60 PROFESSOR W- THOMSON ON THE POSSIBLE DENSITY OF THE 



of any volume of the luminiferous ether, we have for the mechanical value of the 

 disturbance in the same space, 



W 2 



_ V 2 , 



9 



where g is the number 32*2, measuring in absolute units of force, the force of 



83 

 gravity on a pound. Now we found above, from observation, ~ for the mechani- 

 cal value, in foot-pounds, of a cubic foot of sunlight ; and therefore the mass, in 

 pounds, of a cubic foot of the ether, must be given by the equation, 



„ r 32-2 x 83 



If we assume v - — V, this becomes 



W = 



n 

 32-2 x 83 , 32-2 x 83 



~V^ (192000 x5280) 3 3899 x 10 20 ; 



and for the mass, in pounds, of a cubic mile we have 



32-2 x 83 2 n 2 



v n = 



(192000) 3 2649 x 10 9 • 



It is quite impossible to fix a definite limit to the ratio which v may bear to V ; 

 but it appears improbable that it could be more, for instance, than ^, for any kind 

 of light following the observed laws. We may conclude that probably a cubic 

 foot of the luminiferous medium in the space traversed by the earth contains not 

 less than 156Q x xqu of a pound of matter, and a cubic mile not less than 



1060 x 10 6 ' 



If the mean velocity of the vibrations of light within a spherical surface con- 

 centric with the sun and passing through the earth were equal to the earth's 

 velocity — a very tolerable supposition — since this is^^o of the velocity of light, the 

 whole mass of the luminiferous medium within that space would be 35509 of the 

 earth's mass, since the mechanical value of the light within it, being as much as 

 the sun radiates in about 8 minutes, is about jg^o of the mechanical value of the 

 earth's motion. As the mean velocity of the vibrations might be many times 

 greater than has been supposed in this case, the mass of the medium might be con- 

 siderably less than this ; but we may be sure it is not incomparably less, not 

 100,000 times as small for instance. On the other hand, it is worth remarking 

 that the preceding estimate shows that what we know of the mechanical value 

 of light renders it in no way probable that the masses of luminiferous medium in 

 interplanetary spaces, or all round the sun in volumes of which the linear dimen- 

 sions are comparable with the dimensions of the planets' orbits, are otherwise 

 than excessively small in comparison with the masses of the planets. 



But it is also worth observing, that the luminiferous medium is enormously 

 denser than the continuation of the terrestrial atmosphere would be in interplane- 



