|DAHLGREN BREECH-STRAP: A strap connecting the
breech with a separate trunnion-ring, in order to
avoid longitudinal weakness in a gun, without
disturbing the usual and convenient
preponderance. The strap is made of bronze, and
cast in two pieces: one piece constituting the
strap, half the trunnion-ring, and the greater
part of the trunnions; the other constituting the
opposite half of the trunnion-ring and the
remainder of the trunnions. The two parts are
riveted together at the trunnions. This strap
remedies another and greater defect of cast-iron
guns than longitudinal weakness the
unsoundness of the casting around the trunnions.
DAHLGREN GUN: The Dahlgren guns of large caliber are made of cast-iron, solid, and cooled from the exterior. To produce uniformity in the cooling, the piece is cast nearly cylindrical, and then turned down to the required shape, which is shown in the drawing. The thickness of metal around the seat of the charge is a little more than the diameter of the bore, which rule holds good for nearly all cast iron guns. The chase, however, tapers more rapidly than in other cast-iron guns, which gives the appearance of greater thickness of metal at the reinforce. The chamber is of the Gomer form. The principal guns of this system are of 9- and 11-inch caliber. A piece of 10-inch caliber has, however, been introduced into the Navy, on Admiral Dahlgrens plan, for firing solid shot with 40 pounds. of powder. The 15-inch and 20-inch naval guns are shaped exteriorly after the Dahlgren pattern, but are cast hollow and have the elliptical chamber of the Rodman system. The following table shows the principal dimensions, etc., of Dahlgren guns:
See Cast-iron Guns and Ordnance.
DANA PROJECTILE: This projectile consists of a cast-iron body having a conical base, to which is attached a cup-shaped ring of brass. Upon discharge, the ring is driven forward and upon the base, and by this movement the soft metal expanded into the grooves and rotation communicated to the projectile. As the front end of the sabot passes the shoulder it is crowded down into the groove cut round the body of the shot, and thus "clinched," as it were. The same end is sought in the arrangement at the bottom of the sabot, where the gas, acting in the cannelure, presses the lip into the groove cut in the cast-iron. Such is the provision designed to secure the sabot from stripping. Turning upon the projectile is prevented by wedge-shaped projections and recesses upon the base of the projectile and the under surface of the sabot. See Expanding Projectiles.
DEFILADING BATTERY: A battery placed on a raised parapet which offered some protection from artillery on a commanding height.
DEMOLITION OF ARTILLERY: The destruction of ordnance by artificial or other means. This is performed, if the gun is an iron one, by half filling the piece with powder, and jamming in one or two shot with stones, bits of iron, etc.; over this a complete tamping with stones and earth till the bore is filled. To break off the trunnions is not always an infallible mode of destroying ordnance, as they can still be fired from the ground. When time admits of only partially crippling guns by removing one of the trunnions, it is best done by laying the end of the trunnion on a block of wood, the blow being given by a sledge-hammer, or (if that he not at hand) by heavy shot. A gun may be destroyed by firing a shot at it behind one of the trunnions, which, if it should not break it, would render it unsafe. The first method, however, particularly if the muzzle is partly buried in the ground, will be found certain to burst the gun. To render bronze guns unserviceable, fire a shot into them from some other piece, behind the trunnions, which will prevent the possibility of their being used again. See Demolition.
DEVIATION OF FLIGHT: See Drift.
DEVIATION OF PROJECTILES: The term deviation must be understood to mean not only the deflections, right or left, of the line of fire, but also the differences between the ranges of similar projectiles fired under like condition from the same guns. Very great irregularities occur in the paths of spherical projectiles. If a number of projectiles be fired from the same gun, with equal charges and elevations, and with gunpowder of the same quality, the gun-carriage resting upon a platform, and the piece being pointed with the greatest care before each round, very few of the projectiles will range to the same distance; and, moreover the greater part will be found to deflect considerably, unless the range be very short, to the right or left of the line in which the gun is pointed. With elongated projectiles the fire is far more accurate, but still the ranges and deflections are subject to variations of greater or less amount. The causes of the deviations of projectiles, whether fired from smooth-bore or rifled guns, and independent of inaccuracy in pointing, and variable position of the gun-carriage, are wind, variable projectile force, and rotation of the earth.
Should the wind be blowing in gusts and be changeable in direction, it is difficult to allow for it in pointing the gun; but with a steady breeze in a pretty constant direction, a few rounds will generally be sufficient to show the allowance necessary. The velocity of the wind is very low compared with that of the projectiles, but it remains usually nearly the same throughout its flight, whereas the velocity of the projectile decreases rapidly; it therefore frequently happens that the wind appears to have greater effect towards the end of the range, and it may be often noticed in practice that projectiles deviate in a rapidly increasing curved line. The wind, if strong, will greatly affect the ranges of projectiles; decreasing or increasing the range according as it may be blowing with or against the projectile. The lower the velocity of a projectile, the greater will be its deflection caused by the wind, as, for instance, upon mortar-shells, on which, having low velocities and long times of flight, the wind exercises a very disturbing influence. The greater the density of the projectile, the less will its motion, during flight, be affected by the wind; and thus shells are more influenced by wind than shot. The wind exercises a very great deflecting influence upon an elongated projectile during its flight, rendering it difficult to obtain accuracy of fire at long ranges, even from rifled guns, excepting in very calm weather. If the center of gravity be placed very near the center of the long axis, the force of the wind will be pretty evenly distributed over the whole length of the projectile. Should, however, the center of gravity be placed far in advance of or behind the center of figure, the force of the wind will press unequally upon the shot, and uncertain deflections will most probably occur.
It is impossible with our present facilities to manufacture large quantities of powder of a perfectly uniform quality; but supposing it could be accomplished, the force from a given charge would be liable to variation according to the state of the atmosphere, and the condition of the powder as affected by the time it has been in store; it will also be frequently found in practice that the charges have not been weighed out with perfect accuracy, nor the gun loaded so that the projectile is always in the same position with reference to the charge. The consequence is that very few projectiles fired from the same gun with what are called equal charges leave the bore with exactly the same initial velocity.
The deviation of a projectile caused by the rotation of the earth is a complicated problem. The principle that this rotation will impress upon the projectile a tendency, upon leaving the bore, to move with the same velocity in the same direction as the point upon the surface from which the gun is fired is readily comprehended, but not its application to some particular cases. The deviation due to this cause is too slight to be regarded in practice.
The line of sight may be improperly placed and situated out of the vertical plane, either in consequence of the construction of the gun or its carriage, or by the effect of the inclination of the plan upon which it is placed. In these two cases the line of fire maintaining a fixed and determined position in respect to the axis of the gun and the vertical plane of fire, the deviations are constant for equal distances and equal inclinations, and it becomes easy to correct them after a few trials.
The barometric state of the atmosphere may also produce an effect upon the ranges; for the greater the density and elasticity of the displaced fluid, the greater will be the retardation of the projectile.
Spherical Projectiles The principal causes of the deviations of projectiles fired from smooth-bore guns are: 1st. Windage 2d. The imperfect form and roughness of the surface of the projectile. 3d. Eccentricity of projectiles arising from their not being homogeneous.
