328 
The volume of the Sea above the datum level = the area of the 
whole surface of the globe x the depth of the datum level below the 
sea level —the volume of rock displacing water between those 
levels, 
Assuming then that the continents have been shaped out of 
the master elevations, and that the oceans indicate the positions 
of the master depressions, and that both are ultimately due to 
lateral pressure, an estimate of 2he for the whole globe is 
obtained from the above relation upon the following data :— 
(1) The area of the ocean is 146 millions of square miles. 
(2) That of the land is 51 millions. 
(3) The mean depth of the ocean is three miles, 
(4) Its deepest parts are about four miles. 
(5) The mean height of the land is 900 feet (as shewn by 
Mr Carrick Moore)’, 
From these data, as a probable value, 2 ke = 9504 feet, which 
appears more likely to be too small than too large, 
The meaning of this in plain language is, that if all the 
inequalities of the earth’s surface were levelled down, they 
would form a coating 9504 feet thick over the whole globe 
above the datum level; the datum level being such a surface 
as has been already defined. 
Having thus obtained a value for the thickness of the coat 
ing which all the inequalities of the earth’s surface would form, 
if levelled down, a measure of the same thing is sought on 
physical grounds. For this purpose Sir W. Thomson’s paper 
“On the Secular Cooling of the Earth,” is used as a basis to 
work from®. From Mr R. Mallet’s late investigations on the 
contraction of slag from an iron furnace’, a probable coefficient 
of contraction for melted rock is deduced, viz. 0°0000217 for 1° 
Fahr.; and with this is obtained a value for 2 ke, or the thickness 
t Nature, 1872, Vol. v. p. 479. 
® Edin. Trans. 1862; and Natural Philosophy, p. 711. 
3 Royal Soc. Trans. 1873. 
