be taken to ventilate them as much as possible during dry weather, by
opening all doors, windows, loopholes, &c. Magazines are generally made
bomb-proof, and are furnished with lightning conductors. They are
divided into chambers, and these again divided by uprights into bays. At
Purfleet, which is the grand depôt for gunpowder in England, there are
five magazines capable of containing 9,600 whole barrels each. Each
magazine is divided into two chambers, and each chamber into 24 bays,
and in each bay is placed 200 whole, 400 half, or 800 quarter barrels of
powder. Total in the five Magazines, 48,000 barrels, equal to 4,800,000
pounds.
LIGHTNING CONDUCTORS.
_Principles and Instructions relative to their application to Powder
Magazines, by_ SIR W. SNOW HARRIS, F.R.S. _Extracted from Army List
for July, 1859._
1.--Thunder and lightning result from the operation of a peculiar
natural agency through an interval of the atmosphere contained between
the surface of a certain area of clouds, and a corresponding area of the
earth’s surface directly opposed to the clouds. It is always to be
remembered that the earth’s surface and the clouds are the terminating
planes of the action, and that buildings are only assailed by Lightning
because they are points, as it were, in, or form part of, the earth’s
surface, in which the whole action below finally vanishes. Hence
buildings, under any circumstances, will be always open to strokes of
Lightning, and no human power can prevent it, whether having Conductors
or not, or whether having metals about them or not, as experience shows.
2.--Whenever the peculiar agency, (whatever it may be), active in this
operation of nature, and characterized by the general term Electricity,
or Electric Fluid, is confined to substances which are found to resist
its progress, such, for example, as air, glass, resinous bodies, dry
wood, stones, &c., then an explosive form of action is the result,
attended by such an evolution of light and heat, and by such an enormous
expansive force, that the most compact and massive bodies are rent in
pieces, and inflammable matter ignited. Nothing appears to stand against
it. Granite rocks are split open, oak and other trees, of enormous size,
rent in shivers, and masonry of every kind frequently laid in ruins. The
lower masts of ships of the line, 3 feet in diameter, and 110 feet long,
bound with hoops of iron half an inch thick and 5 inches wide, the whole
weighing about 18 tons, have been, in many instances, torn asunder, and
the hoops of iron burst open and scattered on the decks. It is, in fact,
this terrible expansive power which we have to dread in cases of
buildings struck by Lightning, rather than the actual heat attendant on
the discharge itself.
3.--When, however, the electrical agency is confined to bodies, such as
the metals, which are found to oppose but small resistance to its
progress, then this violent expansive or disruptive action is either
greatly reduced, or avoided altogether. The explosive form of action we
term Lightning, vanishes, and becomes, as it were, transformed into a
sort of continuous current action, of a comparatively quiescent kind,
which, if the metallic substance it traverses be of certain known
dimensions, will not be productive of any damage to the metal. If,
however, it be of small capacity, as in the case of a small wire, it may
become heated and fused. In this case, the electrical agency, as before,
is so resisted in its course as to admit of its taking on a greater or
less degree of explosive and heating effect, as in the former case. It
is to be here observed, that all kinds of matter oppose some resistance
to the progress of what is termed the Electrical Discharge, but the
resistance through capacious metallic bodies is comparatively so small,
as to admit of being neglected under ordinary circumstances; hence it is
that such bodies have been termed Conductors of Electricity, whilst
bodies such as air, glass, &c., which are found to oppose very
considerable resistance to electrical action, are placed at the opposite
extremity of the scale, and termed Non-conductors or Insulators.
The resistance of a metallic copper wire to an ordinary electrical
discharge from a battery, was found so small, that the shock traversed
the wire at the rate of 576,000 miles in a second. The resistance
however, through a metallic line of Conduction, small as it be,
increases with the length, and diminishes with the area of the section
of the Conductor, or as the quantity of metal increases.
