FORCE 681 



We then find that the motion of its centre of gravity is nearly uni- 

 form and rectilinear in conformity with Newton's Third Law. This 

 is an experimental fact, which cannot be invalidated by a more accu- 

 rate experiment. What, in fact, would a more accurate experiment 

 teach us? It would teach us that the law is only approximately true, 

 and we know that already. Thus is explained how experiment may 

 serve as a basis for the principles of mechanics, and yet will never 

 invalidate them. 



Anthropomorphic Mechanics. It will be said that Kirchoff has 

 only followed the general tendency of mathematicians towards nom- 

 inalism; from this his skill as a physicist has not saved him. He 

 wanted a definition of a force, and he took the first that came handy; 

 but we do not require a definition of force ; the idea of force is primi- 

 tive, irreducible, indefinable; we all know what it is; of it we have 

 direct intuition. This direct intuition arises from the idea of effort 

 which is familiar to us from childhood. But in the first place, even 

 if this direct intuition made known to us the real nature of force in 

 itself, it would prove to be an insufficient basis for mechanics; it 

 would, moreover, be quite useless. The important thing is not to 

 know what force is, but how to measure it. Everything which does 

 not teach UP how to measure it is as useless to the mechanician as, 

 for instance, the subjective idea of heat and cold to the student of 

 heat. This subjective idea cannot be translated into numbers, and is 

 therefore useless: a scientist whose skin is an absolutely bad con- 

 ductor of heat, and who, therefore, has never felt the sensation of 

 heat or cold, would read a thermometer in just the same way as any 

 one else, and would have enough material to construct the whole of 

 the theory of heat. 



Now this immediate notion of effort is of no use to us in the 

 measurement of force. It is clear, for example, that I shall experi- 

 ence more fatigue in lifting a weight of 100 Ib. than a man who is 

 accustomed to lifting heavy burdens. But there is more than this. 

 This notion of effort does not teach us the nature of force; it is 

 definitely reduced to a recollection of muscular sensations, and no 

 one will maintain that the sun experiences a muscular sensation when 

 it attracts the earth. All that we can expect to find from it is a sym- 

 bol, less precise and less convenient than the arrows (to denote 

 direction) used by geometers, and quite as remote from reality. 



Anthropomorphism plays a considerable historic role in the genesis 

 of mechanics; perhaps it may yet furnish us with a symbol which 

 some minds may find convenient; but it can be the foundation of 

 nothing of a really scientific or philosophical character. 



The Thread School. M. Andrade, in his Legons de Mecanique 

 physique, has modernized anthropomorphic mechanics. To the school 



