70 On Atmospheric Magnetism. 



presence of the magnetic line of force and its direction are' reti 

 dered 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 ov^er the equatorial regions into the other 

 hemisphere, there enter into the earth to complete the known 

 circuit of power. A free needle shows 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 op- 

 posite 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 produced, just as the lines 

 which originate in a magnet are held by it, though not in the 

 same degree; and that any disturbance from above affecting 

 them will cause a greater change in their place and direction in 

 the atmosphere and space above, than in the earth beneath. 



The system of lines of magnetic force around a magnet or 

 the earth is related by a lateral tension of the whole, analogous 

 in some degree to the lateral tension of lines of static electrical 

 force ; both the one and the other being easily made manifest by 

 experiment. The disturbance of the tension in one part is ac- 

 companied instantly by a distiubance of the tension in every 

 other part ; for as the sum of the external powers of a system^ 

 unaltered at its origin, is definite and cannot be changed; so any 

 alteration either of tensity or direction amongst the lines of force 

 at one place, must be accompanied by a corresponding change at 

 every other. So if a mass of soft iron on tlft east side of a 

 magnet causes a concentration of the lines of force from the 

 magnet on that side^ a corresponding expansion or opening out 



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