soon
adopted in England by the Trinity House of London; and in Scotland the
first work of the Northern Lights Board, in 1787, was to light a lantern
on the Old Castle of Kinnaird Head, in Aberdeenshire, by means of
parabolic reflectors and lamps. These reflectors were formed of facets of
mirror-glass placed in hollow paraboloidal moulds of plaster. The more
complicated arrangement of lenses placed round a centre in concentric
circles is due to the great Fresnel, a practical man of science, whose
abilities are acknowledged as fully in England as in France.
The oil used in the lighthouses of the United Kingdom has generally been
sperm. Colza, the expressed oil of the wild cabbage (_Brassica oleracea_),
was very generally used in France, and occasionally in Great Britain. Gas
is used in a few places, where its application is easy. There can hardly
be any doubt now, however, that the coming light will be the electric,
since its steady production is becoming a matter of scientific certainty.
As early as 1857 Professor Holmes submitted to the Trinity House a method
of employing this light, which was submitted to Faraday, and approved. The
Board then allowed a trial at the South Foreland Lighthouse. The light was
first displayed on the 8th of December, 1858. In June, 1862, it was
permanently fixed at Dungeness. In Faraday’s Report to the Trinity House,
published in 1862, he says: “Arrangements were made on shore by which
observations could be made at sea, about five miles off, on the relative
light of the electric lamp and the metallic reflectors with their Argand
oil-lamps, for either could be shown alone, or both together. At the given
distance the eye could not separate the two lights, but by the telescope
they were distinguishable. The combined effect was a glorious light up to
five miles; then, if the electric light was extinguished, there was a
great falling off in the effect, though, after a few moments’ rest to the
eye, it was seen that the oil-lamps and reflectors were in their good and
proper state. On the other hand, when the electric light was restored, the
glory rose to its first high condition.... During the day-time I compared
the intensity of the light with that of the sun, and both looked at
through dark glasses. Its light was as bright as that of the sun, but the
sun was not at its brightest.”
The number of lights on a well-frequented coast being considerable, it is
of the utmost importance to arrange them so as to enable the mariner
easily to distinguish them from each other. Catoptric lights admit of nine
separate distinctions:—1, fixed; 2, revolving white; 3, revolving red and
white; 4, revolving red with two whites; 5, revolving white with two reds;
6, flashing; 7, intermittent; 8, double fixed lights; 9, double revolving
white lights. Mr. Stevenson thus defines their distinctive features:—“The
first exhibits a steady and uniform appearance which is not subject to any
change, and the reflectors used for it 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 produced by the revolution of a frame whose
different sides present red and white lights, and these 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 revolutions, which shows 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 further 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. 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
Gulf 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 four-sevenths to five-sixths 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 four-sevenths 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.”
The construction of the lantern is a point of importance; and one of the
first order will cost about £1,260. On the level of the top of the lower
glass a narrow gangway is usually built for the keeper to stand upon in
order to clean the panes, an operation which in snowy weather may have to
be frequently repeated during the night. At some of the lighthouses on the
Mediterranean the lantern is at certain seasons so completely covered with
moths as to obscure the light and to require the attendance of men with
brooms. Mr. Tomlinson was informed by the keepers at the Eddystone that
bees and other insects were much attracted by the light, and collected
round the lantern in great numbers. Larks and other birds flew against it,
and, becoming stunned with the blow, were picked up on the balcony and
were cooked by the men for breakfast. The lantern is very liable to injury
in high winds, or the glass may be broken by large sea-birds coming
against it on a stormy night, or by small stones violently driven against
it by the wind. Extra plates of glass are always kept to take the place of
broken panes. The number of light-keepers employed varies, ranging from
two to four, and in the latter case one is usually allowed to remain on
shore, the men taking the privilege in turns. When the situation admits,
it is usual to have the keeper’s rooms in a building outside the
lighthouse to avoid dust, which is most injurious to the delicate
apparatus of the light-room. Great cleanliness is enforced in all that
belongs to a lighthouse, the reflectors and lenses being constantly
burnished, polished, and cleansed.
And so we have traced the history and progress of lighthouses, and it is
hard to believe that any great change can be advantageously made in their
construction, though their mode of illumination will doubtless be greatly
improved. As we have seen, the electric light was used practically in a
lighthouse long before it was in the streets of the great metropolis, and
not in a merely experimental way, but with the most successful results.
[Illustration: BREAKWATER AT VENICE.]