INTRODUCTORY REMARKS. 21 



true, and in this its ordinary sense I shall use it in these pages. 

 But though the term has a potential meaning, to depart from 

 which would render language unintelligible, we must guard 

 against supposing that we know essentially more of the phe- 

 nomena by saying they are produced by something, which 

 something is only a word derived from the constancy and 

 similarity of the phenomena we seek to explain by it. The 

 relations of the phenomena to which the terms force or forces 

 are applied give us real knowledge ; these relations may be 

 called relations of forces ; our knowledge of them is not there- 

 by lessened, and the convenience of expression is greatly in- 

 creased, but the separate phenomena are not more intimately 

 known ; no further insight into why the apple falls is acquired 

 by saying it is forced to fall, or it falls by the force of gravita- 

 tion ; by the latter expression we are enabled to relate it 

 most usefully to other phenomena, but we still know no more 

 of the particular phenomenon than that under certain circum- 

 stances the apple does fall. 



In the above illustrations, force has been treated as the 

 producer of motion, in which case the evidence of the force is 

 the motion produced ; thus we estimate the force used to pro- 

 ject a cannon ball in terms of th# mass of matter, and the 

 velocity with which it is projected. The evidence of force 

 when the term is applied to resistance to motion is of a some- 

 what different character ; the matter resisting is molecularly 

 affected, and has its structure more or less changed ; thus a 

 strip of caoutchouc to which a weight is suspended is elonga- 

 ted, and its molecules are displaced as compared with their 

 position when unaffected by the gravitating force. So a piece 

 of glass bent by an appended weight has its whole structure 

 changed ; this internal 'change is made evident by transmit- 

 ting through it a beam of polarised light : a relation thus 

 becomes established between the molecular state of bodies 

 and the external forces or motion of masses. Every particle 

 of the caoutchouc or glass must be acting and contributing to 



