A tractor is arranged to pull its load in two different ways, first by the draw bar, as when pulling plows, and secondly by a belt from the engine pulley as in driving a threshing machine or circular saw. In the first case it is necessary to drive the road wheels through the gear train, and in the second case it is necessary to disconnect the road wheels while driving the thresher or saw. As the engine cannot be started while under load it is also necessary to disconnect the road wheels to free the engine while turning it over to get the first explosion. The device that connects and disconnects the engine from the road wheels is known as the =CLUTCH=. This usually consists of two or more friction surfaces that form a part of the driving gear, which may be brought into frictional contact with the engine pulley, when it is necessary to drive the road wheels. When the two members of the clutch are brought into contact they revolve together, thus connecting the engine with the driving gear. Reverse Gears. [Illustration: The Reverse Gear of the “Big Four” Tractor. ] As it is not practicable to reverse the direction of rotation of the gas engine, the rotation of the road wheels is reversed by means of gears contained in the driving train. In some tractors the reverse gears are similar to those in an automobile, being located in the transmission. In other tractors two bevel pinions are provided that fit loosely on the engine shaft and engage with a large bevel wheel that forms part of the driving gear. A sliding jaw clutch that revolves on the engine shaft is arranged so that it can connect with either of the bevel pinions causing them to rotate with the engine shaft and drive the main wheel. As the two pinions are on opposite sides of the large bevel wheel, they run in opposite directions in regard to it, so that it is possible to reverse the large wheel by engaging the clutch with either one or the other of the bevel pinions. The Differential Gear. The differential gear makes it possible to apply the same amount of power to each of the road wheels, and also allows one wheel to rotate faster than the other when turning around a corner. If both road wheels were rigidly fastened to a single rotating axle it would be practically impossible to turn a corner for it would be necessary for the engine to slip one or the other of the wheels because of their difference in speed, as the outer wheels turn faster than the inner. [Illustration: Differential Gear of the “Big Four” Tractor. ] The Driving Gear. The driving gear consists of a series of spur gears arranged for the purpose of reducing the high speed and small “pull” of the motor into the low speed and heavy pull of the road wheels. This reduction in speed is generally brought about by a double system of shafts, the second shaft from the motor carrying the differential gear and meshes directly with the master gear on the bull wheel. The first shaft is an idler. [Illustration: