666 PROTECTION AGAINST LIGHTNING. 



positively electrified cloud induces the separation of the 

 electricity in a tree, driving the positive electricity into its 

 roots and the earth, whilst the tree becomes charged with 

 negative electricity. The strength of this charge becomes 

 weakened by gradual discharge into the atmosphere from the 

 numerous twigs and leaves in the crown of a vigorous tree. 

 On the contrary, a tree with many dry branches and scanty 

 foliage becomes thoroughly charged with negative electricity, 

 and when struck by lightning receives a more violent shock 

 than a sound tree. 



e. Season. 



In Central and Western Europe the most frequent thunder- 

 storms are in June and July, between 3 and 5 p.m. or 1 and 

 2 a.m. These storms usually pass from S.W. to N.E. or 

 from W. to E. 



In the case of heavy rain before the lightning-stroke, the 

 trees become better conductors, and are more liable to be 

 struck. 



The average rate at which thunder-storms travel in South 

 Germany is given by C. Lang, the result of five years' obser- 

 vation, as 25^ miles an hour, which agrees with French 

 observations. In 1886, the greatest rate was 49 miles, the 

 least 6| miles, an hour. 



3. Register of Damage by Lightning. 



The frequency of thunder-storms in Central Europe decreases 

 as the latitude increases and in proximity to the Atlantic 

 Ocean, as the following average figures show : — 



