INTERNAL-COMBUSTION ENGINES 33 



which is only a twentieth of an inch in length, or we can 

 make it bend the elbow through its full range from com- 

 plete extension to complete flexion. We can make it 

 work the forearm backwards and forwards at any point 

 of its range of movement, quickly or slowly, gently or 

 strongly. It is true that engineers can alter the strength 

 and rate of the stroke of a mechanical engine by opening 

 or closing the throttle valve, thus regulating the amount of 

 explosive mixture admitted ; but there is only one length 

 of stroke — that to which the engine is set. Think for a 

 moment what our case would be if the biceps had only 

 one length of stroke. We should set it, 1 suppose, so 

 that it could just reach the mouth in feeding. To make 

 a delicate or short movement at the elbow would be im- 

 possible. We should have to bring the hand as far as 

 the mouth or not at all. In the muscle-engine a method 

 has been discovered of regulating the rate of combustion 

 as well as length of stroke. Work is not performed by 

 a series of unregulated explosions as in a motor engine, 

 but by a regulated process of oxidation which we do not 

 rightly understand as yet. 



There is another difficulty which all who drive motor 

 cycles or motor cars have to contend with. When driven 

 very hard they may get overheated, and so damaged as to 

 become unworkable. Engineers have striven to find out 

 a perfect way of keeping the cylinder from becoming 

 overheated. They have contrived methods of circulating 

 cool water round it, or of causing draughts of air to 

 blow upon it, and many other ways, but all of them 

 are apt to fail. Then, in winter, there is another mis- 

 chance which may happen to a motor engine. It may 

 become so cold that the petrol will not vaporise properly 

 -—will not ignite and will not explode. Hence we notice 

 that a careful driver, when the weather is cold, always 

 throws a thick rug over the engine when his motor car 

 has to stand still for some time. The engines fitted to 

 aeroplanes, which are exposed to great cold when high 

 flights are undertaken, are kept warm by means of a 

 coat heated by electricity. Drivers not only know that 



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