102 Prof. John Milne—On the Cooling of the Earth. 
where the loss during any given period is greatest and this is travel- 
ling towards the centre. 
At the present time, up to a depth of about 200,000 feet, we see 
that the loss of temperature is nearly proportional to the depth, 
At the commencement the temperature was supposed to be that of | 
melted rock, or as an extreme limit about 7000° Fahr. In a short 
period the surface solidified ; in fact, points near the surface obtained 
very quickly a temperature nearly the same as it is at present. 
Parts near the centre, however, hardly cooled at all, and even now 
the temperature at the centre is probably the same as it was one 
hundred miilion years ago. 
From all this it would appear, as is pointed out by Sir William 
Thomson, that because in early times the increase in temperature as 
you descended into the earth was more rapid than it is at present, 
all changes due to plutonic action must have been more active in 
early times than they are now. 
Also we see that it is very probable that the main features of the 
earth’s surface, like the continents and ocean-basins, are of great 
antiquity, having in fact been formed at or about the time of first 
consolidation. 
Heat received from the sun.—From experiments with the Pyrhelio- 
meter it has been calculated that every year the earth receives from 
the sun a quantity of heat sufficient to melt a layer of ice covering 
the whole surface of the globe 100 feet in thickness. This is 
equivalent to 92 feet of water raised 79° C. or 72:7 feet raised from 
0° to 100° C. (Tyndall, in his “ Heat a Mode of Motion,” gives this 
as 66 miles.) 
Heat lost by the earth.—The heat which is lost per year by the 
earth may be determined from the present gradient, which is about 
1° for 50 feet of descent, and the conductivity of rock, which we will 
assume as being ‘0059. If we calculate this, we shall find that it is 
sufficient to raise a layer of water covering the whole surface of the 
earth -677 centimeters in thickness from 0° to 100° C.! 
This is the absolute loss. The energy received from the sun is 
partly spent in the growth of trees, the elevation of water, etc., 
and part is radiated off into space as soon as it is received, but 
whatever becomes of it, we can definitively say that the above 
quantity of heat is being actually lost by the earth. 
' Let the heat gradient of the earth be 1° F. for 50 feet of descent, or in Centi- 
grade scale and centimeters 8° C. for 50 x 30°5 centimeters in descent. Let the area of 
the earth in square centimeters = §. Let the conductivity of the rocks = H = -0059. 
Then the amount of heat lost by the earth per year 
= § x -0059 x (§ + 50 x 30°5) x 365 x 24 x 60 x 60. 
An ocean over the earth d centimeters deep would be heated from 0° C. to 100° C. if 
Sxdx 100 =8 x *0059 x (§ = 50 x 30°5) x 365 x 24 x 60 x 60. 
Whence d = ‘677. 
That is to say, an ocean covering the whole surface of the earth 677 centimeters 
deep, might be raised from 0° to 100° C. by the heat which flows from the earth every 
year. This heat is heat of a low temperature. 
