MAGNETISM. 



427 



pole of the earth, in the direction of Hudson's Bay. It was visited by Sir J. 

 lloss in 1832, in his voyage of Arctic discovery. The south magnetic pole is 

 situated in the antarctic continent, and has been approached within 170 miles. 



If the ordinary compass be carried to either 



If a compass n ■, .,..., 



needle be car- of thc magnetic poles, it Will lose its power 



Tied to the mag- . . ,• ,,. i . ■,. • t /• • 



netjc pole what and point indiiierently ID anv dircction. It it 



wiU occur? . -11 ,1 . 



IS carried beyond the magnetic pole, to any 

 point between it and the true pole, the poles of the needla 

 become reversed, the end called the north pole pointing to 

 the south, and the south to tlie north. 



H d th '^^^ position assumed by tlie dipping needle varies in di& 



position of the ferent latitudes. If it were carried directly to the north mag- 

 varv'?'° ^^^^^^ netic pole, its south pole would be attracted downwitrd, and 

 the needle would stand perfectly upright. At the south mag- 

 netic pole, its position would be exactly reversed.* If the dipping needle bo 

 taken to the equator of the earth, or to a point midway between the north 

 and south magnetic poles, it will be attracted equally by both, and will re- 

 main perfectly horizontal, or cease to dip at 

 all : as we go north or south, however, it dips 

 more and more, until at the magnetic poles, 

 as before stated, it becomes perpendicular— 

 the end which was uppermost at the north 

 being the lowest at the south.f 

 ♦ I / / \M-.\ II \ If ■^'?- 357 represents the position assumed 

 j~L^MuyT^--y_ Z i^^Li-; T ~j| by the magnetic needle in various latitudes. 



The magnetic poles of the earth are not 

 stationar\', but change their position grad- 

 ually during long intervals of time. 



Observations on the temperature of the 

 earth have afiforded some reason for believ- 



♦ Like the declination and dip, the intensity of the earth's magnetism varies 

 rery much in different parts of the earth ; at the magnetic equator being the most feeble, 

 and gradually increasing as we approach the poles. Theintensity of terrestrial magnetism 

 in different places may be measured by the number of vibrations made by a magnetic 

 needle in a given time. 



t As the directive tendency of the horizontal needle arises from its poles being attracted 

 by those of the earth, it is evident from the rotundity of the earth, that its poles will not 

 be attracted by those of the earth horizontally, but downward, so that the needle cannot 

 tend to be horizontal, except when it is acted upon by both poles equall)— that is, when 

 midway between them. When nearer the north magnetic pole than the south, its north 

 end must be attracted downward, and the contrary when it is nearest the south pole. 

 Accordingly, a needle which was accurately balanced on its support before being mag- 

 netized, will no longer baUance itself when magnetized, but in this country its north pole 

 will appear to dip, or appear to be the heavier end. Tliis circumstance has to be corrected 

 in ships' compasses by a small sliding weight attached to the southern hnlf, which weight 

 has to be removed on approaching the equator, and shifted to the other side of the ueedle 

 vhen iu the northern hemisphere. 



