18(3 Mr. N. Campbell on the JEther. 



of significance. If a man tells me that his watch weighs 

 100 grammes, his statement is for me a significant proposi- 

 tion, because the ordinary definition of " weight " can be 

 applied to a watch ; but if he tells me that the colour of his 

 watch weighs 100 grammes, and refuses to tell me how a 

 colour is to be weighed, I can only conclude that he is 

 uttering meaningless nonsense, or, if this explanation should 

 be excluded by the fact that be is a learned professor, that he 

 means to inform me that, for some reason which may be 

 quite satisfactory, he wishes me to understand " the colour of 

 his watch" when he says " that which weighs 100 grammes." 



Accordingly, wdien one who rejects the principle of relativity, 

 writes down Maxwell's equations, or the simple deduction 

 from them given above, without stating distinctly what is the 

 relative velocity between axes " fixed in the sether," and 

 some material system (relative to which other velocities can 

 be measured), the only meaning which he can convey is that 

 he proposes to call by the term " velocity u relative to the 

 aether." the state of motion of a body bearing a charge e 

 when its magnetic effect measured by any observer is 

 equivalent to a current element of strength eu, More- 

 over, it follows that, if he deduces propositions from his 

 fundamental hypotheses and compares the result with experi- 

 ment, the only valid information which he can attain by 

 his endeavours is with what velocity relative to the aether 

 (according to his definition) some one or more of the bodies 

 which he observes is moving. He cannot possibly confirm 

 or refute any assumptions which he has made in forming his 

 hypotheses. He is in the position of a mathematician treat- 

 ing equations in which there are one or more unknown 

 variables. The most that he can do is to find the values of 

 those variables ; he cannot attain to an identity or non- 

 identity, which will prove that his original equations were 

 either true or false. 



§ 8. It may be suggested that I have overlooked an alter- 

 native meaning of "velocity" which can be defined inde- 

 pendently of the propositions of electromagnetism. There is 

 a quantity termed " absolute velocity " introduced by 

 dynamics, and it may be thought that it is possible to assert 

 that the velocity of a charged body relatively to the aether is 

 its " absolute velocity." Such an assertion is possible and 

 would remove the objections raised in the last paragraph, but 

 it raises far more serious difficulties. For, as is shown in 

 the paper on the " Principles of Dynamics " in this number 

 of the Magazine, "absolute velocity" (or rather Absolute 

 Velocity) is meaningless unless the fundamental propositions 





