THE ETHER OF SPACE 343 



is what we mean by absolute motion, as distinguished from rela- 

 tive motion with reference to masses of matter. Such relative 

 motion is not all that we can ever know. Motion with respect to 

 a universal medium is not meaningless and is not as undis- 

 coverable as you imagine. It has not been discovered but we 

 have by no means abandoned the attempt. 



Besides, surely you admit some meaning in absolute motion 

 beyond your present arbitrary definition ! For what did Coper- 

 nicus and Galileo contend? Does it after all matter nothing 

 whether the earth is spinning on its axis or whether the sun, 

 moon and planets and all the hosts of heaven are revolving 

 round it in twenty-four hours ? On the principle of relativity 

 it makes no difference, no practical difference, nothing that can 

 be detected by experiment. So long as their relative motions 

 are all that can be perceived, it will be said, it is plain that 

 of their absolute motions we know and can know nothing. 

 Whether Arcturus revolves round the earth or whether the 

 earth rotates is merely a matter of convenience of expression. 



If there be an Ether, we grant you that Arcturus could not 

 have such a speed as would be needed ; but in absolute void, 

 what is to prevent it from going at an infinite speed — or rather 

 what meaning can be attached to speed at all in such a case ? 



What have we to oppose to such a contention, beyond 

 popular prejudice and scientific or perhaps unscientific instinct? 



Well, we might urge the theory of electromagnetism. A 

 charge in motion is an electric current. The earth is charged 

 so that if in motion its line of advance is surrounded by 

 magnetic lines of force and essentially every electron on it is 

 also a linear current. With the speed of the earth estimated 

 as twenty or thirty miles a second the current is insignificantly 

 weak. Currents due to charged bodies moving with the earth 

 have never been observed. But if the earth had a speed such 

 as that we just now postulated as thinkable for Arcturus or let 

 us say for the famous Nova of 1901, what kind of current 

 would it equal then ? Three hundred light years multiplied 

 by 27r and divided by a day gives a speed 1800 x 365 times 

 that of light or 2 x io 16 centimetres per second. If the earth 

 had a speed like that, a charged pith ball might be a current 

 of a million amperes. 



The anticipated reply is that we should never know. We 

 should be moving with it and no effect due to its motion could 



