gun which has its strength proportioned to the strain it is called upon
to bear in all its parts. All parts of the sides of a cannon are not
strained equally, and are therefore not brought to the breaking-point at
the same time. Any arrangement of the parts by which the explosive
strain is distributed equally over the entire thickness of the piece,
necessarily brings a greater amount of resistance into play to prevent
rupture. There are two general plans for accomplishing this, viz.:
First, by producing a strain of compression on the metal nearest the
surface of the bore. This is termed an “initial strain,” and is brought
about by shrinking heated bands or tubes around the part to be
compressed, or by slipping a tube into the bore, which has been slightly
enlarged by heat. In either case it is apparent that the extent of the
strain depends on the relative size of the fitting surfaces, and the
amount of heat used to produce expansion. Sometimes the parts are forced
together by hydraulic pressure after they have been carefully bored and
turned to the proper size. The second plan is based on “varying
elasticity,” and is accomplished by placing that metal which stretches
most within its elastic limit around the surface of the bore, so that by
its enlargement the explosive strain is transmitted to the outer parts.
By the selection of suitable materials and their proper management, both
of these plans may be combined in the same gun, and thereby give it
increased strength. See ORDNANCE, CONSTRUCTION OF.
The best-known cannon of the _built-up_ class are:
_Ames Gun._--The rifled guns made by Mr. Horatio Ames, of Falls Village,
Conn., are made of wrought iron on the built-up principle. The wrought
iron is in the form of rings, made by bending a bar around a mandrel and
welding the ends. After turning them in a lathe, two or more of these
rings are fitted one within another to form a disk. These disks are
welded in succession to a concave breech-piece. Some of these guns have
shown remarkable endurance. They are weakest against longitudinal
strains.
_Armstrong Gun._--Is so much like the _Woolwich_, which it preceded,
that a separate description is unnecessary. See WOOLWICH GUN.
_Blakely Gun._--The most approved pattern of the gun invented by Capt.
Blakely combines in its construction the principles of “initial tension”
and “varying elasticity,” the object of which is to bring the strength
of all the metal of the piece into simultaneous play to resist
explosion. It is made of several tubes or barrels, the inner one of
which is of low steel, having considerable but not quite enough
elasticity. The next tube is made of high steel with less elasticity,
and is shrunk on the barrel with just sufficient tension to compensate
for the insufficient difference of elasticity between the two tubes. The
outer cast jacket, to which the trunnions are attached, is the least
elastic of all, and is put on with only the shrinkage by warming it over
a fire. The steel tubes are cast hollow and hammered over steel mandrels
under steam-hammers; by this process they are elongated, and at the same
time the tenacity of the metal is increased, all the steel parts are
annealed. Other combinations of iron and steel are used, except wrought
iron, which is regarded as objectionable on account of its tendency to
stretch permanently. Blakely guns were rifled with one-sided grooves,
and are fired with expanding projectiles. This gun is no longer made
under that name. As now made it is called the
_Vavasseur Gun_, and is manufactured by Messrs. J. Vavasseur & Co. of
the London Ordnance-Works. It is made entirely of the best Sheffield
cast steel, except the trunnions, which are wrought iron, and consists
of an interior tube and outer tube and a number of hoops. The inner tube
is forged from a solid ingot. It is rough bored and turned and then oil
tempered. The outer tube and rings are cast hollow and hammered over
steel mandrels. They are heated and shrunk on. Theoretically, it is
difficult to pick a flaw in the construction of this gun. The rifling
used is anomalous. It consists of three _ribs_ instead of grooves
projecting into the bore. The projectile has corresponding grooves.
These guns have found quite a market in the South American republics.
_Brooke Gun._--This gun was made after the plan of Capt. Brooke for the
Confederate service; it resembles Parrott’s in shape and construction,
except that the reinforcing band is made up of iron rings not welded
together. The rifling is similar to that used in the Blakely guns.
_Fraser Gun._--See WOOLWICH GUN.
_Gatling Gun._--See GATLING GUN.
_Krupp Gun._--See KRUPP GUN.
_Lancaster Gun._--This gun is now little used; it was made of wrought
iron. The bore was cut in a spiral form with an elliptical
cross-section, and the projectile shaped to fit it, by which means a
rotary motion was imparted.