4.--It follows from these established facts, that if a building were
metallic in all its parts, an iron magazine for example, then no damage
could possibly arise to it from any stroke of Lightning which has come
within the experience of mankind; e.g., a man in armour is safe from
damage by Lightning; in fact, from the instant the electrical discharge
in breaking with disruptive and explosive violence through the resisting
air, seizes upon the mass in any point of it, from that instant the
explosive action vanishes, and the forces in operation are neutralized
upon the terminating planes of action, viz., the surface of the earth,
and opposed clouds.
5.--All this plainly teaches us, that in order to guard a building
effectually against damage by Lightning, we must endeavour to bring the
general structure as nearly as may be, into that passive or
non-resisting state it would assume, supposing the whole were a mass of
metal.
6.--To this end, one or more conducting channels of copper depending
upon the magnitude and extent of the building should be systematically
applied to the walls; these conducting channels should consist either of
double copper plates united in series one over the other, as in the
method of fixing such Conductors to the masts of Her Majesty’s Ships,
the plates being not less than 3¹⁄₂ inches wide, and of ¹⁄₁₆th and ¹⁄₈th
of an inch in thickness, or the Conductors may with advantage be
constructed of stout copper pipe not less than ³⁄₁₆ths of an inch thick,
and 1¹⁄₂ to 2 inches in diameter: in either case the Conductors should
be securely fixed to the walls of the building, either by braces, or
copper nails, or clamps; they should terminate in solid metal rods
above, projecting freely into the air, at a moderate and convenient
height above the point to which they are fixed, and below they should
terminate in one or two branches leading outward about a foot under the
surface of the earth; if possible, they should be connected with a
spring of water or other moist ground.
It would be proper in certain dry situations, to lead out in several
directions under the ground, old iron or other metallic chains, so as to
expose a large extent of metallic contact in the surface of the earth.
7.--All the metals in the roof and other parts of the building of
whatever kind, should so far as possible have metallic communication
with these Alarm Conductors, and in case of any prominent elevated
chimney, it would be desirable to lead a pointed conducting tube along
it to the metals of the roof; all of which satisfies the conditions
above specified.
8.--Remark 1.--It is now proved beyond all questions, that the
electrical discharge never leaves perfect conducting lines of small
resistance, in order to pass out upon bad conducting circuits, in which
the resistance is very great, that is an established law of nature;
hence a stroke of Lightning upon such conducting lines will be confined
to the Conductors as constituting a line of discharge of less resistance
than any other line of discharge through the building, which can be
assigned. The apprehension of “Lateral Discharge” therefore, from the
Conductor, is quite absurd; and is not countenanced by any fact
whatever; if any doubt could possibly exist, it would be now most
completely set at rest by the experience of the permanent Conductors,
applied to the masts of Her Majesty’s ships. In very many instances
furious discharges of Lightning have fallen on the masts with a crash as
if the ship’s broadside had been fired, and the solid point aloft has
been found melted; in all these cases electrical discharge robbed by the
Conductor of its explosive violence, has traversed the line of action to
the sea, through the ship, and through the copper bolts, driven through
the ship’s solid timbers, without the least damage to the surrounding
masses, whether metallic, as in the case of the massive iron hoops on
the lower masts, or not. Persons have either been close by or actually
leaning against the Conductors at the time, without experiencing any ill
consequence.
9.--Remark 2.--It has also been incontestably shown, that metallic
bodies have not any specific attractive force or affinity for the matter
of Lightning; metals are as little attractive of lightning as wood or
stone. All matter is equally indifferent to Electricity so far as
regards a specific attraction, hence the idea that metals attract or
invite Lightning is a popular but very unlearned error contradicted by
the most satisfactory evidence, and the whole course of experience; in
short, we find that Lightning falls indiscriminately upon trees, rocks,
and buildings, whether the buildings have metals about them or not.