Windage causes irregularity in the flight of a projectile, from the fact of the elastic gas acting in the first instance on the upper portion of the projectile and driving it against the bottom of the bore. The projectile reacts at the same time that it is impelled forward by the charge, and strikes the upper surface of the bore some distance in advance, and so on, by a succession of rebounds until it leaves the bore in an accidental direction and with a rotatory motion, depending chiefly upon the position of the last impact against the bore. Thus, should the last impact against the bore. Thus, should the last impact of a concentric projectile, when fired from a gun, be on the right-hand side of the bore, as represented in Fig. 1, it will have a tendency to deflect to the left in the direction b, while at the same time a rotation will be given to it in the direction indicated by the arrows, or to the right; the effect of this rotation being to cause the projectile itself to deviate to the right during its flight, so that the deflection will not be to the left, but to the right, unless the range is very short. If the projectile leave the gun, rotating on a vertical axis, with its forward part moving from left to right supposing the observer to be behind the piece there will be a diminished pressure on the right side, and an increased one on the left side, which will therefore cause it to deviate to the right. If a projectile strike the bottom of the bore, the rotation of the fore-part would be from up downwards, and instead of deflecting to the right, the range would be decreased. Suppose the projectile to rotate in an opposite direction, the results would be reversed. Should it, on leaving, strike any intermediate part of the bore, a compound effect would be produced, according to the position of the point of impact. It appears from these explanations that a projectile leaving the gun, rotating on any axis except one parallel to that of the bore, will deviate according to the direction of the rotation.
Should the center of gravity of a projectile not coincide with the center of figure, it is termed eccentric, and is found to deviate according to the position of the center of gravity when the ball is placed in the bore of the gun; should the line joining the center of gravity and the center of figure of a projectile be not parallel to the axis of the bore, the charge of powder will act on a larger surface on one side of the center of gravity than on the other, so that there will be a rotation from the lighter towards the heavier side.
It is found in practice that projectiles deviate in a curved line, either to the right or to the left, the curve rapidly increasing towards the end of the range. This probably occurs from the velocity of rotation decreasing but slightly compared to the velocity of translation; or if a very strong wind is blowing steadily, across the range during the whole time of its flight, this deflecting cause being constant, while the velocity of the projectile diminishes, the curve will manifestly increase with the range; the trajectory is, therefore, a curve of double curvature, its projection on either a horizontal or vertical plane being a curved line.
From the foregoing considerations it follows that the smoother the surface of the projectiles and the less their windage and eccentricity, other things being equal, the greater will be their accuracy. Experiments show that the preponderating side should be put next the charge, and the line joining the center of gravity and the center of figure should be parallel to the axis of the bore. The position of the preponderating side is found by floating the projectile in a bath of mercury, and the degree of promptness with which an eccentric shot, floated as above, assumes the position due to its preponderance is regarded as the measure of the preponderance.
Elongated Projectiles If the projectile comes out of the gun perfectly centered, that is, rotating round its longest axis, and having that axis coincident with the line of flight, there will be no tendency, either of the axis rotation or of the projectile itself, to deflect, so long as the motion is in a straight line, because the resistance of the air will act uniformly all around. As soon, however, as the trajectory has begun to curve downwards under the influence of gravity, the resistance of the air acts more on the under side than on the upper, and effects will be produced depending on the resultant direction of the resistance of the air in relation to the center of gravity. Practically, the path of the projectile is found to result in a deviation increasing uniformly with the distance from the gun, and depending, as to its direction, on the first application. If the deflecting force act on the projectile in a vertical direction upwards, the horizontal projection of, the line of flight will be a line deviating to the right or left of the plane of fire, according as the twist is right-or left-handed. If the deflecting force act in the opposite direction, the projectile will be deflected to the left or right, according as the twist is right or left; and whatever be the direction of the deflecting force, the deviation will be a uniformly increasing one at right angles to it.
These effects may be illustrated experimentally by means of a gyroscope provided with a small elongated projectile instead of the disk used for ordinary experiments. The projectile must be made with the greatest care, so that its center of gravity coincided exactly with that of the two rings within which it is placed; the rings are so arranged that one can turn round a vertical axis, and the other round a horizontal axis, the projectile being therefore free to turn in any direction. A cylindrical portion of metal extends beyond the base of the projectile, in prolongation of its longer axis, round which the string is wound to give the required rotatory motion. As the projectile in the gyroscope has no motion of translation, a strong current of air must be directed upon it, so as to represent the resistance of the atmosphere to a projectile moving with a high velocity. The diameter of the nozzle of the blower should be equal to, or rather larger than, that of the projectile, and the center of the blast should be directed below the point of the projectile.
The line of flight is not absolutely a straight line, but becomes a curve of double curvature; and if projected on a vertical plane at right angles to the plane of fire, would consist of a series of cycloidal curves, were the time of flight sufficiently great, increasing the distance of the projectile from the plane of fire by the length of one of them at each revolution. The length of these curves depends upon the amount of the deflecting force, and their number is equal to the number of revolutions made by the projectile in its flight.
When elongated projectile is fired from a rifled gun, it leaves the bore rotating rapidly round its longer axis; and if the initial velocity were very low, the projectile experiencing but slight resistance from the atmosphere, the larger axis would remain (as in vacuo) during the whole time of flight parallel or nearly so to its primary direction, as shown in Fig. 2. While explaining the effect produced by the resistance of the air upon an elongated projectile moving with a high velocity, the projectile will be supposed to have what is termed a right-handed rotation; that is, the upper part turns from left to right, with reference to an observer placed behind the gun: for the direction of the grooves of rifled pieces is almost invariably so as to give such rotation. After the projectile has left the bore, the resultant of the resistance of the air will, unless the center of gravity be very far forward, act upon a point in front of the center of gravity and below the longer axis, at all angles of elevation given in practical gunnery. The effect produced by this pressure will depend chiefly upon the form of the head of the projectile; therefore let us in the first place consider the effect upon a conoidal head.
Of course the longer axis of an elongated projectile does not remain, during flight, continually a tangent to the trajectory, unless the center of gravity, as in an arrow or rocket, is very near the face-end; yet, practically, on account of the drooping of the point, the longer axis may throughout a considerable portion of the time of flight approximate very nearly to a tangent to the trajectory. The effects on targets furnish most satisfactory evidence of this; it is almost invariably found that the holes made in targets are circular, even when elongated projectiles descend at considerable angles. The most probable explanation of this fact must evidently be that the point of the projectile has dropped during flight, so that, on striking, the longer axis is nearly perpendicular to the plane of the target.
This drooping of the point is of importance; for did the axis remain parallel during flight to its primary direction, the projectile would most probably, when fired at any but a very low angle, on striking an object of hard material and solid structure, turn up against it lengthways, and therefore produce but trifling effect. This has not, however, been found to take place in practice; but on the contrary the penetration of elongated projectiles at considerable ranges is always remarkably great. There is little fear of the projectile turning up against an object unless the velocity of translation and rotation be very low, and the angle of fire very high.
It is found in practice that conoidal-headed projectiles fired from rifled guns giving a right-handed rotation always deviate to the right; and in the few cases tried with guns giving left-handed rotation the deviation is to the left; with flat-headed projectiles these deviations are reversed. This peculiar deviation is called drift, and is generally constant for the same ranges; so that it can be allowed for in pointing the gun, by using a horizontal slide graduated an attached to the tangent scale, or by inclining the tangent scale to the left.
