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, 
Ww v2, 
g 
where g is the number 32:2, measuring in absolute units of force, the force of 
: 83 : 
gravity on apound. Now we found above, from observation, 5; 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, 
_ 82:2 x 83 
Wen reer 
1 : 
If we assume 0 = — V, this becomes 
654 83-2-x:8B. ge. fs BBP 8S pipe n2 
W = —yva— *™ = 792000x 5280)? *” = 3899 x 10%; 
and for the mass, in pounds, of a cubic mile we have 
32:2 x 83 n? 
(192000)? * w =9619 x10. 
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 36, 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 deo" of a pound of matter, and a cubic mile not less than 
1 
1060 x 108 ° 
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 zo}7p of the velocity of light, the 
whole mass of the luminiferous medium within that space would be 3gd99 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 zg459 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- 
