light is intended, for here it is not needed. Nor is it for densely
foggy weather, for here it is ineffectual. But it is for the
intermediate stages of hazy, snowy, or rainy weather, in which a
powerful light can assert itself, while a feeble one is extinguished.
The usual first-order lamp is one of four wicks, but Mr. Douglass, the
able and indefatigable engineer of the Trinity House, has recently
raised the number of the wicks to six, which produce a very noble
flame. To Mr. Wigham, of Dublin, we are indebted for the successful
application of gas to lighthouse illumination. In some lighthouses
his power varies from 28 jets to 108 jets, while in the lighthouse of
Galley Head three burners of the largest size can be employed, the
maximum number of jets being 324. These larger powers are invoked
only in case of fog, the 28-jet burner being amply sufficient for
clear weather. The passage from the small burner to the large, and
from the large burner to the small, is made with ease, rapidity, and
certainty. This employment of gas is indigenous to Ireland, and the
Board of Trade has exercised a wise liberality in allowing every
facility to Mr. Wigham for the development of his invention.
The last great agent employed in lighthouse illumination is
electricity. It was in this Institution, beginning in 1831, that
Faraday proved the existence and illustrated the laws of those induced
currents which in our day have received such astounding development.
In relation to this subject Faraday's words have a prophetic ring. 'I
have rather,' he writes in 1831, 'been desirous of discovering new
facts and new relations dependent on magneto-electric induction than
of exalting the force of those already obtained, being assured that
the latter would find their full development hereafter.' The labours
of Holmes, of the Paris Alliance Company, of Wilde, and of Gramme,
constitute a brilliant fulfilment of this prediction.
But, as regards the augmentation of power, the greatest step hitherto
made was independently taken a few years ago by Dr. Werner Siemens and
Sir Charles Wheatstone. Through the application of their discovery a
machine endowed with an infinitesimal charge of magnetism may, by a
process of accumulation at compound interest, be caused so to enrich
itself magnetically as to cast by its performance all the older
machines into the shade. The light now before you is that of a small
machine placed downstairs, and worked there by a minute steam-engine.
It is a light of about 1000 candles; and for it, and for the
steam-engine that 'works it, our members are indebted to the
liberality of Dr. William Siemens, who in the most generous manner has
presented the machine to this Institution. After an exhaustive trial
at the South Foreland, machines on the principle of Siemens, but of
far greater power than this one, have been recently chosen by the
Elder Brethren of the Trinity House for the two light-houses at the
Lizard Point.
Our most intense lights, including the six-wick lamp, the Wigham
gas-light, and the electric light, being intended to aid the mariner
in heavy weather, may be regarded, in a certain sense, as fog-signals.
But fog, when thick, is intractable to light. The sun cannot
penetrate it, much less any terrestrial source of illumination. Hence
the necessity of employing sound-signals in dense fogs. Bells, gongs,
horns, whistles, guns, and syrens have been used for this purpose; but
it is mainly, if not wholly, with explosive signals that we have now
to deal. The gun has been employed with useful effect at the North
Stack, near Holyhead, on the Kish Bank near Dublin, at Lundy Island,
and at other points on our coasts. During the long, laborious, and I
venture to think memorable series of observations conducted under the
auspices of the Elder Brethren of the Trinity House at the South
Foreland in 1872 and 1873, it was proved that a short 5.5-inch
howitzer, firing 3 lbs. of powder, yielded a louder report than a long
18-pounder firing the same charge. Here was a hint to be acted on by
the Elder Brethren. The effectiveness of the sound depended on the
shape of the gun, and as it could not be assumed that in the howitzer
we had hit accidentally upon the best possible shape, arrangements
were made with the War Office for the construction of a gun specially
calculated to produce the loudest sound attainable from the combustion
of 3 lbs. of powder. To prevent the unnecessary landward waste of the
sound, the gun was furnished with a parabolic muzzle, intended to
project the sound over the sea, where it was most needed. The
construction of this gun was based on a searching series of
experiments executed at Woolwich with small models, provided with
muzzles of various kinds. A drawing of the gun is annexed (p. 309).
It was constructed on the principle of the revolver, its various
chambers being loaded and brought in rapid succession into the firing
position. The performance of the gun proved the correctness of the
principles on which its construction was based.
