20 CORRELATION OF PHYSICAL FORCES. 



grounds to those which apply to the word cause, as it repre- 

 sents a subtle mental conception, and not a sensuous percep- 

 tion or phenomenon. The objection would take something 

 of this form. If the string of a bent bow be cut, the bow 

 will straighten itself ; we thence say there is an elastic force 

 in the bow which straightens it ; but if we applied our expres- 

 sions to this experiment alone, the use of the term force 

 would be superfluous, and would not add to our knowledge 

 on the subject. All the information which our minds could 

 get would be as sufficiently obtained from the expression, 

 when the string is cut, the bow becomes straight, as from the 

 expression, the bow becomes straight by its elastic force. 

 Do we know more of the phenomena, viewed without refer- 

 ence to other phenomena, by saying it is produced by force? 

 Certainly not. All we know or see is the effect ; we do not 

 see force we see motion or moving matter. 



If now we take a piece of caoutchouc and stretch it, when 

 released it returns to its original length. Here, though the 

 subject-matter is very different, we see some analogy in the 

 effect or phenomenon to that of the strung bow. If again 

 we suspend an apple by a string, cut the string, the apple falls. 

 Here, though it is less striking, there is still an analogy to 

 the strung bow and the caoutchouc. 



Now when the word force is employed as comprehending 

 these three different phenomena we find some use in the term, 

 not by its explaining or rendering more intelligible the modus 

 agendi of matter, but as conveying to the mind something 

 which is alike in the three phenomena, however distinct they 

 may be in other respects : the word becomes an abstract or 

 generalised expression, and regarded in this light is of high 

 utility. Although I have given only three examples, it is 

 obvious that the term would equally apply to 300 or 3,000 ex- 

 amples. 



But it will be said, the term force is used not as express- 

 ing the effect, but as that which produces the effect. This is 



