VI.—WNote on the Possible Density of the Luminiferous Medium and on the Mechani- 
cal Value of a Cubic Mile of Sunlight. By Professor WitLiamM THomson. 
(Read Ist May 1854.) 
That there must be a medium forming a continuous material communication 
throughout space to the remotest visible body is a fundamental assumption in 
the undulatory Theory of Light. Whether or not this medium is (as appears to me 
most probable) a continuation of our own atmosphere, its existence is a fact that 
cannot be questioned, when the overwhelming evidence in favour of the undula- 
tory theory is considered ; and the investigation of its properties in every possible 
way becomes an object of the greatest interest. A first question would naturally 
occur, What is the absolute density of the luminiferous ether in any part of space ? 
Iam not aware of any attempt having hitherto been made to answer this ques- 
tion, and the present state of science does not in fact afford sufficient data. It 
has, however, occurred to me that we may assign an inferior limit to the density 
of the luminiferous medium in interplanetary space by considering the mechani- 
cal value of sunlight as deduced in preceding communications to the Royal 
Society from PovitLet’s data on solar radiation, and Joute’s mechanical equiva- 
— lent of the thermal unit. Thus the value of solar radiation per second per square 
foot at the earth’s distance from the sun, estimated at -06 of a thermal unit 
centigrade, or 83 foot-pounds, is the same as the mechanical value of sunlight in 
the luminiferous medium through a space of as many cubic feet as the number 
of linear feet of propagation of light per second. Hence the mechanical value of 
the whole energy, actual and potential, of the disturbance kept. up in the space 
: 83 819 og 
192000 x 5280, °* 07 © 
a foot-pound. The mechanical value of a cubic mile of sunlight is consequently 
12050 foot-pounds, equivalent to the work of one-horse power for a third of a 
minute. This result may give some idea of the actual amount of mechanical 
energy of the luminiferous motions and forces within our own atmosphere. 
of a cubic foot at the earth’s distance from the sun,* i 
* The mechanical value of sunlight in any space near the sun’s surface must be greater than in 
an equal space at the earth’s distance, in the ratio of the square of the earth’s distance to the square 
of the sun’s radius, that is, in the ratio of 46,400 to 1 nearly. The mechanical value of a cubic foot of 
sunlight near the sun must, therefore, be about ‘0038 of a foot-pound, and that of a cubic mile 
560,000,000 foot-pounds. 
VOL XXI. PART I. Q 
