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 characterized 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 color, 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 occurred, conveying the impression of balls of fire to the observer. An instance is given by Mr. Chalmers while on board the Montague, of seventy-four guns, bearing the flag of Admiral Chambers. In the account read to the Royal Society, he states, that “on November 4th, 1749, while taking an observation on the quarter-deck, one of the quarter-masters requested him to look to windward, upon which he observed a large ball of blue fire rolling along on the surface of the water, as large as a mill-stone, at about three miles distance. Before they could raise the main-tack, the ball had reached within forty yards of the main-chains, when it rose perpendicularly with a fearful explosion, and shattered the main-topmast to pieces.” In an account of the fatal effects of lightning