It consists of two cheeks, connected together and with a stock by assembling bolts. The front part supports the piece, and rests upon an axle-tree furnished with wheels, the rear end of the stock or trail resting on the ground. [Sidenote: 124]] 7. What are the _cheeks_? The parts of the carriage between which the piece is placed, and upon which the trunnions are supported. 8. What is the wheel composed of? Of a nave into which the axle-tree enters; of a certain number of spokes fastened in the nave; and a circumference which is composed of a number of fellies equal to half the number of spokes. 9. What is the _dish_ of a wheel? The inclination outward of the spokes, when fastened in the nave. 10. What is the advantage of this obliquity of the spokes? It gives elasticity to the wheel, and protects it from the effect of shocks, which would destroy it, if the spokes were in the same plane. 11. What is the object of giving dish to a wheel? For the purpose of making the body of the carriage wider; to diminish the length of the axle-tree, thus increasing its strength; to throw the mud and water outside the wheels; and to keep the wheel close against the carriage, and prevent any tendency to run off the axle. 12. How are movable gun-carriages distinguished? As field, mountain, and siege-carriages. [Sidenote: 125]] 13. What are the principal considerations to be kept in view in the construction of movable carriages? In firing, the carriage should yield to the recoil. Were it fixed immovably, it would soon be destroyed, no matter how great its solidity. Its weight should be proportional to that of the piece. If too heavy it would soon be destroyed by the shocks of the piece. If too light, the recoil would be immoderate. Its weight should always be less than that of the piece. A heavy piece upon too light a carriage will perform better service than the reverse arrangement, since the effort exerted by a piece depends upon its mass multiplied into the square of the velocity. 14. What are the principal considerations to be kept in view in the construction of field carriages? Lightness and strength combined, great mobility and flexibility, and a low centre of gravity, in order to surmount all difficulties in the field which must frequently arise while artillery is acting with other troops,—to resist the concussion in firing, and the severe jolting produced when moving rapidly over uneven ground. 15. How many kinds of field gun-carriages have we? Three, viz: One for the 6 pdr. gun and 12-pdr. howitzer; another for the 24-pdr. howitzer; and the third for the 12-pdr. gun and 32-pdr. howitzer. 16. In what respect are these carriages similar? In all having the same kind of limber and the same-sized wheels, so that any limber or wheel may be used with any carriage; though, if possible, the heaviest wheel (No. 2) should be used on the carriage of the three heaviest pieces, 12-pdr. gun and 24 and 32-pdr. howitzers.