THE LIGHT AND ILLUMINANTS
While it is the tower that probably creates the deepest impression
upon the popular mind, owing to the round of difficulties overcome
associated with its erection, yet, after all, it is the light which
is the vital thing to the navigator. To him symmetry of outline in
the tower, the searching problems that had to be solved before it was
planted in a forbidding spot, the risks that were incurred in its
erection--these are minor details. His one concern is the light thrown
from the topmost height, warning him to keep off a dangerous spot and
by its characteristic enabling him to determine his position.
I have described the earliest type of light, the open wood or coal
fire blazing on an eminence. In due course the brazier gave way to
tallow candles. This was an advance, certainly, but the range of the
naked light was extremely limited. Consequently efforts were made to
intensify it and to throw it in the desired direction. The first step
was made with a reflector placed behind the illuminant, similar to that
used with the cheap wall-lamp so common in village workshops. This, in
its improved form, is known as the “catoptric system,” the reflector
being of parabolic shape, with the light so disposed that all its rays
(both horizontal and vertical) are reflected in one direction by the
aid of a highly polished surface. While the catoptric system is still
used on some light-vessels, its application to important lighthouses
has fallen into desuetude, as it has been superseded by vastly improved
methods. But the reflector, made either of silvered glass set in a
plaster-of-Paris mould or of brightly polished metallic surfaces,
held the field until the great invention of Augustin Fresnel, which
completely revolutionized the science of lighthouse optics.
[Illustration: FIG. 2.--FIXED APPARATUS OF 360 DEGREES.
Shows one ray throughout the complete circle.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
Fresnel was appointed a member of the French Lighthouse Commission in
1811, and he realized the shortcomings of the existing catoptric method
only too well. Everyone knows that when a lamp is lighted the luminous
rays are diffused on every side, horizontally as well as vertically. In
lighthouse operations the beam has to be thrown in a horizontal line
only, while the light which is shed towards the top and bottom must be
diverted, so that the proportion of waste luminosity may be reduced to
the minimum. While the parabolic reflector achieved this end partially,
it was far from being satisfactory, and Fresnel set to work to condense
the whole of the rays into a horizontal beam. Buffon, a contemporary
investigator, as well as Sir David Brewster, had suggested that the end
might be met by building up a lens in separate concentric rings, but
neither reduced his theories to practice.
Fresnel invented a very simple system. He took a central piece of
glass, which may be described as a bull’s-eye, and around this
disposed a number of concentric rings of glass. But these rings
projected beyond one another. Each constituted the edge of a lens
which, while its radius differed from that of its neighbour, owing to
its position, yet was of the same focus in regard to the source of
illumination. The parts were shaped with extreme care and were united
in position by the aid of fish glue, the whole being mounted in a
metal frame. The advantage of the system was apparent in the first
demonstrations. The lenses being comparatively thin, only one-tenth of
the light passing through was absorbed, whereas in the old parabolic
reflectors one-half of the light was lost.
[Illustration: FIG. 3.--SINGLE FLASHING APPARATUS (ONE PANEL AND
MIRROR).
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
This revolutionary development was perfected in 1822, and in the
following year it was submitted to its first practical application
on the tower of Cordouan in the Gironde. Several modifications were
made by the inventor for the purpose of adapting his system to varying
conditions. One of the most important was the disposition of lenses
and mirrors above the optical apparatus for the purpose of collecting
and driving back the rays which were sent out vertically from the
illuminant, so that they might be mingled with the horizontal beam,
thereby reinforcing it. At a later date similar equiangular prisms
were placed below the horizontal beam so as to catch the light thrown
downwards from the luminous source, the result being that finally none,
or very little, of the light emitted by the illuminant was lost, except
by absorption in the process of bending the rays into the desired
direction.
[Illustration: FIG. 4.--A TWENTY-FOUR PANEL LIGHT, WHICH WAS INTRODUCED
INTO CERTAIN FRENCH LIGHTHOUSES.]
In this ingenious manner the circle of light is divided into sections,
called “panels,” each of which comprises its bull’s-eye and its
group of concentric rings and prisms. The extent of this division
varies appreciably, as many as sixteen panels being utilized in some
instances. In this direction, however, subdivision can be carried too
far. Thus, in some of the French lighthouses no less than twenty-four
panels were introduced. The disadvantage is obvious. The total volume
of light emitted from the luminous source has to be divided into
twenty-four parts, one for each panel. But the fewer the panels, the
more light is thrown through each, and the correspondingly greater
power of the beam. Thus, in a four-panel light each beam will be six
times as powerful as that thrown from a twenty-four panel apparatus of
the same type.
Fresnel also introduced the system of revolving the optical apparatus,
and by the introduction of suitable devices was able to give the light
a flashing characteristic, so that it became possible to provide
a means of identifying a light from a distance entirely by the
peculiarity of its flash. The French authorities were so impressed with
the wonderful improvement produced by Fresnel’s epoch-making invention
that it was adopted immediately for all French lights. Great Britain
followed suit a few years later, while other countries embraced the
system subsequently, so that the Fresnel lens eventually came into
universal use.
