2i 4 SUMMER 



a distance of at least a hundred miles. At forty or fifty 

 miles' range the display is vivid, and we may stand in the 

 calm though sultry night and watch the flicker of several 

 storms in opposite quarters over far-distant counties. The 

 sense of distance gives a kind of awe to the scene, and close 

 at hand the nightingale sings fitfully between the flashes. 



Apart from the soundless display of distant storms, light- 

 ning and thunder are inseparable. We see the flash before 

 we hear the crash, because light travels far more rapidly than 

 sound ; and we can reckon the distance of the flash by the 

 interval at which we hear the thunder, and its length by the 

 time which the clap of thunder lasts. The ratio is one mile 

 for every five seconds. Lightning flashes of three miles in 

 length are not uncommon; and their shape, which can be 

 accurately registered by photography, is very variable, and 

 depends on the local resistance of the air, or the occurrence 

 of solid particles which divert the path of the discharge. It 

 follows the line of least resistance, like a crack in a film of 

 drying mud. This linear formation is particularly well 

 marked in the case of what is commonly known as forked 

 lightning. The zigzag line which stands as the conventional 

 form of lightning in popular imagination is scientifically 

 accurate in a certain proportion of flashes, which impress 

 themselves even on a naked eye by their bold and definite 

 shape. But photographs show that lightning more usually 

 takes a veined or rootlike form, sometimes very intricate, 

 and of great beauty, though very unlike the sharply refracted 

 outline of the occasional zigzag flash. The spark of fire 

 heats the air, and the rush into the vacuum so caused sends 

 out the rumbling crash of the thunder. 



Fireballs or thunderbolts are in many cases incandescent 

 meteorites flying from outer space and finding a target in the 

 earth, but some of them are genuine electric discharges. 



