508 THUNDER-STORMS. 



a violent thunder-storm. Meanwhile the heavens remained perfectly clear, the 

 sun shining with unusual splendor. No one thought for a moment of danger; 

 nevertheless, a flash of lightning, ascending from the cloud, struck the church, 

 and killed seven persons who were in company with Werioschnigg. 



It is, therefore, clearly established that lightning may issue upward from 

 thunder-clouds. 



LIGHTNING. 



Lightnings are resolved by M. Arago into three classes : First, the zigzag, 

 which present the appearance of narrow, well-defined threads or lines of light, 

 following a course which is clearly enough expressed by their name. In 

 color they vary, being often white, sometimes purple, blue, or violet. Second, 

 those lightnings which appear diffused over extensive surfaces, and which are 

 commonly called sheet-lightning. In color these also vary, being often an in- 

 tense red, but occasionally white, blue, or violet. This lightning has an ap- 

 pearance of a momentary light seen through a plate of glass rendered semi- 

 transparent by having its surface ground. Third, lightning which moves 

 through the air at a comparatively slow rate, appearing like a luminous ball or 

 sphere, or like a globe of fire. Let us call this ball-lightning. 



The almost incredible velocity, as will hereafter appear, of lightning of the 

 first class, would hardly seem compatible with the sudden and extreme changes 

 of direction to which its motion is subject. This frequent reversion of direc- 

 tion has been more especially observed in the lightning which traverses vol- 

 canic clouds. Minute and circumstantial accounts of such appearances have 

 been supplied by Sir WILLIAM HAMILTON and others, who have observed the 

 eruptions of^Vesuvius. In the eruption of 1707, described by SORRENTINO, 

 the lightnings which issued from the crater traversed the cloud of ashes as far 

 as the cape Pausillippo, where the cloud terminated. After attaining that point 

 the lightning retraced its course, and struck the summit of the volcano. 



Sir William Hamilton states, that in the eruption of 1779 the lightning was 

 generally confined in its play to the cloud of ashes which extended toward 

 Naples ; that in traversing that cloud from the crater to its limits, it seemed to 

 menace the city with destruction ; but it, nevertheless, after reaching the limit 

 of the cloud, returned toward the crater, where it rejoined the ascending col- 

 umn whence it originally issued. 



Zigzag lightning seldom flashes between two clouds. It is generally mani- 

 fested between a cloud and some terrestrial object. 



It has been supposed that the extremity of the lightning of the first class has 

 a barbed form, like the point of an arrow. Of this there is no sufficient evi- 

 dence. It is, however, sufficiently ascertained that it is often attended by the 

 effect which has given it the name of forked lightning. Thus, when a single 

 luminous line issuing from a cloud has traversed a certain distance it will 

 sometimes divide itself into two lines, which, diverging at an angle more or \ 

 less considerable, will strike distant objects. In some cases it has been seen ) 

 to separate into three perfectly distinct lines. The former may be called bi- 

 cuspidated, and the latter tri-cuspidated lightning. 



Well-ascertained examples of these phenomena are rare ; the occasional 

 occurrence is not, however, the less certain. The abbe RICHARD states that 

 he witnessed a flash of lightning which left the cloud in a single line of light, 

 and at some distance from the earth dividing into two, and each part struck a 

 separate object. 



NICHOLSON states, that, in a storm which broke over the west end of London, 

 on the 19th of June, 1781, being at Battersea, he saw distinctly several flashes 

 of bi-cuspidated lightning. 



