58- Electric Currents on the Earth. 



meridian rendering the air rising there and flowing from 

 thence to that pole highly charged with vapour, and conse- 

 quently with electricity ; while the great extent of land, in 

 the meridian of the Siberian pole, may cause the air flowing 

 over towards that pole to be comparatively dry, and conse^ 

 quently with little electricity. 



The change, observed in England, of the declination of the 

 needle from 11° 15' east in 1580 to its present westward posi- 

 tion, has been attributed to a gradual motion of the magnetic 

 pole from east to west. / believe that no such motion has 

 taken place, and that the magnetic poles have never changed 

 their position in any considerable degree ; the change in de- 

 clination has probably been caused by an increase of power 

 in the American pole, or decrease of power in the Siberian 

 pole, or perhaps a combination of both causes, thus gradually 

 bringing England under the influence of the American or 

 stronger pole. 



It is obvious, that as there are two magnetic poles in the 

 northern hemisphere, one causing a westward and the other 

 an eastward declination, there must be a line of no-variation 

 where the influences of both poles are equal (that is, a di- 

 verging line of no-variation?) This line of no- variation is now 

 a little to the east of St Petersburg, and [it must have been 

 this line of no-variation which passed over London from west 

 to east in 1657 ; for had the line of no-variation been caused 

 by a motion of the poles from east to west, it would have been 

 at Paris (2° 20' east longitude) before it could be at London ; 

 whereas it was at Paris in 1666, or 9 years later than at 

 London ; and as a proof that it was a diverging line of no- 

 variation, the variation at Bristol (2° 50' west longitude) 

 was 1° 27' west at the time there was no declination at Paris, 

 whereas it must have been an eastward variation, had the 

 magnetic pole been at that time in the meridian of Paris. 



It is difficult to assign a cause for the change of strength 

 in the poles. It is not improbable that the increase of the 

 American pole may be from an accumulation of ice in that 

 part of the earth, or that the Siberian pole may have de- 

 creased in strength from an increase of temperature in those 

 regions; or, from a decrease in the evaporation from the 



