362 POPULAR LECTURES AND ADDRESSES. 



the sun radiates out heat from every square foot 

 of his surface, at only about 7,000 horse power. 1 

 Coal, burning at a rate of a little less than a 

 pound per two seconds, would generate the same 

 amount ; and it is estimated (Rankine, Prime 

 Movers, p. 285, Ed. 1852) that, in the furnaces of 

 locomotive engines, coal burns at from one pound 

 in thirty seconds to one pound in ninety seconds, 

 per square foot of grate-bars. Hence heat is 

 radiated from the sun at a rate not more than 

 from fifteen to forty-five times as high as that at 

 which heat is generated on the grate-bars of a 

 locomotive furnace, per equal areas. 



The interior temperature of the sun is probably 

 far higher than that at his surface, because direct 

 conduction can play no sensible part in the trans- 

 ference of heat between the inner and outer 

 portions of his mass, and there must be an 



1 One horse power in mechanics is a technical expression (follow- 

 ing Watt's estimate), used to denote a rate of working in which 

 energy is involved at the rate of 33,000 foot pounds per minute. 

 This, according to Joule's determination of the dynamical value of 

 heat, would, if spent wholly in heat, be sufficient to raise the 

 temperature of 23! Ibs. of water by i Cent, per minute. 



