814 REPORT—1896. 
that there has been almost no cooling. If, however, we take one of the cases 
put by Professor Perry, and assume that below a depth of four miles there is ten 
times the conductivity, we find that after a period of 10,000,000,000 years the 
gradient at the surface is still 1° F. for every 50 feet; but that we have to 
descend to a depth of 1500 miles before we find the initial temperature of 
7000° 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 
enabling 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 Professor Perry’s criticisms, quite admits that the age 
at which he had arrived by the use of this argument may be insufficient. Thus, 
he says, in his letter: ! ‘I thought my range from twenty millions to 400 millions 
was probably wide enough, but it is quite possible that I should have put the 
superior limit a good deal higher, perhaps 4000 instead of 400.’ 
The third argument was suggested by Helmholtz, and depends on the life of 
the sun. Ifthe 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 contraction to his present volume would have been sufficient to last eighteen 
million years at his present rate of radiation.’* Lord Kelvin rejects the assump- 
tion of uniform density, and is, in consequence of this change, able to offer 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 radiation 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 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 away. But such changes in the amount of 
radiation received would follow from a greater distance from the sun of 
15} 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 
allinthe 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 of tidal retardation and of the second of 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 conclusion of other sciences must be wrong, and in 
disregarding the evidence brought forward by workers in other lines of research. 
We must freely admit that this third argument has not yet fully shared the fate 
1 Nature, January 3, 1895. 
2» Newcomb’s Popular Astronomy, p. 523. 
