MECHANICS 43 



they can be compressed or stretched or bent or twisted ; 

 as to the degree of elasticity they possess, or as to the 

 ease with which they can be broken. They differ, besides, 

 as to the amount of resistance (or friction) caused by the 

 movement of one upon another. 



In the elementary theoretical, as distinguished from 

 practical, science of mechanics, however, the consideration 

 of such differences is omitted in order to reduce problems 

 to their simplest form. Solid bodies are supposed to be 

 incapable of any change of form and perfectly in- 

 flexible, while cords are treated as perfectly supple and 

 entirely devoid of any rigidity. Therefore the solids, 

 fluids, and aeriform bodies of mechanics, are imaginary 

 substances, and not such as we actually find in nature. 

 But the consideration of these qualities and properties 

 (of which abstraction is thus, for convenience, made) can 

 always be added, and so the results of the science (as 

 we saw * was the case with mathematics) correspond, 

 with practical exactness, to the characters and properties 

 of real material things. 



Now every such thing may, for convenience, be 

 supposed to be made up of an immense number of most 

 minute and uniformly distributed particles; and the 

 influence that makes it fall which is known as the/brce 

 of gravity, or gravitation might be represented by lines 

 drawn from every such particle towards the centre of the 

 earth. But as such lines would thus converge towards a 

 point enormously distant, they may be treated as if they 

 were all parallel to one another. 



But all such parallel forces (so represented by lines) 

 may be replaced by a single force also represented by a 

 line applied to a certain point, and such point is called 



* See ante, p. 33. 



