EARTH’S CRUST RESULTING FROM SECULAR COOLING. 
235 
If the time that has elapsed since consolidation be taken as 174,240,000 years, the 
depth for which 2 = 4’00 is 400 miles. For simplicity, let us adopt this value for 
the present. 
(9) The radius of a sphere equal to the Earth in volume being about 3959 miles, I 
therefore suppose the Earth at the present time to be made up as follows :—A central 
globe, 3559 miles in radius, at a temperature of 7000° F., which as yet has not 
sensibly cooled; surrounded by 400 concentric spherical shells, each one mile in 
thickness, the rate of cooling in each shell being uniform throughout, and equal to its 
value at the outer surface of that shell. On these assumptions I have calculated, from 
the last expression in § (6), the proportional amounts by which the four hundred 
shells are stretched or folded. 
The results of the calculation are shown in the accompanying figure, in which the 
dotted curve represents the rate of cooling, at the time considered, from the surface 
down to the depth for which z = 2'55, and the continuous curve represents, at the 
same time, the amount by which a great circle of any shell is being stretched or 
folded, the folding by the part of the curve to the left of the axis Ox, and the 
stretching by the part to the right. 
2 h 2 
