150 Diurnal Variations of the Magnetic Needle. 



sing rarefied and more or less humid air until it reaches the polar 

 regions*, where finding an atmosphere saturated with humidity, it 

 will combine readily with the negative electricity accumulated on the 

 earth. We have thus the circuit formed; each annular stratum of 

 the atmosphere gives rise to a current, which proceeds, in the elevated 

 regions, from the upper portion of the stratum towards the pole, re- 

 descends to the earth through the atmosphere surrounding the poles, 

 and returns by the surface of the globe from the pole to the lower 

 part of the stratum from which it started. These currents will 

 constantly be the more numerous, and the more concentrated, the 

 nearer we approach the pole ; and as they all proceed in the same 

 direction, that is to say, from south to north, in the upper portion of 

 the atmosphere, and from north 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 currents pro- 

 duced by the equatorial strata are individually stronger than those 

 proceeding from more northerly strata, the difference, 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 hemisphere ; 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 observer 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 sur- 

 face 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 vari- 

 able ; it is, according to my theory, the parallel 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 variations of 

 the magnetic needle. In conformity with the laws established by 

 Limpere, 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 occurs 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 elec- 

 tric conditions between the lower and the upper stratum of the atmo- 

 sphere ; thus the deviation increases from the morning to l h 30 m 

 P.M. It is 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 , 



