30 THE ENGINES OF THE HUMAN BODY 



combustion chamber of the cylinder and carried an electric 

 current which fired the explosive mixture and set the 

 engine in motion. A cable of most peculiar "wires" 

 also enters the muscle ; we call the cable a nerve, and the 

 current or messages which it conveys to the engine are 

 not electric but of a different kind, yet they serve the 

 same purpose : they set the muscle in motion. 



If we look more closely we shall find that not only 

 have the engines of the motor cycle and of the human 

 machine pipes of a corresponding kind, but that through 

 these pipes there passes a circulation which serves a similar 

 purpose. Petrolised air is pumped through the circulatory 

 system of the engine of the motor cycle ; arterialised 

 blood is pumped through the corresponding system of 

 the muscular engines of the human machine. But the 

 system of pumping employed in muscular engines is by 

 far the superior. We have seen that in the 4-cycle 

 internal-combustion engine only one stroke in four is 

 really effective ; the other three are spent on pumping or 

 circulating the explosive mixture. The first stroke we 

 saw drew in petrolised air and thus charged the cylinder 

 with an explosive mixture ; the second compressed the 

 mixture into the combustion chamber ; the third stroke, 

 which is the only effective or driving one, is caused by the 

 explosion ; the fourth one sweeps out the waste gases 

 through the exhaust (figs. 2a-2d). Thus three strokes out 

 of every four are used to keep up the circulation of petrolised 

 air through the engine. So far as driving power is con- 

 cerned these three strokes are lost or wasted. Engineers 

 are trying hard to make each stroke of the engine an 

 effective or driving one. Dr. Dugald Clerk has succeeded 

 in getting rid of two of them. He managed to do this 

 in a very simple manner. He attached a pump to the 

 engine in such a way that it forced a charge of petrolised 

 air into the combustion chamber just when the piston had 

 swept all the waste gases through the exhaust and was 

 ready to begin a new stroke. Thus the first or suction 

 stroke and the second or compression stroke became 

 unnecessary ; in this way two strokes were saved. 



