Variation of the Magnetic Needle. 321 



question for consideration would then be ; — If by some geo- 

 logical phenomenon the southern parts of Hudson's Bay 

 became exposed to the Atlantic Ocean by an opening similar 

 to the British Channel, and from the 66th to the 70th degree 

 of latitude the land was upheaved so as to cut off all com- 

 munication from the Arctic Seas, how long a time, under 

 these circumstances, would it require for the whole of the 

 north-eastern parts of America, together with the neighbour- 

 ing Arctic regions, to arrive at the highest temperature 

 they would ultimately acquire in consequence of these 

 changes ? For as long as any elevation of temperature was 

 going on, so long would the pole of cold continue to recede 

 towards the west; so long, also, would the variation of the 

 needle go on, and the line of no declination would move from 

 its present situation, near St Petersburg, towards the west. 

 England might thus again be brought under the influence of 

 the Asiatic pole, and the needle throughout Europe again 

 have an eastern declination. 



In considering this subject, it is necessary to bear in mind 

 how slowly heat is conducted by some earths, and also the 

 fact, that 140 degrees of heat are absorbed by ice in melting 

 without any increase of temperature. Many proofs might 

 be given of the slow transmission of heat through earth, but 

 I will only refer to one, as stated by Mr Nasmyth, in the 

 Journal of the Geological Society, vol. iii., p. 233. " The 

 instance in question" (says that gentleman), " was that of a 

 large plate-iron pot, containing 11 tons of white-hot melted 

 cast-iron — a temperature so high as to be quite beyond all 

 thermometric certainty, but well known to be the highest 

 intensity of furnace heat, being quite equal to that of w r elding 

 hot iron. 



" This vast mass of white-hot melted cast-iron, stood in 

 the pot for upwards of 20 minutes, and but for a thin coating 

 of clay and sand, of about half an inch thick, would have soon 

 melted the bottom and sides of the pot. 



■ - This half-inch thickness of mineral substance, however, 

 was quite sufficient to prevent the conduction of the heat to 

 the exterior ; so completely so, that after this mass of hot 

 iron had remained for upwards of 20 minutes in the pot, you 



