MEASURE OF THE SUN'S HEAT. 265 



C. in temperature, a cubic mile of heat. Since one cubic 

 mile of water weighs 408*54 billions of kilogrammes, a cubic 

 mile of heat contains 408*54 billions of units of heat. The 

 effect produced by the rays of the sun on the surface of the 

 earth in one minute is therefore 5*5 cubic miles of heat. 



Let us imagine the sun to be surrounded by a hollow 

 sphere whose radius is equal to the mean distance of the 

 earth from the sun, or 20,589,000 geographical miles ; the 

 surface of this sphere would be equal to 5326 billions of 

 square miles. The surface obtained by the intersection of 

 this hollow sphere and our globe, or the base of the cone of 

 solar light which reaches our earth, stands to the whole sur- 

 face of this hollow sphere as 9 -^^ : 5326 billions, or as 1 to 

 2300 millions. This is the ratio of the heat received by 

 our globe to the whole amount of heat sent forth from the 

 sun, which latter in one minute amounts to 12,650 millions 

 of cubic miles of heat. 



This amazing radiation ought, unless the loss is by some 

 means made good, to cool considerably even a body of the 

 magnitude of the sun. 



If we assume the sun to be endowed with the same capa- 

 city for heat as a mass of water of the same volume, and its 

 loss of heat by radiation to affect uniformly its whole mass, 

 the temperature of the sun ought to decrease 1'8 C. yearly, 

 and for the historic time of 5000 years this loss would conse- 

 quently amount to 9000 C. 



A uniform cooling of the whole of the sun's huge mass 

 cannot, however, take place ; on the contrary, if the radiation 

 were to occur at the expense of a given store of heat or ra- 

 diant power, the sun would become covered in a short space 

 of time with a cold crust, whereby radiation would be brought 

 to an end. Considering the continued activity of the sun 

 through countless centuries, we may assume with mathemati- 

 cal certainty the existence of some compensating influence to 

 make good its enormous loss. 

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