THE MUSCLES u 



cylinder. Just before the feed-pipe actually reaches the 

 space in the head of the cylinder known as the " ex- 

 plosion or combustion chamber," there is placed a regu- 

 lating contrivance or " carburettor." By this contrivance 

 the petrol is mixed with air, and made into an explosive 

 mixture ; it has a valve by which the amount of air let in 

 may be increased or decreased. The rate at which the 

 petrol mixture enters the engine is regulated by the 

 " throttle " valve ; by opening or shutting it the driver can 

 regulate exactly the rate at which the engine is fed with 

 petrol. The explosive mixture passes into the cylinder 

 by the inlet pipe (fig. i). Just where the inlet pipe 

 opens into the combustion chamber is placed a valve, 

 which opens to let the mixture in, but once in, the valve 

 becomes closed in a way we need not stop to describe. 

 Then at the opposite side of the explosion chamber there 

 is represented another opening, also guarded by a valve — 

 the exhaust valve — leading into the exhaust pipe (fig. i). 

 We shall see that, after the explosion takes place, this 

 valve opens and the exploded or burnt gas is forced out 

 through the exhaust pipe. You will note, too, projecting 

 into the head of the cylinder, the " sparking plug " by 

 which the electric spark is discharged and the explosive 

 mixture of petrol and air is fired. I need not remind 

 you that Faraday knew nothing of internal-combustion 

 engines ; they were invented long after he was dead, but 

 the dynamo or magneto, which makes the sparking plug 

 a working thing, we owe to the discoveries he made in 

 the workshops of the Royal Institution. 



We have now to see how this motor cycle or gun is 

 loaded, discharged, turns the main shaft and drives the 

 hind wheel. We must place ourselves in the position 

 of the inventor who discovered this very wonderful 

 machine. In fig. I the piston is shown resting at the upper 

 end of the cylinder, just under the explosion chamber. It 

 fits the cylinder so exactly that if it be drawn down, by 

 turning the crank of the main shaft of the engine, it will 

 act as the piston of a pump, and, supposing the inlet valve 

 to be open, will draw in and fill the cylinder with an 



