Mr. N. Campbell on the JEtlier. 191 



Velocity of sach a particle by means of relation A. The 

 " velocity of the settler/' defined by those who reject this 

 possibility, must be meaningless without assuming the truth 

 of the first theory in which it is mentioned (Maxwell's 

 equations), just as the quantity " b" is meaningless without 

 assuming the truth of Van der Waals's equation. 



On solving the equations by which the " velocity of the 

 aether" is defined, it is found that different values for this velo- 

 city for any particle are found in different cases — a conclusion 

 which shows that this " velocity " has properties different 

 from those of Relative Velocity. It would be analogous if 

 the quantity " b " were found to be negative or imaginary, 

 showing that " b *' has different properties from those attri- 

 buted to a Volume by definition. In the last case two alter- 

 natives would be open : the conclusion might be accepted, 

 or a new theory might be stated which would lead to a 

 different conclusion. In the case of the "aether/* all are 

 agreed that the conclusion is to be rejected and a new theory 

 stated. Adherents to the principle of relativity point out 

 that a new theory can be stated which avoids all necessity 

 for any such quantity as "velocity of the aether "; it can be 

 stated in terms of quantities which are related to measure- 

 ments by relation A alone. The " aetherialists," on the other 

 hand, propose a new theory which introduces again a quantity 

 of the same nature as before, but avoid the possibility of 'the 

 occurrence of fresh undesirable conclusions about it by stating 

 the theory so that the value of the quantity cannot be found 

 by any experiment that is ever likely to be performed. 



My contention is that the former procedure is the more 

 satisfactory, and to what I have said I will only add one 

 argument derived from the analogy of dynamics. I imagine 

 that physicists would agree that if dynamics could be stated 

 in terms of Relative Motion only, without complicating the 

 equations so that they would be unamenable to mathematical 

 treatment, that course should certainly be adopted. tC Absolute 

 Motion" is a disagreeable necessity forced on us by the in- 

 sufficiency of our powers of mathematical treatment. The 

 case against " velocity of the aether " is stronger than that 

 against Absolute Motion, for we can find the value of Absolute 

 Velocity by assuming the truth of the equations by which it 

 is defined, and we cannot find the value of " velocity of the 

 aether," even by assuming the truth of those equations. On 

 the other hand, there is no argument in favour of " velocity 

 of the aether " derived from the necessities of mathematics, 

 because the equations based on the principle of relativity are 

 just as simple as those based on the conception of the aether. 

 September 1909. 



