FOG-SIGNALS
Notwithstanding the wonderful ingenuity that is displayed in the
concentration of light into powerful beams, these all count for nothing
when fog settles upon the sea. The ray of 1,000,000 candle-power is
almost as futile then as the glimmer from a tallow dip.
Fog is the peril of the sea which the mariner dreads more than any
other. The blanket of mist, descending upon the water, not only shuts
everything from sight, but deadens every sound as well. The sea is
absolutely calm, so that no intimation of danger ahead is conveyed by
the breaking of the waves upon rock, shoal, sandbank, or iron-bound
coast.
It is in times of fog that the navigator must be given the greatest
protection. As this is impossible to accomplish visually, appeal must
be made to his ear. In the early days of lighthouse engineering the
methods of conveying audible warning were very crude. The discharge
of a gun was the most popular, but it was neither serviceable nor
reliable, and was made upon somewhat haphazard lines. Thus, in the
case of a dangerous headland on the North American coast, which the
Boston steamer had to round on its journey, the keepers mounted guard
at the probable time of the vessel’s arrival off this point. They
listened eagerly for the steamer’s whistle, and when it came screaming
over the water they began hurriedly firing a carronade, keeping up
the blank-cartridge bombardment until another shriek told them that
those on the vessel had heard their signals. Sometimes the whistle
was heard from a distance of six miles; at others from not more than
two miles away. It depended upon circumstances. Obviously, such a
primitive system was attended with considerable danger, as an accident
was liable to happen to the men in their feverish haste to load and
discharge the gun, while the plight of the boat was far from being
enviable at times.
[Illustration:
_By permission of Messrs. Chance Bros. & Co., Ltd._
A MODERN LIGHTHOUSE SIREN PLANT.
Showing gas engines and air-compressors in duplicate, with siren at
side.]
In the early days every lighthouse tower was provided with a heavy
bell. Indeed, the ponderous dome of metal projecting from the lantern
gallery was considered indispensable. The bell varied in weight from
1,200 to 2,240 pounds, was fitted with a massive clapper, and when
struck emitted a deep musical note. In order to enable the seafarer
to gain some idea of his whereabouts, the fog-signals were given a
sound-characteristic somewhat upon the lines of those in connection
with the light. Thus, one lighthouse would give one stroke every ten
seconds; another would give two strokes in quick succession, followed
by a long silence, and so on. This system suffers from the severe
handicap that the sound does not travel very far during foggy weather.
Another ingenious engineer recommended the utilization of the
locomotive whistle, giving a high-toned, ear-piercing shriek, but
the same objection as attended the use of the bell prevailed: the
sound could not be heard more than a short distance away. The British
lighthouse authorities submitted the idea to a series of searching
investigations to ascertain its possibilities, but eventually were
compelled to conclude that it was not superior to, if as good as, the
other systems then in vogue. The United States authorities, as a result
of their independent experiments, expressed a similar opinion; but in
Canada practical application gave this whistle a favourable verdict.
Rockets also have been adopted, and are highly successful. Indeed, this
method of conveying audible warning prevails still in many countries.
The practicability of such a means of throwing sound over a wide area
was advanced by Sir Richard Collinson, when Deputy-Master of Trinity
House, and his idea comprised the insertion of a gun-cotton charge,
timed to explode at a given height, in the head of the rocket. The
height could be varied up to about 1,000 feet, and the weight of
the charge fluctuated according to requirements. The rocket system
was tested very severely, and in some instances the report was heard
as many as twenty-five miles away. It received the approbation of
Professor Tyndall, and, although superior methods of signalling have
been devised since, there remain one or two lighthouse stations where
it is considered to be the most satisfactory fog-signalling device,
notably the station on the island of Heligoland, where the rocket is
hurled into the air to explode at a height of nearly 700 feet.