_Palliser Gun._--Maj. Palliser of the British service is the inventor of
a system which has been successfully applied in England to utilize
smooth-bore cast-iron guns by converting them into rifles. By his plan
the gun is first bored to a cylinder or finely tapering cone, then lined
with a tube of coiled wrought iron, the breech end of which is shrunk
on; the exterior of the barrel has a uniform diameter throughout. The
tube is double at this part to obtain the benefit of the tension and to
enable any fracture of the inner layer to be made known without bursting
the gun. The bottom of the barrel is closed by a wrought-iron cup
screwed in. The tube is inserted into the gun from the muzzle without
the application of heat. A small amount of play is allowed between the
barrel and the cast-iron body; this disappears, or is much reduced by a
“setting up charge,” which expands the barrel against the cast iron. The
end of the barrel is made to bear accurately against the cast-iron
breech. A collar screwed into the muzzle secures the tube in position,
and prevents it from being thrust forward by the compression of the
metal by repeated firing. In front of the trunnions a pin is screwed in
through the cast iron, to resist the tendency of the tube to be turned
by the bearing of the projectile in the grooves. On the exterior of that
portion of the inner tube that is covered by the second tube is cut a
spiral gas channel; this communicates with a tell-tale hole drilled
through the cast-iron breech, by which gas can escape and announce the
fracture of the inner tube. The venting and rifling are similar to those
employed in the Woolwich guns. In the larger guns Maj. Palliser proposes
to use two or more concentric tubes, in some the exterior one to be of
steel. This system is being applied in the United States with the most
promising results in the conversion of 10-inch Rodman guns into 8-inch
rifles. The rifles thus obtained, though giving to a projectile a less
muzzle velocity than does the 10-inch smooth-bore, has, on account of
the increased weight of shot, greater penetrating power at all ranges,
being doubled at some and trebled at others. Its accuracy is three times
greater, and the capacity of its shell twice that of the original gun.
_Parsons Gun._--The system upon which Mr. Parsons makes his guns is
similar to that of Maj. Palliser. (See PALLISER GUN.) It depends upon
the principle of varying elasticities, and is based upon the fact that
wrought iron may be stretched three times as much as cast iron, and will
offer three and a half to six times the resistance within the limit of
its elasticity. These well-known gun constructions, known as _converting
systems_, both consist in lining a cast-iron case with a wrought-iron or
steel tube. In the Palliser or English method the tube is inserted from
the muzzle. In the Parsons or American method, through the breech. In
both nearly the whole of the longitudinal strain is transferred to the
cast-iron case. Both systems were first perfected in England. Col.
Crispin (U. S. Ordnance Corps) deserves the credit of introducing them
into the U. S. service in constructing the new _experimental rifles_.
The Parsons system is better adapted to constructing breech-loaders.
_Parrott Gun._--The Parrott rifled gun is a cast-iron piece of about the
usual dimensions, strengthened by shrinking a coiled band or barrel of
wrought iron over that portion of the reinforce which surrounds the
charge. The body of the larger Parrott guns are cast hollow, and cooled
from the interior on the Rodman plan. The barrel is formed by bending a
rectangular bar of wrought iron spirally around a mandrel, and then
welding the mass together by hammering it in a strong cast-iron
cylinder, or tube. In bending the bar, the outer side being more
elongated than the inner one, is diminished in thickness, giving the
cross-section of the bar a wedge shape, which possesses the advantage of
allowing the cinders to escape through the opening, thereby securing a
more perfect weld. The barrel is shrunk on by the aid of heat, and for
this purpose the reinforce of the gun is carefully turned to a
cylindrical shape, and about one-sixteenth of an inch to the foot larger
than the interior diameter of the barrel in a cold state. To prevent the
cast iron from expanding when the barrel is slipped on to its place, a
stream of cold water is allowed to run through the bore. At the same
time, and while the band hangs loosely upon it, the body of the gun is
rotated around its axis to render the cooling uniform over the whole
surface of the barrel. The proof of the Parrott guns consists in firing
each piece 10 rounds with service charges.
