CHAPTER XXI 



A POSTAL SYSTEM OF A PECULIAR KIND 



In previous chapters we have examined the contrivances 

 which have been fitted within the human machine for the 

 preparation of the fuel it consumes. In this chapter we 

 begin the consideration of another system of structures 

 altogether, one which is needed to link up the various 

 parts of the machine so that they will move at the right 

 time, in the right order, and at the right rate. Without 

 a co-ordinating or timing system a machine cannot be 

 made to perform efficient work. The engine of a motor 

 cycle is provided with a co-ordinating or timing system. 

 The engine cannot run unless certain events take place in 

 a certain order and at a certain time and rate. The inlet 

 valve must open at the beginning of the first or suction 

 stroke and close at its end ; the combustion mixture 

 must be fired when the second or compression stroke is 

 completed ; the outlet valve must open as the fourth or 

 exhaust stroke sets in and must become promptly shut 

 when that stroke is finished. The engineer has co- 

 ordinated these events by means of a timing gear — a 

 system of toothed wheels driven from the crank-shaft. 

 On one side of these revolving wheels are set movable 

 fingers or|cams, so placed that they open and shut the 

 valves and break the electric current to the sparking-plug 

 at the instant of time which is found to be most effective 

 for the working of the engine. The co-ordinating or 

 timing system of a motor cycle, then, consists of a series 

 of revolving toothed wheels set so as to be turned by the 

 crank-shaft at fixed rates. 





