244 MAGNETIC LINES OF NO VARIATION. 



The miner or the surveyor finds in the magnetic com- 

 pass the surest guide in his labours, and the experiment 

 is for ever studying its indications. 



" True as the needle to the pole," 



has passed into a proverb among mankind, but the 

 searching inquiry of modern observers has shown that 

 the expression is correct only with certain limitations. 

 There are two lines on the surface of the earth along 

 which the needle points true north, or where the mag- 

 netic and the geographical north correspond. These 

 are called lines of no variation, or, as they have also 

 been designated, agonic lines, and one is found in the 

 eastern and the other in the western hemisphere. The 

 American line is singularly regular, passing in a south- 

 east direction from the latitude 60 to the west of Hud- 

 son's Bay, across the American lakes, till it reaches the 

 South Atlantic ocean, and cuts the meridian of Green- 

 wich in about 65 south latitude. The Asiatic line of 

 no variation is very irregular, owing, without doubt, to 

 local interferences ; it begins below New Holland, in 

 latitude 60 south, it bends westward across the Indian, 

 ocean, and from Bombay has an inflection eastward 

 through China, and then northward across the sea of 

 Japan, till it reaches the latitude of 71 north, when it 

 descends again southward, with an immense semicircular 

 bend, which terminates in the White Sea. 



Hansteen has thought that there are two points in 

 each hemisphere which may be regarded as stronger 

 and weaker poles on opposite sides of the poles of revo- 

 lution. These are called the magnetic poles of the 

 earth, or by Hansteen magnetic points of convergence. 

 These four points are considered to have a regular 

 motion round the globe, the two northern ones from 

 west to east, and the two southern ones from east to 

 west. By the assistance of recorded observations, 