_Rodman Gun._--The principal difficulty formerly experienced in
manufacturing very large cast-iron cannon was the injurious strain
produced by cooling the casting from the exterior. Gen. Rodman of the
U. S. Ordnance Department developed a theory of the strains produced by
cooling a casting like that of a cannon (see ORDNANCE, STRAINS UPON),
and as a remedy for them proposed that cannon should be cast on a hollow
core and cooled by a stream of water or air passing through it. This new
mode of casting was afterwards adopted by the War Department. By this
system of casting, guns of greatly increased size and endurance are
fabricated. The largest guns employed in the U. S. service (20-inch) are
made on the Rodman plan, as well as many of the guns employed in the
field service.
_Whitworth Gun._--These guns are made of a species of low steel; the
smaller are forged solid, the larger are built up with coils or hoops;
the hoops are forced on by hydraulic pressure, and for this purpose are
made with a slight taper and with the design to secure initial tension.
The ends of the hoops are joined by screw-threads. The hoops are first
cast hollow, and then hammered out over a steel mandrel. Before
receiving their final finish they are subject to an annealing for some
three or four weeks, which makes the metal very ductile, but at the same
time slightly impairs its tenacity. The system differs from Krupp’s in
the smaller masses used and the greater number of hoops. The process for
making the hoops is better calculated to develop their tensile strength.
The breech-pin is made with offsets in such a way as to screw into the
end of the barrel and the next two surrounding hoops. The cross-section
of the bore of the Whitworth gun is a hexagon with rounded corners. The
twist is very rapid and the projectiles are made very long.
_Woodbridge Gun_ (invented by Dr. Woodbridge, of Little Falls, New
York).--The system of construction consists essentially of a thin steel
barrel over which wire is wound, barrel and wire being subsequently
consolidated into one mass by a brazing solder melted and poured into
the interstices. The following brief description is extracted from one
of the inventor’s letters to the chief of ordnance: “Square wire is
wound upon a steel core somewhat longer than the intended bore of the
gun, a sufficient number of wires being wound at once side by side to
produce the required obliquity of the turns. The successive layers have
opposite twists. When the mass has reached the required dimensions, it
is inclosed in an air-tight case to protect it from oxidation, and is
heated therein to a temperature somewhat above that required for the
fusion of the soldering metal. The soldering metal having been melted is
run in, filling all the interstices of the mass. When cooled the gun is
bored and finished as usual.” The invention dates back to about 1850. A
small gun made in this way was tested by Maj. Laidley (U. S. Ordnance
Corps) in 1865. It endured 1327 rounds with excessive charges, when the
attempt to burst it was abandoned on account of the breaking off of the
trunnions. The only large gun ever made--a 10-inch gun--was fabricated
at Frankford Arsenal. It was not entirely finished till April, 1876,
soon after which it was displayed at the Centennial Exhibition in
Philadelphia. Certain defects in its manufacture prevent it from fairly
representing the Woodbridge system.
_Woolwich Gun._--The Woolwich or Fraser gun is in its construction a
modification of the Armstrong plan, which latter had been previously
used in Great Britain; the principal difference is in substituting for a
number of single coils and a forged breech-piece a few long double and
triple coils, and in using a cheaper quality of wrought iron. The number
of pieces employed in the construction depends upon the size of the gun;
an 8-inch rifled gun is composed of the inner tube (barrel) of steel,
the muzzle-coil (trousers), the breech-coil (jacket), and the
cascabel-screw. The barrel is made from a solid forged cylinder of cast
steel, drawn by heating and hammering; it is turned, bored, and
chambered; then heated to a uniform temperature in a vertical furnace
and plunged into a covered tank of rape-oil, where it cools and soaks.
