ON EXPLOSIVES.
EXPLOSION may be defined as the sudden or extremely rapid conversion of
a solid or liquid body of small bulk into gas or vapour, occupying very
many times the volume of the original substance, and which in addition
is highly expanded by the heat generated during the action.
This sudden or very rapid expansion of volume is attended by an
exhibition of force which is more or less violent, according to
the constitution of the original body and the circumstances of the
explosion.
Any substance capable of undergoing such a change on the application of
heat or other disturbing cause is called an "explosive."
_Explosive Force._--Explosive _force_ is _directly_ proportional to
the heat of combustion and the volume of gas, and _inversely_ to the
specific heat of the mixed products.
Explosive _effect_ is _directly_ proportional to the volume of gas
produced and the temperature of the explosion, and _inversely_ as the
time required for the change to take place.
_Explosive Effect and Force compared._--Explosive effect depends upon
the rapidity with which the conversion is effected, while the same
amount of explosive force may act suddenly or gradually.
As before stated, explosions are more or less violent according to the
_circumstances_ under which they take place. These may be considered as
follows:--
1.--The physical state of the explosive substance.
2.--The external conditions under which the explosive body is fired.
3.--The mode of firing.
_The Physical State of the Explosive Substance._--Numerous instances
may be cited to show the influence the physical condition of an
explosive body has upon its explosion.
Thus, gunpowder may, by merely varying the size, shape, and density of
the grain, be made to ignite rapidly but burn comparatively slowly, or
be made to ignite more slowly, but once inflamed to burn very rapidly.
Again, gun-cotton in a loose, uncompressed state, will, if ignited,
only flash off; if it is spun into threads or woven into webs, its rate
of combustion may be so much reduced that it can be used in gunnery
or for a quick fuze; while if powerfully compressed and damp it burns
slowly. Wet gun-cotton requires a primer of dry gun-cotton and a
fulminate fuze to explode; dry, it may be exploded by a fulminate fuze,
&c.
Then nitro-glycerine, when exploded by 15 grains of fulminate of
mercury, and at a temperature above 40° F., is very violently
detonated; below 40° F. it freezes and cannot be similarly exploded.
To obtain the full effect of all explosives, confinement is absolutely
necessary.
The more rapid the explosion the less confinement required, approaching
in the case of some explosives to so small an amount that it need not,
for practical purposes, be considered.
Thus a charge of nitro-glycerine or gun-cotton, when detonated in the
open air, will destroy wrought iron rails, large blocks of stones,
balks of timber, &c.
In the case of the former body, the confinement of the atmosphere is
sufficient.
In the latter, the mechanical cohesion due to compression is sufficient
restraint.
Abel states that if the film of atmosphere surrounding the
nitro-glycerine, not exceeding 1/1000 inch in thickness, be removed,
the explosive effect is much lessened.
A large charge of gunpowder fired in the ordinary way under water
requires a strong case to retain the gases until the action has become
general, or, owing to its slow rate of burning, the case would be
broken before the whole of the charge had been ignited, and part of the
charge drowned.
This is often to be noticed when firing fine-grained powder in heavy
guns.
Igniting the charge at several points diminishes the confinement needed.
_Mode of Firing._--The application of heat, directly or indirectly, is
the principal means of causing an explosion.
The flame from a percussion cap or primer, or a platinum wire heated to
incandescence by an electric current, will _directly_ ignite a charge.
Friction, concussion, &c., will _indirectly_ ignite a charge due to the
conversion of mechanical energy into heat.
It would appear that when one explosive body is used as a means of
firing another, the resultant explosion is due to the blow suddenly
formed by the gas of the firing charge acting percussively upon the
mass to be exploded. If such were the case, then the most powerful
explosive would be the best agent for causing an explosion. But it is
not so.
For example, nitro-glycerine, which is far more powerful than fulminate
of mercury, requires more than 1000 grains to explode gun-cotton, while
only 15 grains of the latter is needful for the same work, &c.
A small quantity of an explosive substance which is sensitive to
friction or percussion is often used to ignite the original charge.
_Detonation._--The instantaneous explosion of the whole mass of a body
is defined as "detonation."
