AND HORTICULTURE. 5 
In the Paper’in which I treated these phenomena I have 
found the explanation of this periodicity in the different 
state of the heat radiating from the sun, either without 
spots or supplied with them. In the former.case the 
greater part of the heat rays are light, and of shorter wave 
length ; in the latter case the greater number are dark, and 
of greater wave length. The rays must pass through the 
atmosphere, and the darker heat rays will be absorbed 
during this passage rather than the lighter ones, and hence 
there will be more heat stored up in the atmosphere in the 
case when spots appear on the sun. This heat, generally 
contained in watery vapours, is transported through the 
air currents or winds into higher latitudes, and will there 
exercise its beneficent effect. Though I continue to regard 
this explanation as highly probable, I must introduce some 
modification, in order to ascribe a large influence to the 
Polar light, or rather its cause, the electric current from 
the atmosphere, 
In consequence of this modification of my ideas, I must 
give it an important place in order to explain the perio- 
dicity of the harvests. 
§ 8. Everyone who has given attention to the question 
of the causes of the.electrical conditions of the atmosphere, : 
knows that several theories on this matter have been 
brought forward. The most widely known suppositions 
are the following :— 
(a) Unipolar induction, caused by the earth, as a magnet, 
rotating around its axis, produces a force component, which 
carries the positive electricity up in the atmosphere. 
(4) Evaporation, which, according to the opinion of 
several physicists, produces electricity, the vapours being 
positive. 
(c) Unipolar induction working together with the evapora- 
tion in making every particle, great or little, carried from 
the earth, positively electric. 
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