i 3 o GRAVITATION— THE LAW 



Newtonian conception which singled out 32 feet per 

 second per second as the true acceleration and invented 

 a disturbing agent of this particular degree of strength. 



It will be instructive to consider an objection brought, 

 I think, originally by Lenard. A train is passing through 

 a station at 60 miles an hour. Since velocity is relative, 

 it does not matter whether we say that the train is 

 moving at 60 miles an hour past the station or the 

 station is moving at 60 miles an hour past the train. 

 Now suppose, as sometimes happens in railway acci- 

 dents, that this motion is brought to a standstill in a 

 few seconds. There has been a change of velocity or 

 acceleration — a term which includes deceleration. If 

 acceleration is relative this may be described indiffer- 

 ently as an acceleration of the train (relative to the sta- 

 tion) or an acceleration of the station (relative to the 

 train). Why then does it injure the persons in the train 

 and not those in the station? 



Much the same point was put to me by one of my 

 audience. "You must find the journey between Cam- 

 bridge and Edinburgh very tiring. I can understand 

 the fatigue, if you travel to Edinburgh; but why should 

 you get tired if Edinburgh comes to you?" The answer 

 is that the fatigue arises from being shut up in a box 

 and jolted about for nine hours; and it makes no differ- 

 ence whether in the meantime I move to Edinburgh or 

 Edinburgh moves to me. Motion does not tire anybody. 

 With the earth as our vehicle we are travelling at 20 

 miles a second round the sun; the sun carries us at 12 

 miles a second through the galactic system; the galactic 

 system bears us at 250 miles a second amid the spiral 

 nebulae; the spiral nebulae. ... If motion could tire, 

 we ought to be dead tired. 



Similarly change of motion or acceleration does not 



