viz., zigzag-lightning, sheet-lightning, and ball-lightning.

348. ZIGZAG-LIGHTNING. This kind is so called from the peculiarity of its figure, which is thus explained. As the electricity passes through the atmosphere, the air is supposed, at length, to be so powerfully compressed before it, that a great resistance is presented, and the electric fluid then finds an easier route in some other direction. In this it proceeds, until it once more meets with a like opposition, and is compelled again to change its course; and thus it continues glancing from side to side, until at last it reaches the place it seeks.

Zigzag-lightning appears as a narrow, jagged line of intensely vivid light, traversing space with extreme velocity. On account of the unequal conducting power of different portions of the atmosphere, the flash sometimes divides, branching out in several different directions; the lightning is then said to be forked. A division into three distinct lines is of rare occurrence; but even more have been seen, for Kaemtz beheld, at Halle, in June, 1834, a flash of lightning which threw out numerous branches at the sides; the whole presenting the figure of a spine, with its supporting ribs.

It is said that zigzag-lightnings usually pass between the clouds and the earth, seldom flashing from cloud to cloud.

349. SHEET-LIGHTNING. This kind is the most common, and appears during a storm as a diffuse glow of light, illuminating the edges of the clouds; and at times breaking out from the central mass. When it occurs, the clouds are said to open. The flashes of sheet-lightning often follow each other in rapid succession, for the space of many hours; their intensity is by no means great, and the thunder which attends them is low and distant.

350. BALL-LIGHTNING. Lightning of this class is

What are they?

To what is the peculiar figure of zigzag-lightning owing?

What is its appearance?

Describe sheet-lightning. Describe ball-lightning.

extremely rare, and so singular are its attendant phenomena, that we might well doubt its existence, were not the instances of its occurrence fully authenticated. In a storm that happened at Steeple Aston, Wiltshire, in 1772, the Rev. Messrs. Pitcairne and Wainhouse, while in the vestry of the church, saw suddenly before them, at the distance of a foot, and at about their own height from the floor, a ball of fire, nearly the size of a man's fist, surrounded by a black smoke. It burst with an explosion like the discharge of several cannon Pitcairne was dangerously wounded, and his person and clothes showed the usual marks of lightning.

During a thunder-storm that occurred in 1809, at Newcastle on Tyne, the house of David Sutton was struck the lightning descending the chimney. After the explosion, several persons who were assembled in a room, saw at the door a globe of fire, which, after remaining stationary for some time, advanced into the middle of the room, where it burst into fragments, with a report like a rocket.

351. On the fourth of November, 1749, in 42° 48′ N. Lat., 2° W. Long., the crew of the ship Montague beheld, a little before noon, and beneath an unclouded sky, a globe of bluish fire, like a millstone, rolling rapidly upon the sea. At a short distance from the vessel, it rose perpendicularly from the water, and struck the masts with an explosion louder than the discharge of a hundred cannon. Five sailors were thrown senseless upon the deck, one of whom was severely burned.

In the midst of a storm in Scotland, two globes of fire, connected together like chained cannon-shot, were seen by a Mr. Lumsden, passing through the sky revolving one about the other, and striking at last upon the summit of a hill. Philosophers have not yet been enabled to account for lightning of this description; it has, however, been supposed to arise from an unintermitted discharge of electricity.

352. HEAT-LIGHTNING.

It not unfrequently hap

Relate instances. How is ball-lightning supposed to arise?
What is heat-lightning?

pens, during the serene evenings of summer, that the horizon is illumined for many hours with successive flashes of light, unattended with thunder. This is called heat-lightning, and has much perplexed meteorologists. It is affirmed by some, that this illumination is the reflection from the atmosphere of the lightnings of remote storms; the storms themselves being so far distant, that their thunders cannot be heard. Others assert, that during warm, sultry weather, when the air is highly rarefied, its pressure upon the clouds is so much diminished, that the electric fluid can never accumulate upon their surface beyond a certain point, when it escapes in noiseless flashes to the earth.

