CHAP XV. ORDNANCE. GUNS are composed of Iron or Gun metal. Iron guns, being less expensive than others, are used where lightness is not of consequence.* Gun Metal is an alloy of 10 parts of tin to 88 of copper. Bronze is an alloy of 11 parts of tin to 100 of copper; and bell metal consists of 22 parts of tin and 78 of copper. Iron ordnance is valued at about 157. per ton, and Gun metal at about 4 d. per lb. Guns are distinguished from each other by the nature of their metal and weight of their shot. All guns are cast solid. A gun is divided into five principal parts, viz. the cascable, first reinforce, second reinforce, chase, and muzzle. These as well as the minor divisions of both gun and carriage will be better understood by the adjoining sketches. * For much of the information presented in this chapter, the author is indebted to the " Aide Mémoire." The carriage and slide of 68-pounders and 10-inch guns weigh about 33 cwt.; the common carriages about 9 cwt. As the elastic force of the powder is greatest at the breach, that part of the gun is made largest; consequently the outside of the gun is not parallel to the axis of the bore; but if a line be drawn from the cascable along the top of the gun parallel with the axis of the bore, and a projection which will touch that line be fixed on the gun anywhere in the direction of the muzzle, it will be as easy to direct the bore towards an object, as if the gun were the same thickness everywhere. This projection is the Dispart, and may be found by taking the diameter of the base ring, and any other part of the gun where it is desired to place the dispart, subtracting the less from the greater, half of which difference will be the height of the dispart on that part where the less diameter was taken. This may be done by taking the circumference with two pieces of thread, dividing each into three parts for the diameter, half the difference of these diameters being the required height. Howitzers have no dispart; a patch on the muzzle making that part equal in height to the base ring. The Angle of Dispart is that number of degrees which the axis of the bore would point above the object aimed at when laid by the surface of the gun. The Sights are two notches cut on the upper part of the base ring and swell of the muzzle. The Line of metal is an imaginary line drawn along the surface of the metal between the two sights. The Centre of metal is indicated by notches on the side of the base ring and swell of the muzzle, corresponding with the axis of the piece. The Tangent scale is marked with degrees for elevation, found by multiplying the length of the piece in inches from the base ring to the swell of the muzzle by 0.17455, the product giving the length of each nearly. 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. |