390 POPULAR LECTURES AND ADDRESSES. 



of the 78,000 horse-power which we have taken, 

 and diminishes each of our times in the ratio 

 of i to 17. Thus, instead of Helmholtz's 

 twenty million years, which was founded on 

 Pouillet's estimate, we have only twelve millions, 

 and similarly with all our other time reckonings 

 based on Pouillet's results. In the circumstances, 

 and taking fully into account all possibilities 

 of greater density in the sun's interior, and of 

 greater or less activity of radiation in past ages, 

 it would, I think, be exceedingly rash to assume 

 as probable anything more than twenty million 

 years of the sun's light in the past history of 

 the earth, or to reckon on more than five or 

 six million years of sunlight for time to come. 



We have seen that the sun draws on no 

 external source for the heat he radiates out 

 from year to year, and that the whole energy 

 of this heat is due to the mutual attraction 

 between his parts acting in conformity with the 

 Newtonian law of gravitation. We have seen 

 how an ideal mechanism, easily imagined and 

 understood, though infinitely far from possibility 



