CHAP, ii.] ELECTRICITY AND MAGNETISM. 113 



affect the compass, which has therefore to be corrected 

 by placing compensating masses of iron near it, or by 

 fixing it high upon a mast. 



135. The Earth a Magnet. Gilbert made the great 

 discovery that the compass needle points north and 

 south because the earth is itself also a great magnet. 

 The magnetic poles of the earth are, however, not 

 exactly at the geographical north and south poles. The 

 magnetic north pole of the earth is more than 1000 

 miles away from the actual pole, being in lat. 70 5' 

 N., and long. 96 46' W. In 1831, it was found by 

 Sir J. C. Ross to be situated in Boothia Felix, just 

 within the Arctic Circle. The south magnetic pole of 

 the earth has never been reached ; and by reason of 

 irregularities in the distribution of the magnetism there 

 appear to be two south magnetic polar regions. 



136. Declination. In consequence of this natural 

 distribution the compass-needle does not at all points 

 of the earth's surface point truly north and south. 

 Thus, in 1881, the compass-needle at London points at 

 an angle of about i833' west of the true north. This 

 angle between the " magnetic meridian " 1 and the geo- 

 graphical meridian of a place is called the magnetic 

 Declination of that place The existence of this 

 declination was discovered by Columbus in 1492, though 

 it appears to have been previously known to the Chinese, 

 and is said to have been noticed in Europe in the early 

 part of the I3th century by Peter Pellegrinus. The 

 discovery is also claimed, though on doubtful authority, 

 for Sebastian Cabot of Bristol. The fact that the 

 declination differs at different points of the earth's sur- 

 face, is the undisputed discovery of Columbus. 



In order that ships may steer by the compass, mag- 



1 The Magnetic Meridian of any place is an imaginary plane drawn 

 through the zenith, and passing through the magnetic north point and mag- 

 netic south point of the horizon, as observed at that place by the pointing of 

 a horizontally-suspended compass-needle. 

 I 



