THUNDER AND LIGHTNING. 



soil under which the subterranean reservoir lies, a discharge 

 takes place, and the lightning penetrates the strata, fusing the 

 materials of which it is composed, and leaving a tubular hole with 

 a hard vitrified coating. 



Tubes thus formed have been called fulgurites, or thunder 

 tubes. 



19. The well known properties of points, edges, and other 

 projecting parts of conductors, will render easily intelligible the 

 influence of mountains, peaked hills, projecting rocks, trees, 

 lofty edifices, and other objects, natural and artificial, which 

 project upwards from the general surface of the ground. 

 Lightning never strikes the bottom of deep and close valleys. 

 In Switzerland, on the slopes of the Alps and Pyrenees, and in 

 other mountainous countries, multitudes of cultivated valleys are 

 found, the inhabitants of which know by secular tradition that 

 they have nothing to fear from thunder-storms. If, however, the 

 width of the valleys were so great as twenty or thirty times their 

 depth, clouds would occasionally descend upon them in masses 

 sufficiently considerable, and lightning would strike. 



Solitary hills, or elevated buildings rising in the centre of an 

 extensive plain, are peculiarly exposed to lightning, since there 

 are no other projecting objects near them to divert its course. 



Trees, especially if they stand singly apart from others, are 

 likely to be struck. Being from their nature more or less im- 

 pregnated with sap, which is a conductor of electricity, they 

 attract the fluid, and are struck. 



The effects of such objects are, however, sometimes modified by 

 the agency of unseen causes below the surface. The condition of 

 the soil, subsoil, and even the inferior strata, the depth of the 

 roots and their dimensions, also exercise considerable influence on 

 the phenomena, so that in the places where there is the greatest 

 apparent safety there is often the greatest danger. It is, never- 

 theless, a good general maxim not to take a position in a thunder- 

 storm either under a tree or close to an elevated building, but to 

 keep as much as possible in the open plain. 



20. Lightning falling upon buildings chooses by preference the 

 points which are the best conductors. It sometimes strikes and 

 destroys objects which are non-conductors, but this happens 

 generally when such bodies lie in its direct course towards con- 

 ductors. Thus lightning has been found to penetrate a wall 

 attracted by a mass of metal placed within it. 



Metallic roofs, beams, braces, and other parts in buildings, are 



liable thus to attract lightning. The heated and rarefied air in 



chimneys acquires conductibility. Hence it happens often that 



lightning descends chimneys, and thus passes into rooms. It 



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