LIGHTNING AND THUNDER. 215 



than imperfect conductors, like wood, brick, and stone; 

 both from their superior conduct! vity, and their projecting 

 edges and points ; and when connected with a building 

 and not connected by a metallic conductor with the earth, 

 greatly increase the liability of the building, both to re- 

 ceive the electric discharge, and to sustain injury from it, 

 by making the building its terminus instead of the earth. 



DISCHARGE FROM THE EARTH TO THE CLOUDS. 

 As already shown, the electricity of a large cloud, like 

 that of a cylinder, may be so distributed by the prox- 

 imity of one end to another cloud, at a lower potential, 

 or to an elevated portion of the earth's surface, that the 

 potential of this end shall be higher than that of the 

 remote end. The potential of the earth's surface, be- 

 neath it, must also be similarly affected by induction, in 

 reverse order; being negative where the cloud is positive, 

 and positive where the cloud is negative. If, under 

 these circumstances, the difference of potential between 

 the negative end of the cloud and the earth becomes 

 greater than the resistance of the air, a discharge from 

 the earth to the cloud must occur; the discharge in this, 

 as in all other cases, being from higher to lower potential. 



These conditions are similar to those of the three 

 clouds already referred to : so that a discharge from 

 the positive end to another cloud, or to the earth, may 

 increase the difference of potential between earth and 

 cloud at the negative end. 



The resistance of the earth, also, over such an exten- 

 sive area, retards the restoration of surface equilibrium 

 after the discharge from the positive end; and increases 

 the liability of the return discharge from the earth to 

 the cloud, in the ratio of this resistance to that of the 

 vapor of the cloud. 