DISABLING CANNON: If necessary to abandon materiel, it must be disabled or destroyed, so as to be useless to the enemy. Guns are permanently disabled by bursting, bending the chase, breaking off the trunnions, or by scoring the surface of the bore; they are temporarily disabled by spiking, breaking off the sights and the seat for the hausse, or in breech-loaders by carrying off or permanently destroying the breech-blocks, etc.
To burst a cast-iron gun, load with a heavy charge, fill the bore with sand or shot, and fire at a high elevation. To bend the chase of a bronze gun, fire a shotted piece against another, muzzle to muzzle or muzzle to chase; or kindle a fire under the chase and strike on it with a sledge-hammer. To break off a trunnion of a cast-iron cannon, strike on it with a heavy hammer or fire a shotted gun against it. To score the surface of the bore and injure the rifling, cause shells to burst in the gun or fire broken shot from it with high charges.
To spike a gun, drive into the vent a jagged and hardened steel spike with a soft point, break it off flush with the vent-field, and clinch it in the bore with the rammer; a nail without a head, a piece of ramrod, or even a plug of hard wood may be used in the absence of a spike. To prevent the spike from being blown out, make a projectile fast in the bottom of the bore by wrapping it with cloth or felt, or by means of iron wedges driven in with a rammer or with an easily burnt out by a charcoal-fire lighted with a pair of bellows.
When it is expected to retake a gun, use a spring spike with a shoulder to prevent its coming out too readily. Mitrailleurs are permanently disabled by bending the barrels, etc.; they are made temporarily useless by removing the crank-handles, locks, etc. Carriages are destroyed by piling them up and burning them; to prevent them from moving, the spokes and poles may be cut or sawed off. Ammunition-chests are blown up or water is poured over their contents. Implements are carried off or destroyed.
To unspike a gun, try to drive the spike into the bore with a punch; if there be a shot wedged in the bore, expel it by powder inserted through the vent. When it is impossible to drive down the spike into the bore with a punch; if there be a shot wedged in the bore, expel it by powder inserted through the vent. When it is impossible to drive down the spike, if the bore be unobstructed, insert a charge of one third the weight of the projectile, and ram down junk wads with a handspike, first placing on the bottom of the bore a strip of wood with a groove on the lower side containing a strand of quick-match by which the charge is ignited; this plan will not answer when the spike is screwed or riveted into the vent. In a bronze gun, remove some of the metal at the upper orifice of the vent, and pour sulphuric acid into the cavity before firing. Should the preceding methods fail, after several trials, drill out the spike, or drill a new vent if the gun be iron; if it be bronze, unscrew the vent-piece.
To drive out a shot wedged in the bore, unscrew the vent-piece, if there be one, and drive in wedges so as to start the shot forward, then ram it back again, and with a hook withdraw the wedges that may have held it; or pour in powder and fire it after replacing the vent-piece. As a last resort, bore a hole in the bottom of the breech, drive out the shot, and stop the hole with a screw. See Spike and Unspike.
DISMOUNT: The removal of a gun from its carriage either by intention or by shot, thus rendering it unfit for service.
DISPART: Half the difference of a guns diameter, measured at the muzzle and the base ring.
DOLPHINS: Two handles placed over the center of gravity of the piece in order to assist in mounting or dismounting the piece. In the heavy seacoast mortars a clevis was attached to a projection on the piece rather than handles. In earlier weapons these handles were ornamental and cast to represent dolphins, which gave them their name.
DRAG ROPE: Device used by artillerists to drag pieces and to extricate carriages from different positions. It consisted of a 4-inch hemp rope with a wooden thimble attached to each end. A hook was secured to one thimble and six oak or ash handles were put in between the strands of the rope and secured. The handles provided a secure grip for the men pulling the rope.
DREDGING BOX: A sheet copper box used to sprinkle mealed powder over the fuzes of mortar shells after the shell had been placed in the mortar. This made the fuzes more certain of taking fire. The box had a top fitted over it and pierced with holes to allow the escape of the powder.
DRIFT: The movement of rifled projectiles, either to the right or left, while in flight. The direction of the drift was dictated by the curvature of the grooves in the upper side of the bore rifling. If the grooves curved to the right, the projectile drifted to the right. If curved to the left, the drift was to the left. Deviations of smoothbore spherical projectiles were caused by windage, imperfect form and roughness of the surface, and the lack of uniformity in casting.
EARS: Also called Tong Holes. These were indentations found on mortar projectiles and some spherical smoothbore projectiles. The purpose of the ears was to aid in loading and aligning the projectile so the fuze was in the center of the bore of the mortar.
EARTHWORKS: See Field Works.
ECHINUS: See Molding.
EFFECTS OF PROJECTILES: A knowledge of the destructive effects of projectiles on iron, wood, earth, and masonry, the materials of which covering masses are made, is of very great importance in a military point of view. In general, these effects, and particularly that of penetration, depend on the nature of the projectile, its initial velocity, and the distance of the object. The following deductions have been made from trials with armor-plates, extending over several years: 1st. The best material to resist projectiles is soft, tough wrought-iron; and to attain these qualities it should be pure free from sulphur, phosphorus, and carbon. Steely iron, commonly known as homogenous iron, puddled steel, etc., when in large masses, is easily cracked by shot, and is not, therefore, suitable for armor-plates. Thin plates afford a greater comparative resistance. Soft steel may be used for armor-plates; but when cost is taken into consideration it is doubtful if it possesses any advantages over wrought-iron. 2d. Rolled iron does not offer quite so much resistance as hammered iron, yet if the size of the plate admit of it, it is to be preferred on the score of economy. Plates should be as large as possible to reduce the number of joints, which are lines of weakness. 3d. A solid plate offers, for the same thickness, a greater resistance to a projectile than a laminated one, or one made up of several thinner plates; but when the surface is rounded in shape and of small extent, as in the Monitor turrets, the latter may be used to great advantage, as great thickness of metal may thereby be easily obtained. 4th. The resistance of a plate to perforation is very much increased by a suitable backing. Cast-iron, granite, and brick in masses, while they enable a plate to offer a very great resistance, are soon broken up by the blows of heavy projectiles, and their fragments thrown off with great force. Oak and teak are the most suitable timbers for backing plates, and are used as such on vessels. A yielding backing is found to occasion less strain on the fastenings than a very hard one. 5th. Where projectiles are made of the same material and are similar in shape, their penetration into unbacked plates is nearly in proportion to their living force, or their weight multiplied by the square of the velocity of impact. The resistance which an unbacked plate offers to penetration is nearly in proportion to the square of its thickness, provided this thickness be confined within ordinary limits. In the case of oblique plates the penetration diminishes nearly with the sine of the angle of incidence. 6th. The most suitable material for shells to be used against iron plates is tempered steel. These projectiles should be made of cylindrical shape, with thick sides and bottom, to direct the explosive effect of the charge forward after penetration if effected. The most suitable material for solid shot is hard and tough cast-iron. Round shells made of cast-iron will be broken in passing through an inch plate, and an ordinary cast-iron shot will be broken in passing through a two-inch plate. Late experience shows that the pointed, or ogival, is better than the flat form of head for penetration of iron plates. 7th. It follows from the preceding that the most suitable covering or shield for cannon is a conical-shaped turret made of wrought-iron plates, as large as it is practicable to make them, backed with oak or teak. To protect the gunners from the fragments of projectiles which may penetrate completely through this covering, there should be an "inner skin" of thick boiler-plate placed behind the wood.
