EXHAUSTIVE inquiry has established the fact that light- 

 ning ranks next to railroads as a source of forest fires. 

 Forest officers say that the increasing care with fire on 

 the part of the railroads and the public generally tends to 

 make lightning the largest single contributing cause. 



This statement represents a change of view from that held 

 less than a decade ago in this country, when forest journals 

 gravely argued whether lightning caused forest fires, though 

 it was known that trees were the objects most often struck. 

 Trees are said to be oftenest struck simply because they are 

 numerous, and extending upward they shorten the distance 

 jtween the ground and the clouds; further, their branches 

 the air and roots well into the earth invite electrical dis- 

 larges. 



While certain trees are said to invite lightning, and others 

 be immune from stroke, it seems to be a fact that any kind 

 tree will be struck, and the most numerous tree species in 

 y locality is the one most likely to suffer. 

 Other things being equal, lightning seeks the tallest tree, 

 an isolated tree, or one on high ground. A deep-rooted 

 ree is a better conductor than a shallow-rooted one, and a 

 tree full of sap, or wet with rain, is of course a better con- 

 ductor than a dry one. 



Lightning sets fires by igniting the tree itself, particularly 

 when it is dead, or partly decayed and punky, or by igniting 

 the dry humus or duff at its base. The forest soil, when 

 dried out, ignites readily, because it is made of partly de- 

 cayed twigs and leaves, and it can hold a smouldering fire for 

 a considerable period. It is probable that most of the light- 

 ning-set fires start in the duff. 



In the mountains of Southern California, Arizona, and New 

 Mexico there are likely to be each year a number of electrical 



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