COMPARISON OF TERRESTRIAL DIRECTIVE FORCES. 53 



They converge to a north pole, north of Hudson's Bay, 

 and a south pole, in South Victoria: but these poles 

 are not opposite. From E. longitude 70 to 150, the 

 northern parts agree nearly with Geographical Meri- 

 dians: the same remark also applies to a large portion 

 of the southern part in longitude 150. 



The system of Magnetic Meridians has undergone 

 considerable changes in the times of modern accurate 

 science. The southern point of Africa received from 

 the Portuguese voyagers in the fifteenth century the name 

 of L'Agulhas (the needle), because the direction of the 

 compass-needle, or the Local Magnetic Meridian, coin- 

 cided there with the Geographical Meridian : it now 

 makes with it an angle of about 30. In the sixteenth 

 century, the compass-needle in Britain pointed east of 

 north : it now points from 20 to 30 (in different parts 

 of the British isles) west of north. At the present time, 

 a change of the opposite character is going on : in 1819 

 the westerly declination at Greenwich was about 24 23', 

 which was probably its maximum ; in the last thirty 

 years it has diminished from 23J to 20, nearly. It is 

 believed that the magnetic poles are rotating round the 

 geographical poles from East to West. 



25. Imperfect method of measuring the horizontal 

 directive force of terrestrial magnetism at any locality, 

 by vibrations of a magnetic needle. Correction for the 

 torsion-power of the suspending-thread. 



In Article 21, it was found that the angular momen- 

 tum, which the Earth's horizontal magnetic action im- 