The essential difference between an explosion and a detonation is the
comparative suddenness of the transformation of the solid or liquid
explosive substance into gas and vapour.
Some explosive bodies, such as the fulminates, &c., always detonate,
while the detonation of others depends on the mode of firing.
Nitro-glycerine always explodes violently, but when fired with an
initiatory charge of fulminate of mercury it is much more powerful than
when fired with gunpowder.
Compressed gun-cotton in the air-dry state can be detonated by 2 grains
of fulminate of mercury embedded in the material, but when it contains
3 per cent. of water over and above the 2 per cent. which exists
normally in the air-dry substance, 15 grains of the fulminate will not
always do so.
_Theory of Detonation._--The theory of detonation is not yet thoroughly
understood. That it is not alone due to the heat caused by the impact
of the mechanical energy of the particles of gas, set free from the
initiatory charge on the principal mass, is proved by the fact of its
being possible to detonate wet gun-cotton.
Professor Bloxam terms detonation to be "sympathetic" explosion.
Experiments carried on in England by Professor Abel, and in France by
MM. Champion and Pellet, tend to show that it is due to the vibratory
action of the detonating agent.
Thus a glass may withstand a strong blow, though a particular note or
vibration will smash it.
All explosive compounds and mixtures, including gunpowder, are
susceptible of violent explosion through the agency of a detonation.
_Roux and Sarrau._--Roux and Sarrau divide explosions into two orders:--
1st order.--Detonations.
2nd order.--Simple explosions.
Simple explosions are produced by direct inflammation, or by a small
charge of gunpowder.
Detonations are obtained from nitro-glycerine, gun-cotton, &c., by
exploding with fulminate of mercury.
They state that fulminate of mercury does not detonate gunpowder; but
if the exploding charge is a small amount of nitro-glycerine, itself
detonated by fulminate of mercury, then an explosion of the first order
is obtained.
The relative effects were approximately measured by determining the
quantities necessary to rupture small cast iron shells of supposed
equal strength.
_Results of their Experiments._--The following are some of the
results:--
+---------------+---------------------------+
| | Explosive Effect. |
| +---------------------------+
| | 2nd Order. | 1st Order. |
+---------------+-------------+-------------+
|Gunpowder | 1·00 | 4·34 |
|Gun-cotton | 3·00 | 6·46 |
|Nitro-glycerine| 4·80 | 10·13 |
+---------------+-------------+-------------+
According to the above table, nitro-glycerine is more than ten times,
and gun-cotton more than six times, as powerful as gunpowder fired in
the ordinary way (2nd order).
The want of reciprocity between two detonating agents is shown in
a remarkable degree by the following experiments, carried out by
Professor Abel:--
1.--The detonation of 1/4 ounce of gun-cotton (the
smallest quantity that can be thus applied) induced the
simultaneous detonation of nitro-glycerine, enclosed
in a vessel of sheet tin, and placed at a distance of 1
inch from the gun-cotton.
2.--The detonation of 1/2 ounce of gun-cotton produces
the same effect with an intervening space of 3 inches
between the substances.
3.--The detonation of 2 ounces of nitro-glycerine in
_close contact_ with compressed gun-cotton failed to
accomplish the detonation of the latter, which was
simply dispersed in a fine state of division, in all
the instances but one, in a large number of experiments.
Explosive agents are divided into explosive mixtures and compounds.
In the former the ingredients are mechanically mixed, and can be
separated by mechanical means.
In the latter the ingredients are chemically combined, and can only be
separated by chemical change.
_Torpedo Explosive Agents._--The explosive agents that are practically
the most important, as far as their employment as torpedo charges are
concerned, are as follows:--
_Explosive Mixtures._--A.--Explosive mixtures.
1.--Gunpowder. } Nitrate class
2.--Ammonium picrate, or picric powder. }
_Explosive Compounds._--B.--Explosive compounds.
1.--Nitro-glycerine.
2.--Dynamite (No. 1).
3.--Gun-cotton.
4.--Fulminate of mercury.
A.--EXPLOSIVE MIXTURES.
_Gunpowder._--This explosive mixture is composed of seventy-five parts
of nitre (saltpetre), fifteen parts of charcoal, and ten parts of
sulphur.
