150 



THE GEOLOGIST. 



ture is sensibly above that point. In fact, there is no reason 

 why waves of cold of any intensity below 32^ may not be pro- 

 pagated downwards into the interior of the ice ; but waves of 

 heat above that point, of course, never can. Thus the cold 

 of winter, and the frost produced by radiation in the clear 

 nights of summer, will enter the mass, and lower its internal 

 temperature ; while the heat of the summer air, and that im- 

 parted by solar radiation, will mainly be employed in melting 

 the surface, and will run off with the water produced. 



In the concluding part of the paper, Sir John Herschel 

 applies the same mode of reasoning to explain the existence 

 of ice caves below the limits of perpetual snow. If, he says, 

 the surface, during the whole or the greater part of the year, 

 be covered with ice, the mean annual temperature of the 

 interior will be materially less than that due to elevation, and 

 which it would have, were it not covered. 



The paper is concluded by an example, exhibiting the pro- 

 pagation of periodical as well as diurnal waves of heat and 

 cold, throughout a mass ; proving, that, in the case of diurnal 

 fluctuations, the effect would cease to be sensible at a depth 

 300 or 400 times greater than that at which the diurnal 

 effects are neutralized ; and thus that the winter waves of 

 cold (consisting of many diurnal waves of greater or less in- 

 tensity) may not travel down to a cavern till the hottest 

 period of next summer, or of many summers. 



MANCHESTER GEOLOGICAL SOCIETY. 



March Z\st. — Dr. Black, F.G S., in the chair. — Amongst 

 the donations announced at this meeting, those presented by 

 J. Heywood, Esq. and R. Thicknesse, Junr., Esq., were the 

 most considerable, the former consisting of several thousand 

 specimens of British fossils. 



A paper ivas read on the salt of Cheshire'' by Mr, 

 Ormerod. 



