THE MUSCLES 9 



engines which are used in the propulsion of the human 

 machine. I hope that you will agree with me, then, that 

 there is this degree of resemblance between a motor cycle 

 and the human body : each is fitted with a propelling 

 engine or a set of propelling engines. 



When we compare the engine which carried you uphill 

 on your motor cycle with those engines or muscles which 

 carried me up on foot, we shall find that they are alike in 

 many ways. As every boy knows, the engine of a motor 

 cycle is a kind of gun, only the chief part of it is called a 

 cylinder, not a barrel. This gun is charged or loaded, not 

 with gunpowder but with an explosive mixture of petrol 

 and air ; the explosive mixture is fired, not with a per- 

 cussion cap but by an electric spark, which enters by the 

 " sparking plug " set in the upper end of the barrel or 

 cylinder (fig. 1). We load a cylinder, not with a bullet 

 but with a piston ; were the piston free it would be shot 

 from the cylinder as a bullet is from the barrel of a gun. 

 But the piston is not free ; it is tethered by a " connecting 

 rod " to the short lever or " crank-pin " of a shaft or axle 

 which can turn or revolve (fig. 1). Now, when the ex- 

 plosive mixture is fired, the piston or missile being yoked 

 to the shaft spends its force in turning it, instead of 

 shooting free into the air to kill people. And then the 

 main axle of the engine being connected with the hind 

 wheel of the motor cycle turns it and sends the machine 

 forwards. The engine of a motor cycle is thus really a 

 kind of gun which clever men, after endless study and 

 experiment, have made to turn wheels instead of shooting 

 out bullets. Men have found out, too, how to make 

 this kind of gun reload and discharge all by itself, or 

 automatically, so that it can fire two thousand shots or 

 more every minute. We shall find that muscles also 

 reload and discharge automatically. 



Presently we shall come to speak of the muscle-engines, 

 and compare them point by point with the internal-com- 

 bustion engine of the motor cycle. Now you may be 

 familiar with one make of engine and I with another, so 

 in order that there may be no confusion between us I 



