o( 



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 positive. 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 C 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 



