LIGHTNING. 



289 



as a conductor. Bed is the safest place, as blankets are non-conductors. 

 Cellars are not the safest by any means. Lightning may, and it frequently 

 does, strike the house and descend to the basement. If the air be very full 

 of electricity, and a flash be near, a 

 person running away may conduct 

 the lightning to himself by creating a 

 vacuum into which the flash may 

 dart. 



Arago classified lightning into 

 three kinds : zig-zag, globular, and 

 sheet. The first we call forked 

 lightning, and frequently this kind 

 branches out at the end, so that 

 although there may be only one flash, 

 it may strike out in two or three 

 directions at the same time. This 

 may be accounted for by the unequal 

 power of the air strata to conduct the 

 electricity. The forked flashes are 

 of very great length, extending fre- 

 quently for miles, and the bifurcations 

 also are often miles apart. The dura- 

 tion of the flash is less than the 

 thousandth part of a second 



Fig. 289. Nimbus, or rain cloud. 



a second ; so 



instantaneous is it that no motion 

 can be perceived even in . a most 

 rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes 

 fancy that the flash lasts longer, but the impression received by the eye 

 quite accounts for the apparently prolonged sight of the lightning. 



Sheet lightning, the faint flashes frequently seen upon the horizon, 

 are quite harmless. Sheet lightning is that which is seen reflected bchjnd 

 clouds or from far-distant storms. It is sometimes very beautiful. Ball, 

 or globular lightning, is dangerous, and globes of fire have been seen to 

 descend, and striking the ground, bound onwards for some distance. The 

 descent of these forms of electric discharge has given rise to the popular 

 notion of "thunderbolts." The "Mariner's Lights," or St. Elmo's fire, is 

 frequently observed in ships. It is usually regarded as a fortunate occurrence. 

 It was noticed by Columbus. One voyager describes the phenomena as 

 follows : " The sky was suddenly covered with thick clouds. . . . There were 

 more than thirty of St. Elmo's fires on the ship. One of them occupied 

 the vane of the mainmast. I sent a sailor to fetch it. When he was aloft 

 he heard a noise like that which is made when moist gunpowder is burned. 

 I ordered him to take off the vane. He had scarcely executed this order, 

 when the fire quitted it and placed itself at the apex of the mainmast, 

 whence it could not possibly be removed." 



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