and on the Cause of Terrestrial Magnetism, ^f 



through it, and that the end of the needle, at which the posi- 

 tive electricity enters, becomes its north-pole (unless the 

 needle at the time of the shock is lying in or about the line 

 of the magnetic meridian, in which case the end pointing north 

 becomes its north-pole). It is, therefore, probable, that these 

 currents give direction to the magnetic-needle, and the in- 

 terruption of these currents will account for the occasional 

 influence of the aurora on the needle. 



As the pressure of the air from the colder regions, owing 

 to its superior density, is the cause of the trade-winds, the 

 upper current, or superior trade-winds, must be from the 

 equatorial towards the polar regions, and must be strongest 

 to the coldest parts of the earth ; thus rendering those parts 

 the magnetic poles,* by carrying to them a greater quantity 

 of vapour, and consequently electricity, than to other parts. 



If more electricity be received by the earth at the magne- 

 tic poles than at any other part of the earth, the electricity 

 in passing off towards the equatorial or negative parts of the 

 earth would diverge east and west from the direct line of 

 longitude, owing to the excess of electricity at the magnetic 

 poles over other parts of the earth in the same latitude ; the 

 declination of the needle may be thus fully accounted for. 



A large portion of the ice formed during the winter in some 

 parts of the Polar regions, such as the neighbourhood of 

 Greenland or Spitzbergen, must be dispersed during the sum- 

 mer by being drifted into lower latitudes ; but the ice formed 

 in winter in the higher latitudes of America is so blocked in 

 by the land, that the greater part of it must remain unmelted 

 during the summer season, thus rendering that part con- 

 stantly one of the coldest in the northern hemisphere, and 

 consequently one of the magnetic poles of the earth. The 

 high latitudes above the European and Asiatic continents, 

 being under similar circumstances, will account for the mag- 

 netic pole in those regions also. 



The stronger influence of the American magnetic pole is 

 probably owing to the vast extent of tropical seas in that 



* Sir D. Brewster, some years ago, pointed out the connection between the 

 magnetic poles of the earth, and the greatest degree of cold on its surface. 