[Illustration: FIG. 5.--A FOUR-PANEL LIGHT.
The ray thrown through each panel is six times as powerful as the beam
thrown through a twenty-four panel apparatus.]
But the Frenchman’s ingenious invention has been developed out of
recognition. To-day only the fundamental basis is retained. Marked
improvements were made by Mr. Alan Stevenson, the famous Scottish
lighthouse engineer. In fact, he carried the idea to a far greater
degree than Fresnel ever contemplated, and in some instances even
anticipated the latter’s subsequent modifications and improvements.
This was demonstrated more particularly in the holophotal revolving
apparatus, the first example of which he designed for the North
Ronaldshay lighthouse in 1850, a similar apparatus being devised some
years later by Fresnel. In 1862 another great improvement was made by
Mr. J. T. Chance, of the well-known lighthouse engineering firm of
Birmingham, which proved so successful that it was incorporated for
first and third order apparatuses in the New Zealand lights designed by
Messrs. Stevenson in the same year.
[Illustration: FIG. 6.--SINGLE APPARATUS IN FOUR PANELS.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
The French and British investigators, however, were not having things
entirely their own way. The United States played a part in these
developments, although they did not enter very successfully into the
problem. The first lighthouse at Boston Harbour carried candles until
superseded by an ordinary lamp, which was hung in the lantern in much
the same way as it might have been suspended behind the window of a
private dwelling. An inventor, Mr. Winslow Lewis, who confessed that
he knew nothing about lighthouse optics, patented what he called a
“magnifying and reflecting lantern” for lighthouse work, which he
claimed was a lamp, a reflector, and a magnifier, all in one. It was as
crude a device as has ever emanated from an inventive brain, but the
designer succeeded in impressing the Government so effectively that
they gave him £4,000, or $20,000, for his invention. The reflector was
wrought of thin copper with a silvered surface, while the magnifier,
the essence of the invention, was what he called a “lens,” but which
in reality comprised only a circular transparent mass, 9 inches in
diameter, and varying from 2½ to 4 inches in thickness, made of
bottle-green glass. The Government considered that it had acquired a
valuable invention, and was somewhat dismayed by the blunt opinion
of one of its inspectors who held contrary views concerning the
magnifier, inasmuch as he reported cynically that its only merit was
that it made “a bad light worse.”
[Illustration: FIG. 7.--DOUBLE FLASHING APPARATUS: TWO PANELS AND
MIRROR.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
[Illustration: FIG. 8.--DOUBLE FLASHING APPARATUS: TWO GROUPS EACH OF
TWO PANELS.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
The inventor did not manifest any antagonism to this criticism, but
immediately pointed out the great economy in the consumption of oil
that was arising from the use of his idea. Indeed, he prosecuted his
claims so successfully that he clinched a profitable bargain to himself
with the Government. His apparatus had been fitted to thirty-four
lights, and he contracted to maintain them on the basis of receiving
one-half of the oil previously consumed by the lamps which his
invention superseded. This arrangement was in vogue for five years,
when it was renewed, with the difference that on this occasion the
Government, concluding that the inventor was making too much out of
the transaction, reduced the allowance to one-third. Subsequently the
invention received higher commendation from the officials than that
advanced by the critical inspector, although it must be pointed out
that meanwhile the magnifying bull’s-eye had been abandoned, and a
new type of reflector introduced, so that the sole remaining feature
of the wonderful invention was the lamp. Even that had been modified.
When the Lighthouse Board was established in 1852 it abolished the
much-discussed invention, and introduced the Fresnel system, bringing
the United States into line with the rest of the world.
[Illustration: FIG. 9.--TRIPLE FLASHING APPARATUS: THREE PANELS AND
MIRROR.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
One feature of the subject cannot fail to arrest attention. This
is the possibility of producing a variety of combinations by the
aid of the lenses to fulfil different requirements. The Fresnel,
Stevenson, and Chance developments in the science of lighthouse optics
facilitated this work very significantly. Accordingly, to-day a
variety of lights, evolved from the variations in the mounting of the
lenses, is in vogue. For purposes of identification they have been
divided into a number of classifications, and, for the convenience
of the navigator, are described as lights of the first order, second
order, and so on. Broadly speaking, there are seven main groups, or
orders, the rating only applying to dioptric or catadioptric lights,
indicating the bending of the luminous rays in the desired direction,
either by refraction and reflection through the medium of prisms, or a
combination of both. Actually there is a distinction between these two,
the true dioptric system referring only to refraction, where the ray is
bent in the desired direction by a glass agent, known as a “refracting
prism.” In the catadioptric system, on the other hand, both methods are
employed, since the prism performs the dual purpose of reflecting and
refracting the rays. However, in modern lighthouse parlance both are
grouped under the one distinction “dioptric.”
The rating or classification of the lights varies according to the
inside radius or focal distance of the lens--in other words, the
distance from the centre of the light to the inner surface of the lens.
The main groups are as follows:
Hyperradial, 1,330 millimetres (52·3 inches) focal distance.
