LIGHTNING AND THUNDER 129 



southern Colorado blew all the leaves off a Cot- 

 tonwood clump without other visible injury. 



Neither the wood in lightning-struck trees nor 

 the chunks of exploded ones as a rule show signs 

 of heat or fire injury. Limbs of a lightning-struck 

 oak in southern Colorado, however, were shattered 

 and frayed out so that they appeared more like 

 shredded hemp than anything else. 



On examining a tree that I saw struck, there were 

 two parallel lines of rupture grooves about four in- 

 ches apart down the trunk. Either the bolt had 

 divided before striking the tree, or else two bolts 

 had struck the tree at about the same spot and 

 instant. 



Apparently a bolt striking a tree-top follows 

 down the grain of the wood — follows even the 

 intensive twists of a tree from the top, where it 

 strikes, to the earth. In some trees this twist 

 of the grain was so spiral that the bolt passed three 

 times around the tree trunk in its descent to the 

 earth. 



Usually the bolt plows a tiny U-shaped groove 

 through the bark without otherwise injuring the 

 tree. The lightning-struck tree, unless shattered 

 to pieces, usually survives, but the openings which 

 the lightning makes through the bark allow the 

 entrance of insect enemies which frequently are 

 detrimental. 



There is not a complete agreement as to just 

 what produces this wrecking explosiveness of light- 



