FOKCE 683 



definition in all cases, and to regard the experiments in question, not 

 as verifications of that law, but as verifications of the principle of 

 action and reaction, or as proving the deformations of an elastic body 

 depend only on the forces acting on that body? Without taking 

 into account the fact that the conditions in which your definition 

 could be accepted can only be very imperfectly fulfilled, that a thread 

 is never without mass, that it is never isolated from all other forces 

 than the reacticn of the bodies attached to its extremities. 



The ideas expounded by M. Andrade are none the less very interest- 

 ing. If they do not satisfy our logical requirements, they give us a 

 better view of the historical genesis of the fundamental ideas of 

 mechanics. The reflections they suggest show us how the human 

 mind passed from a naive anthropomorphism to the present concep- 

 tion of science. 



We see that we end with an experiment which is very particular, 

 and as a matter of fact very crude, and we start with a perfectly 

 general law, perfectly precise, the truth of which we regard as abso- 

 lute. We have, so to speak, freely conferred this certainty on it by 

 looking upon it as a convention. 



Are the laws of acceleration and of the composition of forces only 

 arbitrary conventions ? Conventions, yes ; arbitrary, no they would 

 be so if we lost sight of the experiments which led the founders of 

 the science to adopt them, and which, imperfect as they were, were 

 sufficient to justify their adoption. It is well from time to time to 

 let our attention dwell on the experimental origin of these conventions. 



Relative and Absolute Motion 



TJic Principle of Relative Motion. Sometimes endeavors have 

 been made to connect the law of acceleration with a more general 

 principle. The movement of any system whatever ought to obey the 

 same laws, whether it is referred to fixed axes or to the movable axes 

 which are implied in uniform motion in a straight line. This is 

 the principle of relative motion; it is imposed upon us for two rea- 

 sons: the commonest experiment confirms it; the consideration of the 

 contrary hypothesis is singularly repugnant to the mind. 



Let us admit it then, and consider a body under the action of a 

 force. The relative motion of this body with respect to an observer 

 moving with a uniform velocity equal to the initial velocity of the 

 body, should be identical with what would be its absolute motion if 

 it started from rest. We conclude that its acceleration must not de- 

 pend upon its absolute velocity, and from that we attempt to deduce 

 the complete law of acceleration. 



For a long time there have been traces of this proof in the regula- 

 tion? for the degree of B.Sc. It is clear that the attempt has 