1st order, 920 ” (36·2 ” ) ”
2nd ” 700 ” (27·6 ” ) ”
3rd ” 500 ” (19·7 ” ) ”
3½ ” 375 ” (14·7 ” ) ”
4th ” 250 ” ( 9·8 ” ) ”
5th ” 187·5 ” ( 7·4 ” ) ”
6th ” 150 ” ( 5·9 ” ) ”
The most powerful apparatus used to-day, however, is that known as the
“hyperradiant,” and it is the largest which has yet been devised. For
this, lighthouse engineering is indebted to Messrs. Stevenson, the
engineers to the Commissioners of Northern Lighthouses. It was first
suggested as far back as 1869, and experiments were carried out which
emphasized the fact that such an apparatus was required, since it
was found that when large gas-burners were used much of the light in
revolving apparatuses was out of focus and escaped condensation. The
Scottish engineers thereupon suggested that an apparatus should be
used having a focal distance of 1,330 millimetres, or 52·3 inches. In
fact, they went farther and suggested even larger apparatuses, but this
idea has not matured. But it was not until 1885 that Messrs. Stevenson
had such a system manufactured, and then it was tested at the South
Foreland beside the powerful lenses which had just been built for
the new Eddystone and the Mew Island lighthouses. The merits of the
theories advanced by Messrs. Stevenson were then completely proved, for
it was found that with a ten-ring gas-burner the hyperradiant apparatus
threw a light nearly twice as powerful as that given by the rival
lenses with the same burner.
[Illustration: FIG. 10.--QUADRUPLE FLASHING APPARATUS: FOUR PANELS.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
At the present moment the hyperradiant is regarded as the _ultima
thule_ of lighthouse optical engineering, and Messrs. Chance Brothers
and Co., of Birmingham, have built some very magnificent apparatuses of
this order. At present there are not more than a dozen such powerful
lights in operation. Three are on the English coast, at Bishop Rock,
Spurn Point, and Round Island, respectively; two in Scotland, at
Fair Isle and Sule Skerry; two in Ireland, at Bull Rock and Tory
Island; one in France, at Cap d’Antifer; one in China, at Pei Yu-shan;
one in India, at Manora Point, Karachi; and the Cape Race light in
Newfoundland. The hyperradiant apparatus is a massive cage of glass,
standing some 12 feet in height, and, as may be supposed, is extremely
expensive.
There is another point in lighthouse optics which demands explanation.
This is the term “divergence,” which plays an important part in the
duration of the flash. In speaking about focus, the engineer follows
somewhat in Euclid’s footsteps in regard to the definition of a point;
in a way it is equally imaginary. The focal point does not mean the
whole of the flame, but the centre of the luminous source, and, as is
obvious, it is impossible to secure a flame without dimensions. It may
be an attenuated, round, oval, or fan-shaped light--the result is the
same. The focal point is the theoretical centre of the luminous source,
and the rays, coming from the top, sides, and bottom of the flame
cannot come from the true focus. If they did, all the light from one
panel would be emitted in absolutely parallel lines, and therefore in a
revolving apparatus the beam would pass any given point on the horizon
in an infinitely short period of time--to be precise, instantaneously.
But the ex-focal rays of the flame, in passing through the lens, emerge
at an angle to those coming from the absolute centre, so that the whole
beam becomes “diverged,” and throws a cone of light from the lens.
Consequently the beam occupies an appreciable period of time in passing
a given point on the horizon.
As may be supposed, the intricate character of the lenses constituting
the optical apparatus of the modern lighthouse demands the highest
skill and infinite care in their preparation, while the composition
of the glass itself is a closely guarded secret. There are less than
half a dozen firms in the world engaged in this delicate and highly
specialized work, of which France claims three, Germany one, and
Great Britain one. All the lighthouse authorities of the various
nations have to secure their requirements from one or other of these
organizations. The industry commenced in France, and for many years the
French reigned supreme. Then it contrived to make its entrance into
England, and was taken up by the family of Chance in Birmingham, who
soon proved themselves equal to their French leaders.
[Illustration: FIG. 11.--RED AND WHITE FLASHING APPARATUS.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
The British firm has established a unique reputation, as it has been
responsible for the majority of the great lights of the world, some of
which are not only of huge dimensions and weight, but also of novel
form. The hyperradial apparatuses which have been placed recently in
the towers of Manora Point and Cape Race probably rank as the most
powerful and the finest in existence. These are used in conjunction
with the petroleum vapour incandescent burner. The Cape Race light,
for instance, comprises a revolving optic of four panels, subtending
a horizontal angle of 90 degrees, with a vertical angle of 121½
degrees. Each lens comprises the central disc, or bull’s-eye, around
which are placed nine rings of glass, giving a total refracting angle
of 57 degrees. In order to bend the vertical rays into a horizontal
path twenty-two catadioptric reflecting prisms are disposed above
the lens, while below are thirteen similar prisms. The total amount
of glass worked into the four panels is about 6,720 pounds, and the
prisms are mounted in gun-metal frames, which weigh approximately
4,800 pounds, so that the total weight of the glass portion and its
mounting alone, standing some 12 feet in height, is over 11,500 pounds.
