Prof. John Milne—On the Cooling of the Earth. 101 
temperature, and maintaining that temperature. constant. This 
assumption is supported by the well-known instance of the rapid 
cooling of a mass of lava, and by the fact that the result obtained 
satisfies observations of the present temperature of the earth. 
Assuming, then, that Sir William Thomson’s result is correct, it 
would seem that we should be hardly justified in employing his 
result for very great depths in the earth, because in his calculations 
he did not consider the cooling of a globular mass, but of an infinite 
mass of rock with a plane surface. In Fig. 2, the curve OM is 
Temperature = 7000° Fahr, 
| CCE aL een Bev REE Oe Lae Cera 
Fic. 2. 
reduced from Thomson and Tait’s Natural Philosophy, vol. i. p. 719. 
It represents the distribution of temperature in the earth one hun- 
dred million years after the commencement of cooling, the initial 
temperature being 7000° Fahr., and it also, so far as we know, repre- 
sents the distribution of temperature in the earth as determined by 
experiment. The direction OY measures temperature, and the 
direction OX distance, from the surface at O towards the centre at 
X. The curves OK and OL are sketched to show approximately 
what the distribution of temperature was previous to this period, and 
the curve ON to show what it will be subsequently, say for example 
10,000 years hence. 
Up to a depth of about 100,000 feet, according to Thomson, or 
according to the curve he has given approximately to a depth of 
nearly 80 miles, the temperature increases directly with the depth 
(1° F. for 51 feet descent). After, say 10,000 years, the temperature 
will, according to the curve ON, increase directly with the depth 
for a depth greater than 80 miles. 
MN represents the loss of temperature in 10,000 years at a depth 
of 100,000 feet, m’ n’ at 400,000 feet, m” n” at 600,000 feet, ete. 
By comparing together the losses of heat which have occurred at 
different points, it will be seen that there will be a spherical belt 