Effect on Wood The effect of a projectile fired against wood varies with the nature of the wood and the direction of the penetration. If the projectile strike perpendicular to the fibers, and the fibers be tough and elastic, as in the case of oak, a portion of them are crushed, and others are bent under the pressure of the projectile, but regain their form as soon as it has passed by them. It is found that a hole make in oak by a ball 4 inches in diameter closes up again, so as to leave an opening scarcely large enough to measure the depth of penetration. The size of the hole and the shattering effect increases rapidly for the larger calibers. A 9-inch projectile has been found to leave a hole that does not close up, and to tear away large fragments from the back portion of an oak target representing the side of a ship of war, the effect of which on a vessel would have been to injure the crew stationed around, or if the hole had been situated at or below the water-line, to have endangered the vessel. If penetration take place in the direction of the fibers, the piece is almost always split, even by the smallest host, and splinters are thrown to a considerable distance. In consequence of the softness of white pine nearly all the fibers struck are broken, and the orifice is nearly the size of the projectile; for the same reason the effects of the projectile do not extend much beyond the orifice; pine is therefore to be preferred to oak for structures that are not intended to resist cannon-projectiles, as blockhouses, etc.
Effect on Earth Earth possesses advantages over all other materials as a covering against projectiles; it is cheap and easily obtained, it offers considerable resistance to penetration, and to a certain extent regains its position after displacement. It is found by experience that a projectile has very little effect on an earthen parapet unless it passes completely through it, and that injury done by day can be promptly repaired at night. Wherever masonry is liable to be breached, it should be masked by earthworks with natural slopes. The size of the openings formed by the passage of a projectile into earth is about one third larger than the projectile, increasing, however, towards the outer orifice. Rifle-projectiles are easily deflected from their course in earth. They are sometimes found lying in a position at right angles to their course, and sometimes with the base to the front; hence their penetration is variable. Unless a shell be very large in proportion to the mass of earth penetrated, its explosion will produce but little displacement; generally a small opening is formed around an exploded shell by the action of the gas in pressing back the earth. Experience at Fort Wagner showed that it took one pound of metal to permanently displace 3.27 pounds. of the sand of which the fort was made. Time-fuzes, being liable to be extinguished by the pressure of the earth, are inferior to percussion-fuzes, which produce explosion when the projectile has made about three fourths of its proper penetration. The penetration in earth of the oblong compared to round projectiles, when fired with the service-charges and at a distance of about 400 yards, is at least one fourth greater. This difference, however, is less at short and greater at long distances. The penetration of the smallest: or 3-inch, cannon-projectile, at a distance of 400 yards, in a newly made parapet of loam mixed with gravel, is about 6 feet. The 100-pdr. Projectile, under similar circumstances, penetrates about 16 feet. A penetration as great as 31 ½ feet has been obtained at the Washington Navy Yard by firing a 12-inch rifle-projectile into a natural clay-bank at a short distance. The greatest penetration of a 15-inch solid shot, fired with 50 pounds. of powder, in well-rammed sand, at a distance of 400 yards, is 20 feet.
Effect on Masonry The effect of a projectile against masonry is to form a truncated conical hole, terminated by another of a cylindrical form, as shown in the drawing. The material in front of and around the projectile is broken and shattered, and the end of the cylindrical hole even reduced to powder. Pieces of the masonry are sometimes thrown 50 or 60 yards from the wall. The elasticity developed by the shock reacts upon the projectile, sometimes throwing it back 150 yards, so as to be dangers to persons in a breaching-battery. The exterior opening varies from 4 to 5 times the diameter of the projectile, and the depth, as we have seen, varies with the size and density of the projectile and its velocity. With charges of ½, 1/3, ¼, and 1/6, a projectile ceases to rebound from a wall of masonry when the angles formed by the line of fire and the surface of the wall exceed 20, 24, 33, 43 degrees, respectively. With these angles, the angle of reflection is much greater than the angle of incidence, and the velocity after impact is very slight. When a projectile strikes against a surface of oak, as the side of a ship, it will not stick if the angle of incidence be less than 15 degrees, and if it does not penetrate to a depth nearly equal to its diameter. Solid cast-iron shot break against granite, but not against freestone or brick. Shells are broken into small fragments against each of these materials.
Effect of Bullets The penetration of the new breech-loading rifle-musket bullet in a target made of pine boards one inch thick is as follows: at 100 yards, 13 inches; at 500 yards, 9 inches. If bullets are hardened by the addition of a little tin or antimony to the lead, their penetration is very much increased. From the experiments made in Denmark, the following relations were found between the penetration of a bullet in pine and its effects on the body of a living horse, viz.: 1st When the force of the bullet is sufficient to penetrate .31 inch into pine, it is only sufficient to produce a slight contusion of the skin; 2d. When the force of penetration is equal to .63 inch, the wound begins to be dangerous, but does not disable; 3d. When the force of penetration is equal to 1.2 inch, the wound is very dangers. A plate of wrought-iron three sixteenths of an inch thick is sufficient to resist a rifle-musket bullet at distances varying from 20 to 200 yards. That a rope mantlet may give full protection against rifle-musket bullets, it should be composed of five layers (three vertical and two horizontal) of 4 ½ inch rope. See Breaching, Projectiles, Punching, and Racking.
ELEVATING ARC: Graduated into degrees and parts of a degree, this device was attached to the rear part of the cheek of a gun carriage. The axis of the piece was placed horizontal and the breech was marked at any one of the divisions on the arc. The required elevation or depression of the piece was noted by the number of degrees above or below this mark. When not in use it was placed inside the cheek to which it was attached.
ELEVATING SCREW: Threaded metal cylinder with a four pronged handle attached to one end. The other end was screwed into the bottom of the breech. The gunner would use this screw to elevate or depress the tube.
ELEVATION: The vertical angle which the axis of a gun or mortar made with the horizon.
ELONGATED PROJECTILE: Also known as rifled ordnance, the elongated projectile had a length of two to three calibers of the bore and was fitted with a sabot or had a bore-shaped body. This caused the projectile to have greater accuracy in flight and increased range over spherical projectiles. The length varied among the different projectiles for the same gun. Rotary motion around the long axis of the projectile in a rifled bore was obtained from the sabot, usually located at the base. Bore-shaped projectiles, such as the Whitworth, did not require a sabot since the twist needed was supplied by the body. Elongated spherical projectiles were occasionally manufactured for smoothbore guns. A projectile, fitted with a sabot or bore-shaped body, with a length of two to three calibers of the bore of the gun, attained a greater accuracy of flight and increased range over spherical projectiles.
ELONGATED PROJECTILES: The great improvements which have been made of late, in the accuracy and range of cannon, consist simply in the use of the elongated instead of the spherical form of projectile. To attain accuracy of flight and increase of range with an elongated projectile, it is necessary that it should move through the air in the direction of its length. Experience seems to show that the only sure method of effecting this is to give it a rapid rotary motion around its long axis by the grooves of the rifles. The length necessarily varies in the different descriptions of projectiles for the same gun, inasmuch as it is to some extent subordinate to the consideration of bringing them all, with certain exceptions, to the same weight; but experiments go to prove that a length of two calibers at least is necessary for very accurate firing, and it is desirable for good vis viva, or destructive effect on impact at any but very short ranges, to have the weight great in proportion to the caliber, or, in fact, to the surface of resistance, and of course this is favored by an increased length of projectile. As a rule, the best length for accurate firing with any ordinary twist has been found to be from two to three calibers.
