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V. Note on the Possible Density of the Luminiferous Medium 

 and on the Mechanical Value of a Cubic Mile of Sunlight. 

 By Professor William Thomson*. 



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 evi- 

 dence in favour of the undulatory 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 aether in 

 any part of space ? I am not aware of any attempt having 

 hitherto been made to answer this question, 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 mechanical value of sunlight as deduced in 

 preceding communications to the Royal Society from Pouillet's 

 data on solar radiation, and Joule's mechanical equivalent 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 me- 

 chanical value of the whole energy, actual and potential, of the 

 disturbance kept up in the space of a cubic foot at the earth's 



CO ,Q1 Q 



distance from the sunf, is 1920Q0x528(y or -^ of 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 lumini- 

 ferous motions and forces within our own atmosphere. Merely 

 to commence the illumination of thi'ee cubic miles, requires an 



* From the Transactions of the Royal Society of Edinburgh, vol. xxi. 

 part 1 ; read May 1, 1854. 



f 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. 



