352 



James Croll — On the Glacial Epoch. 



they will approach towards the cap to a certain extent, sinking, as 

 it were, into the lighter materials, and displacing them towards the 

 opposite pole. This displacement will of course tend to shift the 

 earth's centre of gravity in the direction of the ice-cap, because 

 the heavier materials are shifted in this direction, and the lighter 

 materials in the opposite direction. This process will perhaps be 

 better understood from the following figures. 



Fig. 2. Fig. 3. 



0. The Ocean. S. Solid Crust or Shell. 



F, F^, F^, F^. The various concentric layers of the fluid interior. The layers 

 increase in density towards the centre. 



1. The Ice-cap. C. Centre of gravity. 

 C^. The displaced centre of gravity. 



In Figure 2, where there is no ice-cap, the centre of gravity of the 

 earth coincides with the centre of the concentric layers of the fluid 

 interior. In Figure 3, where there is an ice-cap placed on one pole, 

 the concentric layer F' being denser than layer F, is attracted 

 towards the cap more forcibly than F, and consequently sinks to a 

 certain depth in F, Again, F^ being denser than F', it also sinks to 

 a certain extent in F^ And again, F^ the mass at the centre, being 

 denser than F^, it also sinks in F^. All this being combined with 

 the effects of the ice-cap, and the displaced ocean outside the shell, 

 the centre of gravity of the entire globe will no longer be at C, but 

 at C^ a considerable distance nearer to the side of the shell on which 

 the cap rests than C, and also a considerable distance nearer than it 

 would have been had the interior of the globe been solid. There 

 are here three causes tending to shift the centre of gravity, (1) the 

 ice-cap, (2) the displaced ocean, and (3) the displaced materials in 

 the interior. Two of the three causes mutually re-act on each other 

 in such a way as to increase each other's effect. Thus the more the 

 ocean is drawn in the direction of the ice-cap, the more effect it has 

 in drawing the heavier materials in the interior in the same direc- 

 tion, and in turn the more the heavier materials, in the interior, are 

 drawn towards the cap, the greater is the displacement of the earth's 