The installation completed for the equipment of the Manora Point
lighthouse, Karachi, is very similar.
In some cases the demand for a powerful light has been met with a
system differing from the “hyperradiant.” The lenses and respective
groups of refractors are superimposed, each tier having its individual
burner and flues for carrying off the products of combustion. In this
way we have the biform, comprising two such panels arranged one above
the other, as in the Fastnet and Eddystone lights; and the quadriform,
wherein four tiers are built one above the other, as installed at the
Mew Island light in Ireland. The advantage of this arrangement is that
a beam of great intensity is secured with a lantern of comparatively
small diameter.
The French authorities adopted a modification of this system. Instead
of placing two lenses and refractors one above the other, they ranged
them side by side, the effect being analogous to a couple of squinting
eyes, the panels being parallel and therefore throwing out parallel
beams. But these adaptations have not come into extensive use, as
they have been superseded by more simple means of achieving similar
requirements with an even more powerful ray. The hyperradiant stands
as the finest type of apparatus yet devised, and therefore is employed
when an extremely powerful light is required.
While the design and arrangement of the optical apparatus is certainly
a most vital and delicate task, the mounting thereof upon a substantial
support in such a way that it may perform its work with the highest
efficiency is equally imperative, since the finest apparatus might be
very adversely affected by being improperly mounted.
[Illustration: FIG. 12.--APPARATUS SHOWING A DOUBLE FLASH, FOLLOWED BY
A SINGLE FLASH.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
Obviously, owing to the great weight of the glass, the support must
be heavy and substantial. A massive cast-iron pedestal is employed
for this purpose. When the light is of the revolving character, means
have to be incorporated to secure the requisite rotation. In the early
days the turntable upon which the lens is mounted ran upon rollers,
but now a very much better system is universally employed. This has
been brought to a high standard of perfection by Messrs. Chance of
Birmingham, who have carried out unceasing experiments in this field.
The objection to rollers was the enormous friction that was set up,
and the great effort that was required, not only to set the lenses
revolving, but to keep them rotating at a steady pace. In the modern
apparatus the rollers are superseded by an iron trough filled with
mercury, upon which floats the turntable carrying the lenses. When
the apparatus is properly built and balanced, the friction is so
slight that the turntable can be set in motion by the little finger,
notwithstanding that several tons have to be moved. Although the
optical part of the apparatus floats upon the bed of quicksilver in
the same way as a cork lifebelt floats upon water, it is provided with
rollers which serve to hold the whole apparatus steady and to overcome
any oscillation.
In the case of an immense apparatus such as a hyperradiant lens,
which, together with the turntable, may have a total weight of 17,000
pounds, an enormous quantity of mercury is required. The trough of the
Cape Race hyperradiant light carries 950 pounds of quicksilver, upon
which the lantern is floated. In such an instance, also, the pedestal
is a weighty part of the apparatus, representing in this case about
26,800 pounds, so that the complete apparatus utilized to throw the
1,100,000 candle-power beam from the guardian of the Newfoundland coast
aggregates, when in working order, some 44,000 pounds, or approximately
20 tons.
Within the base of the pedestal is mounted the mechanism for rotating
the optical apparatus. This is of the clockwork type driven by a
weight. The latter moves up and down a tube which extends vertically
to a certain depth through the centre of the tower. The weight of the
driving force and the depth of its fall naturally vary according to
the character of the light. In the Cape Race light the weight is of
900 pounds, and it falls 14½ feet per hour. Similarly, the length of
time which the clock will run on one winding fluctuates. As a rule it
requires to be rewound once every sixty or ninety minutes. A longer run
is not recommended, as it would demand a longer weight-tube, while many
authorities prefer the frequent winding, as the man on duty is kept
on the alert thereby. As the weight approaches the bottom of its tube
it sets an electric bell or gong in action, which serves to warn the
light-keeper that the mechanism demands rewinding.
[Illustration: FIG. 13.--THE CLASSIFICATION OF LIGHTS, SHOWING
THE RESPECTIVE RADIUS OR FOCAL DISTANCE OF LENS FROM 150 TO 1,330
MILLIMETRES.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
The weight and clockwork mechanism perfected by Messrs. Chance is
regarded as one of the best in service. The rotation is perfect and
even, owing to the governing system incorporated, while the steel wire
carrying the weight is preferable to the chain, which is subject to
wear and is noisy in action. In the Chance clockwork gear the weight
is just sufficient to start the apparatus from a state of rest, the
advantage of such a method being that, should the apparatus be stopped
in its revolution from any untoward incident, it is able to restart
itself.
Of course, the clockwork mechanism is required only in those cases
where the lenticular apparatus has to be revolved. This introduces
the question of avoiding confusion between lights. When beacons were
first brought into service, the lights were of the fixed type, and the
navigator, although warned by the glare to keep away from the spot
so marked, was given no information as to his position. Accordingly,
lighthouse engineers sought to assist him in this direction during
the blackness of the night by providing a ready visual means of
identification. Owing to the ingenuity which has been displayed, it
has been rendered possible to ring the changes upon a light very
extensively.
