A military dictionary

by William Duane

Now the extent of the vibration made by the pendulum may be increased by the riband: for if the pressure of the steel edges on the riband be regulated by the screw, so as to be free and easy, though with some minute resistance to hinder it from slipping itself; then setting the pendulum at rest, let the part of the riband between the pendulum and the steel edges be down straight, but not strained, and fixing a pin in the part of the riband contiguous to the edges, the pendulum, swinging back by means of the impulse of the ball, will draw out the riband to the just extent of its vibration, which will be determined by the interval on the riband between the edges and the space of the pin.

The computation by which the velocity of the shot is determined from the vibration of the pendulum, after the stroke, is founded on the principle of mechanics; that if a body in motion strikes another at rest, and they are not separated after the stroke, but move on with one common motion, then that common motion is equal to the motion with which the first body moved before the stroke; whence, if that common motion and the masses of the two bodies are known, the motion of the first body before the stroke is thence determined. On this principle it follows, that the velocity of a shot may be diminished in any given ratio, by its being made to impinge on a body of weight properly proportioned to it.

It is to be observed, that the length to which the riband is drawn, is always near the chord of the arc described by the ascent; it being so placed, as to differ insensibly from those chords which must frequently occur: and these chords are known to be in the proportion of the velocities of the pendulum acquired from the stroke. Hence it follows, that the proportion between the lengths of the riband, drawn out at different times, will be the same with that of the velocities of the impinging shots.

Now from the computations delivered by Mr. Robins, it appears, that the velocity of the bullet was 1641 feet in one second of time, when the chord of the arc described by the ascent of the pendulum, in consequence of the blow, was 17¹⁄₄ inches, the proportion of the velocity with which the bullets impinge, to the known velocity of 1641 feet in one second, will be determined.

Mr. Robins was (till of late) the only author who attempted to ascertain the velocity of a military projectile by experiment; yet his conclusions seem to be unsatisfactory. Perhaps he was too much attached to the forming of a system, and warped his experiments a little in favor of it. The resisting power he assigns to the air is probably too great; and his notion of the tripling of this power when the velocity of the projectile exceeds that of sound, seems to be rather an ingenious theory than a well-grounded fact. However, experiment alone must decide these points.

The great importance of the art of gunnery is the reason that we distinguish it from the doctrine of projectiles in general; for in truth it is no more than an application of those laws which all bodies observe when cast into the air, to such as are put in motion by the explosion of guns or other engines of that sort: and it matters not whether we talk of projectiles in general, or of such only as belong to gunnery; for, from the moment the force is impressed, all distinction, with regard to the power which put the body first in motion is lost, and it can only be considered as a simple projectile.

Every body cast into the air moves under the influence of two distinct forces. By the one it is carried forward with an equal motion, and describes equal spaces in equal times, in the direction in which it was projected; and by the other, which we call gravity, is drawn downwards in lines perpendicular to the surface of the earth, with a motion continually accelerated, or whose velocity is always increasing. If either of these forces were destroyed, the body would move according to the direction of the other alone, so far as its motion was not hindered by the interposition of other bodies; but as both continue to act, the course of the projectile must be determined by a power compounded of those two forces.

Gunnery is also the province of the artillerist, and comprehends, in an active sense, the perfect knowlege of the power of the machine, and the proportions of powder to be employed in order to produce any required effect. It also comprehends a knowlege of the properties and composition of gunpowder, and the various kinds of shot, which are employed in the practice of gunnery; the metal best adapted to make guns, the proper weight and corresponding proportions between the calibre of the gun and the shot fired from it, and also the dimensions fitted for the various services in which gunnery is employed: for batteries of permanent works, for ships, for field service, and the light or flying artillery. Gunnery indeed comprehends all the duties of the able artillerist and bombardier.

Gunnery. By the assistance of good tables of practice, and the tables of amplitudes, sines, tangents, and secants, all the cases in gunnery in a nonresisting medium may be easily solved; and perhaps the solution may be sufficiently correct for practice, if the initial velocity of the projectile be not so great as to make the air’s resistance considerable.

For the tables of ranges with ordnance, see the different natures, as [Gun], [Mortar], &c. and for the tables of amplitudes, sines, tangents, and secants, see [pages 247] and [248].