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of the coating above defined, Sir W. Thomson's investigation 
proceeds upon the supposition, founded upon Bischoff’s experi- 
ments upon the contraction of melted rocks in cooling, that, 
if the earth, or an outer coating of it, were once in a molten 
state, then, as soon as a crust began to form, it would break up 
and sink, and thus the whole would be reduced to the tempera- 
ture of incipient solidification before it could be permanently 
crusted over. From the time of such incipient solidification 
it has gone on cooling, subject to the laws of cooling of a solid. 
He then proves that upon this supposition the temperature 
would increase from the surface downwards, at first at a nearly 
uniform rate, but at a greater depth much more slowly, until at 
a certain point such a temperature would be arrived at, as 
would be about sufficient to induce fusion under the pressure 
existing at that depth. Now the rate at which the tempera- 
ture first begins to increase is known to be about 1° Fahr. for 51 
fect, Sir W. Thomson has determined, by observation on the 
rocks at Edinburgh, that their conductivity on an average is 
400. With these data he proves that if, for the sake of illus- 
tration, the temperature at which the crust began to solidify be 
taken at 7000° Fahr., then the time since such solidification 
commenced will have been about one hundred millions of years, 
and that at about 100 miles below the surface the melting 
temperature would be reached. 
Proceeding upon these assumptions, with the coefficient of 
contraction for rock above mentioned, the value of 2ke is 
calculated, or the thickness of the coating which all the eleva- 
tions would form if they were levelled down, and it is found to 
come out less than 800 feet. 
Still further, if instead of 7000° Fahr. 4000° is assumed to be 
the temperature for melting rock, which seems to be justified 
by Mr Mallet’s experiments, then the value of 2ke, or the 
thickness of the coating referred to, would be less than 150 
feet. In the latter case the time since solidification commenced 
would be about thirty-three millions of years. 
