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XLV. On the Physical Cause of the Submergence and Emer- 

 gence of the Land during the Glacial Epoch. By James 

 Croll*. 



IN the Philosophical Magazine for last month Mr. Heath has 

 favoured us with a long and elaborate paper on the glacial- 

 submergence question. In that paper he arrives at the conclu- 

 sion that an ice-cap placed upon the arctic regions would not 

 only attract the water of the ocean towards those regions, but 

 would also cause a similar flow of water to the south pole, pro- 

 ducing a rise in the sea-level there also, the solid nucleus of the 

 globe meanwhile remaining undisturbed in the centre of the 

 ocean. That an ice-cap resting on the solid ground should by- 

 its attraction cause the waters of the ocean to recede to the oppo- 

 site side of the globe, is a conclusion so diametrically opposed to 

 the received principles of mechanics, that we fear there are few 

 physicists who will not be apt to suspect that Mr. Heath in his 

 reasoning must have gone astray somewhere or other. We are 

 inclined to think that he has been misled by adopting an erro- 

 neous theory regarding the cause of the tides. Mr. Heath ex- 

 plains the cause of tides as follows : — 



"The common explanation," he says, "of the phenomena of 

 the tides, by reference to the tendency of the ocean, under the 

 influence of the moon or sun, to assume an elliptical shape with 

 the long axis pointing towards the luminary and the solid nucleus 

 of the earth at the centre, would not be in anywise affected by 

 supposing either luminary to be attached to the l solid mass of 

 the earth ' by a rod of insensible weight ; whereas Mr. CrolFs 

 axiom would lead to the gathering up of the waters, not into 

 two opposite semidiurnal tides respectively under and antipodal 

 to the moon, but into one globular mass under it." 



It is perfectly true that the ordinary phenomena of the tides 

 would occur just as they do at present, though the earth and 

 moon were connected by a "rod of insensible weight," pro- 

 vided those orbs be permitted to revolve around their common 

 centre of gravity. But stop their motion and allow them to be 

 held separate by the "rod," which we presume is the idea 

 Mr. Heath wishes to convey, and there would not then be two 

 semidiurnal lunar tides, as he supposes, but only one on the 

 side under the moon, just as my " axiom " would lead us to 

 conclude. 



I am of opinion that Mr. Heath has been misled at the very 

 outset by overlooking the fact that the rise of the waters by the 

 attraction of the ice-cap and the rise of the tidal wave are not 



* Communicated by the Author. 



