cause of a cloud being at certain times in a different state of 
electric excitation to the earth and other clouds, and of this 
difference being sometimes on the positive side and some- 
times on the negative, that is to say, why a cloud should 
sometimes appear to us on the earth to be positively charged, 
sometimes negatively, and at others not to be charged 
at all. 
The assumed condition of the sun and earth may be repre- 
sented by two conductors S and E acting on one another by 
induction, the sun being negative and the earth positiv e. The 
distance between these bodies is so great that the induc- 
tive action would not be confined to those parts which are 
opposed, but would in a greater or less degree extend all 
over their surfaces, though it would still be greater on that 
side of E which is opposite to S than on the other side. 
The conductor E must be surrounded by an imperfectly 
insulating medium to represent damp air. The formation 
of a cloud may then be represented by the introduction of 
a conductor C near to the surface of E. Such a conductor 
at first having no charge would attract the positive elec- 
tricity in E and appear by reference to E to be negatively 
charged. If it was near enough to E, a spark would at 
once pass, which would represent a flash of forked lightning. 
If it were not near enough for this it would obtain a charge 
through the imperfect insulation of the medium. Such a 
charge might pass quietly or by the electric brush. When 
the cloud had obtained a charge it would not exert any 
influence on the earth, unless it altered its position. But if 
the heat of the sun caused part of the cloud to evaporate the 
remainder would be surcharged and appear positive. Or if 
C approached E then G would be overcharged, and a part of 
its electricity would return, and on its return it might cause 
positive lightning. Thus, suppose that after a cloud had 
