184 Mr. N. Campbell on the SEtlier. 



proviso in the proposition that the axes of reference are 

 " fixed in the sether/' The statement that the gether is 

 " the body ..." undoubtedly suggests, and has been com- 

 monly taken to mean, that the sether, in so far as the relative 

 motion of its parts is concerned, resembles a block of some 

 solid material : that, except so far as it is disturbed by the 

 vibrations which it transmits, its parts have no relative 

 motion : that the motion of a body relative to the aether is 

 uniquely determined and is, in general, unrelated to the 

 motion of that body relative to any material system. Until 

 quite lately it seems to have been assumed almost uni- 

 versally that the velocity to which the magnetic effect of 

 a moving charge is proportional is not its velocity relative 

 to some material system, but to some system independent 

 of all material bodies, extended throughout the universe 

 and having no relative motion between its parts. That such 

 a proposition is dubitable will not be disputed when it is 

 stated explicitly. My present object is to show that it is so 

 far from being even inherently probable that it would never 

 have been accepted for a moment, if it had not been for the 

 unfortunate invention of so attractive a word as " the settier." 

 It seems to me certain that if "the aether" had been replaced 

 by a word in the plural number, or if to the definition 

 offered above the words " or bodies " had been added, one of 

 the most difficult problems of modern physics would never 

 have been presented. 



§5. Axes "fixed in the aether" involve the idea of the 

 motion of a material system relative to the ?ether, or con- 

 versely, of the motion of the aether relative to a material 

 body. Let us inquire what can be meant by such a velocity 

 of the aether. When we speak of the velocity of a material 

 body A relative to a body B, one of two definitions of the word 

 " velocity w is implied, according as the bodies are solid or 

 fluid. In the former case the velocity is the rate of change 

 of the distance of a point on A, identified by some property 

 distinguishing it from neighbouring points, from a point on 

 B similarly identified * ; in the latter case velocity means the 

 rate of transference of the body (measured by volume) across 

 unit cross-section. It will probably be admitted that the latter 

 definition (which is connected with the former and funda- 

 mental definition only by our belief in quasi-solid molecules) 

 is not relevant in the case of the aether,, but the former might 

 seem to be applicable. Consider the simple case of two or 

 more electrically charged bodies moving with different 

 uniform velocities relative to some observer. Round each 



* See note at the end of the paper. 



