The only motion of the carriage which takes place is that due to the elastic bending and rebound of its parts under the cross strains set up on discharge. These strains are inevitable since the direction of recoil cannot be always exactly in the line of the resistance of the earth behind the spade. This movement of the axis is known as jump and must be determined by experiment for the individual piece in its particular mounting. The principal parts of the typical gun carriage are the cradle, a device for mounting the cradle—called in the different models rocker, pintle yoke, and top carriage, the trail, the wheels and axle. The gun slides in recoil on the upper surface of the cradle and the cradle contains the recoil controlling parts. In the design of the carriage the constructional difficulty lies not so much in preventing the carriage from recoiling but in preventing the wheels from rising off the ground on the shock of discharge. The force of the recoil of the gun, acting in the line of motion of the center of gravity of the recoiling parts, tends to turn the carriage over backwards about the point of the trail or center of the spade. This force is resisted by the weight of the gun and carriage, which tends to keep the wheels on the ground. The leverage with which the overturning force acts is due to the distance of its line of action above the center of the spade; the leverage with which the overturning force acts is that due to the horizontal distance of the center of gravity of the gun and carriage from the center of the spade. It follows that the steadiness of the carriage for a given muzzle energy may be promoted by four factors. (a) Increasing the weight of the gun and recoiling parts. This reduces the recoil energy. (b) Increasing the length of recoil allowed. This reduces the overturning pull. (c) Keeping this gun as low as possible either by reducing the height of the wheels, or by cranking the axle downwards. This reduces the leverage of the overturning force. (d) Increasing the length of the trail. This increases the leverage of the steadying force. The well designed gun carriage is one that combines these factors in a practical way so as to give the greatest possible steadiness to the carriage at the same time keeping within the limits of weight imposed by the necessity of mobility.