The form of head is governed by two considerations, flight and penetration. The latter gives different forms in different instances. The question of flight affects all equally, and on this many experiments have been made, which have resulted in the general adoption of what is termed an ogival head, or figure generated by the revolution of an ogival or pointed arch about its axis. It has been found that the total pressure on a nine-inch spherical projectile, moving with a velocity of 1150 feet per second, is about 555 pounds. ANBM representing the spherical nine-inch projectile, and the total pressure on a hemispherical-headed, elongated projectile of the same diameter represented by ACDBM, and moving with the same velocity, is 487 pounds, thus showing a difference of 68 pounds. Total pressure. Now supposing the elongated projectile to move steadily, point first, the pressure on the respective heads, A, M, B, must be the same; therefore the difference of the total pressure, viz., 68 pounds, must be due to the difference of minus pressure on the bases ANB, ACDB respectively, thus showing that the form of base of a projectile materially influences the total pressure which it meets with when moving through the air at a high velocity. The total pressure on an ordinary ogival-headed projectile of 9 inches diameter, represented by ACDBM, is only 389 pounds, thus showing the great difference of pressure, viz., 166 pounds, on an elongated ogilval-headed projectile and a spherical projectile of the same diameter when moving at the same velocity through the air. Another great advantage which the elongated projectile possesses over the spherical is that, for the same caliber, the momentum of the former is much greater, varying, of course, in proportion to their respective weights, which would be nearly three to one, depending on the length of the elongated projectile. Piobert says that the figure experiencing the least resistance from the air has a length five times its greatest diameter, with its largest section placed 2/5 of the length from the hind part. The shapes of some of the Whitworth projectiles approach more nearly to this form than those of any elongated projectiles hitherto uses. See Projectiles.
ELSWICK GUN-WORKS: The Elswick Works of Sir William Armstrong, at Newcastle-upon-Tyne, have produced the largest constructions in England of their well-known type, and from which sprang the modified form known as the Woolwich gun. We cannot here attempt to give a description of the Works in any general sense, but merely to notice a few features such as characterize them, or should be noticed by visitors to Elswick, especially engineers. We suppose the works to be traversed in the order adopted, as far as we understand on the last Public Day.
Commencing at the northeast corner of the Works, the first objects of interest are the 6-inch and 40-ton breech-loading gun mounted in barbette. It is well to observe the system in action and the cover afforded to the detachment. Close to these guns is a shrinking-pit for ordnance from the 100-ton gun downwards, also nineteen gas-producers for furnaces. The shops then may be taken in the following order.
Coiling The largest section of bar has been 12 to 10 inches; length of coiling-furnace, 180 feet; gas-furnace for heating barrels, also for tempering, with an oil-well 50 feet deep, over which stands a hydraulic hoist.
Forge The large hammer here, made by Thwaites & Carbutt, Bradford, has a 48-inch cylinder and 12 feet stroke; weight of piston and hammer-head, 35 tons. Blast smelting-furnaces, one furnace building, two in work, and running from 900 to 1000 tons a week, chiefly no. 1, 2, and 3 pig, made from Spanish and Elba ores, most of it sold for steel-making. The blast is at present heated by horseshoe pipes, but Cowpers heating-stoves are in course of erection; temperature of blast, from 750 degrees to 800 degrees about the melting-point of zinc. The engine for the furnaces is made by the firm.
Carriage-shed There are band-saws cutting iron which may be noticed, and Albini carriage on short recoil and self-running-up system.
Projectile-store Containing the finished projectiles. These are chiefly made with bands only up to full diameter, which saves work, and leaves to the projectile the strength of the uninjured skin of the casting. The Palliser chilled projectiles will be generally found with sharp-pointed heads struck with two diameters ogival.
Foundry Containing the ten cupola-furnaces, of which four are generally in work. Forty tons is about the maximum weight of casting made in the foundry a much larger one, such as the bed of the steam-hammer, weighing 137 tons, being cast on its own ground. The system of hydraulic cranes should be noticed. They are fixed so as to work in pairs, or three together, for heavy work.
Engines Near this are the engines for the East Works, and also those for the West Ordnance-works. Horizontal double Corliss engines are employed, with four boilers three working at a time. Jukes bars and system of stoking is applied to all. The Jetty may probably be conveniently visited next, near which are more horizontal engines, 100 horse-power, working on the accumulators; the water-pressure maintained is about 700 pounds per square inch. Five or six locomotives are generally employed in the works. On the east end of the jetty are two fixed hydraulic cranes for lifting 5 tons and 30 hundred-weight; and between them large hydraulic shears, made by Day & Summers, worked by a direct-acting hydraulic cylinder, 40 feet stroke, lifting 120 tons. The bag-leg moves so as to bring the lifting-cylinder about 30 feet out, 15 feet inboard of a vessel. The foot is moved by a screw 50 feet long, with hydraulic engine and gear, with three different powers. Along the jetty run pipes with hydrants from 18 to 36 feet apart, on which work five movable cranes, each lifting about 30 hundred weight, being placed in position to suit the holds of the vessels by means of telescope-tubes attached to the nearest hydrants.
The finishing-shop may be taken next in order. The proportions of new-type guns should be noticed also; the breech-loading fittings, and apparatus for firing by electricity and also mechanically. One shop is for small machine-work, completing Gatling machine-guns, hydraulic valves, etc. Another contains planing-machines, etc. Others are constructed for turning, finishing, and boring work, commencing on the solid ingot. At the east end guns are bored vertically in a pit 23 feet deep. The finest lathe is one of Whitworths for turning, boring, screw-cutting, and rifling, taking a job 44 feet in length, 36-inch centers. There is also a convenient one made by Fairbairn, Kennedy & Naylor, modified at Elswick, taking a chuck job 20 feet in diameter, 4 feet 6 inches long, or a job 34 feet long and 8 feet in diameter. It is fitted with slide-rests on independent beds. There are also chambering and rifling machines. In another shop, crank-shaft and gun work, coil-welding, etc., are performed. The steam-hammers, from 24 tons to 15 hundredweight, are chiefly Morrisons make. There is a great variety of small machinery, for turning and boring out short coils; also a large endless band-saw, 1 ¾ inch wide, which cuts directly through iron cylindrical work about 16 inches in diameter. Its speed is from 76 feet to 129 feet per minute.
It will be seen that the facilities of these works are ample in every respect for ordnance-constructions; and when we come to consider the decidedly advanced progress in the adaptation of steel in its strongest form ribboned in gun-constructions of light weight combined with great power, it must be admitted that in the pure question of the building up of guns to resist the drafts upon them, especially by tangential strains, far beyond standard limits in England, Sir William Armstrong & Co. are farthest advanced as the pioneers in Great Britain of a system destined, probably, to solve in the most satisfactory manner the problem of all heavy gun-construction, in the present state of the art, in producing the metals deemed most suitable for making sound and reliable ordnance.
EMBRASURE: Opening cut through the parapet to allow the artillery to command a certain extent of the surrounding country.
