_Solid shot_ are now almost invariably made of cast iron,[35] formed in moulds of sand or iron. This projectile is used under almost every circumstance, whether in the battle-field or in the attack and defence of places, and is the only one that is effectual against the stone walls of forts. _Hot shot_ are used against shipping and wooden structures of every description. Red-hot balls were first employed by the king of Poland, in 1575, but, on account of the difficulty of heating them with rapidity, and the danger of loading the piece with them, this kind of projectile was not in general use till a much later period. It was at first supposed that the expansion of the metal would be so great, when heated to a red or white heat, as to prevent the ball from entering the piece; it is found, however, that the windage is still sufficient for loading with facility. These red-hot balls are principally used to fire wooden buildings, ships, and other combustible matter. They are therefore much used as a projectile for coast defence, and all fortifications on the seaboard should be provided with furnaces and grates, arranged so as to heat them with facility and rapidity. [Footnote 35: In Mexico, where iron is scarce, copper is used for shot and shells; but it is a poor substitute.] There are several kinds of _hollow-shot_ and _shells_, called _bombs, howitzes, grenades_, &c. They are made of cast iron, and usually in a spherical shape, the cavity being concentric with the exterior surface. The cavity was formerly made eccentric with the exterior, under the belief that the heavier side would always strike first. The rotary motion of the shell during its flight rendered this precaution of no use. Fire is communicated to the combustible matter within the shell by means of a fuse, which is so regulated that the explosion shall take place at the desired moment. Hollow-shot are used with advantage to destroy ordinary buildings, ships, earthwork, and thin walls of masonry; they, however, are of little avail in breaking the massive walls of well-constructed forts. Howitzes and grenades are particularly effective against cavalry and columns of infantry, and are much employed on the battle-field; they are also much used in the attack and defence of places.