In many lighthouses the detonation of gun-cotton constitutes the means
of conveying warning to passing vessels, but is accomplished in a
different manner. The charge, instead of being sent into the air to
be exploded, is attached to a special device which is supported upon
a simple frame at a point above the lantern, so that no damage may
be inflicted upon the glass of the latter from the concussion. The
apparatus is fitted with a safety device which prevents premature
explosion, so that the keeper is preserved from personal injury, and,
unless culpable negligence is manifested, the charge cannot be ignited
until it has been raised to its designed position. The report is of
great volume, and as a rule can be heard a considerable distance; but
in this, as in all other cases, the atmosphere plays many strange
tricks. Still, it has not been superseded yet for isolated sea-rock
lighthouses, such as the Eddystone, Skerryvore, and Bell Rock, where
there is lack of adequate space for the installation of any other
equally efficient fog-signalling facilities.
[Illustration:
_Photo, Paul, Penzance._
THE SIRENS OF THE LIZARD.
Owing to the importance of the Lizard Station and the fact that the
coast often is obscured by fog, a powerful fog-signalling station is
imperative.]
In the early seventies an American investigator, Mr. C. L. Daboll,
contrived an entirely new system, which developed into the foundation
of one of the most successful fog-signalling devices for lighthouses
which has been discovered--the siren. The Daboll invention was a huge
trumpet, recalling a mammoth phonograph horn. It was 17 feet in length,
and its mouth was 38 inches in diameter. In the lower end of this
trumpet--the throat--was placed a tongue of steel measuring 10 inches
in length and secured at one end to form a reed. It was blown by air
compressed in a reservoir to the desired degree, and then permitted to
escape through the trumpet. The mad rush of the expanding air through
the constricted passage set the reed vibrating violently, causing the
emission of a penetrating, discordant bellow. When Daboll commenced his
experiments, he suffered from the lack of a suitable mechanical means
for compressing the air, and made shift with a donkey for this purpose
until the hot-air engine was improved, when the latter was substituted.
Trinity House adopted the idea and found it serviceable; but the
Canadian authorities, after four years’ experiment, dissented from this
view, remarking that the trumpet was expensive to maintain, unreliable
in working, and liable to break down when most urgently needed. In
fact, they characterized the Daboll trumpets which they had installed
as “sources of danger instead of aids to navigation.”
From the trumpet to the siren was not a very big step. The history
of the latter’s invention is somewhat obscure, but it was brought
before the United States Government in a primitive form. The American
engineers, recognizing its latent possibilities, took it up, and
endeavoured to improve it to such a degree as to render it suitable
for lighthouse work. Their efforts were only partially successful.
The solution of the many difficulties attending its perfection
was effected in Great Britain by Professor Frederick Hale Holmes,
whose magneto-electric machine brought electricity within reach
of the lighthouse as an illuminant, and it was due to the efforts
of this scientist that the siren became one of the most efficient
sound-producing instruments which have been discovered for this class
of work.
The reason that made Professor Holmes bring his energies and knowledge
to bear upon this subject was somewhat curious. The siren in its
first form made its way from the United States to Great Britain. The
British Admiralty realized the power and penetration of its sound,
and forthwith adopted it in the navy, operating it by steam instead
of by air. At this there arose a great outcry from the mercantile
marine. Captains argued that the similarity of the signals confused
and often misled them, as they could not tell in the fog whether the
sound proceeded from a warship or a lighthouse. The Board of Trade was
forced to intervene, but, as it had no jurisdiction over the Admiralty,
it sought to extricate itself from an awkward situation by inviting
Professor Holmes to perfect a siren which would emit a distinctive
sound. His efforts were crowned with complete success.
[Illustration: FIG. 15.--THE FIXED (A) AND REVOLVING (B) PARTS OF THE
SIREN.]
Professor Holmes exhibited his wonderful device at the Paris Exhibition
of 1867. He installed it in working order, and the visitors displayed
an anxiety to hear it. It was brought into action, and those around
never forgot the experience. It was the most diabolical ear-splitting
noise which had been heard, and, apprehensive that serious results
might arise from its demonstration when the buildings were thronged
with sight-seers, the authorities refused to permit it to be sounded
again. The humorous illustrated papers did not suffer such a golden
opportunity to escape. Grotesque and laughable cartoons appeared
depicting the curious effects produced by the blast of the instrument,
one showing the various statues being frightened off their pedestals
proving exceptionally popular.
