the Unipolar Induction of the Earth. 501 



intensity 0*0231 D, a force causing positive electricity to move 

 upwards. The sum of electromotive forces in a layer 1000 

 metres in thickness is 23*1 D, as against 231 D in a layer 

 of 10,000 metres. Now, if the air were an absolute non- 

 conductor, the production of a difference in electric tension 

 between the air at the surface of the earth and the air of the 

 upper regions would be totally impossible. But this is only 

 the case when the air is perfectly dry. If, on the contrary, it 

 is saturated with moisture, it forms a comparatively good 

 conductor of electricity, as is proved, amongst other things, 

 by the impossibility of maintaining an electric charge, how- 

 ever feeble, on a body surrounded by air of which the relative 

 humidity is considerable. Thus, when the air is moist, the 

 difference of tension may soon attain the limits mentioned ; 

 but when the electricity has reached a certain distance from 

 the equatoreal plane, it is conveyed, by the horizontal force of 

 unipolar induction, towards more elevated latitudes — towards 

 the north in the northern hemisphere, and towards the south 

 in the southern hemisphere. The force of unipolar induction 

 is always the same, where not subject to small variations 

 produced by those of the terrestrial magnetism; but the con- 

 ductibility of the air depends upon the changes which take 

 place in its humidity. The result is a perpetual variation in 

 the electric condition of the air. If the electric tension, at 

 any point under a high latitude, becomes greater than that 

 which corresponds to the vertical component of unipolar induc- 

 tion at the same point (which is possible, since electricity 

 is brought there from points situated in lower latitudes), part 

 of the electricity will flow from the air to the earth. As is 

 shown in the preceding memoir, this takes place especially 

 in the zone surrounding the magnetic poles, where we meet 

 with the aurora borealis. If the air is highly saturated with 

 moisture, and if this saturation extends through some con- 

 siderable space, the air will become more highly electric than 

 when the air is relatively dry. Consequently the air will 

 generally be greater in winter than in summer, after cooling 

 at night. The air usually attains its maximum relative 

 humidity in the early hours of the day, diminishing in the 

 morning as the air becomes heated, attaining its minimum 

 soon after noon, and then increasing in intensity towards the 

 evening. All this is in complete agreement with well-known 

 observations. On the other hand, it appears more difficult to 

 explain the minimum of aerial electricity which, according to 

 certain observations, is produced in the course of the night ; 

 at least if we do not admit that, in those regions where this 

 minimum has been observed, local causes have produced a 



