The action of the differential is illustrated by an experiment that requires a pair of wheels on an axle, like buggy wheels, and a stick long enough to reach from one to the other. With the wheels on smooth ground, put the ends of the stick through the wheels at the top, each end pressing against a spoke. Hold the stick at its center and push it forward; the stick will transmit the pressure to the spokes, and the wheels will turn. The wheels being on smooth ground, there is equal resistance to their movement, and they will run straight forward. Now repeat the experiment with the wheels so placed that one is on a smooth roadway and the other on sand; as the wheel on the smooth surface meets with less resistance than the other does, it moves faster, and the pair of wheels circles, although the stick applies equal pressure to both. The power developed by the engine is transmitted by the differential to both rear wheels; when the wheels meet with equal resistance, they turn equally, but when one wheel meets with greater resistance than the other, it slows down, while the other speeds up to correspond. A tractor with two driving wheels must use a differential in order to make turns easily. Without a differential, the wheels would run always at equal speed, and in making a turn one would be obliged to slip. The use of a differential has a disadvantage, however. If one wheel is in a mudhole and the other is on hard ground, the wheel in the mud meets with little resistance, and all of the power of the engine goes to it; it spins without moving the tractor, while the other wheel remains stationary. In such a case all of the power should be applied to the wheel that has traction in order to move the tractor, but this the differential fails to allow. In some tractors the differential is so made that the parts may be locked together. This lock is used when one wheel is in a mud hole, and as by its use power is transmitted equally to both wheels, the tractor moves. Great care must be taken to unlock the differential as soon as the need for the lock has passed, for otherwise the wheels would slip on a turn, and the parts of the transmission might be strained or broken. A differential is usually made with two bevel gears placed face to face; between them is a frame holding three or more small bevel gears that are in mesh with them both. The engine revolves the frame with its small gears; each of the large bevel gears revolves a driving wheel. When the tractor moves straight ahead the differential turns as if it were one solid piece. When there is less resistance to one driving wheel than to the other, the small bevel gears, in addition to revolving with the frame that carries them, turn on their shafts. This transmits the power of the engine to one wheel more than the other, according to the resistance of the wheels. [Illustration: FIG. 64.—“I. H. C.” CHAIN DRIVE, SHOWING THE DIFFERENTIAL]