The muzzle-coil is constructed of two single coils welded together
endways. Each coil is formed by heating a long bar and wrapping it about
a mandrel; this is next heated in a reverberatory furnace and welded
under a steam-hammer. Before being united the two cylinders are turned
and bored. The breech-coil is composed of a triple coil, a
trunnion-ring, and a double coil welded together. The double coil is
formed by placing a single coil, when cold, on a mandrel and winding
over it, but in the reverse directions to break joints, a second bar; if
over this a third bar is immediately wound in the same direction as the
first, a triple coil will result. These coils are welded by being heated
and hammered on the end and on the sides. The trunnion-ring is made by
welding slabs of iron together on the flat end of a bar, and gradually
forming a ring by driving through the centre wedges and mandrels
increasing in size; the trunnions, one of which comes from the bar, are
at the same time hammered into shape. The coils and the ring having been
turned and bored, the latter is placed on a shoulder of the triple coil,
the double coil is dropped through the trunnion-ring on the triple coil,
and the joints welded in this position. The cascabel is forged of good
scrap-iron; the different parts having been formed are accurately turned
and bored with a slight taper. The muzzle-coil tube being heated is
dropped over the barrel, which is stood in a pit, a stream of cold water
circulating through the bore. The half-formed gun is then placed on its
muzzle, water forced through the bore, and the breech-coil heated and
slipped into position. The cascabel is screwed into the breech-coil
abutting against the barrel, great care being taken that the contact is
perfect. A tell-tale hole is cut along the thread on the cascabel to
give warning by the escape of gas should the barrel break in firing. The
vent is bored through hardened copper; it enters near the centre of the
service cartridge. This gives greater velocity, but also greater
pressure. The large guns have from seven to ten grooves. The twist is
uniformly increasing; the shape of the grooves is circular, with curved
edges.
_Sutcliffe Gun._--This invention, by E. A. Sutcliffe of New York City,
relates to a breech-mechanism for cannon. See BREECH MECHANISM.
_Griffin Gun._--Name sometimes given to the 3-inch rifled field-piece in
the U. S. service. It is made of wrought iron. The method of fabrication
is to wrap boiler-plate around a mandrel and to weld it.
=Ordnance, Metals for.= The only metals ordinarily used for cannon are
cast and wrought iron, steel, and an alloy of copper and tin, or a
combination of these metals. Cannon metals should be able to resist the
corroding action of the atmosphere, the heat and the products of
combustion of the powder; should be susceptible of being easily bored
and turned, and should not be too costly. The qualities necessary in
cannon metals are strength to resist the explosion of the charge, weight
to overcome severe recoil, and hardness to endure the bounding of the
projectile along the bore. The shape of the bore would otherwise be
rapidly altered by the action of the projectile. This quality is
particularly necessary in rifled cannon. The term strength as applied to
cannon metal is not confined to tensile strength alone, but embraces
also elasticity, ductility, and crystalline structure, which affect its
power to resist the enormous and oft-repeated force of gunpowder. (See
ORDNANCE, STRAINS UPON.) Each discharge of a cannon, however small,
impairs its strength, and repeated a sufficient number of times, will
burst it; this arises from the fact that the feeblest strains produce a
permanent elongation or compression of iron; this is technically known
as the permanent set, and the same is probably true of all other metals.
The property of ductility is of importance in enabling a metal to resist
rupture after it has passed its elastic limit. The size and arrangement
of the crystals of a metal have an important influence in its strength
to resist a particular force. A metal will be strongest when its
crystals are small, and the principal faces parallel to the straining
force, if it be one of extension, and perpendicular to it, if it be one
of compression. The size of the crystals of a particular metal depends
on the rate of cooling; the most rapid cooling giving the smallest
crystals.
_Cast iron_ is very generally employed, notably in the United States, in
the fabrication of heavy cannon for siege and sea-coast purposes. It
possesses the very important qualities of tenacity, hardness, and
cheapness, and with proper care is not seriously affected by rust. Its
principal defect is an almost entire want of elasticity, in consequence
of which its tenacity is destroyed after a certain number of
applications of the straining force. But little is known of the causes
which affect the quality of the cast iron used for cannon metal. The
amount of carbon, the state of its combination, together with the ore,
fuel, and fluxes, and the process of manufacture, all materially affect
the quality of the iron. All that is known is, that certain ores treated
in a certain way make cast iron suitable for cannon, and the fitness of
a particular kind of cast iron for artillery purposes can only be
determined by submitting it to the tests of the service. After this is
known, a knowledge of certain physical properties, such as tenacity,
hardness, density, and color, form and size of crystals presented in a
freshly fractured surface, will be useful in keeping the metal up to the
required standard. The pig-iron from which cannon are made should be
soft, yielding easily to the file and chisel; the appearance of the
fracture should be uniform, with a brilliant aspect, dark gray color,
and medium-sized crystals. When remelted and cast into cannon, it should
have about sufficient hardness to resist the file and chisel, but not to
be so hard as to be bored and turned with much difficulty; its color
should be a bright gray, crystals small, structure uniform, close, and
compact. The density of gun metal should be about 7.25, and its tenacity
about 30,000. There are several varieties of cast iron differing from
each other by almost insensible shades. The principal divisions are,
however, gray and white. Gray iron is softer and less brittle than the
white, is slightly malleable and flexible, and does not resist the file.