On being ignited, the oxygen which is feebly held by the nitrogen
combines with the carbon, forming carbonic oxide gas, whilst the
sulphur unites with the potassium of the nitre, the whole combination
being accompanied by a great evolution of heat and expansion of gas,
and the nitrogen is set free.
_Properties, &c._--A spark, friction between hard bodies, or a
temperature of 572° F., are any of them sufficient to cause an
explosion of gunpowder.
Slight moisture, due to damp air, &c., produces caking and
deterioration.
Wetting causes permanent destruction.
Frost does not injure it.
It can be fired by ordinary methods.
It can be transported and handled with safety and great ease.
It is not a suitable explosive agent for torpedoes, on account of its
liability to be injured by damp, as well as its not being sufficiently
violent, though for the sake of convenience, &c., it is often employed
for such work.
The effect produced by the explosion of a charge of gunpowder,
ignited by the ordinary method, is that of an uplifting rather than a
shattering effect.
This evil may be greatly remedied, when gunpowder is used as the charge
of a torpedo, by firing it with a detonator, by which means its fullest
explosive effect is developed.
_Picric Powder._--The picrates are salts of picric acid.
Picric acid is formed by the action of nitric acid on carbolic acid.
The picrate employed by Professor Abel is prepared from picric acid and
ammonium. This preparation, or salt mixed with nitre (saltpetre), forms
Abel's picric powder.
_Properties, &c._--It is prepared for use in a similar manner to
gunpowder, and it can be handled in the same way.
It is less violent than dynamite or gun-cotton, though much more so
than gunpowder.
It is difficult to explode it by blows or friction.
If flame be applied to it, the part touched burns, but the combustion
does not become general.
This explosive agent will probably be used for spar torpedoes, when
gun-cotton or dynamite are not employed.
B.--EXPLOSIVE COMPOUNDS.
_Nitro-glycerine._--Nitro-glycerine is formed by the action of nitric
acid upon glycerine at a low temperature.
The manufacture of this compound consists, first, in the slow mixture
of the glycerine with the acid, at a low temperature; secondly, in
washing the nitro-glycerine from the excess of acid with water.
The nitric acid before use is mixed with a certain proportion of strong
sulphuric acid, so that the water formed during the reaction may be
taken up, and thus any dilution of the nitric acid is prevented.
Nitro-glycerine is composed of carbon, hydrogen, nitrogen, and oxygen,
as indicated by the equation _C_{3}H_{5}N_{3}O_{9}_.
_Properties, &c._--At ordinary temperatures nitro-glycerine is an oily
liquid, having a specific gravity of 1·6. Freshly made it is creamy
white and opaque, but clears and becomes colourless on standing for a
certain time, depending on the temperature.
It does not mix with, nor is it affected by, water. It has a sweet,
aromatic taste, and produces a violent headache if placed upon the
tongue.
The opaque, freshly made nitro-glycerine does not freeze until the
temperature is lowered to 3°-5° below zero, F., but, when cleared, it
freezes at 39°-40° F. Nitro-glycerine freezes to a white crystalline
mass, and in this state it can be thawed by placing the vessel
containing it in water, at a temperature not over 100° F.
If flame is applied to freely exposed nitro-glycerine, it burns slowly
without explosion.
Nitro-glycerine in a state of decomposition becomes very sensitive,
exploding violently when struck, even when unconfined.
Pure nitro-glycerine does not spontaneously decompose at any ordinary
temperature, but if it contains any free acid, then decomposition
may happen. When pure, it is not sensitive to friction, or moderate
percussion. If struck with a hammer, only the particle receiving the
blow explodes, the remainder being scattered.
The firing point of nitro-glycerine is about 356° F., though it begins
to decompose at a lower temperature.
The mode of firing nitro-glycerine usually employed is that of a
fulminate of mercury detonating fuse.
Nitro-glycerine in the frozen state cannot be fired even by large
charges of fulminate.
In one instance, 1600 lbs. of liquid nitro-glycerine exploded in a
magazine containing 600 lbs. of the same substance in a frozen state,
but failed to fire the latter, only breaking it up and scattering it in
every direction.
