The London, Edinburgh and Dublin philosophical magazine and journal of science . h one has chosen that form which appeared to himbest adapted to simplify calculations; and of course we are at per-fect liberty to do the same. M. Adhemar, who was the first todirect his attention to the subject, and Mr. Heath, and alsoMr. Fisher, assume the cap to be of uniform thickness. I shall,however, assume it to be thickest at the pole, and to thin awaytowards the equator. We shall begin with a period of glaciation on the southernhemisphere. Let WNES be the solid part of the earth, Fig. 1. and c its centre
The London, Edinburgh and Dublin philosophical magazine and journal of science . h one has chosen that form which appeared to himbest adapted to simplify calculations; and of course we are at per-fect liberty to do the same. M. Adhemar, who was the first todirect his attention to the subject, and Mr. Heath, and alsoMr. Fisher, assume the cap to be of uniform thickness. I shall,however, assume it to be thickest at the pole, and to thin awaytowards the equator. We shall begin with a period of glaciation on the southernhemisphere. Let WNES be the solid part of the earth, Fig. 1. and c its centre of let ESW be an ice-capcovering the southern hemi-sphere. Let us in the firstcase assume the earth to beof the same density as thecap. The earth with its capforms now a perfect spherewith its centre of gravity ato; for WNES is a circle,and o is its centre. Sup-pose now the whole to becovered with an ocean a few miles deep. The ocean will assume the spherical form, and willbe of uniform depth. Let the southern winter solstice begin * Reader, March 3, 1866. f and Emergence of the Land during the Glacial Epoch, 303 now to move round from the aphelion. The ice-cap will alsocommence gradually to diminish in thickness, and another capwill begin to make its appearance on the northern the northern cap may be supposed, for simplicity of calcula-tion, to increase at the same rate that the southern will dimi-nish, the spherical form of the earth will always be the time that the northern cap has reached a maximum, thesouthern cap will have completely disappeared. The circleWNES will now represent the earth with its cap on thenorthern hemisphere, and o will be its centre of gravity; foro1 is the centre of the circle W N E S. And as the distancebetween the centres o and o is equal to N N, the thickness ofthe cap at the pole, therefore N N will represent the extent towhich the centre of gravity has been displaced. It will alsorepresent the exten
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