214 ELEMENTS OF STATIC ELECTRICITY. 



cloud, must iii itself have great energy, greatly intensi- 

 fies these effects. 



DISCHARGE FROM THE CLOUDS TO THE EARTH. 

 We have already seen that the potential of the atmos- 

 phere, and hence of the clouds, is almost invariably 

 positive with reference to that of the earth. Hence the 

 earth's surface under a thunder cloud, and all objects 

 connected with it, become negatively electrified by in- 

 duction, to the same degree that the cloud is positive ; 

 electricity, equal to the charge of the cloud, being re- 

 pelled from the earth's surface to its interior. A result of 

 this difference of potential is a strong attraction between 

 the earth and cloud, by which the cloud is drawn towards 

 the earth; and, unless its potential is reduced by discharge 

 into another cloud, a discharge to the earth is inevitable, 

 whenever, from reduction of distance, the resistance of 

 the air becomes less than the electric tension of the cloud. 



When there are two clouds at different altitudes, and 

 a discharge takes place from the upper to the lower 

 cloud, the difference of potential between the latter and 

 the earth, being thus increased, the liability of a dis- 

 charge from it to the earth is increased in the same ratio. 



If there are elevated objects, such as trees and build- 

 ings, on the surface below, the resistance between them 

 and the cloud is less than that of the surrounding flat 

 surface ; not only on account of reduced distance, but 

 also on account of the points and angles which they 

 present. Hence, we find, that trees, flag-staffs, tele- 

 graph poles, church spires, chimneys, and projecting 

 corners of roofs are much more frequently struck by 

 lightning than flat surfaces. 



Good conductors, such as tin gutters, metal cornices, 

 and ornamental iron work, also offer far less resistance 



