C. Davison—Secular Straining of the Earth. 221 
Leaving its details out of account, the fundamental idea in the 
contraction theory may be stated as follows: The whole earth was 
originally at a high temperature throughout, its present distribution 
of temperature being the result of cooling since the initial epoch. 
The surface of the earth has now practically ceased to cool, and 
the interior at and below a depth of two or three hundred miles has 
not yet begun sensibly to lose its heat. The intermediate layer in 
cooling contracts, and the outermost crust, being deprived of its 
support, is crushed and folded by the tremendous pressures thus 
brought into action. The ridges and wrinkles, into which the crust 
is thrown, constitute our mountain-chains. 
It is curious that, until recently, attention has been wholly 
concentrated on the behaviour of the outer crust with respect to the 
cooling layer beneath it, and that the behaviour of the cooling layer 
itself with respect to the uncooled nucleus within has passed 
unnoticed. Taking this latter relation into account, however, it will 
be found that new light is thrown upon several points of the 
theory, and especially on the comparatively superficial nature of the 
mountain-making forces. 
I shall assume in this paper the truth of the doctrine, which 
recent researches agree in indicating with a high degree of proba- 
bility, that the earth is practically a solid body; also of Sir W. 
Thomson’s celebrated investigation on the secular cooling of the 
earth. Further, in order to simplify the problem, I shall suppose 
the earth to be a sphere and its surface perfectly smooth. 
Taking the initial temperature of the earth at 7000° F., and the 
average rate near the surface at which the temperature increases 
with the depth at 1° F. for every 51 feet, Sir W. Thomson shows 
that the date at which the earth solidified cannot have been less 
than 20 million, nor more than 400 million, years ago, that it was 
probably not far from 100 million years ago. In his well-known 
paper, he gives a curve which repre- 
sents the temperature at different 
depths, and which is reproduced 
in the accompanying figure. The 
depths from the surface are repre- 
sented by lengths measured from 
A along the line as. The tempera- 
ture at the depth an is represented 
by the length of the line np 
drawn at right angles to as. If 
lines like np be drawn from every 
point in aB, and if each line be 
made proportional in length to the 
temperature at the corresponding 
depth, the other ends will all lie 
on the curve apc. At the depth 
represented by as, about 150 miles, the temperature is very nearly 
the same as it must be in the entire mass below. 
At a subsequent date, every point, through cooling, is at a lower 
D 
B 
