the Magnetic Needle and on Aurora Boreales. 289 



mosphere and from nortli to south on the surface of the earth, 

 their effect will become the more perceptible in proportion as 

 we leave the equator and approach the pole. But as the cur- 

 rents produced by the equatorial strata are individually stronger 

 than those proceeding from more northerly strata, the dif- 

 ference, although real, will notwithstanding be less than 

 would be believed. What passes in our northern hemisphere 

 must occur in exactly the same manner in the southern hemi- 

 sphere; the currents proceed equally from the equator to the 

 pole in the upper regions of the air, and from the pole to the 

 equator on the surface of the earth ; consequently, for an ob- 

 server travelling from the north pole to the south, the current 

 would proceed in the same direction from the northern pole 

 to the equator, and in a contrary direction from the equator 

 to the southern pole : I speak here of the current circulating 

 on the surface of the earth. I ought moreover to observe, 

 that the limit which separates the regions occupied by each of 

 these two great currents is not the equator properly so called, 

 for it must be variable; it is, according to my theory, the pa- 

 rallel between the tropics which has the sun at its zenith ; it 

 changes consequently each day. 



Now it is easy to conceive the cause of the diurnal varia- 

 tions of the magnetic needle. In conformity with the laws esta- 

 blished by Ampere, the current which proceeds from the 

 northern pole to the equator ought to cause the north pole of 

 the needle to deviate to the west, which is what takes place in 

 our hemisphere ; and the current which proceeds from the 

 southern pole to the equator should cause the north pole of 

 the needle to deviate to the east, which is precisely what oc- 

 curs in the southern hemisphere. The deviation should be, 

 in one and the same place, the more considerable the greater 

 the difference of temperature, and consequently of the electric 

 conditions between the lower and the upper stratum of the 

 atmosphere ; thus the deviation increases from the morning to 

 jh 3om p jyj^ j[ jg more considerable in those months during 

 which the sun is longer above the horizon ; it is at its minimum 

 in the winter months. Lastly, these diurnal variations increase 

 in magnitude in proportion as we recede from the equator and 

 approach the pole, a result which again perfectly agrees with 

 what I have stated respecting the increase in number of the cur- 

 rents towards the polar regions. In these regions themselves 

 the variations may be very irregular, and may be entirely 

 absent if the magnetic needle happens to be [)lacedin those very 

 localities where the electric currents traverse the atmosphere 

 to reach the earth; in fact, a needle surrounded thus on all 

 sides by currents is no longer affected by them, or at least is 



Phil. Mag. S. 3. .Vol. 34-. No. 229. April 1849. U 



