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To point a piece, is to place it in such a position that the shot may reach the object it is intended to strike. To do this, the axis of the trunnions, being horizontal, the line of metal, called also the.natural line of sight, must be so directed as to pass through the object, and then the elevation given to the piece to throw the shot the required distance. The direction is given from the trail, and the elevation from the breech; the trail being traversed by a handspike, and the breech raised or depressed by an elevating screw.
The axis of the piece coincides with that of the cylinder of the bore.
The line of sight in pointing is the line of direction from the eye to the object. It lies in a vertical plane, passing through, or parallel to the axis of the piece.
The angle of sight is the angle which the line of sight makes with the axis of the piece.
The natural line of sight is the straight line passing through the highest points of the base ring, and the swell of the muzzle, muzzle sight, or muzzle band.
The natural angle of sight is the angle which the natural line of sight makes with the axis of the piece.
The dispart of a piece, is half the difference between the diameters of the base ring and swell of the muzzle, or the muzzle band. It is therefore the tangent of the natural angle of sight, to a radius equal to the distance from the highest point of the swell of the muzzle or muzzle band, to the plane passing through the rear of the base ring.
By range is commonly meant the distance between the piece and the object which the ball is intended to strike; or, the first graze of the ball upon the horizontal plane on which the carriage stands. Point-blank range is the distance between the piece and the point-blank. Extreme range is the distance between the piece and the spot where the ball finally rests.
Theory of pointing. The point-blank is the second point of intersection of the trajectory or curve described by the projectile in its flight with the line of sight. As the angle of sight is increased, the projectile is thrown farther above the line of sight, and the trajectory and point-blank distance becomes more extended.
The point-blank range increases with the velocity, the diameter and the density of the ball. It is also affected by the inclination of the line of sight; but with the angles of elevation used in field service, this effect is too small to be taken into account.
A piece is said to be aimed point-blank when the line of metal, which is the natural line of sight, is directed upon the object. This must be the case when the object is at point-blank distance. When at a greater distance the pendulum-hausse, or the tangent scale, is raised upon the breech until the sight is at the height which the degree of elevation for the distance may require. An artificial line of sight, and an artificial point-blank are thus obtained, and the piece is aimed as before.
The different lines, angles, etc., which an artilleryman has to take into account in pointing, will be best understood by the following figure:
A B is the axis of the piece. B I F L is the trajectory or curve described by the projectile in its flight. C D F is the natural line of sight. C D A is the natural angle of sight.
The projectile, thrown in the direction of the axis A B D G, is acted upon by the force of gravity, and begins to fall at once below the line at the rate of 16 1/2 feet for one second, 64 1/3 for two, 144 3/4 for three, and so on in proportion to the time. It cuts the line of sight at D, a short distance from the muzzle of the piece, and descending, again cuts it at the point F. This second point of intersection is the point-blank.
Pendulum-hausse. The instrument at present in most general use in pointing field guns at objects beyond the natural pointblank, is called a pendulum-hausse, of which the component parts are denominated the scale, the slider, and the seat. The scale is made of sheet brass: at the lower end is a brass bulb filled with lead. The slider is of thin brass, and is retained in any desired position on the scale by means of a brass set screw with a milled head. The scale is passed through a slit in a piece of steel, with which it is connected by a brass screw, forming a pivot on which the scale can vibrate laterally: this slit is made long enough to allow the scale to take a vertical position in any ordinary cases of inequality of the ground on which the wheels of the carriage may stand. The ends of this piece of steel form two journals, by means of which the scale is supported on the seat attached to the piece, and is at liberty to vibrate in the direction of the axis of the piece. The seat is of iron, and is fastened to the base of the breech by three screws, in such manner that the centres of the two journal notches shall be at a distance from the axis equal to the radius of the base ring.
A muzzle sight of iron is screwed into the swell of the muzzle of guns, or into the middle of the muzzle ring of howitzers. The height of this sight is equal to the dispart of the piece, so that a line from the top of the muzzle sight to the pivot of the scale is parallel to the axis of the piece. Consequently the vertical plane of sight passing through the centre line of the scale and the top of the muzzle sight will be also parallel to the axis in any position of the piece: the scale will therefore always indicate correctly the angle which the line of sight makes with the axis. The seat for suspending the hausse upon the piece is adapted to each piece according to the varying inclination of the base of the breech to the axis. The hausse, the seat, and the muzzle sight, varying as they do, in their construction and arrangement, according to the configuration of the piece upon which they are intended to be used, are marked for the kind of piece to which they belong. The graduations on the scale are the tangents of each quarter of a degree, to a radius equal to the distance between the muzzle sight and the centre of the journal-notches, which are, in all cases, one inch in rear of the base ring.
