OPTICAL PHENOMENA. 519 



each side of the line where the bell-wires had run, was blackened. The metal had been 

 fused, and although the room was about fifteen feet high, the globules, dropping on the 

 chairs and furniture, had drilled in them a chain of minute holes. A part of the wall 

 was shattered as if by gunpowder, and the fragments had been blown off with force 

 sufficient to indent the wall on the opposite side of the room. The frame of a looking-glass 

 was blackened ; the gilding must have been volatilized, for a smelling-bottle, which stood 

 on the chimney-piece, was coated with bright metallic particles, which adhered as firmly 

 as if they had been enamelled." Near the shores of the Rio Plata, in a broad band of 

 sand hillocks, he found those singular specimens of electric architecture, a group of 

 vitrified siliceous tubes, formed by the lightning striking into loose sand. These tubes 

 had a glossy surface, and were about two inches in circumference, the thickness of the 

 wall of each tube varying from the twentieth to the thirtieth part of an inch. Four sets 

 were noticed, probably not produced by successive distinct discharges, but by the 

 lightning dividing itself into separate branches before entering the ground. Similar 

 cylindrical formations have been noticed in other places. Dr. Priestley has described, in 

 the Philosophical Transactions, some siliceous tubes, which were found in digging into the 

 ground, under a tree, where a man had been killed by lightning ; and at Drigg in Cum- 

 berland, three were observed, within an area of fifteen yards, one of which was traced to 

 a depth of not less than thirty feet. In the temperate climates, electrical phenomena 

 are most common and usually most energetic in the summer season, and the displays are 

 grander and more formidable in mountainous than in level countries. As we approach 

 the poles, they become less striking ; thunder is rarely heard in high northern latitudes, 

 and only as a feeble detonation ; and though lightning is more common, it is seldom 

 destructive. In Iceland, in the winter, it often plays in the impressive but harmless 

 manner which the natives call laptelltur. This is a fluctuating appearance of the whole sky. 

 as if on fire, accompanied by a strong wind and drifting snow, but inflicting no further 

 damage than that arising from the terrified cattle falling over the rocks in their efforts to 

 escape from the phenomenon. 



The rapidity of lightning, as measured by means of the camera lucida, M. Halvig estimates 

 at probably eight or ten miles in a second, or about forty times greater velocity than 

 that of sound ; and according to M. Gay Lussac, a flash sometimes darts more than three 

 miles at once in a straight direction. M. Arago distinguishes three classes of lightning : 

 First, luminous discharges characterised by a long streak of light, very thin, and well 

 defined at the edges, of a white, violet, or purple hue, moving in a straight line, or 

 deviating into a zigzag track, frequently dividing into two or more streams in striking 

 terrestrial objects, but invariably proceeding from a single point. Secondly, he notices 

 expanded flashes spreading over a vast surface without having any apparent depth, of a 

 red, blue, or violet colour, not so active as the former class, and generally confined to the 

 edges of the clouds from which they appear to proceed. Thirdly, he mentions concentrated 

 masses of light, which he terms globular lightning, which seem to occupy time, to endure 

 for several seconds, and to have a progressive motion. Mr. Hearder of Plymouth describes 

 a discharge of lightning of this kind, on the Dartmouth hills, very near to him. Several 

 vivid flashes had occurred before the mass of clouds approached the hill on which he was 

 standing ; and before he had time to retreat from his dangerous position, a tremendous 

 crash and explosion burst close to him. The spark had the appearance of a nucleus of 

 intensely ignited matter, followed by a flood of light. It struck the path near him, and 

 dashed with fearful brilliancy down its whole length to a rivulet at the foot of the hill, 

 where it terminated. Analogous to the discharges described as globular lightning are the 

 fire-balls so often noticed, about which there has been no little scepticism ; but the 

 evidence cannot reasonably be doubted, that displays of electrical light have repeatedly 



