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XLI. 1 lie Sun's Motion loitli respect to the yEther. 

 By C. V. Burton, D.Sc* 



1. QOME time ago, in a letter to ' Nature/ I ventured to 

 O point out that the Sun's velocity with respect to the 

 sether could be deduced from the observed times o£ the 

 eclipses of Jupiter's satellites. At that time it was unknown 

 to me that the following remark on the same subject had 

 been made by Maxwell t : — 



" The only practicable method of determining directly the 

 relative velocity of the aether with respect to the solar 

 system is to compare the values of the velocity of light 

 deduced from the observation of the eclipses of Jupiter's 

 satellites when Jupiter is seen from the earth at nearly 

 opposite points of the ecliptic." 

 If for brevity we speak of the motion of the aether with 

 respect to the solar system as a wind, the essence of the 

 matter might be expressed by saying that light from Jupiter 

 would reach us more quickly when that planet was to wind- 

 ward of us than when it was to leeward ; the wind-velocity 

 being in the former case added to, and in the latter case 

 subtracted from, the velocity of propagation. Maxwell's 

 reference to Jupiter as 4; seen from the earth at nearly 

 opposite points of the ecliptic " is evidently a mere slip. 

 The context makes it clear that the very simple essentials of 

 the problem had been fully realized. 



2. Though some thirty years have elapsed since Maxwell 

 wrote, attention does not seem to have been directed to the 

 subject in any effective degree ; yet from the standpoint of 

 electromagnetic theory, great interest attaches to any physi- 

 cally possible method of determining motion with respect to 

 the aether; while from an astronomical point of view the 

 results to be looked for are no less important. Unfortunately, 

 owing to the small inclination of Jupiter's orbit to the 

 ecliptic, we are virtually restricted to a problem in two 

 dimensions. The present would seem to be a favourable 

 time for directing the attention of astronomers and physicists 

 to these questions anew ; on the one hand because ProE. 

 Kapteyn's discovery of two principal star-drifts makes it 

 important to determine the motion of each drift with respect 

 to the aether, so far as we can ; and on the other hand because 

 the material needed for the computations will soon be avail- 

 able. The Harvard photometric observations of the eclipses 

 have been very fully discussed by Prof. R. A. Sampson, in 



* Communicated by the Physical Society : read November 26, 1909. 

 t Encyclopcedia Britannica, 9th ed., article "Ether." 



Phil. Mag. S. 6. Vol. 19. No. 111. March 1910. 2 E 



