EXPERIMENTS ON ETHER-DRIFT 



By A. O. RANKINE, B.Sc. 



Assistant in the Department of Physics, University College, London 



There are few branches of experimental inquiry which have so 

 uniformly failed to disclose definite information as that which 

 has aimed at determining, by wholly terrestrial methods, the 

 motion of the earth through space. It is, of course, possible, 

 by means of astronomical observations, to find out how the 

 earth moves relatively to the sun or other celestial bodies ; 

 but there is no reason for regarding any one of these, more 

 than another, as fixed. No meaning can be attached to numbers 

 specifying the movements of a body, unless the system with 

 reference to which measurements are made is also known. 

 The selection of the particular system of reference is a matter 

 of choice ; nevertheless, when considering stellar and planetary 

 motion, it is surely desirable to choose something less arbitrary 

 than the so-called fixed stars. It has become customary to 

 regard the ether as a suitable and most natural standard 

 structure relatively to which all motion of matter should be 

 defined. The nature of the ether is at present, in the main, 

 hypothetical. It is postulated in order to account for the 

 propagation of light and other electro-magnetic waves through 

 space ; but of its actual structure and the exact method by 

 which these effects are carried through it, nothing is known 

 with certainty. It is sometimes convenient to think of it as 

 a frictionless fluid filling all space, or, at other times, to suppose 

 it possessing the properties of a perfectly elastic solid. 



These assumptions may, or may not, be true. If its mere 

 existence be admitted, however, it may be described as " at 

 rest," provided that there is no relative movement between the 

 parts of which it consists. This may not, indeed, be the case. 

 There is considerable evidence of modification of ether in 

 matter, and possibly in the near neighbourhood of matter. 

 Such modification may amount to a difference of velocity 

 between the parts of the ether affected by the presence of 



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