_Dynamite._--This explosive compound is merely a preparation in which
nitro-glycerine is itself presented for use, its explosive properties
being those of the nitro-glycerine contained in it, as the absorbent is
an inert body.
Dynamite is formed of seventy-five parts of nitro-glycerine absorbed by
twenty-five parts of a porous siliceous earth or "kieselguhr."
The best substitute for "kieselguhr" is ashes of bog-head coal.
Dynamite is a loose, soft, readily moulded substance, of a buff colour.
The preparation of dynamite is very simple.
The nitro-glycerine is mixed by means of wooden spatulas with the fine
white powder (kieselguhr) in a leaden vessel.
It freezes at 39°-40° F., and when solidly frozen cannot be exploded,
but if in a pulverised state it can be exploded, though with diminished
violence.
It can be easily thawed, by placing the vessel containing it in hot
water.
Friction or moderate percussion does not explode it.
Its firing point is 356° F.
If flame be applied to it, it burns with a strong flame.
It is fired by means of fulminate of mercury, and its explosive force
is about seven times that of gunpowder.
For ground and buoyant mines, where actual contact between the hostile
vessel and the torpedo will be rarely achieved, this being next to
nitro-glycerine the most violent of all known explosive agents, and
being cheaply and readily procured, is the very best explosive for such
torpedoes.
That it is not generally adopted is owing to its containing a large
proportion of that seemingly dangerous substance, nitro-glycerine,
which makes the handling of dynamite a somewhat hazardous operation.
According to Professor Abel, there are now as many as fifteen dynamite
factories in different parts of the world (including a very extensive
one in Scotland) working under the supervision of Mr. Nobel, the
originator of the nitro-glycerine industry; and six or seven other
establishments exist where dynamite or preparations of very similar
character are also manufactured.
The total production of dynamite in 1867 was only eleven tons, while in
1878 it amounted to 6140 tons.
This explosive compound is most extensively used for general blasting
purposes all over the world, and for this purpose, owing to its
cheapness and the convenience in manipulating it, is far superior to
compressed gun-cotton.
Gun-cotton is formed by the action of concentrated nitric acid
on cotton, its composition being indicated by the formula
_CH_{7}(NO_{2})_{3}O_{5}_.
Professor Abel's process for manufacturing pulped and compressed
gun-cotton is as follows:--
Cotton waste is the form of cotton used; it is picked and cleaned,
thoroughly dried at 160° F., and then allowed to cool.
The strongest nitric and sulphuric acids are employed, mixed in the
proportion of one part of the former to three of the latter by weight.
These are mixed in large quantities, and stored in cast-iron tanks.
The cotton in 1-lb. charges is immersed in the acid mixture, which is
contained in a trough surrounded by cold water. After being subjected
to the action of the acid for a short space of time, the cotton is
taken up, placed upon a perforated shelf, and as much as possible of
the acid squeezed out of it. It is then put into jars, covered with
fresh acid, and the jars placed in fresh water, remaining there for
twenty-four hours.
To remove the acid, the gun-cotton from the jars is thrown into a
centrifugal strainer, by which nearly all the acid is expelled. It is
then diffused quickly in small quantities through a large volume of
water, and again passed through a centrifugal machine.
The next process is that of thoroughly washing the gun-cotton, for the
purpose of removing the traces of the acid still adhering to it. By
pulping, which operation is performed in pulping engines or beaters,
the washing is expeditious and thorough.
A _beater_ is an oblong tub in which is placed a revolving wheel
carrying strips of steel on its circumference. From the bottom under
the wheel project similar steel strips.
The action of this machine is as follows:--
By the rotation of the wheel, the gun-cotton which is suspended in
water circulates around the tub, and is drawn between the two sets of
steel projections, by which it is reduced to a state of _pulp_.
The bottom of the tub is movable, and thus the space through which the
gun-cotton must pass may be contracted, as the operation proceeds.
The pulping being complete, the contents are run into _poachers_ for
the final washing.
A _poacher_ is a large oblong wooden tub. On one side at the middle is
placed a wooden paddle-wheel, which extends half way across the tub.
