THE MUSCLES 13 



finished. Secondly, he placed on the crank shaft of the 

 engine a fly-wheel which was sent spinning by the driving 

 stroke, and thus, when the driving stroke was over, kept 

 the crank shaft turning and the piston moving. The 

 piston, in its return stroke, acting like the piston of a 

 force-pump, sweeps out the exploded gas from the cylinder 

 through the exhaust valve. . Thus the second upward or 

 "exhaust" stroke serves to clear the cylinder (fig. 2d). 

 The moment the exhaust stroke is completed, the exhaust 

 valve shuts, and as it shuts, the inlet valve opens to let 

 in a fresh charge of petrolised air. 



The spin or momentum of the fly-wheel, imparted by 

 the " driving " stroke, is not only sufficient to return the 

 piston and sweep out the burnt gases from the cylinder, 

 but is also enough to begin the cycle of strokes again — 

 the first downward stroke which charges the cylinder, as 

 well as also the first upward or compression stroke which 

 squeezes the charge into the combustion chamber. Now 

 here is really a clever invention ; in each set or cycle of 

 an internal-combustion engine there are four strokes, but 

 only one of these — the second downward or explosion 

 stroke — is really effective. The explosion stroke has to 

 do all the work, not only turn the hind wheel and thus 

 drive the bicycle, but has also to empty and to charge the 

 engine. For one stroke the engine acts as a gun ; during 

 the other three it acts as a pump. It was a flash of genius 

 which showed how the effective stroke could be made to 

 perform the other three. But we shall find that Nature in 

 designing muscle-engines has shown even greater genius — 

 a genius so great that we have discovered as yet only some 

 of the secrets and patents of her inventions. 



Having thus looked at the plan on which the engine of 

 a motor cycle is made, we are now to set out to see how the 

 engines of the human body are shaped and fixed. They 

 are fixed on a framework of bones known as a skeleton. 

 Any day you may see a motor cycle and make out all the 

 parts of its skeleton, but although men, women, boys, and 

 girls may be seen on the roads or on the streets we cannot 

 see their skeletons for two reasons : firstly because they 



