100 Prof. John Milne—On the Cooling of the Earth. 
ference may be seen in fig. 6 of the paper from which these two 
have been reduced. 
From this we see that when a ball is made hot, and we may 
imagine this ball to be the earth, when it cools it does so at very 
different rates in its different parts. 
Temperature. 
‘Time. 
Fig. 1. 
Thus the external part, if the ball is at a high temperature, will 
cool exceedingly rapidly at first, and this rapidity will grow less 
and less. The cooling of the centre, however, will be different. For 
some period it will cool slowly, after which it will cool with a 
rapidity which continually approaches the rate at which the circum- 
ference cools, until eventually it cools even faster than the circum- 
ference. Intermediate places at the beginning cool more slowly, as 
they are nearer the centre. For each of these, however, there is a 
certain interval, after which they cool faster than places in the circum- 
ference. From what we know of the earth, its interior is probably 
yet very much hotter than points near the surface; points near the 
surface are therefore probably still cooling at a rate much quicker 
than points near the centre, and the cooling which has so far taken 
place in the earth may be represented by the parts of these two 
curves, which are included between OP and some line like op. 
Messrs. Perry and Ayrton, in this series of curves, have given the 
true conditions for a cooling globe, whose surface had a given 
emissivity ; but Sir William Thomson, whose calculations satisfy very 
well the known conditions of the earth, used a much greater 
emissivity. ; 
In fact, Sir William Thomson assumed so great an emissivity for 
the surface of the earth, that after 10,000 years from the commence- 
ment of superficial solidification of the earth there was no farther 
change of temperature at the surface, and this is practically equi- 
valent to an instantaneous cooling of the earth’s surface to its present 
