3rd. By excentric projectiles; recent experiments having shown that if the centre of gravity is placed directly above the centre of figure the range is greatly increased.[5]
A Tangent scale is affixed to the breech of Guns and Howitzers, by means of which the requisite elevation may be given, and the object seen at the same time. This scale has divisions, called degrees, marked on it, and it is placed in a groove at the breech, from which it can be raised (being fastened by a screw) to give the necessary elevation.
The divisions on the Tangent scale are found by multiplying the length of the piece in inches, from the base ring to the swell of the muzzle, by ·017455, and the product will give the length nearly of each degree or division on the tangent scale. By subtracting the dispart from this product, the length of the tangent scale above the base ring for one degree of elevation will be obtained.
The Sights of a gun are two small notches marked on the upper part of the base ring (or on the top of the tangent scale) and swell of the muzzle. Their situation is ascertained by means of a spirit level.
The line of metal is an imaginary line drawn along the surface of the metal between the two sights.
The line of metal Elevation is obtained by laying a Gun at an object by means of the sights, without giving any elevation; from the thickness of metal at the breech, the line of metal elevation varies from one to two degrees.
The Centre of metal is indicated by a line drawn through the uppermost point of the base ring and swell of the muzzle; and is ascertained by means of a spirit level.
Windage is the difference between the diameter of the bore, and that of the shot. The windage formerly allowed was one-twentieth the diameter of the shot, but it is now reduced considerably: (Field Guns having only one-tenth of an inch) and this diminution of windage is very beneficial, longer ranges being obtained with the same charges of powder, and also greater precision of fire.
The Vent, for every nature of Ordnance, is two-ninths of an inch in diameter.
Bouching a gun is fixing a pure copper vent into it; which is done by drilling a hole in the piece, where the vent is usually placed, about one inch in diameter, and screwing therein a piece of wrought copper with a vent of two-ninths of an inch through the centre of it.