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184 equal—the simpler the gear the better and more efficient will it be.

That being so, it may not unnaturally be asked why the simplest method is not always used. If it were only the matter of conveyance of the power from the motor to the road wheels, doubtless this would be done; but where the petrol or internal combustion motor is used another matter has to be provided for, and that is the variation of the ratio of engine speed to wheel speed; for with the great majority of motors of this type, unless the speed rate of the motor can be maintained, it will stop, so that 'variable gearing' has to be adopted and the power sent through this to the wheels. By this I mean that the means of transmission may be so altered at will that whereas when on level ground the engine may make, say, only two revolutions to each one of the road wheels, for hilly or heavy work it may make, say, four, six, or eight, and so, whilst the car travels slower, the engine speed may remain the same. Where steam engines are used this is not usually required, as the steam engine obtains more power for heavier work by the use of more steam.

Now the simplest method possible would be the driving of the road wheels or wheel direct by the piston-rods of the engine, a plan only possible where a very small wheel is used, and only actually employed in the Holden bicycle as described in the chapter devoted to this type of machine. Next to this comes the use of gear wheels—'cog' wheels—as employed upon tricycles and some light forms of car where the motor is set close to the axle. In these we have one gear wheel fixed to the shaft of the motor, gearing into a similar one upon the axle. And here we may halt for a moment to consider the action of gear wheels. As will be seen by fig. 1, we have two wheels, the edges of which are cut into a number of equal-sized teeth, and these wheels are so fixed in relation to each other that the two sets of teeth mesh or interlock with each other. Now it will be seen that any movement of one will be imparted to the other through the teeth, but in an opposite