To keep a tractor running is a matter only of understanding and of common sense; common sense to realize that any piece of machinery needs some degree of care and attention, and understanding of where the care and attention should be applied. The more thoroughly a tractor operator understands his machine, the more work he will be able to get out of it, and the more continuously it will run. This is only another way of saying that understanding and knowledge pay a direct return in work done and money earned. In the early days of the automobile there were as many types of cars as there were manufacturers. As time has gone on, the unsatisfactory ideas have been weeded out, and automobiles have approached what may be called a standard design. At the present time, tractor designs are varied, and it is hardly possible to speak of any type as standard. The reason for this lies in the fact that many manufacturers start with a design for one special part, and build the tractor around it. For example, a manufacturer may develop a method of driving the wheels that he feels is especially good for tractor work. In applying it he may find that the engine must be so placed on the frame that when the power pulley is in position the belt will interfere with the front wheels unless they are small; he therefore uses small front wheels, and advocates them for tractors. Another manufacturer with a patent steering gear may be able to place the power pulley so that there is ample clearance for the belt; he finds that by using high front wheels he can get a better support for the frame, and therefore claims that high front wheels are an advantage. Other designs may be based on having three wheels, or two; advantages are claimed for each type, and each type undoubtedly has them. The selection of a tractor is based on one’s own experience or on that of neighbors, or on the ability of the salesman to bring out the advantages of the make that he sells; but when the tractor is bought and delivered, its ability to do the work promised for it depends solely on the care with which it is handled and looked after. Whatever the design of a tractor may be, there are certain parts that it must have in order to do the work required of it. These parts, or groups of parts, are as follows: =Engine.=—This furnishes the power by which the tractor operates. =Clutch.=—By means of a clutch the engine may be connected with the mechanism, so that the tractor moves, or it may be disconnected, so that it may run without moving the tractor. =Change Speed Gear.=—As will be explained in later chapters, an engine, in order to work most efficiently, should run at a fixed speed; the tractor should be able to run fast or slow, according to conditions. A change speed gear is therefore provided, by which the speed of the tractor may be changed, although there is no change in the speed of the engine.