RELATIVITY OF ACCELERATION 131 



injure anyone, even when it is (according to the New- 

 tonian view) an absolute acceleration. We do not even 

 feel the change of motion as our earth takes the curve 

 round the sun. We feel something when a railway train 

 takes a curve, but what we feel is not the change of 

 motion nor anything which invariably accompanies 

 change of motion; it is something incidental to the 

 curved track of the train but not to the curved track of 

 the earth. The cause of injury in the railway accident 

 is easily traced. Something hit the train; that is to say, 

 the train was bombarded by a swarm of molecules and 

 the bombardment spread all the way along it. The 

 cause is evident — gross, material, absolute — recognised 

 by everyone, no matter what his frame of reference, 

 as occurring in the train not the station. Besides injur- 

 ing the passengers this cause also produced the relative 

 acceleration of the train and station — an effect which 

 might equally well have been produced by molecular 

 bombardment of the station, though in this case it was 



not 



The critical reader will probably pursue his objection. 

 "Are you not being paradoxical when you say that a 

 molecular bombardment of the train can cause an accel- 

 eration of the station — and in fact of the earth and the 

 rest of the universe? To put it mildly, relative accelera- 

 tion is a relation with two ends to it, and we may at 

 first seem to have an option which end we shall grasp 

 it by; but in this case the causation (molecular bom- 

 bardment) clearly indicates the right end to take hold 

 of, and you are merely spinning paradoxes when you 

 insist on your liberty to take hold of the other." 



If there is an absurdity in taking hold of the wrong 

 end of the relation it has passed into our current 

 speech and thought. Your suggestion is in fact more 



