On Atmospheric Maynetism. 



65 



from other parts as shewn by P P iu the figure, or if a 

 worse conductins: substance is introduced, the lines are 



opened out as at D D. In both cases the lines 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 Faraday's paper in the Philosophical Transac- 

 tions for 1851, Part I. Par. 2843, &c. 



Now this by the hypothesis is assumed to take place in 

 the atmosphere. Supposing it all at mean temperature, the 

 lines of force would have the direction determined by the 

 arrangement of the power within the earth. Then the sun's 

 presence in the east would make all the atmosphere in that 

 region a worse conductor, and cause it to assume the cha- 

 racter of D ; and as the sun came up to and passed over the 

 meridian and away to the west, the atmosphere under his 

 influence would bring up changes in direction like those 

 shewn in either D or D ; it would therefore manifestly set 

 a needle in a given latitude in opposite directions as it passed 

 by ; and as evidently set two needles in north and south 

 latitudes in opposite directions at the same moment of time. 

 As the night came on and a temperature lower than the 

 mean came up from the east and passed over, the lines of 

 force would be inflected as in P or P, and a reverse variation 

 of the needle to that which occurred before would now take 

 place. 



That natural efi^ects of variation must be produced con- 

 sequent upon the magnetic nature of oxygen and its daily 

 variations of temperature is manifest; but whether they 

 cause the observed variations, or are competent to do so, is 

 a question that can only be decided after very careful inquiry. 

 Observations are now made on the surface of the earth with 

 extreme care in many places, and these are collated, and the 



VOL. LI. NO. CI. — JULY 1851. E 



