=Drive.=—The drive is the mechanism that applies the power of the engine to the wheels, and makes them turn. =Differential.=—When a tractor makes a turn, the outside wheels cover a larger circle than the inside wheels, and therefore must run faster in order to get around in the same time. It is usually the case that the power of the engine is applied to both driving wheels; if both were solid on the axle, like the wheels of a railroad car, one would be forced to slip when making a turn, which would waste power. By applying a differential, the engine can drive both wheels, but the wheels may run at different speeds when conditions require it. The clutch, change speed gear, drive and differential form the _transmission_. =Steering gear.=—By means of the steering gear the direction in which the tractor moves may be changed. =Supports.=—A tractor moves on broad-tired wheels, or on crawlers, which are so formed that they grip the ground and do not slip. They give so broad a support that even on soft ground the weight of the tractor will not pack the soil sufficiently to injure it as a seed bed. =Frame.=—The frame is the foundation of the tractor, and holds the parts in the proper relation to each other. It is usually made of channel steel, the parts being bolted to it; in some tractors, however, the parts are so attached to each other that they form their own support, and no other frame is needed. Tractor manufacturers make these parts in different ways; all accomplish the same result, but do it by different methods. The main principles are much the same, and should be known and understood. They are described and explained in the succeeding chapters. CHAPTER II ENGINE PRINCIPLES The working part of a tractor is the engine; it is this that furnishes the power that makes the machine go. The engine gets its power from the burning of a mixture of fuel vapor and air. When this mixture burns, it becomes heated, and, as is usual with hot things, it tries to expand, or to occupy more room. The mixture is placed in a cylinder, between the closed end and the piston; it is then heated by being burned, and, in struggling to expand, it forces the piston to slide down the cylinder. This movement of the piston makes the crank shaft revolve, which in turn drives the tractor. The first step in making the engine run is to put a charge of mixture into the cylinder, and it is clear that if the burning of the charge is to move the piston, the piston must be in such a position that it is able to move. When the mixture is burned, the piston must therefore be at the closed end of the cylinder. After the charge of mixture has been burned, the cylinder must be cleared of the dead and useless gases that remain, in order to make room for a fresh charge.