254 



mile at the earth's distance being 83 x 6280 x 5280 foot pounds ; 



the mechanical value of all the energy potential and actual, kept up in 



^ , . ., , ,. , ... 83 X 5280 X 5280 

 the space of a cubic mile by sunlight crossing it, is ^ 



= 1200 ft. lbs. at the earth's distance. Similarly the mechani- 

 cal value of a cubic mile of sunlight near the sun is found to be 

 1200 X 46,000 = 55,000,000 foot pounds. 



If A be the excursion on each side of its position of equilibrium, 

 which any particle would have if the mean effect of the solar radia- 

 tion at the earth's distance were produced by plane polarised vibra- 

 tions of wave length X, the mass of a cubic foot of the luminiferous 

 -medium, in pounds, is shown to be 



g X 83_ / X Y _ A / 



2^ X V^ V A / "" 77 X 10^3 



where g is the number 32-2, measuring the force of gravity, and V 

 the velocity of light in feet per second. Similarly, if A and X relate 

 to sunlight near the sun, the mass of a cubic foot of the vibrating 

 medium in that locality is found to be 



iil 



166x1018 



The possibility of great variation in density of the luminiferous medium 

 at different distances from the sun, depending on solar gravitation, and 

 heat, and centrifugal force of the vortices kept up in it by planetary 

 and meteoric motions, is indicated ; and it is suggested that a re- 

 fraction of this inter-planetary atmosphere may produce annual ap- 

 parent motions in the stars, which may be sensible, although not yet 

 discovered. — As to the preceding expressions for the density of the 



vibrating medium, all that is known of the values of — r- is that they 



A. 



must in all probability be large. If nothing less than 100 be ad- 

 missible, the mass of a eubic foot of the vibrating medium at the 

 earth's distance could not be less than 



1 lb. 



77 X 1019 



