356 POPULAR LECTURES AND ADDRESSES. 



stance is less, and very certain that it cannot be 

 much greater, than that of water. If it were equal 

 to the specific heat of water we should only have 

 to divide the preceding number (6 x io 30 ), derived 

 from Herschel's and Pouillet's observations, by the 

 number of pounds (4*3 x io 30 ) in the sun's mass, 

 to find i*4 Cent, for the present annual rate of 

 cooling. It might therefore seem probable that 

 the sun cools more, and almost certain that he 

 does not cool less, than a centigrade degree and 

 four-tenths annually. But, if this estimate were 

 well-founded, it would be equally just to assume 

 that the sun's expansibility 1 with heat does not 

 differ greatly from that of some average terrestrial 



" mean specific heat " of the sun, in the text, signifies the total 

 amount of heat actually radiated away from the sun, divided by his 

 mass, during any time in which the average temperature of his 

 mass sinks by i, whatever physical or chemical changes any part 

 of his substance may experience. 



1 The "expansibility in volume," or the "cubical expansibility," 

 of a body, is an expression technically used to denote the proportion 

 which the increase or diminution of its bulk, accompanying a rise or 

 fall of i in its temperature, bears to its whole bulk at some stated 

 temperature. The expression, "the sun's expansibility," used in 

 the text, may be taken as signifying the ratio which the actual 

 contraction, during a lowering of his mean temperature by i Cent., 

 bears to his present volume. 