10.--Remark 3.--A building that is hence clear, may be struck and
damaged by Lightning without having a particle of metal in its
construction; if there be metals in it, however, and they happen to be
in such situations as will enable them to facilitate the progress of the
electrical discharge, so far as they go, then the discharge will fall on
them in preference to other bodies offering more resistance, but not
otherwise; if metallic substances be not present, or if present, they
happen to occupy places in which they cannot be of any use in helping on
the discharge in the course it wants to go, then the electricity seizes
upon other bodies, which lie in that course, or which can help it,
however small their power of doing so, and in this attempt such bodies
are commonly, but not always, shattered in pieces. The great law of the
discharge is,--progress between the terminating planes of action,
viz:--the clouds and earth, and in such line or lines as upon the whole,
offer the least mechanical impediment or resistance to this operation,
just as water falling over the side of a hill in a rain storm, picks out
or selects as it were by the force of gravity, all the little furrows or
channels which lie convenient to its course, and avoids those which do
not. If in the case of Lightning you provide through the instrumentality
of efficient Conductors, a free and uninterrupted course for the
electrical discharge, then it will follow that course without damage to
the general structure; if you do not, then this irresistible agency will
find a course for itself through the edifice in some line or lines of
least resistance to it, and will shake all imperfect conducting matter
in pieces in doing so; moreover it is to be specially remarked in this
case, that the damage ensues, not where the metals are, but where they
cease to be continued, the more metal in a building therefore the
better, more especially when connected by an uninterrupted circuit with
any medium of communication with the earth.
Such is, in fact, the great condition to be satisfied in the application
of Lightning Conductors, which is virtually nothing more than the
perfecting a line or lines of small resistance in given directions, less
than the resistance in any other lines in the building, which can be
assigned in any other direction, and in which by a law of nature the
electrical agency will move in preference to any others.
11.--It follows from the foregoing principles, that a magazine
constructed entirely of iron or other metal, would be infinitely more
safe in Lightning storms than if built with masonry in the usual way;
metallic roofs for magazines, with capacious metallic Conductors to the
earth, would be unobjectionable, and a source of security.
Metallic gutters and ridges having continuous metallic connection with
the earth are also unobjectionable.
A good method of Conductors for magazines built of masonry, would be
such as already described, regard being had to the position of the
building, its extent, and most prominent points, also to the nature,
state, and condition of the soil, whether it be moist or dry, alluvial
calcareous, or of hard rock; we must also consider the extent,
disposition, and peculiar position of the metallic bodies entering into
the general structure of the building, whether the roof be flat,
pointed, or angular in various parts.
The pointed projecting extremities of the two Conductors, one or more as
the case may be, will be commonly sufficient; but, in buildings having
tall chimneys or other elevated prominent points, at a distance from the
Main Conductor, it will be requisite to guard such chimneys or other
parts, by a pointed rod, led along them to the metals of the roof, or
directly connected with the Main Conductors, by metallic connections.
12.--Pointed terminations of the Conductors in the air, are so far
important that they tend to break the force of a discharge of Lightning
when it falls on them. In fact, before the great shock actually takes
place, under the form of a dense explosion, a very large amount of the
discharge, which otherwise would be concentrated, runs off, as it were,
through the pointed Conductor; but they have no other influence.
With respect to these pointed terminations, no great care need be taken
about them, except that they should consist of solid copper rod, of
about three-quarters of an inch in diameter, and about a foot in length,
and be united by brazing to the conducting tube, elevated at such
convenient height above the walls of the building as the case may
suggest.
As a support to the Conductor, when raised above the wall, we may employ
a small staff or spar of wood fixed to the masonry.
13.--Copper linings to the doors and window shutters of magazines are
not objectionable, if requisite, as a precaution against fire; but they
are useless as a means of keeping out Lightning; on the other hand, it
is not easy to conceive a case in which the explosion of the gunpowder
is to be apprehended from the action of Lightning on the doors or
windows. Supposing, however, such metallic linings desirable as a
precaution against common cases of fire, then the masses of metal
should, according to the principles already laid down, have metallic
communication with the general system of conduction in the building and
the Main Conductor.
ON THE EXPLOSIVE FORCE OF GUNPOWDER.
~Advantages of Gunpowder~
The advantages of Gunpowder, as a propelling agent, over any other
explosive material are, the comparative safety attending its manufacture
and transport, and the gradual nature of its decomposition when compared
with those materials, such as fulminating gold, silver, mercury, &c. &c.