These may be subdivided broadly as follows:
+--------------------+---------+-------------------------------------+
| Type of Light. | Symbol. | Characteristics. |
+--------------------+---------+-------------------------------------+
| Fixed | F. | A steady continuous light. |
| | | |
| Flashing | Fl. | A revolving light showing a single |
| | | flash at regular intervals, or a |
| | | fixed light with total eclipses. |
| | | |
| Fixed and flashing | F.Fl. | A fixed light varied at regular |
| | | intervals by a single flash of |
| | | greater brilliancy. |
| | | |
| Group flashing | Gp.Fl. | Various combinations of flashes |
| | | shown at regular intervals. |
| | | |
| Occulting | Occ. | A steady light suddenly and totally |
| | | eclipsed at regular intervals. |
| | | |
| Group occulting | Gp.Occ. | A steady light suddenly and totally |
| | | eclipsed by a group of two |
| | | or more eclipses. |
+--------------------+---------+-------------------------------------+
In the foregoing classifications only a white light is used. But it may
so happen that the lighthouse, owing to its position and the dangerous
character of the spot which it marks, carries a light which changes
colour from white to red or green, which are shown alternately in
various combinations. These characteristics are indicated as follows:
+--------------------+------------+----------------------------------+
| Type of Light. | Symbol. | Characteristics. |
+--------------------+------------+----------------------------------+
| | | |
| Alternating | Alt. | White and colour alternating. |
| | | |
| Alternating | Alt.Fl. | Flashing alternations by |
| flashing | | revolving mechanism. |
| | | |
| Alternating fixed | Alt.F.Fl. | Fixed and flashing alternating. |
| and flashing | | |
| | | |
| Alternating group | Alt.Gp.Fl. | Group flashing alternating. |
| flashing | | |
| | | |
| Alternating | Alt.Occ. | Occulting alternately with |
| occulting | | white and coloured |
+--------------------+------------+----------------------------------+
In timing a revolving or flashing light, the cycle is taken from the
beginning of one flash to the beginning of the next. In these readings
the flash is always shorter than the duration of the eclipse, while
an occultation is shorter than, or equal to, the length of the light
interval. Since flashing and occulting may be carried out with a
fixed light suddenly extinguished or eclipsed, the characterization
is determined solely according to the relative duration of light
and darkness, irrespective of the type of apparatus employed or the
relative brilliancy. There is one peculiarity of the flashing light
which may be remarked. At short distances and in clear weather a faint
continuous light may be shown.
Hand in hand with the development of the optical apparatus has been
the wonderful improvement in regard to the illuminants and the methods
of producing a brilliant clear flame. The fuel first used upon the
introduction of the oil lamp was sperm or colza oil, the former being
obtained from the whale, and the latter from seeds and a wild-cabbage.
Both were very expensive, so that the maintenance of a light was
costly--so much so that the United States authorities devoted their
efforts to the perfection of a high-class lard-oil. This proved highly
satisfactory, possessing only one drawback. In winter it congealed so
much under the low temperature that it had to be heated before it
could be placed in the lamp; but once the light was set going, the heat
radiated from the burner served to keep the oil sufficiently fluid to
enable it to mount the wick to the point of combustion under capillary
action.
So far as the American authorities were concerned, the advantages of
lard-oil sufficed to bring a cheaper medium than colza-oil into vogue.
A company, which had been induced by the Government to install an
elaborate and expensive plant for the production of colza-oil, after
prolonged experiment and efforts to reduce the cost of production,
announced that it could not compete with the lard-oil, and suggested
that the latter should be employed in preference to the colza. The
Government agreed, but, to compensate the company for its trouble,
purchased the plant which the latter had laid down.
The advances in the processes for refining petroleum, and the
exploitation of the extensive resources of the latter, led to
“earth-oil,” in some form or other, being employed for lighthouse
purposes. The attempt was facilitated by the invention and improvement
of the Argand burner, whereby a brilliant white annular sheet of flame
is produced. Various lighthouse engineers devoted their attention to
the improvement of this burner in conjunction with paraffin. Their
results were completely successful, and at last paraffin became
universally utilized as the cheapest and most efficient illuminant
known.
The general method of feeding the lamps was to pump the oil from a low
level to the burner, thereby producing practically a pressure-feed
system in preference to the capillary action which is used in the
ordinary household lamp. By increasing the number of rings the
intensity of the flame was increased, until at last it was thought that
with this development perfection had been attained so far as lamps were
concerned.
Then came another radical revolution. The invention of the incandescent
gas mantle by Dr. von Auer, and the complete change that it wrought
in connection with gas lighting, induced lighthouse engineers to
experiment in this field. As they could not use coal-gas, they devoted
their investigations to the perfection of a gas from petroleum, which
should be capable of combustion with the incandescent burner. Many
years were devoted to these experiments, and many petroleum vapour
systems were devised. One of the best known, most successful, and most
scientifically perfect, is the Chance incandescent light. This burner
is used in many of the most powerful lights of the world and has given
complete satisfaction. The mantle varies in size with the size and
type of the light, ranging from 35 to 85 millimetres in diameter, the
latter, in conjunction with a hyperradial apparatus, producing a light
exceeding 1,000,000 candle-power.
