1880.] Mode of Detecting a Motion of the Solar System, 109 



a priori to the contrary, the motion might be very much greater than 

 what would correspond to this ; and the idea has a value of its own, 

 irrespective of the possibility of actually making the determination. 



In his letter to me Mr. Todd remarks, " I regard the communica- 

 tion as one of extraordinary importance, although (as you will notice 

 if you have access to the reply which I made) it is like to be a long 

 time before we shall have tables of the satellites of Jupiter suffi- 

 ciently accurate to put the matter to a practical test." 



I have not" thought it expedient to delay the publication of the 

 letter on the chance that something bearing on the subject might be 

 found among Maxwell's papers. 



G-. G-. Stokes. 



(Copy.) 



Cavendish Laboratory, 



Cambridge, 



19th March, 1879. 



SlK, 



I have received with much pleasure the tables of the satellites of 

 Jupiter which you have been so kind as to send me, and I am 

 encouraged by your interest in the Jovial system to ask you if you 

 have made any special study of the apparent retardation of the 

 eclipses as affected by the geocentric position of Jupiter. 



I am told that observations of this kind have been somewhat put 

 out of fashion by other methods of determining quantities related to 

 the velocity of light, but they afford the only method, so far as I 

 know, of getting any estimate of the direction and magnitude of the 

 velocity of the sun with respect to the luminiferous medium. Even 

 if we were sure of the theory of aberration, we can only get differences 

 of position of stars, and in the terrestrial methods of determining the 

 velocity of light, the light comes back along the same path again, so 

 that the velocity of the earth with respect to the ether would alter 

 the time of the double passage by a quantity depending on the square 

 of the ratio of the earth's velocity to that of light, and this is quite 

 too small to be observed. 



But if JE is the distance of Jupiter from the earth and I the 

 geocentric longitude, and if I' is the longitude and \ the latitude 

 of the direction in which the sun is moving through ether with 

 velocity v, and if Y is the velocity of light and t the time of transit 

 from J to E, 



JE=[Y-vcos\cos (Z-Z')] t. 



By a comparison of the values of t when Jupiter is in different 

 signs of the zodiac, it would be possible to determine V and v cos A. 



I do not see how to determine \, unless we had a planet with an 

 orbit very much inclined to the ecliptic. It may be noticed that 

 whereas the determination of Y, the velocity of light, by this method 