An incidental point of some interest was decided by the earliest
Woolwich experiments. It had been a widely spread opinion among
artillerists, that a bronze gun produces a specially loud report. I
doubted from the outset whether this would help us; and in a letter
dated 22nd April, 1874, I ventured to express myself thus: 'The
report of a gun, as affecting an observer close at hand, is made up of
two factors--the sound due to the shock of the air by the violently
expanding gas, and the sound derived from the vibrations of the gun,
which, to some extent, rings like a bell. This latter, I apprehend,
will disappear at considerable distances.'
FIG. 8. Breech-loading Fog-signal Gun, with Bell Mouth, proposed by
Major Maitland, R.A. Assistant Superintendent. [Footnote: The carriage
of this gun has been modified in construction since this drawing was
made.]
The result of subsequent trial, as reported by General Campbell, is,
'that the sonorous qualities of bronze are greatly superior to those
of cast iron at short distances, but that the advantage lies with the
baser metal at long ranges.' [Footnote: General Campbell assigns a
true cause for this difference. The ring of the bronze gun represents
so much energy withdrawn from the explosive force of the gunpowder.
Further experiments would, however, be needed to place the superiority
of the cast-iron gun at a distance beyond question.]
Coincident with these trials of guns at Woolwich, gun-cotton was
thought of as a probably effective sound-producer. From the first,
indeed, theoretic considerations caused me to fix my attention
persistently on this substance; for the remarkable experiments of Mr.
Abel, whereby its rapidity of combustion and violently explosive
energy are demonstrated, seemed to single it out as a substance
eminently calculated to fulfil the conditions necessary to the
production of an intense wave of sound. What those conditions are we
shall now more particularly enquire, calling to our aid a brief but
very remarkable paper, published by Professor Stokes in the
'Philosophical Magazine' for 1868.
The explosive force of gunpowder is known to depend on the sudden
conversion of a solid body into an intensely heated gas. Now the work
which the artillerist requires the expanding gas to perform is the
displacement of the projectile, besides which it has to displace the
air in front of the projectile, which is backed by the whole pressure
of the atmosphere. Such, however, is not the work that we want our
gunpowder to perform. We wish to transmute its energy not into the
mere mechanical translation of either shot or air, but into vibratory
motion. We want _pulses_ to be formed which shall propagate themselves
to vast distances through the atmosphere, and this requires a certain
choice and management of the explosive material.
A sound-wave consists essentially of two parts--a condensation and a
rarefaction. Now air is a very mobile fluid, and if the shock
imparted to it lack due promptness, the wave is not produced. Consider
the case of a common clock pendulum, which oscillates to and fro, and
which might be expected to generate corresponding pulses in the air.
When, for example, the bob moves to the right, the air to the right of
it might be supposed to be condensed, while a partial vacuum might be
supposed to follow the bob. As a matter of fact, we have nothing of
the kind. The air particles in front of the bob retreat so rapidly,
and those behind it close so rapidly in, that no sound-pulse is
formed. The mobility of hydrogen, moreover, being far greater than
that of air, a prompter action is essential to the formation of
sonorous waves in hydrogen than in air. It is to this rapid power of
readjustment, this refusal, so to speak, to allow its atoms to be
crowded together or to be drawn apart, that Professor Stokes, with
admirable penetration, refers the damping power, first described by
Sir John Leslie, of hydrogen upon sound.
A tuning-fork which executes 256 complete vibrations in a second, if
struck gently on a pad and held in free air, emits a scarcely audible
note. It behaves to some extent like the pendulum bob just referred
to. This feebleness is due to the prompt 'reciprocating flow' of the
air between the incipient condensations and rarefactions, whereby the
formation of sound-pulses is forestalled. Stokes, however, has taught
us that this flow may be intercepted by placing the edge of a card in
close proximity to one of the corners of the fork. An immediate
augmentation of the sound of the fork is the consequence.
The more rapid the shock imparted to the air, the greater is the
fractional part of the energy of the shock converted into wave motion.
And as different kinds of gunpowder vary considerably in their
rapidity of combustion, it may be expected that they will also vary as
producers of sound. This theoretic inference is completely verified
by experiment. In a series of preliminary trials conducted at
Woolwich on the 4th of June, 1875, the sound-producing powers of four
different kinds of powder were determined. In the order of the size
of their grains they bear the names respectively of Fine-grain
(F.G.), Large-grain (L.G.), Rifle Large-grain (R.L.G.), and
Pebble-powder (P.) (See annexed figures.) The charge in each case
amounted to 4.5 lbs. four 24-lb. howitzers being employed to fire the
respective charges.
