3.30 MISCELLANEOUS PHENOMENA. 



constantly drawn from the transparent aqueous 

 vapour by the extended surface of falling cloud, 

 prevents it from accumulating into a large ball 

 of fire or thunder-bolt, in the same way that a 

 damp atmosphere prevents us from charging the 

 Ley den battery of a common electrical machine. 

 The streaked and forked appearance of ordi- 

 nary lightning are optical illusions, which result 

 from the rapid passage of the electric spark 

 through the air, which makes an impression on 

 the optic nerve that remains during its passage, 

 in the same way that a burning stick when re- 

 volved, presents the appearance of a continuous 

 circle of fire. The fork is owing to a division of 

 the original spark or ball into two or more sparks, 

 which diverge from each other. The zigzag ap- 

 pearance is owing to frequent slight deflections 

 of the spark, as it moves through strata of air, of 

 different densities. Whenever an electric explo- 

 sion takes place near to the spectator, it always 

 presents the appearance of a ball of fire, at- 

 tended by a simultaneous report, like that of 

 artillery. The reason that the report is usually 

 heard many seconds after the flash is seen, is 

 owing to its distance from the point of observa- 

 tion, and to the slowness with which vibrations 

 are propagated through the atmosphere : while 

 its rumbling or prolonged existence is owing to 

 the space over which it passes. The report 

 itself is probably caused by a sudden collapse of 