In the poacher the pulped gun-cotton is stirred for a long time with a
large quantity of water. The revolution of the paddle-wheel keeps up a
constant circulation, and care is taken that no deposit occurs in any
part of the tub.
Having converted the cotton into gun-cotton, reduced it to a state of
pulp, and thoroughly washed it, the next process is to separate the
water from the pulp, and compress it into cakes or discs.
This is accomplished by means of two presses, the first of which has 36
hollow cylinders, in which perforated plungers work upwards.
These plungers having been drawn down, the cylinders are filled with
the water-laden pulp, and their tops covered with a weight; the
plungers are then forced up by hydraulic power, compressing the pulp,
and forcing the water to escape through their perforations.
The second one is used to more solidly compress the cylindrical masses
of gun-cotton formed by the action of the first press, a pressure of 6
tons to the inch being in this case applied.
About 6 per cent. of moisture still remains in the discs, which can be
readily removed by drying.
_Properties._--Cotton converted into gun-cotton is little changed in
appearance, though the latter is harsher to the touch than the former.
If a flame be applied to dry loose gun-cotton, it flashes up, without
explosion; if compressed it burns rapidly, but quietly.
Moist compressed gun-cotton under the same circumstances burns away
slowly.
Gun-cotton containing 12 to 14 per cent. of water is ignited with much
difficulty on applying a highly heated body. As it leaves the hydraulic
press upon being converted from the pulped state to masses, it contains
about 15 per cent. of water; in this condition it may be thrown on to
a fire or held in a flame without exhibiting any tendency to burn;
the masses may be perforated by means of a red-hot iron, or with a
drilling tool, and they may with perfect safety be cut into slices by
means of saws revolving with great rapidity. If placed upon a fire and
allowed to remain there, a feeble and transparent flame flickers over
the surface of the wet gun-cotton from time to time as the exterior
becomes sufficiently dry to inflame; in this way a piece of compressed
gun-cotton will burn away very gradually indeed.
To test the safety of wet gun-cotton, the following two experiments
among many have been made:--
Quantities of wet gun-cotton, 20 cwt. each, packed in one instance in
a large, strong wooden case, and in the other in a number of strong
packing cases, were placed in small magazines, very substantially built
of concrete and brickwork. Large fires were kindled around the packages
in each building, the doors being just left ajar. The entire contents
of both buildings had burned away, without anything approaching
explosive action, in less than two hours.
This comparatively great safety of wet gun-cotton, coupled with the
fact that its detonation in that state may be readily accomplished
through the agency of a small quantity of dry gun-cotton, termed a
"primer," which, by means of a fulminating fuze, or detonator, is
made to act as the initiative detonating agent, gives it important
advantages over other violent explosive agents, when used for purposes
which involve the employment of a considerable quantity of the
material, on account of the safety attending its storage and necessary
manipulation.
From experiments conducted by engineer officers in Austria, it was
found that if boxes containing dry compressed gun-cotton are fired into
from small arms, even at a short range, the gun-cotton is generally
inflamed, but never exploded, the sharpness of the blow essential to
effect an explosion, which the bullet might otherwise give, being
diminished by its penetration through the side of the box before
reaching the explosive. Wet gun-cotton, containing even as little as 15
per cent. of water, is never inflamed on these conditions.
Dynamite, on the other hand, is invariably detonated when struck by a
bullet on passing through the side of the box.
Gun-cotton is insoluble in and unaffected by water.
The firing point of gun-cotton is about 360° F.
The temperature of explosion of gun-cotton is about 8700° F., being
more than double that of gunpowder. Gun-cotton is not sensitive to
friction or percussion.
If not perfectly converted or thoroughly washed, gun-cotton is liable
to spontaneous decomposition, which under favourable conditions may
result in explosion.
Compressed gun-cotton is free from such danger, as it may be kept
and used saturated with water. It is stored in the wet state, care
being taken that it is not exposed to a temperature that will freeze
the water in the cakes, as if this occurs they are liable to be
disintegrated by the expansion of the water in freezing.
