Variation of the Magnetic Needle. 317 



other parts in this hemisphere in the same parallels of lati- 

 tude. 



The opening into the Arctic Sea through Behring's Strait 

 produces similar effects in those regions, but in a lesser de- 

 gree. It is true that the isothermal lines in the North Pa- 

 cific Ocean have not their highest northern limits in the 

 meridian of the Strait, but to the east of it ; this fact supports 

 the theory, as the high temperature of the western parts of 

 North America may fairly be attributed to the great height 

 of the Rocky Mountains intercepting the cold breezes from 

 Hudson's Bay and the frozen seas above it. 



With respect to the changes of temperature which have 

 taken place in Europe, it is evident, from glacial marks and 

 deposits in Switzerland, Scotland, and in almost the whole 

 of North-Western Europe, that the temperature of these 

 regions has been much lower than at the present time. I 

 will not attempt to give an opinion as to the depression of 

 temperature which would produce such an extent of glaciers ; 

 but M. Charles Martins, in an article* " on the Ancient Ex- 

 tent of the Glaciers of Chamounix," calculated that a mean 

 temperature similar to that of the northern part of the State 

 of New York, by lowering the line of perpetual snow, and 

 enlarging the area for its accumulation, would be quite suffi- 

 cient to account for the extent of ancient glaciers in the 

 district around Mont Blanc. On this authority, therefore, I 

 conclude that the extent of ancient glaciers in Europe may 

 be accounted for by a temperature such as now prevails in 

 similar latitudes in the eastern parts of America. 



It has been suggested, that a change in the direction of 

 the Gulf Stream may have been the cause of the change of 

 temperature since the glacial period ; this cause alone seems 

 insufficient to produce such effects, and the supposition of 

 any change in the course of the Gulf Stream is purely hypo- 

 thetical. 



I submit that this change of temperature may be fairly 

 attributed to geological changes, of which we have evidence 

 at the present time. I believe the low temperature of the 



* Edinburgh New Philosophical Journal, No. 85. 

 VOL. LIV. NO. CVIII. — APRIL 1853. 