The hausse, when not in use, is carried by the gunner in a leather pouch, suspended from a shoulder strap.
As it is impossible to point a piece correctly without knowing the distance of the object, artillerymen should be frequently practiced in estimating distances by the eye alone, and verifying the estimate afterwards, either by pacing the distance, or by actual measurement with a tape-line or chain, until they acquire the habit of estimating them correctly.
Shells are intended to burst in the object aimed at: spherical case shot are intended to burst from fifty to seventy-five yards short of it.
Shell or spherical case firing, for long ranges, is less accurate than that of solid shot. At high elevations a solid shot will range farther than a shell or spherical case shot of the same diameter fired with an equal charge. But at low elevations, the shell or spherical case will have a greater initial velocity, and a longer range. If, however, the charges be proportioned to the weights of the projectiles, the solid shot will in all cases have the longest range.
The velocity or range of a shot is not affected in any appreciable degree by checking the recoil of the carriage, by using a tight wad, or by different degrees of ramming.
The principal causes which disturb the true flight of the projectile may be simply stated as follows:
1st. If the wheels of the carriage are not upon the same horizontal plane, the projectile will deviate towards the lowest side of the carriage.
2d. If the direction of the wind is across the line of fire, deviations in the flight of the projectile will be occasioned, and in proportion to the strength of the wind, the angle its direction makes with the line of fire, and the velocity of the projectile.
3d. If the centre of gravity of the projectile be not coincident with the centre of figure, the projectile will deviate towards the heaviest side, that is, in the same direction that the centre of gravity of the projectile, while resting in the piece, lies with regard to the centre of figure. Therefore, if a shot be placed in the piece so that its centre of gravity is to the right of the centre of the ball, the shot will deviate towards the right; and vice versa. If the centre of gravity be above the centre of figure, the range will be increased; if below, it will be diminished.
Should an enemy's cavalry be at a distance of 1000 yards from the battery it is about to charge, it will move over the first 400 yards at a walk, approaching to a gentle trot, in about four and a half minutes; it passes over the next 400 yards at a round trot, in a little more than two minutes; and over the last 200 yards at a gallop, in about half a minute, the passage over the whole distance requiring about seven minutes. This estimate will generally be very near the truth, as the ground is not always even, nor easy to move over. Many losses arise from the fire of the artillery and from accidents, and the forming, and filling up of intervals create disorder; all of which contribute to retard the charge. Now a piece can throw with sufficient deliberation for pointing, two solid shot or three canisters per minute. Each piece of the battery, therefore, might fire nine rounds of solid shot upon the cavalry whilst it is passing over the first 400 yards; two rounds of solid shot and three of canister whilst it is passing over the next 400 yards; and two rounds of canister whilst passing over the last 200 yards-making a total from each gun of eleven round shot and five canisters. To this is added the fire of the supporting infantry. Care should be taken not to cease firing solid shot too soon, in order to commence with canister. If the effect of the latter be very great on hard, horizontal, or smooth ground, which is without obstruction of any kind, it is less in irregular and soft ground, or on that covered with brushwood; for, if the ground be not favorable, a large portion of the canister shot is intercepted. A solid shot is true to its direction, and, in ricochet, may hit the second line if it misses the first.
Solid shot should be used from 350 yards upwards: the use of canister should begin at 350 yards, and the rapidity of the fire increase as the range diminishes. In emergencies, double charges of canister may be used at 150 or 160 yards, with a single cartridge.
Spherical case ought not, as a general rule, to be used for a less range than 500 yards; and neither spherical case nor shells should be fired at rapidly advancing bodies, as for instance, cavalry charging.
The fire of spherical case and of shells on bodies of cavalry in line or column, and in position, is often very effective. To the destructive effects of the projectiles are added the confusion and disorder occasioned amongst the horses by the noise of their explosion; but neither shells nor spherical case should be fired so rapidly as solid shot.
In case of necessity, solid shot may be fired from howitzers.
RANGES OF FIELD GUNS AND HOWITZERS.
The range of a shot or shell in this table is the distance from the piece to the point at which the first graze of the ball is made on horizontal ground, the piece being mounted on its appropriate field carriage.
The range of a spherical case shot is the distance at which the shot bursts near the ground in the time given, thus showing the elevation, and the length of fuze required for certain distances.
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