ENERGY AND ITS TRANSFORMATIONS. 



33 



and nutrition into available form for use by the nervous 

 and muscular systems of the body, by means of inter- 

 mediate processes which are as yet obscure. It is only 

 certain that they cannot all be thermodynamic pro- 

 cesses ; it seems probable that they are, in some cases, 

 at least, related to the electrical methods of transforma- 

 tion. In some instances, as in the carnivora, the final 

 conversion results from a double transformation. 



Thus substantially all utilized natural energy is de- 

 rived, directly or indirectly, from the sun. 



According to Sir William Thomson, "the mechanical 

 value of a cubic mile of sunlight is 12,050 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. 

 Merely to commence the illumination of three cubic 

 miles requires an amount of work equal to that of a 

 horse-power for a minute ; the same amount of energy 

 exists in that space as long as light continues to traverse 

 it ; and, if the source of hght be suddenly stopped, 

 must be emitted from it before the illumination 

 ceases." * 



The same authority says : " Taking the estimate 

 2781 thermal units centigrade, or 3,869,000 foot-pounds, 



* The mechanical value of sunlight in any space near the sun's sur- 

 face 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 i, 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. 

 Similarly we find 15,000 horse-power for a minute as the amount of 

 work required to generate the energy existing in a cubic mile of light 

 near the sun. — Thomson. 



