On Atmospheric Magnetism. 63 



line of magnetic force may vary exceedingly from a straight 

 line to every degree of curvature, and may even have double 

 and complicated curvatures impressed upon it. Its direction 

 is determined by its polarity, the two changing together. 

 Its povi^ers are such, that a magnetic needle placed in it 

 finds its place of rest parallel to it ; a crystal of calcareous 

 spar turns until its optic axis is transverse to it ; and a wire 

 which is unaffected when moved in or along it, has an elec- 

 tric current evolved the instant that it passes across it : by 

 these and by other means the presence of the magnetic line 

 of force and its direction are rendered manifest. 



The Earth is a great magnet : its power, according to Gauss, 

 being equal to that which would be conferred if every cubic 

 yard of it contained six one-pound magnets ; the sum of the 

 force therefore is equal to 8,464,000,000,000,000,000,000 

 such magnets. The disposition of this magnetic force is not 

 regular, nor are there any points on the surface which can 

 be properly called poles : still the regions of polarity are in 

 high north and south latitudes ; and these are connected by 

 lines of magnetic force (being the lines of direction) which, 

 generally speaking, rise out of the earth in one (magnetic) 

 hemisphere, and passing in varied directions over the equa- 

 torial regions into the other hemisphere, there enter into the 

 earth to complete the known circuit of power. A free needle 

 shews the presence and direction of these lines. In London 

 they issue from the earth at an angle of about 69° with the 

 horizon (being the dip or inclination) ; and the plane in which 

 they rise forms an angle of 23° W. nearly with true north, 

 giving what is called west declination. Where the dip is 

 small, as at the magnetic equator, these lines scarcely rise 

 out of the earth and pass but a little way above the surface ; 

 but where it is large, as in northern or southern latitudes, 

 they rise up at a greater angle, and pass into the distant 

 realms of space, from whence they return again to the earth 

 in the opposite magnetic hemisphere ; thus investing the 

 globe with a system of forces like that about an ordinary 

 magnet, which wherever it passes through the atmosphere 

 is subject to the changing action of its magnetic oxygen. 

 There is every reason to believe that these lines are held in 

 the earth, out of which they arise and by which they are 



