14 THE AGE OF THE EARTH 



of a sun. Such accessions may have followed from the 

 convergence of a stream of meteors. Again, it is possible 

 that the radiation of the sun may have been diminished 

 and his energy conserved by a solar atmosphere. 



Newcomb has objected to these two possible modes by 

 which the life of the sun may have been greatly lengthened, 

 that a lessening of the sun's heat by under a quarter 

 would cause all the water on the earth to freeze, while 

 an increase of much over half would probably boil it all 

 into steam. But such changes in the amount of radiation 

 received would follow from a greater distance from the 

 sun of 15I per cent., and a greater proximity to him of 

 18-4 per cent., respectively. Venus is inside the latter 

 limit, and Mars outside the former, and yet it would be 

 a very large assumption to conclude that all the water in 

 the former is steam, and all in the latter ice. Indeed, the 

 existence of water and the melting of snow on Mars are 

 considered to be thoroughly well authenticated. It is 

 further possible that in a time of lessened solar radiation 

 the earth may have possessed an atmosphere which 

 would retain a larger proportion of the sun's heat ; and 

 the internal heat of the earth itself, great lakes of lava 

 under a canopy of cloud for example, may have played 

 an important part in supplying warmth. 



Again we have a greater age if there was more energy 

 available than in Helmholtz's hypothesis. Lord Kelvin 

 maintains that this is improbable because of the slow 

 rotation of the sun, but Perry has given reasons for an 

 opposite conclusion. 



The collapse of the first argument based on tidal 

 retardation, and of the second based on the cooling of 

 the earth, warn us to beware of a conclusion founded on 

 the assumption that the sun's energy depends, and has 

 ever depended, on a single source of which we know the 

 beginning and the end. It may be safely maintained 

 that such a conclusion has not that degree of certainty 

 which justifies the followers of one science in assuming 

 that the conclusions of other sciences must be wrong, 

 and in disregarding the evidence brought forward by 

 workers in other lines of research. 



