22 SCIENCE PROGRESS 



material bodies and the parts not so affected. If relative 

 motion of this kind exists, it is obviously impossible still to 

 take the ether, as a whole, as our standard system of reference. 

 A particular part of it must then be selected. Now, it is 

 extremely unlikely that those parts of the ether which are 

 very remote from all material bodies are in any way disturbed by 

 them, and they can be conveniently defined as stationary. With 

 respect to this free ether, the earth will, in general, be moving. 

 The possibility of determining its motion by terrestrial observa- 

 tions alone will, however, depend on whether the ether in the 

 near neighbourhood of the earth is free or not. For suppose 

 that it is not free — that is, that it is partially, if not wholly, 

 dragged with the earth in its course — then all that could possibly 

 be measured on the earth itself would be the relative motion 

 between it and the neighbouring ether; and this would afford 

 no information as to what may be called its " absolute " motion, 

 i.e. its motion relative to free ether. If, on the other hand, 

 the ether is not modified near the earth and drifts past it as it 

 moves onward, it at first sight seems likely that the velocity 

 of the earth in free ether could be determined without the aid 

 of astronomy. 



The experiments which form the subject of this paper are 

 based upon the assumption that the latter is the case, in the 

 hope that they would afford a means of discovering the resultant 

 motion of the earth through space, and, incidentally, of testing 

 the accuracy of the astronomical estimate. The justification for 

 these attempts is principally based on the well-known phenome- 

 non of the aberration of the stars, first observed by Bradley 

 about 1725. Briefly stated, it is that the stars appear to occupy 

 positions in the heavens different from the calculated positions 

 at various times of the year. Stars on the ecliptic appear to 

 describe straight lines, those at the pole of the ecliptic small 

 circles, and intermediate stars small ellipses about their true 

 positions. These apparent movements can be explained very 

 simply indeed, if it be assumed that the ether near the earth 

 is perfectly free, but no satisfactory explanation has yet been 

 offered accounting for aberration on the supposition that the 

 ethereal medium is dragged either partially or wholly by the 

 earth in its neighbourhood. Stellar aberration may be taken, 

 therefore, as at least suggesting that the ether drifts freely 

 past the earth. It is true that an equally good explanation 



