12 CORRELATION OF PHYSICAL FORCES. 



thereby lessened, and the convenience of expression is greatly 

 increased, 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 gravi- 

 tation ; 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 

 project a cannon-ball in terms of the 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 somewhat 

 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 elongated, 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 transmitting 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 resist 

 or arrest the motion of the mass of matter appended to it. 



We need some word to express this state of tension ; we 

 know that it produces an effect, though the effect be nega- 

 tive in character : although in this effort of inanimate matter 

 we can no more trace the mode of action to its ultimate 

 elements than we can follow out the connection of our own 

 muscles with the volition which calls them into action, we 

 are experimentally convinced that matter changes its state 

 by the agency of other matter, and this agency we call 

 force. 



In placing the weight on the glass, we have moved the 

 former to an extent equivalent to that which it would again 

 describe if the resistance were removed, and this motion of the 

 mass becomes an exponent or measure of the force exerted 



