42 



CHAPTER III. 

 The Mass of the Earth. 



WE shall now consider the adaptation which 

 may, as we conceive, be traced in the amount of 

 some of the quantities which determine the course 

 of events in the organic world ; and especially in 

 the amount of the forces which are in action. 

 The life of vegetables and animals implies a con- 

 stant motion of their fluid parts, and this motion 

 must be produced by forces which urge or draw 

 the particles of the fluids. The positions of the 

 parts of vegetables are also the result of the flexi- 

 bility and elasticity of their substance ; the volun- 

 tary motions of animals are produced by the 

 tension of the muscles. But in all those cases, 

 the effect really produced depends upon the force 

 of gravity also ; and in order that the motions and 

 positions may be such as answer their purpose, 

 the forces which produce them must have a due 

 proportion to the force of gravity. In human 

 works, if, for instance, we have a fluid to raise, or 

 a weight to move, some calculation is requisite, in 

 order to determine the power which we must use, 

 relatively to the work which is to be done : we 

 have a mechanical problem to solve, in order that 

 we may adjust the one to the other. And the 



