172 Archdeacon Pratt on the Level of the Sea during 



The step; by which this reasoning may be justified may be pre- 

 sented as follows : — 



A blindfolded man is told that four points are marked on 

 a large sheet, and is asked the chance that one of them is en- 

 closed by the other three. He requests that the points may be 

 joined by four lines, no three passing through a point. There 

 are now six points ; and if he is allowed now to inspect the figure, 

 there are three original groups of four points from which it is 

 equally probable that the figure originated, and in one only, and 

 one always, of these three groups one point is enclosed by the 

 other three ; so that the probability is \. 



The reasoning may also be justified in the way by which the 

 result of the first question was arrived at. 



This method, which might be called instantaneous, is easily 

 applied to similar problems ; and I have given these remarks in 

 full, in order to remove, if possible, doubt as to its validity. 



XXIV. On the Level of the Sea during the Glacial Epoch in the 

 Northern Hemisphere. By Archdeacon Pratt. 



To the Editors of the Philosophical Magazine and Journal. 



Baitool, Central Provinces, India, 

 Gentlemen, January 3, 1866. 



DURING a journey in these out-of-the-way parts I have 

 lighted upon an interesting letter in a late Number of the 

 'Reader' (September 2, 1865) by Mr. Croll of Glasgow, suggest- 

 ing that the greater depth of the ocean in the upper parts of the 

 northern hemisphere, during the Glacial Epoch, may have been 

 caused by the change of the centre of gravity of the earth by 

 the presence in the northern regions of an enormous accumu- 

 lation of solid matter in an " Ice-Sheet," which has since disap- 

 peared. This idea appears to me so ingenious and so probable, 

 that it deserves a more careful examination. The hypothesis 

 appears to be, that in time past a grand cosmical change has 

 been going on, according to which the northern and the south- 

 ern hemispheres (at any rate the higher portions of them) have 

 been alternately bound up in ice, and have alternately yielded to 

 milder influences, when the ice-sheet has become broken up, 

 moving off in huge fragments which have caused the phenomena 

 of the drift, and has finally disappeared. The centre of gravity 

 of the earth has therefore slightly shifted during these enormous 

 periods, first north and then south, and produced a corresponding 

 effect upon the depth of the ocean. The question is, whether 

 the matter deposited from the air in snow and hail and held fast 

 in a solid mass can have been sufficient to produce the required 



