L/uminiferous Medium, and the Mechanical value of Sunlight. 39 



mechanical value, in actual and potential energy, of the disturb- 

 ance in a certain space traversed by it ; and from all we know of 

 the mechanical theory of undulations, it seems certain that this 

 velocity must be a very small fraction of the velocity of propa- 

 gation in the most intense light or radiant heat which is propa- 

 gated according to known laws. Denoting this velocity for the 

 case of sunlight at the earth's distance from the sun by v, and 

 calling W the mass in pounds of any volume of the luminiferous 

 aether, we have for the mechanical value of the disturbance in the 

 same space, 



W 2 



— tr, 



9 



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



the foi'ce of gravity on a pound. Now we found above, from ob- 



83 

 servation, ~ for the mechanical value, in foot-pounds, of a cubic 



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

 foot of the aether, must be given by the equation, 



32-2 x 83 



W = - 



v*V 



If we assume v = — V, this becomes 

 n 



_ 32-2x83 , 32-2x83 



W= — ™ xr = 



V 3 * (192000 x 5280) 3 3899 x 10 20 ' 



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

 32-2 x 83 2 _ » 2 

 (192000) 3 * n ~2649xl0 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 j^, for any kind of light following the observed 

 laws. We may conclude that probably a cubic foot of the lumi- 

 niferous medium in the space traversed by the earth contains not 



less than , _„,. — tt-p= of a pound of matter, and a cubic mile not 

 looO x 10 17 r 



less than lQC J xW - 



If the mean velocity of the vibrations of light within a sphe- 

 rical surface concentric with the sun and passing through the 

 earth were equal to the earth's velocity — a very tolerable suppo- 

 sition — since this is yn-fnj °f the velocity of light, the whole 

 mass of the luminiferous medium within that space would be 

 JoffUC °f tnc cal '' u ' s " i:iss > s iiice the mechanical value of the 

 fight within it, being as much as the sun radiates in about 