FIG. 9.
There were eleven observers, all of whom, without a single
dissentient, pronounced the sound of the fine-grain powder loudest of
all. In the opinion of seven of the eleven the large-grain powder
came next; seven also of the eleven placed the rifle large-grain third
on the list; while they were again unanimous in pronouncing the
pebble-powder the worst sound-producer. These differences are
entirely due to differences in the rapidity of combustion. All who
have witnessed the performance of the 80-ton gun must have been
surprised at the mildness of its thunder. To avoid the strain
resulting from quick combustion, the powder employed is composed of
lumps far larger than those of the pebble-powder above referred to. In
the long tube of the gun these lumps of solid matter gradually resolve
themselves into gas, which on issuing from muzzle imparts a kind of
push to the air, instead of the sharp shock necessary to form the
condensation of an intensely sonorous wave.
These are some of the physical reasons why guncotton might be regarded
as a promising fog-signal. Firing it as we have been taught to do by
Mr. Abel, its explosion is more rapid than that of gunpowder. In its
case the air particles, alert as they are, will not, it might be
presumed, be able to slip from condensation to rarefaction with a
rapidity sufficient to forestall the formation of the wave. On _à
priori_ grounds then, we are entitled to infer the effectiveness of
gun-cotton, while in a great number of comparative experiments,
stretching from 1874 to the present time, this inference has been
verified in the most conclusive manner.
As regards explosive material, and zealous and accomplished help in
the use of it, the resources of Woolwich Arsenal have been freely
placed at the disposal of the Elder Brethren. General Campbell,
General Younghusband, Colonel Fraser, Colonel Maitland, and other
officers, have taken an active personal part in the investigation, and
in most cases have incurred the labour of reducing and reporting on
the observations. Guns of various forms and sizes have been invoked
for gunpowder, while gun-cotton has been fired in free air and in the
foci of parabolic reflectors.
On the 22nd of February, 1875, a number of small guns, cast specially
for the purpose--some with plain, some with conical, and some with
parabolic muzzles--firing 4 oz. of fine-grain powder, were pitted
against 4 oz. of gun-cotton detonated both in the open, and in the
focus of a parabolic reflector. [Footnote: For charges of this
weight the reflector is of moderate size, and may be employed without
fear of fracture.]
The sound produced by the gun-cotton, reinforced by the reflector, was
unanimously pronounced loudest of all. With equal unanimity, the
gun-cotton detonated in free air was placed second in intensity.
Though the same charge was used throughout, the guns differed notably
among themselves, but none of them came up to the gun-cotton, either
with or without the reflector. A second series, observed from a
different distance on the same day, confirmed to the letter the
foregoing result.
As a practical point, however, the comparative cost of gun-cotton and
gunpowder has to be taken into account, though considerations of cost
ought not to be stretched too far in cases involving the safety of
human life. In the earlier experiments, where quantities of equal
price were pitted against each other, the results were somewhat
fluctuating. Indeed, the perfect manipulation of the gun-cotton
required some preliminary discipline--promptness, certainty, and
effectiveness of firing, augmenting as experience increased. As 1 lb.
of gun-cotton costs as much as 3 lbs. of gunpowder, these quantities
were compared together on the 22nd of February. The guns employed to
discharge the gunpowder were a 12-lb. brass howitzer, a 24-lb.
cast-iron howitzer, and the long 18-pounder employed at the South
Foreland. The result was, that the 24-lb. howitzer, firing 3 lbs. of
gunpowder, had a slight advantage over 1 lb. of gun-cotton detonated
in the open; while the 12-lb. howitzer and the 18-pounder were both
beaten by the gun-cotton. On the end of May, on the other hand, the
gun-cotton is reported as having been beaten by all the guns.