[Illustration:
_By courtesy of Messrs. Chance Bros. & Co., Ltd._
THE HYPERRADIAL APPARATUS FOR THE MANORA POINT LIGHT, KARACHI, INDIA.
Of 1,330 millimetres focus, this is the most powerful and largest
lighthouse apparatus made.]
Not only was a far more powerful light obtained in this manner with
the assistance of the petroleum vapour burner and incandescent mantle,
but the cost of maintaining the light was reduced, owing to the great
economy in oil consumption that was effected thereby, the largest
mantle and burner--85 millimetres--burning only 2½ pints of oil per
hour. The light thus obtained, while being vastly superior to that
derived from a six-wick oil-burner, enables a saving of nearly £48, or
$240, per annum to be recorded, taking the cost of the petroleum at
1s., or 25 cents, per gallon delivered to the lighthouse.
While petroleum is generally used, some countries have adopted other
oil fuels for small permanent lights. Thus, in Germany compressed
oil-gas, water-gas associated with benzine vapour, and Blau liquid gas,
are utilized. The last-named is coming very extensively into vogue,
also, in Holland, Denmark, and Austria. Blau gas has the advantage
that it can be transported in small steel tanks under extremely high
pressure--up to 100 atmospheres, or approximately 1,400 pounds per
square inch. It is an extract of oil-gas produced at a low pressure in
the gas retorts, and then compressed so severely that it liquefies. The
fuel, as it is drawn from the cylinder in which it is stored, has the
pressure reduced by means of a valve, so that it reaches the burner
in a gaseous form at a pressure equivalent to that of the coal-gas used
in private houses, and is burned in the same way with an incandescent
mantle. The advantage of this method lies in the facility with which
large volumes of gas may be transported, a steel cylinder containing
7,500 cubic feet weighing only 132 pounds. It is also inexpensive, a
bottle of the foregoing capacity costing only 12s. 6d., or $3. In some
cases the incandescent mantles, the average life of which is about a
fortnight, are of large diameter, running up to 100 millimetres, or
about 4 inches.
Recently Mr. Gustaf Dalén, of the Gas Accumulator Company of Stockholm,
the inventor of the Dalén flasher and sun-valve, which are described
elsewhere, has introduced a new illuminant, which is coming into vogue,
especially on the Continent. This is called “Daléngas,” and is a
mixture of 9 per cent. dissolved acetylene and 91 per cent. atmospheric
air. Here the dissolved acetylene gas is conducted from a storage
reservoir or high-pressure gas cylinder, of special construction, to
a governor, where the pressure is reduced, and then to the mixing
apparatus, where the acetylene gas is associated with the air in the
above proportions. The idea of this combination and method is to enable
an acetylene gas mixture to be used with the ordinary incandescent
mantles.
[Illustration:
_By courtesy of Messrs. Chance Bros. & Co., Ltd._
FIRST ORDER TRIPLE FLASHING LIGHT OF 920 MILLIMETRES FOCAL DISTANCE FOR
CHILANG LIGHTHOUSE, CHINA.]
The advantage of the Daléngas, according to present experience, is
the increased candle-power that is obtainable as compared with other
systems, the superiority being about 75 per cent. under ordinary
conditions. With the largest Fresnel lenses a lighting power of 200,000
Hefner candle-power is secured, while with revolving lenses of the
latest type a beam of 3,000,000 candle-power can be obtained. The flame
is small, and thus becomes concentrated more in the focus of the lens,
so that the divergence of the light may be diminished if desired. When
a light of a certain range is to be installed, the optical apparatus
can be made smaller for Daléngas than for other illuminants, and the
cost is reduced correspondingly. Similarly, if the system is introduced
into an existing light, the latter can be made appreciably more
powerful, without changing the optical apparatus or affecting the
divergence.
In this system the gas is conducted into the lens apparatus from above,
and the lighting arrangement is quite independent of, and does not
interfere in any way with, the revolving apparatus, while the time
spent in changing the mantle is less than half a minute.
All combustible gases, mixed with air in certain proportions, may
produce more or less violent detonations when fired. But the quantity
of mixed gas in this instance is confined in the length of piping
between the burner and the mixing apparatus, and this quantity is so
small that an explosion cannot be dangerous. In fact, all such danger
has been guarded against completely--is, indeed, impossible in any
circumstances.