In gunpowder, the force resulting from the rapid evolution of gas in a
confined space has sufficient time to overcome the inertia of the
projectile, which is not the case with other explosive materials, the
conversion of which gaseous products is so instantaneous that nothing
can resist the intensity of their explosive action. Other advantages
suggest themselves in the use of Gunpowder, such as the comparative
cheapness of the ingredients composing it, and the ease with which they
may be obtained; for the sulphur and saltpetre are very abundant
productions of nature, and the charcoal can be manufactured cheaply and
with great facility, and if care is taken in the process of the
fabrication of powder, little deterioration will take place on its
exposure to heat or moisture.
~Air & Steam as propellants~
Condensed air and steam have been used as propelling agents; but the
great inconvenience attending their use quite preclude the possibility
of adapting them to war purposes.
~Force of Gunpowder.~
As the force and effect obtained from Gunpowder is the foundation of all
other particulars relating to Gunnery, we will briefly consider these
points.
~Upon what the action of powder depends.~
The action of Gunpowder is dependent upon a purely chemical process. Mr.
Robins proved that the force generated by the combustion of gunpowder,
was owing to an elastic gas which was suddenly disengaged from the
powder, when it was brought to a certain temperature, and further that
this disengaged gas had its elastic force greatly augmented by the heat
evolved by the chemical action.
~Ingredients are charged with a large volume of heated gas.~
The propelling power of Gunpowder is dependent on the rapid
decomposition of the nitre into its component parts; the oxygen forms
carbonic acid with the carbon in the charcoal, and the heat thus
generated by ignition changes both this and the nitrogen into a large
volume of heated gas. In a mixture of nitre and charcoal alone, the
oxidation proceeds with comparative slowness; by the addition of
sulphur, an augmentation of combustibility is gained, in consequence of
its igniting at a very low temperature; the sulphur, also, by its
presence, renders available for the oxidation of the carbon an
additional amount of oxygen, viz: that which is united with the
potassium, the latter being at once converted into sulphite upon
ignition of the powder.
~Weight of gas evolved.~
~Volume of gas evolved.~
~Heat of gas evolved.~
~Pressure of gas generated.~
~Strength of powder not affected by density of air, but by damp.~
It appears that the weight of gas generated is equal to three tenths of
the weight of the powder which yielded it, and that its bulk when cold,
and expanded to the rarity of Common air was 240 times that of the
powder; the barometer standing at about 30 inches. From this Robins
concluded that if the fluid occupied a space equal to the volume of the
gunpowder, its elastic force, when cold, would be 240 times the pressure
of the atmosphere, when the barometer stands as above. Mr. Robins also
considered that the heat evolved was at least equal to that of red hot
iron, and he found by experiments that air heated to this temperature
had its elasticity quadrupled, and therefore, that the force of gas from
powder is at least four times 240 = 960, or in round numbers 1,000 times
as great as the elasticity of the air measured by its pressure on an
equal extent of surface. From the height of the barometer it is known
that the pressure of the atmosphere is about 14³⁄₄lbs. upon the square
inch, so that the pressure of the elastic gas generated by the
combustion of the gunpowder upon the same area would be 14.75 by 1,000
or 14,750lbs. at the moment of explosion. He found that the strength of
Gunpowder was the same whatever might be the density of the atmosphere,
but that the moisture of the air effected it considerably, in fact that
the same quantity of powder which would give a bullet an initial
velocity of 1,700 feet per second on a day when the atmosphere was
comparatively dry, would upon a damp day give no more than 1,200 or
1,300 feet.
~Velocity of gas~
The velocity of the expansion of the gas is a most important point, upon
which depends, chiefly, the peculiar value of the substance as a
propelling agent. Many of the warlike machines of the Ancients produced
a momentum far surpassing that of our heaviest cannon, but the great
celerity given to the bodies projected from guns by gunpowder cannot be
in the least approached by any other means than by the sudden production
of an elastic gas. Mr. Robins found that the flame of gunpowder expanded
itself when at the muzzle of the gun with a velocity of 7,000 feet per
second.
