and Emergence of the Land during the Glacial Epoch, 303 



now to move round from the aphelion. The ice-cap will also 

 commence gradually to diminish in thickness, and another cap 

 will begin to make its appearance on the northern hemisphere. 

 As 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 maintained. 

 By the time that the northern cap has reached a maximum, the 

 southern cap will have completely disappeared. The circle 

 WN'ES' will now represent the earth with its cap on the 

 northern hemisphere, and o' will be its centre of gravity ; for 

 o 1 is the centre of the circle W N' E S'. And as the distance 

 between the centres o and o' is equal to N N', the thickness of 

 the cap at the pole, therefore N N' will represent the extent to 

 which the centre of gravity has been displaced. It will also 

 represent the extent to which the ocean has risen at the north 

 pole and sunk at the south. This is evident; for as the sphere 

 WN'ES' is the same in all respects as the sphere W N E S, 

 with the exception only that the cap is on the opposite side, the 

 surface of the ocean at the poles will now be at the same distance 

 from the centre o' as it was from the centre o when the cap 

 covered the southern hemisphere. Hence the distance between 

 o and o 1 must be equal to the extent of the submergence at the 

 north pole and the emergence at the south. Neglect the attrac- 

 tion of the altering water on the water itself, which later on will 

 come under our consideration. 



We shall now consider the result when the earth is taken at 

 its actual density, which is generally believed to be about 5*5. 

 The density of ice being '92, the density of the cap to that of 

 the earth will therefore be as 1 to 6. 



Let fig. 2 represent the earth with an ice-cap on the north- 

 ern hemisphere, whose thick- 

 ness is, say, 6000 feet at the 

 pole. The centre of gravity 

 of the earth without the cap 

 is at c. When the cap is on, 

 the centre of gravity is shifted 

 to o, a point a little more than 

 500 feet to the north of c. 

 Had the cap and the earth 

 been of equal density, the 

 centre of gravity would have 

 been shifted to o' } the cen- 

 tre of the figure, a point situ- ~e 

 ated, of course, 3000 feet to the north of c. Now it is very 

 approximately true that the ocean will tend to adjust itself as a 

 sphere around the centre of gravity o. Thus it would of course 



