This contains certain gears, which are designed to change the speed of the transmission shaft relative to the engine shaft. TRANSMISSION LEVERAGE.--It is simply using leverage in order to produce a more effective pull, or to attain greater speed, from a shaft which runs at a certain number of revolutions. If we have a motor with a shaft speed of, say, 800 revolutions per minute, and an axle with a speed of 400 revolutions, the ratio would be 2 to 1. Now, to speed up the machine, so that the axle will turn 800 revolutions, would require an engine speed of 1600, which might be impossible. [Illustration: Fig. 42. Progressive Transmission. Low.] ECONOMY OF TRANSMISSION GEARING.--From an economical standpoint, also, it would be undesirable, even though the engine should be able to make the speed. Owing to the explosion impulses of the gasoline motor, a heavy fly wheel is necessary on the engine shaft, in order to store up power by momentum, and also to give a uniform speed. In hill climbing, or in carrying heavy loads, the transmission shaft must have its speed cut down, while permitting the engine to run at full or normal speed. [Illustration: Fig. 43. Neutral Position.] The transmission gearing is, therefore, the most satisfactory solution of the problem, because changing the engine speed destroys its effectiveness, and we shall, therefore, consider some of the types for that purpose. There are two distinct systems of transmission, namely: The Positive, and the Frictional. Of the positive system we have the planetary and the sliding gear types. The sliding gear type has two methods of control, one known as the _progressive_, and the other the _selective_. CHARACTERISTICS OF TRANSMISSION.--The progressive, selective and planetary types, are entirely different from the frictional system, for the reason that they effect the changes by step movements, the speeds being produced at certain ratios, whereas the frictional method has indefinite and infinite ratios. [Illustration: Fig. 44. Intermediate.] The following diagrams will clearly bring out the distinctive features of each. Fig. 42 shows a shaft A, which derives power from the engine, having in line with it a shaft B, which connects with the driven shaft. The shaft B is squared, but it has a round end C, which is socketed axially within the head of the shaft A. THE PROGRESSIVE.--The head D has a small pinion E, and on its side is provided with projecting teeth F. The loosely-revolving squared shaft B has thereon a pair of spur gear G H, separated from each other a trifle more than the width of each gear, and they are united by an intermediate hub so they turn in unison. [Illustration: Fig. 45. High.]