The siren in its simplest form is an enlarged edition of the “Deviline”
toy whistle. There is a Daboll trumpet with a small throat, in which
is placed horizontally, not a reed, but a metal disc, so as to fill
the whole circular space of the throat. The sheet of metal is pierced
with a number of radial slits. Behind this disc is a second plate of
a similar character, and likewise pierced with radial slits of the
same size, shape and number; but whereas the first disc is fixed, the
second is mounted on a spindle. The free disc rotates at high speed,
so that the twelve jets of air which are driven through the throat are
interrupted intermittently by the blanks of the revolving disc coming
over the openings in the fixed disc, while when the two slits are in
line the air has a free passage. If the revolving disc completes 3,000
revolutions per minute, and there are twelve slits in the discs, then a
total of 36,000 vibrations per minute is produced while the instrument
is in operation. The speed of the revolving disc, as well as the number
and size of the openings, varies according to the size and class of the
siren; but in any case an intensely powerful, dense and penetrating
musical tone is emitted, which can be heard a considerable distance
away. The blast of a high-powered large siren has been heard at a
distance of twenty to thirty miles in clear weather, though of course
in thick weather its range is reduced.
While Professor Holmes was experimenting with this device, another
investigator, Mr. Slight, of Trinity House, was wrestling with the same
problem. Indeed, he may be described as the inventor of the modern
siren. Although he effected only an apparently slight modification,
it was the touch which rendered the instrument perfect, while it also
removed the possibility of a breakdown at a critical moment, as he
rendered the moving part freer in its working and eliminated the severe
strains to which it was subjected. The improvement was appreciated by
Professor Holmes, who adopted it immediately.
While these indefatigable efforts were in progress, ingenious attempts
were made to press Nature herself into operation. As is well known,
there are many “blowing-holes” distributed throughout the world, where
the water by erosion has produced a long, narrow cavern in the base
of a rock, with a constricted outlet into the outer air. The waves,
rushing into the cave, compress the air within, which, in its escape at
high velocity through the small vent, produces a bellowing sound. It
was this curious phenomenon which gave the Wolf Rock its name. General
Hartmann Bache, of the United States Engineers, attempted in 1858 to
make use of a blowing-hole on one of the Farallon Isles, lying forty
miles off the entrance to San Francisco Bay. A chimney was built with
bricks above the orifice, through which the air compressed by the waves
below made its escape, and on top of this shaft a locomotive whistle
was placed. The first effort was a dead failure, because the force of
the rush of air was so great that it carried away the chimney; but in
the second attempt success was achieved, and an excellent automatic
whistle blared out night and day almost continuously and was audible
for some distance out to sea. The only drawback was that in foggy
weather, when the most intense sound was required, the signal was dumb
owing to the smoothness of the water. This novel signal was maintained
for some time and then was superseded by a powerful siren.
One of the most interesting fog-signalling installations in service is
that on the bald formidable hump of rock lying in the estuary of the
Clyde, known as Ailsa Craig. For years this rock constituted a terrible
menace to the crowded shipping of this important marine thoroughfare,
and its victims were numerous. While the Commissioners of Northern
Lighthouses mitigated its terrors as far as possible by the provision
of a powerful light, they recognized the fact that a visual warning
did not meet the situation completely. But the installation of a
fog-signal was a somewhat peculiar problem, owing to the configuration
of the rock. A single station would not meet requirements, because it
was necessary to throw the warning from both sides of the obstruction.
The provision of two sound-stations would have been an expensive
matter, even if it had been feasible, which it was not, owing to the
precipitous nature of the cliffs.
An ingenious solution was advanced by Mr. Charles Ingrey, C.E. He
proposed to erect a central power-station and to control the sounding
of two sirens, placed on opposite sides of the island, therefrom,
the compressed air being led through underground piping. The plans
were submitted to Messrs. Stevenson, the engineers to the Northern
Lighthouse Board, who, after examining the proposal thoroughly, gave
it their approval. But when it came to obtaining the sanction for
the requisite expenditure from the Board of Trade, that august body,
despite the fact that the project had been investigated and had
received the approbation of the engineers to the Northern Lighthouse
Commissioners, declined to permit public money to be expended upon
an untried scheme. Such is the way in which pioneering effort and
ingenuity are stifled by Government departments.
[Illustration: THE ACETYLENE FOG-GUN.