Electric light has been adopted in one or two cases; but while the
foremost authorities agree that it throws the best, most brilliant and
most powerful beam of light, the system is generally impracticable
on account of its great cost. When tests with this light were made
some years ago in comparison with the light thrown from oil burners,
it was claimed that the latter, owing to its reddish-yellow tinge,
was the most suitable from the all-round point of view, and that it
could penetrate to a greater distance in foggy weather. I have been
informed by several authorities, who have gone more deeply into this
question since, that this is a fallacy, and that the advantage rests
completely with electric light. Experience in Germany, which has two
magnificent electric lighthouses, and in Scotland, certainly supports
this contention, and I have been assured that the sole reason why
electric lighting has not been adopted more widely is the heavy cost,
both of installation and of maintenance. When electric lighting is
rendered cheaper and is brought more to the level of existing lighting
arrangements, one may expect another complete change in lighthouse
practice. In this direction, as explained in another chapter, the
Germans have carried out practical experiments in their characteristic
manner, and have brought the cost of maintaining a most powerful
electric light to the minimum.
One very great advantage of the electric light is the ease with which
the power of the beam may be increased during thick weather, so as to
secure penetration to the greatest distance, and decreased to suit
easier conditions in clear weather.
This point raises the question, “From how far can a light be seen out
at sea?” This factor is influenced by climatic conditions, and also by
the curvature of the earth. The higher the light, or the spectator,
or both, is elevated above the water, the greater the distance from
which the light can be seen. The table on p. 52, prepared by Mr.
Alan Stevenson, the eminent Scottish lighthouse engineer, gives the
distances at which objects can be seen at sea, according to the
respective elevations of the object and the eye of the observer.
For instance, the passenger on a liner the boat-deck of which is 40
feet above the water, approaching the English Channel, will sight
the Bishop Rock light from a distance of about 22 miles, because the
focal plane--that is, the bull’s-eye of the lens--is 163 feet above
the water, which, according to the following table, equals about 14½
miles, to which must be added the height of the boat’s deck, 40 feet
representing 7·25 miles. Similarly, the ray of the Belle Ile light
will come into view when the vessel is 32½ miles distant--height of
focal plane of light, 470 feet = 25 miles, + eye of observer on board
the liner, 45 feet = 7·69 miles; while the Navesink light, being 246
feet above the water, may be picked up by the captain of a liner from
a distance of 28 miles. The range of many lights, however, owing to
the curvature of the earth, is greatly in excess of their geographical
range, and with the most powerful lights the glare of the luminous
beams sweeping the clouds overhead may be seen for a full hour or more
before the ray itself comes into view.
TABLE OF DISTANCES AT WHICH OBJECTS CAN BE SEEN AT SEA, ACCORDING
TO THEIR RESPECTIVE ELEVATIONS AND THE ELEVATION OF THE EYE OF THE
OBSERVER.
+------------+--------------------+-----------------+
| Heights in | Distances in | Distances in |
| Feet. | Statute or English | Geographical or |
| | Miles. | Nautical Miles. |
+------------+--------------------+-----------------+
| 5 | 2·958 | 2·565 |
| 10 | 4·184 | 3·628 |
| 15 | 5·123 | 4·443 |
| 20 | 5·916 | 5·130 |
| 25 | 6·614 | 5·736 |
| 30 | 7·245 | 6·283 |
| 35 | 7·826 | 6·787 |
| 40 | 8·366 | 7·255 |
| 45 | 8·874 | 7·696 |
| 50 | 9·354 | 8·112 |
| 55 | 9·811 | 8·509 |
| 60 | 10·246 | 8·886 |
| 65 | 10·665 | 9·249 |
| 70 | 11·067 | 9·598 |
| 75 | 11·456 | 9·935 |
| 80 | 11·832 | 10·260 |
| 85 | 12·196 | 10·570 |
| 90 | 12·549 | 10·880 |
| 95 | 12·893 | 11·180 |
| 100 | 13·228 | 11·470 |
| 110 | 13·874 | 12·030 |
| 120 | 14·490 | 12·560 |
| 130 | 15·083 | 13·080 |
| 140 | 15·652 | 13·570 |
| 150 | 16·201 | 14·220 |
| 200 | 18·708 | 16·220 |
| 250 | 20·916 | 18·14 |
| 300 | 22·912 | 19·87 |
| 350 | 24·748 | 21·46 |
| 400 | 26·457 | 22·94 |
| 450 | 28·062 | 24·30 |
| 500 | 29·580 | 25·65 |
| 550 | 31·024 | 26·90 |
| 600 | 32·403 | 28·10 |
| 650 | 33·726 | 29·25 |
| 700 | 35·000 | 30·28 |
| 800 | 37·416 | 32·45 |
| 900 | 39·836 | 34·54 |
| 1,000 | 41·833 | 36·28 |
+------------+--------------------+-----------------+
[Illustration:
_By permission of the “Syren and Shipping.”_
LOOKING UP THE LANTERN OF THE NEEDLES LIGHTHOUSE.]
So far as the candle-power of any light is concerned, the method of
determining this factor, varying according to the calculating methods
adopted, is somewhat misleading. So far as Great Britain is concerned,
the practice of setting out the candle-power of any light in the
official list has been abandoned, the authorities merely stating that
such and such a light is of great power. The United States and Canada,
on the other hand, indicate the approximate candle-power.
[Illustration:
_By courtesy of Messrs. Chance Bros. & Co., Ltd._
FIXED APPARATUS OF THE FOURTH ORDER FOR SARAWAK.
