58 Presidential Address 



absolute (in the only sense in which the 

 term can have a physical meaning) with 

 which we can compare our own motion, 

 when we have learnt how. 



But in ordinary astronomical transla- 

 tion translation as of the earth in 

 its orbit all our instruments, all our 

 standards, the whole contents of our 

 laboratory, are moving at the same rate 

 in the same direction ; under those condi- 

 tions we cannot expect to observe any- 

 thing. Clerk Maxwell went so far as 

 to say that if every particle of matter 

 simultaneously received a graduated 

 blow so as to produce a given constant 

 acceleration all in the same direction, we 

 should be unaware of the fact. He did 

 not then know all that we know about 

 radiation. But apart from that, and 

 limiting ourselves to comparatively slow 

 changes of velocity, our standards will 

 inevitably share whatever change occurs. 

 So far as observation goes, everything 

 will be practically as if no change had 

 occurred at all ; though that may not be 

 the truth. All that experiment estab- 



