ANIMAL MECHANISM. 



61 



will be careful to give to the surface of the piston such an 

 area, that the pressure on this surface may be precisely equal 

 to the resistance which it has to overcome ; then he will give 

 to the cylinder such a length that it will allow the piston to 

 travel just as far as the resistance ought to move. It is only 

 under these conditions that the machine will do the desired 

 work, and utilize all its moving power. On the contrary, 

 in the case in which work answering to a kilogrammetre 

 must be done by lifting 100 kilogrammes to the height of a 

 centimetre, the cylinder must be made so large that the pres- 

 sure of steam on the surface of the piston will develop an 

 effort of 100 kilogrammes, and such a length only must be 

 given to the cylinder, that the movement of the piston may 

 be merely a centimetre. 



One cannot substitute one of these forms of cylinder for 

 the other, for in one case the force would be insufficient, and 

 in the other, the range would be too restricted. 



The only thing which is equal in both is the amount of 

 v/ork that the two machines can do, that is to say, the pro- 

 duct of the force employed multiplied by the space passed 

 through ; this is again the product of the surface of a section 

 of the cylinder multiplied by its length, or, in other terms, 

 it is the volume of steam contained in each machine, this 

 vapour being supposed to be at an equal tension. 



This proportion of the volume of the matter which works 

 to the work performed, is found in every case in which a 

 moving force is employed. 



Two masses of lead falling from the same height will do 

 work proportionate to their volume, or, which is the same 

 thing, to their weight. Two threads of india-rubber of the 

 same length, both of which have been stretched to the same 

 degree, will do work proportionate to their transverse sec- 

 tions, and, consequently, to their respective weights. Lastly, 

 two threads of the same diameter, but of unequal lengths, 

 after having been subjected to the same elongation in pro- 

 portion to their original lengths, will, as they contract, do 

 work proportionate to their respective lengths, that is to say, 

 to their weight. 



This leads to the consideration of muscle, which conforms 