Meanwhile, the parabolic-muzzle gun, expressly intended for
fog-signalling, was pushed rapidly forward, and on March 22 and 23,
1876, its power was tested at Shoeburyness. Pitted against it were a
16-pounder, a 5.5-inch howitzer, 1.5 lb. of gun-cotton detonated in
the focus of a reflector (see annexed figure), and 1.5 lb. of
gun-cotton detonated in free air. On this occasion nineteen different
series of experiments were made, when the new experimental gun, firing
a 3-lb. charge, demonstrated its superiority over all guns previously
employed to fire the same charge. As regards the comparative merits
of the gun-cotton fired in the open, and the gunpowder fired from the
new gun, the mean values of their sounds were the same. Fired in the
focus of the reflector, the gun-cotton clearly dominated over all the
other sound-producers. [Footnote: The reflector was fractured by the
explosion, but it did good service afterwards.]
FIG. 10.
Gun-cotton Slab (1.5 lb.) Detonated in the Focus of a Cast-iron
Reflector.
The whole of the observations here referred to were embraced by an
angle of about 70°, of which 50' lay on the one side and 20° on the
other side of the line of fire. The shots were heard by eleven
observers on board the 'Galatea,' which took up positions varying from
2 miles to 13.5 miles from the firing-point. In all these
observations, the reinforcing action of the reflector, and of the
parabolic muzzle of the gun, came into play. But the reinforcement of
the sound in one direction implies its withdrawal from some other
direction, and accordingly it was found that at a distance of 5.25
miles from the firing-point, and on a line including nearly an angle
of 90° with the line of fire, the gun-cotton in the open beat the new
gun; while behind the station, at distances of 8.5 miles and 13.5
miles respectively, the gun-cotton in the open beat both the gun and
the gun-cotton in the reflector. This result is rendered more
important by the fact that the sound reached the Mucking Light, a
distance of 13.5 miles, against a light wind which was blowing at the
time.
Most, if not all, of our ordinary sound-producers send forth waves
which are not of uniform intensity throughout. A trumpet is loudest
in the direction of its axis. The same is true of a gun. A bell,
with its mouth pointed upwards or downwards, sends forth waves which
are far denser in the horizontal plane passing through the bell than
at an angular distance of 90° from that plane. The oldest bellbangers
must have been aware of the fact that the sides of the bell, and not
its mouth, emitted the strongest sound, their practice being probably
determined by this knowledge. Our slabs of gun-cotton also emit waves
of different densities in different parts. It has occurred in the
experiments at Shoeburyness that when the broad side of a slab was
turned towards the suspending wire of a second slab six feet distant,
the wire was cut by the explosion, while when the edge of the slab was
turned to the wire this never occurred.
To the circumstance that the broadsides of the slabs faced the sea is
probably to be ascribed the remarkable fact observed on March 23, that
in two directions, not far removed from the line of fire, the
gun-cotton detonated in the open had a slight advantage over the new
gun.
Theoretic considerations rendered it probable that the shape and size
of the exploding mass would affect the constitution of the wave of
sound. I did not think large rectangular slabs the most favourable
shape, and accordingly proposed cutting a large slab into fragments of
different sizes, and pitting them against each other The differences
between the sounds were by no means so great as the differences in the
quantities of explosive material might lead one to expect. The mean
values of eighteen series of observations made on board the 'Galatea,'
at distances varying from 1.75 mile to 4.8 miles, were as follows:
Weights 4 oz. 6 oz. 9 oz. 12 oz.
Value of sound 3.12 3.34 4.0 4.03
These charges were cut from a slab of dry gun-cotton about 1.75 inch
thick: they were squares and rectangles of the following dimensions:
4 oz, 2 inches by 2 inches;
6 oz, 2 inches by 3 inches;
9 oz, 3 inches by 3 inches;
12 oz, 2 inches by 6 inches.
The numbers under the respective weights express the recorded value of
the sounds. They must be simply taken as a ready means of expressing
the approximate relative intensity of the sounds as estimated by the
ear. When we find a 9-oz. charge marked 4, and a 12-oz. charge marked
4.03, the two sounds may be regarded as practically equal in
intensity, thus proving that an addition of 30 per cent. in the
larger charges produces no sensible difference in the sound. Were the
sounds estimated by some physical means, instead of by the ear, the
values of the sounds at the distances recorded would not, in my
opinion, show a greater advance with the increase of material than
that indicated by the foregoing numbers. Subsequent experiments
rendered still more certain the effectiveness, as well as the economy,
of the smaller charges of gun-cotton.
