32 WILLIAM H. ALEXANDER Vol. XXII, No. 1 



the lightning discharge is exceedingly variable, ranging from 

 0.0002 second for a single flash to, in rare cases, even a full 

 second or more for a m.ultiple flash consisting of a primary and 

 a series of subsequent flashes. The lightning discharge is 

 direct, not alternating, as shown by the fact that the lightning 

 may operate telegraph instruments, may reverse the polarity of 

 dynamos, both of which requires a direct current. 



The length of the lightning streak also varies greatly. 

 When the discharge is from cloud to earth the length is seldom 

 more than 2 or 3 kilometers, but when from cloud to cloud 

 may be 10 to 20 kilometers (6 to 12 miles). The path of the 

 lightning discharge may extend from the cloud to earth, from 

 one portion to another of the same cloud, or from one cloud to 

 another cloud. Obviously the second case is of the most fre- 

 quent occurrence, that is, from the upper to the lower portion 

 of the same cloud; from cloud to earth is next in point of fre- 

 quency, and from cloud to cloud, relatively rare. Sometimes 

 the discharge from cloud to earth may include in its strange 

 and tortuous path objects that have not sufficient conductivity 

 to carry it and as a result of the sudden and excessive heating 

 many very freakish things may be done, such as shingles blown 

 off, chimneys shattered, trees stripped of their bark or splin- 

 tered, wdres fused, even holes melted through metal, etc. Then 

 there are certain chemical reactions resulting from these elec- 

 trical discharges that play an important part in the economy of 

 nature. For instance the health-giving ozone of the atmosphere 

 is greatly increased by the passage of a thunderstorm, and even 

 the fertility of the soil may be increased by the production of 

 considerable quantities of ammonia and soluble salts. 



Perhaps, a w^ord or two should be said regarding the danger 

 to life incident to the passage of a thunderstorm. That there 

 is danger, even great danger at times, is abundantly shown 

 from the tragic statistics of deaths each year from this cause. 

 While it is not possible, perhaps, to remove this danger, it is 

 possible to reduce it, chiefly by avoiding the points of greatest 

 danger. In general, it is safer inside than outside of a house, 

 especially if the house has a well-grounded rod or metal roof; 

 it is also safer in the valley than on a hill or elevated portion 

 of land, this because the chance for a cloud-to-earth discharge 

 varies inversely as the distance between them; it is also very 

 unsafe to take refuge under a tree and the taller the tree the 