The focal distance is 250 millimetres, and the diameter of lantern
inside glazing 6 feet 7¾ inches.]
By combining and arranging the integral parts of the optical apparatus,
the lighthouse engineer is able to accomplish many astonishing results.
Thus, while the various types generally follow accepted broad lines,
coinciding with the order which they represent, here and there some
very striking divergences are made. The Bell Rock light is perhaps
the most interesting example in this direction. It was designed by
Messrs. D. and T. Stevenson, and built by Messrs. Chance Brothers
and Co. The light is alternating, the colours being white and red.
Externally the optical apparatus appears to be bizarre, yet it is one
of the most perfect which has ever been installed. In its design and
construction almost all the known lighthouse optical elements are
incorporated, including the equiangular refractor, the reflecting
prism, the double-reflecting prism, and the dioptric mirror. Another
noteworthy fact is that, by an exceedingly ingenious arrangement, the
absorption of the rays by the glass used in producing the red flashes
is neutralized to such a vast degree that the white and red flashes are
of equal intensity.
The subsidiary light is another striking feature which the lighthouse
engineer has introduced. For instance, a light may be shown from a
dangerous reef, and give the mariner all the warning desired. But
some distance away may lurk another isolated rock, which it is just
as imperative to indicate, and yet on which another tower cannot be
erected. This necessity is met by the subsidiary light. A portion
of the light from the main apparatus is deflected and thrown to the
desired spot by an ingenious arrangement of the prisms. On the west
coast of Scotland, at Stornoway, a stream of light used to be deflected
from the lantern in a vertical direction down the tower, and there
bent at right angles, to be thrown through a lower window and fall upon
a prism placed on the crest of a rock several hundred feet distant.
From the deck of a vessel, the effect of the light striking the prism
was akin to that produced by a beacon. Similarly in the case of St.
Catherine’s light in the Isle of Wight: a portion of the light, which
would otherwise be wasted over the area on the landward side, is
carried vertically down the tower by a disposal of lenses and prisms,
and is projected horizontally through a small window, after being
coloured into a red ray by passing through some glass of the desired
tint, to mark a danger spot some distance away. This method, however,
is not favoured now, as the peril can be more efficiently marked by
means of an independent beacon, a system which has become feasible
owing to the vast improvements that have been made in automatic lights
requiring no attention for several weeks or months at a time.
But in those instances where the latter expedient is not adopted, the
practice is to cover the danger with a ray thrown from an entirely
different light. When the present Eddystone tower was completed, a
“low-light room,” as it is called, was incorporated, and a low-powered
light was thrown from two Argand burners and reflectors through a
window to mark a dangerous reef some three miles distant. But perhaps
the best example of a subsidiary light is that which was carried out
by Messrs. Chance in connection with the Cap de Couedie lighthouse. In
this instance two dangers had to be indicated in a subsidiary manner,
one being covered with a red, the other with a green, ray. The red
sector marks a danger spot known as Lipson’s Reef, lying 8¾ miles
distant, while the green light indicates Casuarina Island, 1¾ miles
away. This installation, it may be pointed out, has proved highly
successful, and certainly is very economical.
[Illustration: FIG. 14.--THE MEANS WHEREBY THE RAYS ARE DEFLECTED FROM
THE MAIN LIGHT TO FORM A SUBSIDIARY LIGHT.
(_By permission of Messrs. Chance Bros. and Co., Ltd._)]
There is another point which deserves mention--the duration of the
flash in a revolving light. There was considerable discussion and
difference of opinion upon this question some years ago. It was
maintained that the shorter the duration of the flash, and the more
rapidly it were thrown, the better it would be for the mariner. The
Scottish engineers realized the significance of this problem, and,
despite the hostile criticism of contemporary engineers, adopted a
specific principle which was to give a flash of two and three-quarter
seconds’ duration. Subsequently it was reduced to one second. The
introduction of the mercury float enabled the optical apparatus to be
revolved faster, and also facilitated the reduction in the number of
panels or faces, so that ultimately the Scottish engineers reduced the
flash to one of four-tenths of a second.
When Mr. Bourdelles devised the mercury float which enabled rotation to
be accelerated, the French authorities rushed to the opposite extreme.
They reduced the faces to four, and arranged for the apparatus to
be revolved at a high speed, so that the duration of the flash was
only one-tenth of a second at rapidly-recurring intervals. This type
of light was called the _feu-éclair_, and was adopted as a result
of prolonged laboratory investigation. But this was an instance
where laboratory experiments and scientific reasoning failed to go
hand in glove with practical experience and navigation, where the
mariner has to contend with all sorts and conditions of weather. The
seafarer expressed his opinion of the one-tenth of a second flash
in uncomplimentary terms, displaying an indifferent appreciation of
artificially-produced sheet-lightning.
Eventually there was a general agreement, among all those countries
which had investigated the problem closely, that a flash of about
three-tenths of a second was the most satisfactory, and this has
since become tacitly standardized. The French authorities recognized
the fallacy of their idea, and soon came into line with the other
countries.