J. 



On Atmospheric Magnetis7n. 



71 



of thlfe lines on the west side must be and is at the same time 

 produced; or if the siiiij on rising in the east, renders all the 

 oxygen of the air on that side of the globe less magnetic and 

 less able therefore to favor the transition of the lines of terrestrial 

 force theroj a greater number of them will be determined through 

 the western region ; and even though the lines of force may be 

 doubted by some as having a separate existence such as that 

 above assumed, still no error as to the effects on magnetic needles 

 would in that case be introduced, for they by experiment would 

 be and are the same. 



The power of a magnetic body as iron or oxygen to favor the 

 transmission of lines of force through it more than other bodies 

 not magnetic, may be expressed by the term conduction. Dif- 

 ferent bodies, as iron, nickel, oxygen, conduct in various degrees, 

 and not only that, but the same body as iron or oxygen conducts 

 m different degrees at different temperatures. When space trav- 

 ersed by uniform lines of magnetic force is occupied by a uni- 

 form body as air, the disposition of the hnes is not altered; but 

 if a better conducting substance than the air is introduced, so as 

 to occupy part of the space, the lines are concentrated in it, and 

 drawn from other parts as shown by P P, in the figure, or if a 



■^vorse conducting substance is introduced, the lines are opened 

 out as at D D. In both cases the hnes of force are inflected, and 

 a small magnetic needle standing in them at the inflected part 

 Would have its direction changed accordingly. Experimental 

 Illustrations of these changes in direction are given in Mr. Fara- 

 way's paper in the Philosophical Transactions for 1851, Part I, 

 ^ar. 2S43. &,c. 



Now ili'is by the hypothesis is assumed to take place in the at- 

 J^osphere. - Supposing it all at mean temperature, the lines of 

 orce would have the direction determined by the arrangement of 

 tne power within the earth. Then the sun's presence in the 

 east would make all the atmosphere in that region a worse con- 

 ductor, and cause it to assume the character oi D ; and as the 

 ^in came up to and passed over the meridian and away to the 

 6st, the atmosphere under his influence would bring up changes 

 j." "'fection like those shown in either D or D ; it would there- 

 ore manifestly set a needle in a given latitude in opposite direc- 



.'f'V 



tions 



and 



opposite 



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ii 



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