ATMOSPHERICAL ELECTRICITY. 335 



which rise above the crater of the volcano of Toluca, at an 

 elevation of nearly 15300 English feet), but they have also 

 been seen over the lowlands in the temperate zone at a 

 measured vertical elevation of 25000 (26650 English) feet. 

 On the other hand, the stratum of cloud in which thunder is 

 taking place sometimes sinks to a height of only 5000 or 

 even 3000 feet above the plain. 



According to Arago's investigations, which are the most 

 complete and comprehensive that we yet possess on this 

 difficult portion of meteorology, the disengagement of light 

 (lightning) is of three kinds: the zigzag form, sharply 

 defined at the edges ; lightning without definite form, illumi- 

 nating at the same instant the whole cloud, which appeals 

 as it were to open and display its inner recesses; and 

 lightning in the form of balls of fire. ( 416 j The duration 

 of the two first kinds is scarcely the thousandth part of a 

 second; but the globular lightning moves far more slowly, 

 its appearance lasting for several seconds. Recent observa- 

 tions confirm the phenomenon described by Nicholson and 

 Beccaria, in which, without any audible thunder or any indi- 

 cation of storm, isolated clouds remain stationary for 

 some time high above the horizon, and lightning pro- 

 ceeds uninterruptedly both from their interior and from 

 their margins : hailstones, drops of rain, and flakes of 

 snow, have also been seen to appear luminous with 

 electric light when there has been no thunder. In the 

 geographical distribution of thunder-storms, the Peru- 

 vian coast, where thunder and lightning are unknown. ( 417 j 

 presents the most striking contrast to the rest of the torrid 

 zone, in which at certain seasons of the year storms take 



place almost daily, four or five hours after the sun has reached 

 VOL. I. 2 A 



