Peoceedixgs of the Polytechnic Association. 923 



Si:coND. — Sheet Lightning. 



Lightnino-s of this class present a light, which, instead of being 

 concentrated into lines without apparent breadth, embrace on the 

 contrary, immense surfaces. 



They have neither the whiteness nor the vivacity of forked liglit- 

 nings; are more sluggish, occupying ap}>recial)lc time in their dis- 

 chai'ges ; their tint is generally oi intense red, though other colors 

 sometimes prevail. Their bright light sometimes embraces the 

 whole superiicial extent of a cloud, and even of a whole gr<.)np of 

 clouds; at others, only j)art of a cloud, or tiie border; it is fre- 

 quently undulatory and flickering. The discharges appear to be 

 interstitial, darting among the globules or ultimate vesicles of the 

 cloud, which seem to servo a ])urpose in some way resend^ling the 

 metallic particles of the spangled pane of the lecture room. Some- 

 times the discharges appear to be made among the larger individuali- 

 ties of tlie cloud, and are exhil)ited in great numbers of interior 

 flashes and zigzag flames of red light following each other in almost 

 continual succession. 



The lightnings of this second class are the most numerous of any, 

 and are especially interesting on the present occasion on account of 

 the aiualogy existing between their genesis and that of aurora hore- 

 aUs. 



Silent flashes of lightning are frequently seen along a cloudless 

 horizon, formerly called heat lightnings. Since the establishment 

 of the telegraph, we learn that they are the lightnings of real storm, 

 so far away as to be entirely below the plane of the horizon and too 

 far oft' to be heard. 



Third. — Ball Lightning. 



Lightnings of this class are much less numerous than tliose of aiiy 

 other, and are Ijy far tlie most diflicult of comprehension, liaving 

 never l)een satisfactorily accounted for. They are veritable globes 

 of Are, several inches in diameter, and are transported from the 

 cloud to the earth with sufficient slowiiess to be distinctly followed 

 by the eye, being visible for two or three an.d even for ten seconds. 



They sometimes di\-ide and rcbouiul iq)on the gi'ound several 

 times ; sometimes they burst, causing the detoiuitions of heavy 

 ordiumce. 



They sometimes enter dwellings, barns and outdiouses, enveloping 

 the whole building in flames in an instant. 



They seem so pay no regard to conductors either good or bad. 