Gun-cotton is the agent most extensively used for all kinds of
military engineering and submarine operations in Great Britain, it
being especially manufactured by the English government for that
express purpose; but in other countries it is not so manufactured, and
therefore, as it is little used for other than military purposes, it
is not to any extent privately manufactured, as is the case with other
explosives, such as dynamite, dualine, lithofracteur, &c., and thus, in
case of war, would be somewhat difficult to obtain out of England.
Compared with dynamite, it is not so violent, and occupies more space,
weight for weight, and also requires a more complicated means of
detonating it. On the other hand, gun-cotton is infinitely safer to
store and manipulate, and is not so subject to detonation by concussion
(not being so sensitive) as dynamite.
_Fulminate of Mercury._--Fulminate of mercury is formed by the
action of mercuric nitrate and nitric acid upon alcohol. The mode of
preparation is as follows:--
Dissolve one part of mercury in twelve parts of nitric acid, and pour
this solution into twelve parts of alcohol.
Pour this mixture into a vessel which is placed in hot water until it
darkens and becomes turbid and begins to evolve dense white fumes,
then remove it from the water. The reaction goes on, with strong
effervescence and copious evolution of dense white ethereal vapours. If
red fumes appear, cold alcohol should be added to check the violence of
the action.
The operation should be performed at a distance from a fire or flame,
and in a strong draught, so that the vapours may be carried off.
When the liquid clears, and the dense white fumes are no longer given
off, further action is stopped by filling up with cold water. The
fulminate settles to the bottom of the vessel as a grey crystalline
precipitate. The liquid is then poured off, and the fulminate washed
several times by decantation or upon a filter.
Dry fulminate of mercury explodes violently when heated to 367° F.,
when forcibly struck by the electric spark, &c.
When wet it is inexplosive, and therefore it is always kept wet, being
dried in small amounts when required for use.
Fulminate of mercury is applied in many ways, either pure or mixed with
other substances, as in percussion caps, percussion powder, primers,
detonators, &c.
For the purpose of detonating nitro-glycerine or its preparations, 15
grains of the fulminate are sufficient, but to detonate gun-cotton
25 grains are necessary. The fulminate in detonating fuzes should
be enclosed in a copper case or cap, and must never be loose. The
fulminate should be wet when charging the detonators, as it is very
dangerous to handle when dry.
Great care is requisite in handling this explosive compound.
In addition to the foregoing explosive compounds and mixtures, the
following explosive agents have also been employed for the purposes of
submarine operations, though only to a small extent.
_Dualin._--Dualin is a nitro-glycerine preparation formed by mixing
sawdust and saltpetre with that substance.
This preparation, inferior to dynamite, was employed by the Germans as
the explosive agent for their submarine mines during the Franco-German
war (1870-71).
_Lithofracteur._--Lithofracteur is also a preparation
of nitro-glycerine. It is composed of the following
materials:--Nitro-glycerine, kieselguhr, coal, soda, saltpetre, and
sulphur.
This explosive agent, also inferior to dynamite, is used, though not
very extensively, by the French for their submarine mines.
_Horsley's Powder._--Horsley's powder is a chlorate mixture formed of
potassium, chlorate, and galls. This explosive mixture was formerly
used by Captain Harvey for his towing torpedo, but has recently been
discarded for compressed gun-cotton.
_Abel's Detonation Experiments._--The following are the results of
experiments carried out by Professor Abel, C.B., F.R.S., on the subject
of detonation:--
1.--A fuze containing rather more than 1 ounce of
gunpowder, strongly confined, exploded in contact with
a mass of compressed gun-cotton, _only inflames it_,
although the explosion of the fuze is apparently a
sharp one.
2.--45 grains of fulminate of mercury, exploded
unconfined on the surface of a piece of compressed
gun-cotton, only inflames or disperses it.
3.--A fuze containing 9 grains of fulminate of mercury,
strongly confined, exploded in contact with compressed
gun-cotton, or dynamite, detonates it with certainty.
4.--An equal quantity of fulminate of mercury,
similarly confined, does not detonate _uncompressed_
gun-cotton in which it is imbedded, but merely
disperses and inflames it.
5.--150 grains of compressed gun-cotton, detonated in
proximity to dynamite, _detonates the latter_.