It is an obvious corollary from the foregoing experiments that on our
'nesses' and promontories, where the land is clasped on both sides for
a considerable distance by the sea--where, therefore, the sound has to
propagate itself rearward as well as forward--the use of the parabolic
gun, or of the parabolic reflector, might be a disadvantage rather
than an advantage. Here guncotton, exploded in the open, forms the
most appropriate source of sound. This remark is especially
applicable to such lightships as are intended to spread the sound all
round them as from central foci.
As a signal in rock lighthouses, where neither syren, steam-whistle,
nor gun could be mounted; and as a handy fleet-signal, dispensing with
the lumber of special signal-guns, the gun-cotton will prove
invaluable. But in most of these cases we have the drawback that
local damage may be done by the explosion. The lantern of the rock
lighthouse might suffer from concussion near at hand, and though
mechanical arrangements might be devised, both in the case of the
lighthouse and of the ship's deck, to place the firing-point of the
gun-cotton at a safe distance, no such arrangement could compete, as
regards simplicity and effectiveness, with the expedient of a
gun-cotton rocket. Had such a means of signalling existed at the
Bishop's Rock lighthouse, the ill-fated 'Schiller' might have been
warned of her approach to danger ten, or it may be twenty, miles
before she reached the rock which wrecked her. Had the fleet
possessed such a signal, instead of the ubiquitous but ineffectual
whistle, the 'Iron Duke' and 'Vanguard' need never have come into
collision.
It was the necessity of providing a suitable signal for rock
lighthouses, and of clearing obstacles which cast an acoustic shadow,
that suggested the idea of the gun-cotton rocket to Sir Richard
Collinson, Deputy Master of the Trinity House. His idea was to place
a disk or short cylinder of gun-cotton in the head of a rocket, the
ascensional force of which should be employed to carry the disk to an
elevation of 1000 feet or thereabouts, where by the ignition of a fuse
associated with a detonator, the gun-cotton should be fired, sending
its sound in all directions vertically and obliquely down upon earth
and sea. The first attempt to realise this idea was made on July 18,
1876, at the firework manufactory of the Messrs. Brock, at Nunhead.
Eight rockets were then fired, four being charged with 5 oz. and four
with 7.5 oz. of gun-cotton. They ascended to a great height, and
exploded with a very loud report in the air. On July 27, the rockets
were tried at Shoeburyness.
The most noteworthy result on this occasion was the hearing of the
sounds at the Mouse Lighthouse, 8.5 miles E. by S, and at the Chapman
Lighthouse, 8.5 miles W. by N; that is to say, at opposite sides of
the firing-point. It is worthy of remark that, in the case of the
Chapman Lighthouse, land and trees intervened between the firing-point
and the place of observation. This,' as General Younghusband justly
remarked at the time, 'may prove to be a valuable consideration if it
should be found necessary to place a signal station in a position
whence the sea could not be freely observed.' Indeed, the clearing of
such obstacles was one of the objects which the inventor of the rocket
had in view.
With reference to the action of the wind, it was thought desirable to
compare the range of explosions produced near the surface of the earth
with others produced at the elevation attainable by the gun-cotton
rockets. Wind and weather, however, are not at our command; and hence
one of the objects of a series of experiments conducted on December
13, 1876, was not fulfilled. It is worthy, however, of note that on
this day, with smooth water and a calm atmosphere, the rockets were
distinctly heard at a distance of 11.2 miles from the firing-point.
The quantity of gun-cotton employed was 7.5 oz. On Thursday, March 8,
1877, these comparative experiments of firing at high and low
elevations were pushed still further. The gun-cotton near the ground
consisted of 0.5-lb. disks, suspended from a horizontal iron bar about
4.5 feet above the ground.
The rockets carried the same quantity of gun-cotton in their heads,
and the height to which they attained, as determined by a theodolite,
was from 800 to 900 feet. The day was cold, with occasional squalls
of snow and hail, the direction of the sound being at right angles to
that of the wind. Five series of observations were made on board the
'Vestal,' at distances varying from 3 to 6 miles. The mean value of
the explosions in the air exceeded that of the explosions near the
ground by a small but sensible quantity. At Windmill Hill, Gravesend,
however, which was nearly to leeward, and 5.5 miles from the
firing-point, in nineteen cases out of twenty-four the disk fired near
the ground was loudest; while in the remaining five the rocket had the
advantage.