Popular Science Monthly 



torily explained about seven years ago by 

 Dr. George Simpson, of the Indian Meteor- 

 ological Service. The 

 first stage in the pro- 

 duction of a thunder- 

 storm is a violent and 

 turbulent uprush of 

 air, resulting in rapid 

 condensation of mois- 

 ture in the form of the 

 immense clouds that 

 characterize such a 

 storm. The drops of 

 water constituting 

 these clouds are re- 

 peatedly broken up 

 by the air currents, 

 and it has been proved 

 by laboratory experi- 

 ments that this process in- 

 volves the separation of posi- 

 tive from negative electricity. 

 The drops become positively 

 charged; i. e., they retain a 

 greater number of positive 

 than of negative ions. The 

 latter are set free and carried 

 aloft to the upper part of the 

 cloud, giving it a strong nega- 

 tive charge; while the positive 

 ions are carried down with 

 rain. If the process continues 

 long enough, a strong potential 

 gradient is set up between 



A house equipped with properly grounded 

 lightning rods is practically safe from lightning 



cloud and earth, until ultimately the 

 tension is relieved by an electrical discharge. 

 Lightning occurs in 

 many forms and pre- 

 sents many curious 

 features, not all of 

 which are fully un- 

 derstood. Besides the 

 ordinary linear 

 flashes, we have the 

 familiar phenomenon 

 of "sheet lightning," 

 which is, as a rule, 

 merely the reflection 

 on the clouds of dis- 

 tant linear lightning, 

 but may sometimes 

 be an actual diffuse 

 discharge. Dr. 

 Walter Knoche, director of the 

 meteorological service of Chile, 

 has recently described a re- 

 markable form of sheet light- 

 ning that occurs on a vast 

 scale along the crest of the 

 Andes during the warm season. 

 The mountains seem to act as 

 gigantic lightning-rods, giving 

 rise to more or less continuous 

 diffuse discharges between 

 themselves and the clouds, 

 with occasional outbursts sim- 

 ulating the beams of a vast 

 searchlight. These displays 



How a woman was killed 

 when a bolt struck the tin 

 roof of an unprotected house 



The' modern method of fastening lightning rods to buildings. Insulators in clamps are no 

 longer considered necessary except in localities where prejudice in their favor still remains 