ENDURANCE OF GUNS: The principal injuries caused by service are internal, arising from the separate action of the powder and the projectile. They increase in extent with the caliber, whatever may be the nature of the gun, but are modified by the material of which it is made. The injured from the powder generally occur in rear of the projectile. They are: 1st. Enlargement of that portion of the bore which contains the powder, arising from the compression of the metal. This injury is more marked when a sabot or wad is placed between the powder and the projectile, and is greatest in a vertical direction. 2d. Cavities produced by the melting away of a portion of the metal by the heat of combustion of the charge. 3d. Cracks arising from the tearing asunder of the particles of the metal at the surface of the bore. At fist a crack of this kind is scarcely perceptible, but it is increased by continued firing until it extends completely through the side of the piece. It generally commences at the junction of the chamber with the bore, as this portion is less supported than the others. 4th. Furrows or scoring produced by the erosive action of the inflamed gases. This injury is most apparent where the current of the gas is most rapid, or at the interior orifice of the vent, and on the surface of the bore, immediately over the seat of the projectile. Scoring commences very early in large guns; at first it is only a mere roughness, which gradually increases in depth and forms lines along the bore; but it is not until a gun has been fired very considerably that it becomes of importance.
The impressions of deep scoring resemble the bark of an old elm-tree, the metal being eaten away into irregular furrows and ridges. Even when it has reached this extreme case, however, scoring has not caused the destruction of the gun, though in some instances, acting like a wedge, it has split the bore at that part. Some experimental guns, excessively scored on the upper side of the bore, have been turned over, vented and sighted on the under side; but this has not been found necessary until the gun has been used more than is probable under ordinary circumstances.
The injuries arising from the action of the projectile occur around the projectile and in front of it. They are: 1st. Indention in the lower side of the bore, produced by the pressure on the projectile by the escape of gas through the windage, before the ball has moved from its seat. The elasticity of the metal, and the burr, or crowding up of the metal in front of the projectile, cause it to rebound, and, being carried forward by the force of the charge, to strike against the upper side of the bore, a short distance in front of the trunnions. From this it is reflected against the bottom, and again reflected against the top of the bore, and so on until it leaves the piece. The first is called "indentation," and the others are called "enlargements." In pieces of ordinary length there are generally three enlargements when this injury first makes its appearance, but their number is increased as the "indentation" is depressed and the angle of incidence increased. The effect of this bounding motion is alternately to raise and depress the piece in its trunnion-holes, and to diminish the accuracy of fire, until finally the piece becomes unfit for service. It is principally from this injury that bronze guns become unserviceable. Mortars and howitzers are not much affected by it. The principal means used to prevent this injury are to wrap the projectile with cloth or paper, and to shift the seat of the projectile. The latter may be done by a wad or lengthened sabot, or by reducing the diameter and increasing the length of the cartridge. The last of these methods is considered the more practical as well as the more effective; and it has the additional advantage of decreasing the strain on the bore, by increasing the space in which the charge expands before the ball is moved. 2d. Scratches or furrows made upon the surface of the bore by rough projectiles, or by case-shot. 3d. Cuts made by the fragments of projectiles which break in the bore. 4th. Wearing away of the lands of rifled cannon, especially at the driving-edges. A little rubbing of the side of the grooves from the friction of hard bearings is of little importance. 5th. Enlargement of the muzzle, arising from the forcing outward of the metal by the striking of the projectile against the side of the bore as it leaves the piece. By this action the shape of the muzzle is elongated in a vertical direction. 6th. Cracks on the exterior. These are formed by the compression of the metal within, generally at the chase, where the metal is thinnest. This portion of a bronze gun is the first to give way by long firing, whereas cast-iron guns usually burst in rear of the trunnions, and the fracture passes through the vent, if it be much enlarged.
The endurance of a smooth-bore gun with service-charges may be surely predicted by observation of the progressive wear of the interior orifice of the vent. There are certain general forms in which this enlargement takes place. They may be classed as triangular, lozenge, quadrilateral, star circular, and elliptic. With the lateral vent of the Dahlgren system it usually takes the lozenge form, the cracks extending from the opposite angles lengthwise of the bore. With those rifled cannon in which the vent is bouched, the cracks appear around the bouching, and although the bouching preserves the vent, yet the formation of fissures around the enlarged orifice, when once commenced, causes a greater tendency to rupture. With the vent not bouched, the wear in rifled cannon is about double that of the smooth-bore. So long as the wear of the vent is regular and without cracks, a mere enlargement is not indicative of danger; but when it reaches a diameter of four tenths of an inch, the vent should be closed and a new one opened. A gun of large caliber should not in service be expected to stand more than 400 or 500 rounds before it will be necessary to open the new vent, which, however, will be of no advantage unless the old one be closed at its interior orifice, on which the gases otherwise would continue to act as a wedge. The first distinct appearance of the cracks, as shown by the button, is the proper limit. After the gun bursts, a sketch or draught is made showing the lines of fracture, and specimens are reserved for trail of density and tensile strength; and if practicable, a photograph is taken. See Cannon and Ordnance.
ENFILADE: A military term applied to a fire of musketry or artillery made in the direction of the length of a line of troops or a line of rampart. A besieging battery so placed as to send its shot along any part of the line of a fortification, and inside the parapet, does great execution in dismounting the guns which thus present the largest surface to the balls. Hence the lines of ramparts should be planned that their prolongations may fall in situations inaccessible to the enemy. Where this is not possible, the lines are either broken, or are protected by bonnets, or by traverses or blindages. In the siege of a fortress the trenches of approach are cut in a zigzag to prevent the defenders enfilading them from the walls.
ENFILADING BATTERY: Enfilading and counter batteries are used for destroying the artillery and traverses, and silencing the fire of the defenses. Positions are chosen for the enfilading-batteries from which the terre-pleins of the faces, and other lines that bear upon the ground on which the parallels and approaches are laid out, can be swept throughout; the counter batteries are so placed that they can bring a direct or a slant fire against the embrasures of the points to be silenced. The shot from the former is thrown with small charges, under small angles of elevation, so as to ricochet along the terre-pleins, taking the guns of the defenses in flank; the latter fire with full charges directly against the point to be attained. As the effects of both direct and enfilading fire vary greatly with the range, positions should be chosen for these batteries as near the defenses as they can be thrown up without too great a sacrifice of life. Positions which will give ranges between 300 and 700 yards are the best for smooth-bore guns; nearer than 300 yards the workmen would be exposed both to the fire of musketry and case-shot; beyond 700 yards the fire upon the defenses becomes very uncertain.
The greater range of rifled guns gives to the besiegers a greatly enlarged zone in the choice of positions for enfilading and counter batteries over that for the ordinary siege-train of smooth-bore guns. This greater range and the greater certainty of the fire of rifled guns are more favorable to counter-batteries than to those intended for enfilading; as the great angles of elevation under which the guns are fired, to attain the desired ranges, give to the projectile, in the descending branch of its trajectory, a great plunge, which, although more favorable to attaining objects covered by traverses than if the plunge were smaller, is less favorable to the ricochet of the projectile from which the chief advantage of enfilading with round shot is derived. Besides this, the elongated projectiles used in rifled guns from the form given to their point are readily deflected from their course by very slight obstacles, as a fascine even, which also adds to the uncertainty of their effects. At the siege of Fort Wagner it was observed that the heavy projectiles of the smooth-bore navy guns were landed with more accuracy within the enemys works, and were more destructive to their ricochet, than the projectiles from the army rifled guns.
