On the Electrical Condition of the Atmosphere. 127 



producing aurora, by bringing the superior portions of the air 

 nearer to the earth. 



186. On atmospheric electrical maxima and minima. — "It has 

 been ascertained by the observations of De Saussure, Schubler, 

 Arago and others, that the positive electricity of the atmosphere 

 is subject to diurnal variations of intensity, there being two 

 maxima and two minima during the twenty-four hours. The 

 first minimum takes place a little before the rising of the sun ; 

 as it rises, the intensity, at first gradually and then rapidly, in- 

 creases, and arrives at its first maximum a few hours after. This 

 excess diminishes at first rapidly and afterwards slowly, and 

 arrives at its minimum some hours before sunset ; it reascends 

 when the sun approaches the horizon, and attains its second 

 maximum a few hours after, then diminishes till sunrise, and 

 proceeds in the order already indicated. The intensity of the 

 free electricity of the atmosphere has also been found to undergo 

 annual changes, increasing from the month of July to the month 

 of November inclusive, so that the greatest intensity occurs in 

 winter, and the least in summer*." 



187. When the sun has but a small elevation above the horizon, 

 its rays enter the atmosphere so as to traverse large distances of 

 air, and the rays are thereby more freely absorbed by the air at 

 the place of observation than when the sun stands higher ; and 

 at sunrising, the clouds which may have fomied during night 

 will still further intercept the sun's heat ; and it is obvious, that 

 the air at little distances from the ground will be less heated 

 than the su])erior strata, which receive the more undiminished 

 rays. Now the efiiect of this wUl be to produce an expansion 

 of the atmosphere, and the expansion of the higher strata will 

 be greater than that of the lower strata ; consequently that part 

 of the negative atmospheric orb, over-head at the place of obser- 

 vation, becomes lifted up, and more separated from the inferior 

 positive orb than it was before sunrise ; and the appearance pre- 

 sented to us is that of an increased positive tension of the atmo- 

 sphere, just as when the plates of an ordinary condenser are 

 separated. And this, I think, gives rise to the first maximum 

 after sunrise. With regard to the minimum which next occurs, 

 it must be observed, that as the sun ascends, the proportion of 

 its rays absorbed by the ground increases, both absolutely and 

 in comparison with the quantity absorbed by the atmosphere 

 during their passage through it. Therefore the lower strata of 

 the atmosphere now become more especially the seat of the ex- 

 pansion produced by the sun's heat ; the result of which is, that 

 the over-head portion of the positive orb is uplifted, and its influ- 



* Council of tlie Koynl Society, Phil. Mag. S.J. vol. xv. j). 1219. 