The latest ingenious device for giving both audible and visual warning
automatically.]
Many another engineer would have abandoned the project after such
a rebuff, but Mr. Ingrey without any delay laid down a complete
installation upon the lines he contemplated on the island of Pladda,
where a Holmes fog-horn was in service. With the aid of a workman
whom he took from Glasgow, the light-keepers and some farm labourers,
this trial installation was completed, the piping being carried round
the island from the air-compressing plant to the fog-signal. The
work occupied about a fortnight, and then, everything being ready to
convince the sceptical Board of Trade, the inspecting engineers were
treated to a comprehensive and conclusive demonstration. They were
satisfied with what they saw, appreciated the reliability of the idea
and gave the requisite sanction. Forthwith the Ailsa Craig Island
installation was put in hand and duly completed.
This plant possesses many ingenious features. As the light is derived
from gas distilled from crude oil, a small gas-making plant is
installed on the island, and this is used also for driving a battery
of five eight-horse-power gas-engines--four are used at a time, the
fifth being in reserve--to supply the thirty-horse-power demanded to
operate the fog-signal. The energy thus developed drives two sets of
powerful air-compressors, the four cylinders of which have a bore
of 10 inches by a stroke of 20 inches, the air being compressed to 80
pounds per square inch and stored in two large air-receivers which hold
194 cubic feet. From this reservoir pipes buried in a trench excavated
from the solid rock extend to the two trumpets, placed on the north
and south sides of the island respectively. The length of piping on
the north side is 3,400 feet, and on the south side 2,500 feet. At
places where the pipe makes a dip, owing to the configuration of the
rock, facilities are provided to draw off any water which may collect.
Extreme care had to be displayed in connecting the lengths of piping,
so that there might be no leakage, in which event, of course, the
pressure of the air would drop and thereby incapacitate the signal.
[Illustration: THE RATTRAY HEAD LIGHTHOUSE.
A very exposed Scottish rock tower. It is unique because a full-powered
siren fog-signal is installed therein.]
Each signal is mounted in a domed house built of concrete, the mouth
of the trumpet extending from the crown of the roof. Within the house
is an air-receiver 9 feet in height by 4½ feet in diameter, of about
140 cubic feet capacity, which receives the compressed air transmitted
through the piping from the compressing-station. It also contains
the automatic apparatus whereby the signal is brought into action
at the stipulated intervals, so as to produce the requisite sound
characteristic. This is a self-winding clockwork mechanism which admits
and cuts off the supply of air to the trumpets, its chief feature
being that the clock is wound up by the compressed air itself, so that
it is entirely free from human control. However, as a breakdown even
with the best-designed and most-carefully-tended machinery cannot be
circumvented entirely, there is a duplicate electrical mechanism,
also automatically controlled from the power-generating station, the
electric cables for which are laid in the pipe trenches. This acts as
an emergency control.
[Illustration:
_By courtesy of Messrs. D. and C. Stevenson._
SULE SKERRY LIGHT.
A lonely light of Scotland. The nearest land is the Butt of Lewis, 30
miles distant.]
The two signals are not sounded simultaneously; neither are they
alike nor of the same tone. The north signal gives a single blast of
high tone, lasting five seconds, and then is silent for 175 seconds.
On the south side the siren gives a double note, although there are
three blasts--viz., high, low, high--corresponding to the letter R of
the Morse code. The notes are sounded for two seconds, with similar
intervening periods of silence, and silence for 170 seconds between
the groups. The complete signal from the two stations is given once
in three minutes, the north signal commencing to sound ninety seconds
after the south signal has ceased. The high note corresponds to the
fourth E in the musical compass, there being 38,400 vibrations per
minute; while the low note is tuned to the third D in the musical
compass, with 16,800 vibrations per minute. The notes are purposely
timed more than an octave apart and made discordant, as thereby
the sound is more likely to attract attention and to be readily
distinguished.
About eighteen minutes are required to bring the apparatus into
operation--that is, to start compressing and to raise the pressure of
the air to the requisite degree--but, as fogs descend upon the Clyde
with startling suddenness, the signals may be started within five
minutes of the fog-alarm. The air-reservoirs are kept charged to the
working pressure, the machinery being run once or twice for a short
time every week for this purpose and to keep the plant in working order.