The judgment and experience of the officer must in these cases be left full play in the selection of the position of the batteries of these two classes of guns and in their armament; bearing always in mind two very important considerations: first, that with long ranges and high angles of elevation the projectiles will clear all the trenches in front up to a near approach of the besieged work without danger to them, except from unforeseen accidents; and second, that to secure any decided or certain effect from either class of these batteries there must be nothing to obstruct the view of the object to be attained. The batteries may be placed either within the parallels, in advance of, or in rear of them. The positions usually selected are from 20 to 30 yards in front of the parallels; because, if placed within them, there might be mutual interference between the service of the batteries and that of the parallels, which is often a very serious cause of delay to both the service of the batteries and the passage of troops; and, unless placed some distance in the rear of it, the parapet of the parallel might obstruct the shot of the battery, and the troops in the trench be annoyed by the fire.
The most effective positions for batteries of smooth-bore guns are in front of the second parallel of from 300 to 400 yards from the point to be reached; and unless the fire of the defenses is very destructive, it will be best to place them there. If placed in front of the first parallel it may be necessary to shift the most of them to the front of the second parallel soon after the latter is thrown up; for the third parallel and the approaches leading to it from the second parallel run the risk of being attained by shot from batteries at so great a distance in their rear as the first parallel. See Batteries and Counter-battery.
EPAULEMENT: Elevation constructed in order to protect troops and batteries from the fire of the enemy. It was usually composed of gabions filled with earth, or sand bags. In permanent fortifications, the epaulement was considered to be the low stone wall constructed at the top of the rampart.
EPROUVETTE: A small light mortar for testing the projectile force of gunpowder by observing its effects when used in the same quantities as would be used in the field.
EXPANDING PROJECTILES: Projectiles of this class are forced to take the grooves by the action of the charge of powder, and require no other precaution in loading than spherical shell. It is essential, however, that the base-ring of every rifle-projectile, especially the Parrott, shall be greased before entering it into the gun, to prevent the formation of a hard deposit in the grooves. Parrotts projectile is composed of a cast-iron body and brass ring cast into a rabbet formed around its base. The ring is from 1 inch to 1 ½ inch in width, and about 1 inch in maximum depth. The gas presses against the bottom of the ring and underneath it, so as to expand it into the grooves of the gun. To prevent the ring from turning in the rabbet, the latter is recessed at several points of its circumference, like the teeth of gearing. The diameter of the rabbet is greatest at the extreme rear of the shot, so that the brass ring cannot fly off without breaking. The entire projectile is slightly smaller than the bore, so as to be easily rammed home. The projectile has a slight groove turned out of the iron of the base to permit the powder gases to enter and expand the ring. The use of a little grease or other lubricating material on the base of the projectile, before firing, is advantageous. Parrotts shot for iron-clad fighting, as shown in the drawing, is entirely of cast-iron, but is reduced and chilled at the end, which prevents its mashing like strong soft cast-iron.
The new Parrott projectile differs from that just described in that the base is separated from the expanding ring by a cannelure which render its taking the grooves more certain. Those for the 60-pounder and under have one hole for the core-stem, which becomes the fuze-hole. The larger projectiles have a hole in each end in consequence of the necessity of using two core-stems to steady the core. The battering-shell have but one hole in rear which serves as a loading-hole; the hole in rear is closed by a screw-plug. The Hotchkiss projectile is composed of three parts: the body, the expanding ring of lead, and the cast-iron cup. The action of the charge is to crowd the cup against the soft-metal ring, thereby expanding it into the rifling of the gun. The time-fuze projectile has deep longitudinal grooves cut on its sides to allow the flame to pass over and ignite the fuze. The last rifle-projectile submitted by Mr. Hotchkiss has an expanding cup of grass attached to its base in a very peculiar manner. The cup is divided into four parts by thin projections on the base of the projectile. This arrangement is intended to facilitate the expansion of the cup and to allow the flame to pass over to ignite the fuze.
The Butler shell also belongs to this class, and differs from the mode of attaching the expanding ring and in the position of the cannelure. The expanding ring is screwed on to the base, in such a manner that the rotary motion screws in tighter; the rear part is divided by the cannelure into two lips, so that the gases are distributed evenly and the entrance of the gas between the ring and the body of the projectile is prevented; the grip of the inner lip on the projectile being also increased by the wedging action of the gas. See Arrick Projectile, Blakely Projectile, Butler Projectiles, Confederate Projectiles, Dana Projectile, Dyer Projectile, Hotchkiss Projectiles, James Projectile, Parrott Projectiles, Projectiles, Sawyer Projectiles, and Schenkl Projectile.
EXPANDING SABOT PROJECTILE: An elongated projectile designed to take the grooves of the bore by the use of an expanding sabot or forcing cup system. The expansion was accomplished by the force of the gas, produced by the explosion of the powder charge, pushing against the soft metal sabot and forcing the metal into the groove of the weapon. This gave the projectile a twist as it exited the bore and increased the accuracy of flight and trajectory range. Examples of expanding projectiles are the Parrott, Dyer, Hotchkiss, James, Sawyer, and Schenkl, among others.
EXPANSION CUP: A metal cup, ring, or soft metal sabot attached to the base of the projectile. The cup was the same diameter as the projectile when loaded, but when it was fired the cup expanded into the lands and grooves of the bore.
EXPANSIVE SYSTEM OF RIFLING: This system embraces all projectiles which in loading are inserted in the gun without respect to the rifling, but which take the grooves by the action of the gases of discharge upon a device or feature of the projectile, which is readily expanded thereby into the grooves of the gun. This class of projectiles has been so extensively and almost exclusively used in the United States that it is known as the American system. The chief projectiles of this class are: 1. Those where the sabot is of lead or soft metal. In these the windage is apt to be entirely closed. The lead may strip or be forced over the projectile, and balloting or wedging be induced. 2. Those having sabots of copper or brass, cup-shaped on the bottom of the projectile. These seem to suffer from the violence of the explosion within the cup, which is apt to be broken or unevenly set up. 3. Those where a leaden jacket is forced out by the action of the discharge upon a wedge or key. These have small capacity as shell and little strength as shot, strip easily, and are open to many objections. 4. Those where a concave or convex disk is flattened against the base of the projectile, or in addition is provided with a flange or key which is driven by the discharge upon the tapered base of the projectile. 5. Those where the rotating device consists of an annular band or ring attached to the base of the projectile and intended to be expanded into the rifling by the gases of discharge. These have proved most successful in practice. See System of Rifling.
EXPENSE MAGAZINE: See Magazine.
EXPENSE MAGAZINES: The very small gunpowder-magazines, containing the made-up ammunition for the service of the guns on the works, at the rate of so many rounds per gun. In fortifications of the old construction an expense-magazine was made in each bastion and battery, though this was not always the case. Expense-magazines are often made under the earthen ramparts of fortifications, with a passage cut into them in the interior slopes. In the more modern works, such as the Instruction of Fortification at the Royal Military Academy, Woolwich, it is shown that expense-magazines should be placed as near as is practicable to the guns which they have to supply, and may often be conveniently constructed under the traverses and below the level of the terre-plein, with lifts of communication. They can, if so situated, be easily secured against the enemys fire, and be provided with subterranean communications with the main magazine, which would permit them to be replenished without risk, even during action. The first suggestions made as to the size of expense-magazines in fortifications of the present day gave four guns to be supplied by each, but a later recommendations proposes only two guns, in the case of very heavy guns.
EXPLOSION: The term explosion is rather loosely used. Considering it as synonymous with explosive reaction, it may defined as a chemical action causing the sudden or extremely rapid formation of a very great volume of highly expanded gas.