It has a brilliant fracture of a gray or bluish-gray color. This iron
melts at a lower temperature than white iron and becomes more fluid,
contracts less and contains fewer cavities; it fills the mold well, the
edges of a casting are short, and the surface smooth, convex, and
covered with carburet of iron. Gray iron is the only kind suitable for
making castings which require great strength, such as cannon. White iron
is very brittle, resists the file and chisel, and is susceptible of high
polish, the surface of a casting is concave, the fracture presents a
silvery appearance. Its qualities are the reverse of those of gray iron;
it is therefore unsuitable for ordnance purposes. Mottled iron is a
mixture of white and gray; it has a spotted appearance, and flows well.
The casting has a plane surface with edges slightly rounded. It is
suitable for making shot and shells. Besides these general divisions,
there are several other varieties of iron whose qualities depend upon
the proportion of carbon, and the state in which it is found in the
metal. The color and texture of cast iron depend greatly on the size of
the casting and the rapidity of cooling. See ORDNANCE, STRAINS UPON.
_Wrought iron_ was among the earliest metals employed in the
construction of cannon, but in consequence of the defects which almost
invariably accompany the forging of large masses, it was superseded by
bronze and cast iron to a great extent. Wrought iron is softer than cast
iron, and, being pure iron, is more liable to be corroded by the action
of the atmosphere and products of combustion of the powder; it possesses
also considerable ductility. The tensile strength of wrought iron, which
under the most favorable circumstances is double that of the best cast
iron, depends on the character of the crystalline structure, and the
manner of applying the tensile force, or in other words, wrought iron
offers the greatest resistance to a force of extension when the
structure is fibrous, and the force acts in the direction of the fibres.
The practical difficulties of rapidly cooling large masses so as to form
small crystals, and compressing them by hammering, rolling, or otherwise
to develop and give a particular direction to the fibre, have not thus
far been wholly surmounted. On the contrary, large masses are generally
found to contain such internal defects as false welds, cracks, and a
spongy and irregularly crystalline structure, arising from the more
rapid cooling of the exterior surface.
_Steel_ is a compound of iron and carbon, in which the proportion of the
latter seldom exceeds 1.7 per cent. It may be distinguished from iron by
its fine grain, its susceptibility of hardening by immersing it when hot
in cold water, and with certainty by the action of diluted nitric acid,
which leaves a black spot on steel, and on iron a spot which is lighter
colored in proportion as the iron contains less carbon. For the
construction of cannon, steel may be divided into high and low steel,
the difference being that the former contains more carbon than the
latter. High steel is very hard and has great ultimate tenacity. It has
but little extensibility within or without the elastic limit, and is
therefore too brittle for use in cannon, unless used in such large
masses that the elastic limit will not be exceeded by the explosive
force of the powder. It melts at a lower temperature than wrought iron
and is difficult to weld, as its welding temperature is but little less
than that at which it melts. Low steel is often known as “mild steel,”
“soft steel,” “homogeneous metal,” and “homogeneous iron,” and is made
by fusing wrought iron with carbon in a crucible; after which it is
cast into an ingot and worked under a hammer. As it contains less carbon
than high steel, it has greater specific gravity. It can be welded
without difficulty, although overheating injures it. It more nearly
resembles wrought iron in all its properties, although it has much
greater hardness and ultimate tenacity, and a lower range of ductility
depending on its proportion of carbon. It has less extensibility within
the elastic limit than high steel, but greater beyond it, or in other
words, greater ductility. Its great advantage over wrought iron for
general purposes is that it can be melted at a practicable heat, and run
into large masses possessing soundness and tenacity. Its advantages for
cannon are greater elasticity, tenacity, and hardness. Its tenacity when
suitable for cannon is three times as much as cast gun iron, and
one-half more than the best wrought iron. The principal varieties of
steel are:
_Natural Steel._--This is made principally in Germany, and is used for
making files and other tools. It is obtained by reducing the rich and
pure kinds of iron ore with charcoal, and re-fusing the cast iron so as
to bring it to a malleable state. The India steel, or Wootz, is a
natural steel containing a small proportion of other metals.