~Dr. Hutton’s calculation as to:--_Volume, Temperature, Pressure_.~
~Temperature~
~Expansion.~
~How to calculate expansion~
~Absolute force of gunpowder cannot be determined.~
It has been calculated that one cubic inch of powder is converted into
250 cubic inches of gas at the temperature of the atmosphere, and Dr.
Hutton states that the increase of volume at the moment of ignition
cannot be less than eight times; therefore one inch of gunpowder, if
confined, at the time of explosion exerts a pressure of about 30,000lbs.
being 250 by 8 by 15 = 30,000lbs. on the cubic inch, or 5,000lbs. on the
square inch; and which at once accounts for its extraordinary power. The
value of the temperature to which the gases are raised, on the explosion
of the powder, has been variously estimated and it may be concluded to
rise as high as will melt copper, or 4,000° Fahrenheit. All gases expand
uniformly by heat, the expansion having been calculated with great
precision, to be ¹⁄₄₈₀th for each degree of Fahrenheit. If therefore we
take Dr. Hutton’s calculations of one volume of powder expanding into
250 volumes of gas at the temperature of the atmosphere, and if we
suppose 4,000° Fahrenheit to be the heat to which they are raised on
ignition, the expansion of gunpowder would be calculated. Thus, suppose
the gas to be at 60°, the temperature of the atmosphere, we must deduct
60° from 4,000°, which will give 3,940, being the number of degrees
remaining to which it is raised, hence
temp. vol. temp. vol. vol.
1 3940
1° : --- 3,940° : ---- = 8·2
480 480
that is, each volume of gas would at a temperature of 4000° be
increased 8·2 in volume. Gunpowder when at the temperature of the air
being expanded 250 times in volume; therefore 250 by 8·2 = 2,050 as the
increased expansion for each volume of gas generated by the explosion of
gunpowder at the temperature of 4,000° Fahrenheit. Lieut-Colonel Boxer
calculates that the heat generated by good dry powder is not under
3,000° Fahrenheit. It appears with our present knowledge, the absolute
value of the force of gunpowder cannot be determined. Still by careful
and extensive experiments no doubt a near approximation to the truth may
ultimately be arrived at, so that although much has already been done by
various eminent philosophers, there is still more to be accomplished;
and the importance of the subject ought to act as a stimulus to the
exertions of those belonging to a profession the most interested in the
question.
~Loss of velocity by windage.~
It has been found by experiments that in calculating the initial
velocity of a projectile, one third of the whole force was lost with a
windage of ¹⁄₁₀th inch with a shot of 1·96-in. and 1·86-in. in diameter.
The bore of the gun being 2·02-in.
~Definition of ignition and combustion.~
By ignition we understand the act of setting fire to a single grain, or
to a charge of gunpowder, and by combustion we mean the entire
consumption of a grain or of a charge.
~Quickness of combustion.~
Upon the quickness of combustion mainly depends the applicability of
gunpowder for Military purposes.
~Ignition by heat.~
Gunpowder may be inflamed in a variety of ways, but whatever be the
method, one portion of the substance must in the first instance be
raised to a temperature a little above that necessary to sublime the
sulphur, which can be removed from the other ingredients, by gradually
raising the compound to a heat sufficient to drive it off in a state of
vapour. The heat required for this purpose is between 600° and 680°
Fahrenheit.
~Progressive combustion.~
When a charge of powder is exploded in the bore of a gun, to all
appearance there would seem to be an instantaneous generation of the
whole force. But in fact it is not so, a certain time being necessary to
the complete combustion of the substance. This gradual firing is of the
utmost importance, for were it otherwise, the gun, unless of enormous
strength, must be shattered in pieces, as well as the projectile; for in
such a case, this great force being suddenly exerted upon one part only
of the material, there would not be time for the action to be
distributed over the particles, at any great distance, before those in
the immediate vicinity of the explosion, were forced out of the sphere
of action of the cohesive force, and consequently rupture must take
place.
~Substances which have a more violent action than powder.~