Explosive effect is caused by the blow or impulse given by this rapid production of gas in a confined space. The explosive character of the change, then, depends 1st. Upon the great change of state produced; that is, the formation of gas very much greater in volume than the substance from which it is derived, and which is still more expanded by the heat evolved. 2d. Upon the shortness of the time required for the change to take place. Both these causes operate to a greater or less extent in all explosive reactions. When both are fully exerted the most energetic chemical reaction, or, in other words, the most violent explosion, takes place. Also, the differences in explosions and explosive bodies depend upon the differing manner and proportions in which they are exerted. Thus, a nitroglycerine is much more powerful and violent than gunpowder, because it generates a larger volume of gas in a shorter time. Again, fulminating mercy is not more powerful than gunpowder, although the decomposition goes on more quickly, since the quantity of gas given off and the temperature of the reaction are less.
The kinds and quantity of gas given off in an explosive reaction depend upon the chemical composition of the explosive body and the character of the decomposition. The heat evolved during the reaction adds to the effect by increasing the tension (expanding the volume) of the gas formed. The heat given off in a reaction is an absolute quantity, the same whether the reaction goes on slowly or rapidly. But the explosive effect will evidently greatly depend upon the rapidity of the formation and expansion of the gas. Thus, if an explosive undergoes the same change under all circumstances of firing, then total amount of force developed will always be the same; but the explosive effect will be increased as the time of action is lessened. Explosions are greatly affected by the circumstances attending them. Different substances, of course, give different results, from their different compositions and reactions. But we also find that the same substance will exercise a different explosive effect when fired under certain conditions than under others. These may affect either the rapidity or the results of the chemical change. By shortening the time of the reaction the explosion is rendered sharper and more violent. With some explosives the decomposition is different under different circumstances. Thus, gunpowder when fired under great pressure gives different products than when fired unconfined. Circumstances of explosion may be generally considered under 1st. Physical or mechanical condition of the explosive body itself. 2d. External conditions. 3d. Mode of firing. Many instances may be given indicating the influence of its state upon the explosion of a substance. Thus, nitroglycerine at temperatures above 40 degrees Fahr. Exploded by a fuze containing 15 grains of fulminating mercury. Below 40 degrees it freezes and cannot be so fired. The advantage of dynamite over nitroglycerine lies altogether in the fact that the former is presented in another mechanical condition, more convenient and safer to use than the liquid form. The nitroglycerine itself is the same chemically in either case. The same mixture of charcoal, sulphur, and saltpeter gives a very different effect if made up into large grains than if made up into small ones. Gun-cotton presents the most marked example of the effect of mechanical state, since it can be prepared in so many ways. If flame is applied to loose uncompressed gun-cotton it will 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; powerfully compressed and damp, it burns slowly; dry gun-cotton may be exploded by a fulminate-fuze; wet, it requires an initial explosion of a small amount of dry, etc.
Confinement is necessary to obtain the full effect of all explosives. The most rapid explosion requires certain time for its accomplishment. As the time required is less, the amount of confinement necessary is less. Then, with the sudden or violent explosives, the confinement required may be so small that its consideration may be practically neglected. For instance, large stones or blocks of iron may be broken by the explosion of nitroglycerine upon their surfaces in the open air. Here the atmosphere itself acts as a confining agent. The explosion of the nitroglycerine is so sudden that the air is not at once moved. Again, chloride of nitrogen is one of the most sudden and violent of all explosives. In its preparation it is precipitated from a watery liquid, and therefore is, when used, wet or covered with a very thin film of water. This thin film of water, not more than 1/1000 of an inch in thickness, is necessary and sufficient confinement, and if it is removed the explosive effect is much diminished. Gunpowder, on the other hand, requires strong confinement, since its explosion is comparatively slow. Thus, in firing a large charge of gunpowder under water, unless the case is strong enough to retain the gases until the action has become general, it will be broken, and a large amount of the powder thrown out unburned. This is often the case in firing large-grained powder in heavy guns. The ball leaves the gun before all the powder has burned, and grains or lumps of it are thrown out uninjured. The confinement needed by the slower explosives may be diminished by igniting the charge at many points, so that less time is required for its complete explosion.
In any explosive reaction the mode of bringing about the change exercises an important influence. The application of heat, directly or indirectly, is the principal means of causing an explosion. Thus, in gunnery, the flame from the percussion-cap or primer directly ignites the charge; so also a fine platinum wire heated by an electric current will ignite explosive material which is in contact with it. Friction, percussion, concussion, produce the same effect indirectly, by the conversion of mechanical energy into heat, which is communicated to the body to be exploded. When one explosive body is used as a means of firing another, it may be considered that the blow delivered by the gas suddenly formed from the firing-charge acts percussively upon the mass to be exploded. The particles of this gas are thrown out with great velocity; but meeting with the resistance of the mass around them, they are checked, and their energy is converted into heat. It is found, however, that the action of explosives on one another cannot be perfectly explained in this way. If the action were simply the conversion of energy into heat, then the most powerful explosive would be the best agent for causing explosion. But this is not the case. Nitroglycerine is much more powerful than fulminating mercury; but 15 grains of the latter will explode gun-cotton, while 70 times as much nitroglycerine will not do it. Chloride of nitrogen is much more violent than fulminating mercury, but larger quantities of the former than of the latter must be used to cause other explosions. Again, nitroglycerine is fired with certainty by a small amount of fulminating mercury, while with a much large amount of gunpowder the explosion is less certain and feebler. In these cases it is evident that the fulminating mercury must have some special advantage, since it produces the desired effect more easily than the others. It may be considered that the fulminating mercury sets up a form of motion or vibration to which the other bodies are sensitive. Just as a vibrating body will induce corresponding vibrations in others, so the peculiar rate of motion or wave of impulse sent out by the fulminating mercury exerts a greater disturbing influence upon the molecules of some bodies than that derived from other substances.
An explosive molecule is unstable and very susceptible to external influences. Its atoms are in a nicely balanced equilibrium, which is, however, more readily overturned by one kind of blow than another. The explosive molecule takes up the wave of impulse of the fulminate, but the strain is too great, and its own balance is destroyed. So a glass may stand a strong blow; while a particular note or vibration will break it. In the case mentioned above, of gun-cotton affected by nitroglycerine or fulminate, the explosion of the nitroglycerine is strong enough to teat and scatter the gun-cotton, but the blow, though very powerful, is not one that the gun-cotton is sensitive to; on the other hand, the fulminate blow, though weaker, readily upsets the molecule of the gun-cotton. In addition, the explosion proceeds very differently when brought about in this way than when caused by simple inflammation. When a mass of explosive is ignited by a flame, the action extends gradually through it; but if it is exploded by a blow, acting in the manner above described, it is plain that the explosion will be nearly instantaneous throughout, since the impulse will be transmitted through the mass with far greater rapidity than an inflammation proceeding from particle to particle. The explosive reaction will then proceed much more rapidly, and the explosive effect will be more violent.
The phenomenon of the explosion of powder may be divided into three distinct parts, viz., ignition, inflammation, and combustion. By ignition is understood the setting on fire of a particular point of the charge; by inflammation, the spread of the ignition from one grain to another; and by combustion, the burning of each grain from its surface to center. See Combustion, Detonation, Explosive Agents, Gun-powder, Ignition, and Inflammation.