632 



SCIENCE. 



[N. S. Vol. IV. No. 96. 



help, I am able to state it in a few words. 

 Lord Kelvin's assumption of an earth re- 

 sembling the surface rock in its relations to 

 heat leads to the present condition of things, 

 namely, a surface gradient of 1 ° F. for 

 every 50 feet, in 100 million years, more or 

 less. Deeper than 150 miles he imagines 

 that there has been almost no cooling. If, 

 however, we take one of the cases put by 

 Prof. Perry, and assume that below a depth 

 of four miles there is ten times the conduc- 

 tivity, we find that after a period of 10 bil- 

 lion years the gradient at the surface is 

 still 1 ° F. for every 50 feet ; but that we 

 have to descend to a depth of 1,500 miles 

 before we find the initial temperature of 

 7,000° F. undiminished by cooling. In 

 fact, the earth, as a whole, has cooled far 

 more quickly than under Lord Kelvin's 

 conditions, the greater conductivity ena- 

 bling a far larger amount of the internal heat 

 to escape ; but in escaping it has kept up 

 the temperature gradient at the surface. 



Lord Kelvin, replying to Prof. Perry's 

 criticisms, quite admits that the age at 

 which he had arrived by the use of this 

 argument may be insufiicient. Thus, he 

 says, in his letter*: "I thought my range 

 from 20 millions to 400 millions was prob- 

 ably wide enough, but it is quite possible 

 that I should have put the superior limit a 

 good deal higher, perhaps 4,000 instead of 

 400." 



The third argument was suggested by 

 Helmholtz, and depends on the life of the 

 sun. If the energy of the sun is due only 

 to the mutual gravitation of its parts, and if 

 the sun is now of uniform density, " the 

 amount of heat generated by his contrac- 

 tion to his present volume would have been 

 sufficient to last 18 million years at his pres- 

 ent rate of radiation . "f Lord Kelvin rej ects 

 the assumption of uniform density, and is, 

 in consequence of this change, able to offer 



^Nature, January 3, 1895. . 

 fNewcomb's Popular Astronomy, p. 523. 



a much higher upward limit of 500 million 

 years. 



This argument also implies the strictest 

 uniformitarianism as regards the sun. We 

 know that other suns may suddenly gain a 

 great accession of energy, so that their radi- 

 ation is immensely increased. We only 

 detect such changes when they are large 

 and sudden, but they prepare us to believe 

 that smaller accessions may be much more 

 frequent, and perhaps a normal occurrence 

 in the evolution 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 possi- 

 ble modes by which the life of the sun may 

 have been greatly lengthened, that a lessen- 

 ing 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 away. But such 

 changes in the amount of radiation received 

 would follow from a greater distance from 

 the sun of Ib^ per cent., and a greater 

 proximity to him of 18,4 per cent., respec- 

 tively. 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 authenti- 

 cated. 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 