Up to this time it had been the practice to place the siren in close
proximity to the air-compressing machinery, but the installation at
Ailsa Craig proves conclusively that this is not essential to success;
also it demonstrates the fact that a number of signals can be operated
reliably and effectively from a central station. Indeed, this Scottish
plant aroused such widespread interest that the Pulsometer Engineering
Company of Reading, who had acquired Professor Holmes’s patents and who
carried out the above installation, received several inquiries from
abroad with regard to its suitability for similar situations. In one
instance the compressed air was to be transmitted for a distance of
nearly four miles.
While the siren has been adopted and found adequate by the majority
of nations, the Canadian Government has installed a far more powerful
instrument upon the River St. Lawrence, as the ordinary siren signals
originally established near the mouth of the river, although of
great power, were found to be inadequate. The new apparatus, which
is known as the “diaphone,” gives an extraordinarily powerful sound.
It comprises a cylindrical chamber, in the walls of which are cut a
number of parallel slits. Concentrically disposed within the chamber
is a cylindrical hollow piston, with similar slits and a flange at one
end, the whole being enclosed in an outer casing. Air under pressure
is admitted into the outer casing, and drives the piston backwards and
forwards with great rapidity. The result is that the air effects its
escape through the orifices, when they come into line, in intermittent
puffs.
While the broad principle is not unlike that of the conventional
siren, the main difference is that in the latter there is a rotary
motion, whereas in the diaphone the action is reciprocating. The great
advantage of the latter is that all the vibrations are synchronous,
owing to the symmetrical disposition of the slits, and consequently
the note produced is very pure. The mechanism is so devised that the
piston’s motion is controlled to a nicety, and the sound is constant.
Experience has proved that the best results are obtained by using air
at a pressure of 30 pounds per square inch. The sound thus produced
is intensified to a markedly greater degree by means of a resonator
properly attuned.
This instrument has displaced the siren among the stations upon the St.
Lawrence River. The general type of apparatus has a piston 4½ inches
in diameter, and uses 11 pounds of air per second during the sounding
of the blast. But at more important stations a far larger and more
powerful class of apparatus is used, the diaphone at Cape Race having a
piston 8½ inches in diameter and using 27 feet of air per second while
sounding. This does not indicate the limit of size, however, since the
builders of this terrible noise-producer are experimenting with an
apparatus having a piston 14 inches in diameter. The sound issuing from
such a huge apparatus would be almost as deafening as the report of a
big gun and should succeed in warning a mariner several miles away.
The atmosphere, however, plays many strange pranks with the most
powerful sound-producing instruments. To-day, for instance, a
fog-signal may be heard at a distance of ten miles; to-morrow it will
fail to be audible more than a mile away. This aberration of sound is
extraordinary and constitutes one of the unsolved problems of science.
Innumerable investigations have been made with the object of finding
the cause of this erratic action, but no conclusive explanation has
been forthcoming. Another strange trick is that, while a sound may be
audible at distances of two and four miles during a fog, it fails to
strike the ear at three miles. It is as if the sound struck the water
at a range of two miles, bounded high into the air, and again fell upon
the water at four miles, giving a second leap to hit the water again
farther on, in much the same way as a thin flat stone, when thrown
horizontally into the water, will hop, skip, and jump over the surface.
This trick renders the task of the lighthouse engineer additionally
exasperating and taxes his ingenuity to the utmost, as it appears to
baffle completely any attempt towards its elimination.
Recently another ingenious and novel system has been perfected by
Messrs. D. and C. Stevenson. This is an acetylene gun which acts
automatically. Hitherto an unattended fog-signal--except the bell-buoy
tolled by the movement of the waves, which is far from satisfactory,
or the whistling buoy, which is operated upon the same lines and is
equally ineffective except at very short range--has found little
favour. The objections to the bell and whistle buoys are the faintness
of the sounds, which may be drowned by the noises produced on the ship
herself; while, if the wind is blowing away from the vessel, she may
pass within a few feet of the signal, yet outside its range. Thus it
will be recognized that the fog-gun serves to fill a very important gap
in connection with the warning of seafarers during thick weather.
