direction thus if wheel a revolves to the right, wheel b will turn to the left, and vice versa. There is also another peculiarity about these wheels. It will be noticed that they are of different sizes. The result is that if wheel a is the first to receive the power, one turn of the wheel will not cause wheel b to make a complete turn, whilst, conversely, wheel b being the larger of the two, will, if revolved, cause wheel a to turn more than once. Just what their actual relation of movement to each other may be, is determined actually by their respective diameters and, for ease of calculation, by the number of teeth they respectively contain. Thus if wheel a contains 20 teeth and wheel b 50, it will take 212 turns of a to revolve b once,
_0219.jpg)
Fig 1.
whilst one revolution of b as the driver would cause a to go round 212 times. It will thus be seen that by varying the size of the different gear wheels used, the ratio between engine and road wheels can be varied. In an arrangement employing this simple form of transmission only, the engine and axle must be set close together, and we only have the friction in the bearings of the engine and road wheels to be overcome, together with that caused by the teeth of the gear wheels as they engage with and push each other around. When, however, it is found desirable that the engine should be separated from the road wheels, some other form of transmission becomes necessary, and other means have to be adopted; and the simplest and
