388 POPULAR LECTURES AND ADDRESSES. 



hardly likely that the sun can continue to 

 give sufficient heat to support life on the 

 earth (such life as we now are accquainted 

 with at least) for ten million years from the 

 present time." 



In all our calculations hitherto we have 

 for simplicity taken the density as uniform 

 throughout, and equal to the true mean 

 density of the sun, being about 1-4 times the 

 density of water, or about a quarter of the earth's 

 mean density. In reality the density in the 

 upper parts of the sun's mass must be something 

 less than this, and something considerably more 

 than this in the central parts, because of the 

 pressure in the interior increasing to some- 

 thing enormously great at the centre. If we 

 knew the distribution of interior density we 

 could easily modify our calculations accordingly ; 

 but it does not seem probable that the correction 

 could, with any probable assumption as to the 

 greatness of the density throughout a considerable 

 proportion of the sun's interior, add more than a 

 few million years to the past of solar heat, and 