As is well known, even a small charge of acetylene, when fired, will
produce a loud report, and this characteristic of the gas induced
Messrs. Stevenson to apply it to a fog-signal. They have developed
the automatic acetylene system of lighting to a very high degree
around the coasts of Scotland, and there are now more than twenty
lights of this class, mostly unattended, in operation, some of which
have been established for many years. These lights have proved highly
satisfactory. There has never been an accident, a freedom which is due
to the fact that Moye’s system is used, wherein the possibilities of
mishap are surmounted very effectively. Accordingly, the engineers saw
no reason why a similar system should not be adapted to the emission of
sound instead of light signals, or, if desired, of both simultaneously.
Their experiments have been crowned with complete success, and, as the
gun uses no more gas than would be consumed if a flashing light system
were used, the cost of operation is very low.
The general features of the acetylene fog-gun may be observed from the
illustration (facing p. 64). The acetylene, dissolved in acetone, is
contained under pressure in a cylinder, and thence passes through a
reducing valve to an annular space, where it is ignited by an electric
spark. A trumpet is attached to the firing chamber, so that the sound
becomes intensified. If desired, the explosion can be effected at the
burner, so that, in addition to a sound-signal, a flashing light is
given.
The applications vary according to the circumstances. Suppose there is
an unlighted bell-buoy at the bar of a port. Here the procedure is to
install a gun and light combined, so that the flash of the explosion
may give visual and the report audible warning. Or, should there be a
lighted buoy already in position, its effectiveness may be enhanced
by adding the gun, the detonation alone being employed for warning
purposes. The size of the cylinder containing the dissolved acetylene
may be varied, so that renewal need only be carried out once in one,
two, or more months, according to conditions. If the increasing traffic
around a certain rock demand that the latter should be marked, a
combined sound and light apparatus can be installed. It may be that the
head of a pier which is accessible only at certain times, or a beacon
which can be reached only at rare intervals, may require improved
facilities. In this case the gun can be set up and a cable laid to a
convenient spot which may be approached at all times by an attendant.
Then the latter, by the movement of a switch, can bring the gun
instantly into action upon the alarm of fog, and it will keep firing at
the set intervals until, the fog lifting, the gun is switched off.
In some cases, where the apparatus is set upon a lonely rock,
a submarine cable may be laid between the marked point and the
control-station. The cable is not a very costly addition. There are
many lights where wages have to be paid merely for a man to bring the
fog-signalling bell machinery into action. In such cases a fog-gun can
be installed and the annual cost of maintenance decreased enormously,
thereby enabling the outlay on the gun to be recouped within a very
short time; while the light may be improved by using the flashes, so
that the warning can be rendered more distinctive.
The invention is also applicable to lightships, many of which are
manned by four men or more at a large cost per annum. In the majority
of cases an unattended Stevenson lightship--such as described in
another chapter, six of which are in use around the coasts of Scotland,
and which give, not only a first-class light, but, by the aid of
the fog-signal gun, can be made to give an excellent fog-signal as
well--offers a means of reducing the heavy maintenance charges arising
in connection with a manned light-vessel. In many instances existing
lightships can be converted to the automatic system and completed by
the gun. Each case must, of course, be decided upon its merits as
regards the time the gun and light are required to work upon a single
charge of acetylene, but there are no insuperable obstacles to its
utilization.
Of course, in an isolated station lying perhaps some miles off the
mainland, it may be necessary to keep the gun going night and day in
fog and in clear weather alike. In this case, naturally, the great
number of explosions involves considerable expense; but the inventors
are carrying out experiments with a view to switching the gun on and
off, as required, from a distant point by means of wireless telegraphy,
so as to effect a saving in the expenditure of acetylene when there is
no need on account of fine weather to keep the gun going. Still, it
must not be supposed that the detonations even during clear weather
are altogether abortive, inasmuch as a sound-signal at sea, where
the atmosphere has a long-distance-carrying capacity as a rule, in
conjunction with a light, draws double attention to a danger spot.
Under such circumstances the waste of acetylene gas during periods of
clear weather is more apparent than real.
The contest against the elements is still being waged, and slowly but
surely engineering science is improving its position, and is hopeful of
rendering audible signals as completely effective as those of a visual
character.