6.--3 ounces of dynamite, and very much larger
quantities, detonated in contact with compressed
gun-cotton, only disperses it.
7.--A wrought-iron rail can be destroyed by detonating
8 ounces of compressed gun-cotton placed unconfined on
the rail.
8.--A piece of wet gun-cotton, quite uninflammable,
removed from a fire, and detonated upon a block of
granite, using a small primer of dry gun-cotton,
shatters the block.
9.--A submerged charge of wet gun-cotton, open on all
sides to the water, and merely confined around the
dry initiative, or primer, by means of a net, can be
exploded.
_Explosive Agents in Torpedoes._--The explosive agents that at the
present time are most generally used in torpedoes are gunpowder,
gun-cotton in the wet compressed state, and dynamite, and these may be
compared as to their properties and their explosive effects.
_Gunpowder._--Gunpowder is a familiar material, in general use for
all military purposes. It can be handled and transported with safety
and ease, and it can be fired by ordinary methods. But for submarine
purposes it has the disadvantage of being very easily injured by water,
so that it is absolutely necessary to enclose it in water-tight cases.
_Gun-cotton._--Gun-cotton is free from liability to accidents, and in
this matter, and the safety of its manufacture, it compares favourably
with gunpowder.
It is peculiarly adapted to submarine work, being unaffected by water.
And as it may be kept in water, ready for use, it can be safely carried
on board ship in large quantities. It is far more violent in its
action when detonated than gunpowder. The chief objection to its use
is, that being applied only for special purposes, it is not readily
obtained. Also it requires a peculiar and somewhat complicated mode of
firing it.
_Dynamite._--Dynamite is more easily manufactured than the two
foregoing explosives. The fact of it containing nitro-glycerine, which
has a bad reputation, has militated against its use as a torpedo
explosive agent, though for blasting purposes it is most extensively
used. Though not directly affected by water, its firing is hindered
when diffused through water. Another disadvantage is its high freezing
point. Like gun-cotton, it requires special means to fire it, though
much simpler, and also is much more powerful than gunpowder. The
explosive effect of dynamite or gun-cotton is a rending or a shattering
one, while that of gunpowder is an uplifting or heaving one.
Again, it is necessary when using gunpowder that the object be in the
line of least resistance, but with dynamite or gun-cotton the effect is
nearly equal in every direction, therefore for submarine operations,
either dynamite or gun-cotton is the explosive agent that should be
invariably used.
_Size of Torpedo Charges._--For permanent mines, a charge of 700 lbs.
to 1000 lbs. of gun-cotton is quite sufficient, though too large a
charge cannot be employed, except as regards the matter of convenience.
For buoyant mines, 500 lbs. to 700 lbs. of gun-cotton is an ample
charge, and for contact mines, 200 lbs. to 300 lbs. of gun-cotton is
sufficient. In spar torpedoes, where lightness is a consideration,
gun-cotton charges of 30 lbs. to 50 lbs. will be found ample, and
similarly in the case of the towing or locomotive torpedoes. Of course,
with regard to such a submarine weapon as the Lay torpedo boat, any
size charge may be carried, according to the wish of the builder.
_Torpedo Explosions illustrated._--At Fig. 166 is represented a sketch
of a torpedo explosion, from a photograph taken at the moment the
column of water was at its greatest elevation. The torpedo contained
432 lbs. of gun-cotton, and was exploded under 27 feet of water.
The height of the column thrown up measured 81 feet, and the diameter
at the base 132 feet.
[Illustration: SUBMARINE MINE EXPLOSION.
PLATE LII]
[Illustration: SUBMARINE MINE EXPLOSIONS.
PLATE LIII]
At Fig. 165 is shown a sketch of two submarine mine explosions from
an instantaneous photograph; the schooner which is shown in
the sketch happened to be passing at the moment of explosion, thus
affording a comparison as to the size of the columns of water thrown up.
The column on the left was due to the explosion of a submarine mine
containing 100 lbs. gunpowder at a depth of 10 feet below the surface.
That on the right was the result of an explosion of a similar mine, but
at a depth of 41 feet below the surface. Its extreme height was 400
feet.