ON THE AGE OF THE SUN'S HEAT. 351 



PART I. 



ON THE SECULAR COOLING OF THE SUN. 



How much the sun is actually cooled from 

 year to year, if at all, we have no means of 

 ascertaining, or scarcely even of estimating in 

 the roughest manner. In the first place we do 

 not know that he is losing heat at all. For it 

 is quite certain that some heat is generated in his 

 atmosphere by the influx of meteoric matter ; 

 and it is possible that the amount of heat so 

 generated from year to year is sufficient to com- 

 pensate the loss by radiation. It is, however, also 

 possible that the sun is now an incandescent liquid 

 mass, radiating away heat, either primitively created 

 in his substance, or, what seems far more probable, 

 generated by the falling in of meteors in past 

 times, with no sensible compensation by a con- 

 tinuance of meteoric action. 



It has been shown l that, if the former sup- 



1 "On the Mechanical Energies of the Solar System," Trans- 

 actions of the Royal Society of Edinburgh, April, 1854, and 



