Variation of the Magnetic Needle. 315 



pour, and consequently electricity, than the current flowing 

 to the Asiatic pole ; or it may be caused by the ice in Ame- 

 rica being blocked up at much lower latitudes than in Asia, 

 thus producing in a greater degree those currents of air, 

 vapour, and electricity, to which I attribute magnetic phe- 

 nomena. 



It may be observed, that the point of greatest intensity is 

 between Hudson's Bay and the Gulf of Mexico, and that the 

 lines shew the great intensity from the American pole in the 

 direction of the warm equatorial seas between Africa and 

 America. 



The intensity of effect from the Asiatic pole seems to be 

 influenced by like causes, as it is directed towards the Indian 

 Ocean, where the line of maximum temperature has its 

 greatest northern latitude, i. e., 12° 30', and where the tem- 

 perature of that line is at the highest, viz. 85J° of Fahrenheit, 

 and consequently where the evaporation is greatest. Thus, 

 in both cases the greatest intensity is from the poles towards 

 the nearest parts where the evaporation is great. 



In the meridian, where the intensity is the least, we find 

 no such pole of cold, and the tropical region is occupied by 

 Africa, consequently the rate of evaporation is low ; and in 

 the meridian of Behring's Straits the Arctic Sea is open, 

 and the line of maximum temperature is about 7° south of 

 the equator, so that this line is nearly 20° farther to the 

 south in that meridian than it is in the Indian Ocean, and 

 the temperature lower. 



The lines of equal temperature follow the same general 

 direction as the lines of magnetic intensity, shewing that they 

 are both influenced by the same cause, viz. the two great 

 centres or poles of cold ; and generally in the meridians 

 where the temperature is highest, the intensity of magnetism 

 is the least, and where the temperature is lowest the inten- 

 sity is greatest. The lines of equal temperature do not 

 differ so much from the parallels of latitude as the lines of 

 magnetic intensity, but yet the difference is great, especially 

 in high northern latitudes. 



Taking the line of mean temperature of 30° of Fahrenheit, 

 it has its highest limit rather north-west of the North Cape, 