353. Multiplied observations have proved, that heatlightning generally originates in the first-mentioned cause; but the instances are by no means rare, when silent flashes of electric light play between the earth and the clouds. These cases occur when the weather is sultry, the air being then both rarefied and moist ; two conditions which lessen its non-conducting power; the atmosphere thus becomes an imperfect conductor between the clouds and the earth, which are in opposite electrical states, and opposes just sufficient resistance to the passage of the electric fluid as to render it visible.

354. VELOCITY OF LIGHTNING. By a very ingenious piece of apparatus, Prof. Wheatstone, of King's College, London, has been enabled to show that the duration of a flash of lightning is less than the thousandth part of a second, and Arago has demonstrated that it does not exceed the millionth part.

Now the duration of a flash, is the time it occupies in traversing the space between two clouds, or between a cloud and the earth; if we then estimate this distance to be equal sometimes to a quarter of a mile, which is a low computation, the velocity of lightning, in such cases, according to Arago, could not be less than 250 000 miles per second. The electricity developed by the

How does it originate?

What is said in regard to the velocity of lightning?

electrical machine, has been shown by another beautiful contrivance of Prof. Wheatstone, to possess a speed of 288,000 miles per second: the rapidity of lightning is probably not less.

The preceding remarks apply only to lightnings of the first and second class. Ball-lightnings, on the contrary, often move slowly, and are visible for many seconds.

355. Color. When thunder-clouds are near the earth, the flashes are of a brilliant white; but when the storm is high, and the lightnings play through a rarefied atmosphere, their color approaches to violet. A spark of electricity assumes the same hue, when it is made to pass through the exhausted receiver of an air-pump.

356. EFFECTS OF LIGHTNING. These are precisely similar to those of common electricity in kind, though far exceeding them in degree. Life is destroyed by the shock, the stoutest trees shivered to pieces, ponderous weights displaced, combustibles inflamed, metals softened and fused, sand vitrified, and iron and steel rendered magnetic. It is needless to multiply instances in proof of these particular points, but a few cases may tend to impress them upon the mind.

357. On the night of the 21st of June, 1723, a tree in the forest of Nemours was struck by lightning. The trunk was split into two fragments, one seventeen feet long, the other twenty-two; and though the first required four men to lift it, and the second eight, yet both of them were hurled to a distance of seventeen yards. On the 6th of August, 1809, a flash of lightning struck a house at Swinton, near Manchester. The wall of a building attached to the house was loosened from its foundation a foot below the ground, and raised in a mass to the surface, still maintaining its upright position; one end of it was moved nine, and the other four feet from its original place. The wall thus moved was eleven feet high and three feet thick, and contained 7000 bricks, which, exclusive of the mortar, were estimated to weigh nearly twenty-six tons.

What of its color? What of its effects?

On the 20th of April, 1807, at Great Mouton, in Lancashire, a windmill was struck by lightning; the fluid passed along a large iron chain, the links of which were so softened, that by their own weight they became welded together; and the chain was converted into an inflexible bar of iron.

In Sept. 1845, a house at New Haven, Ct., was struck during a thunder-storm. Several articles of steel were rendered magnetic, and a razor, lying in a case near the spot where the lightning entered, was found capable of sustaining a key, weighing half an ounce.

358. FULGURITES. When a flash of lightning falls upon sand, its path below the surface is often marked by a fulgurite, so called from the Latin word fulgur, lightning. It is a tube composed of sand, vitrified by the action of the lightning. Fulgurites were first discovered in Silesia, in 1711, and specimens were forwarded to the museum at Dresden, where they are still preserved : they have since been found in great numbers, in Germany, England, and amid the sands of Bahia, in Brazil.

The fulgurite is winding in its form, often throws out lateral spurs or branches, and contracts in size towards the lower extremity, which usually terminates at a spring of water, or in some substance that is a good conductor of electricity.

359. These tubes are generally hollow, the interior surface being coated with a brilliant glass. Their di ameters vary from four-hundredths of an inch, to three inches and a half, and the thickness of their sides from one-fiftieth of an inch, to nearly an inch.

The branches of the fulgurite, differ in length from three quarters of an inch to a foot, but the main tube often extends to the depth of many yards. Several of considerable length, which had been taken from the sandy plains of Silesia, were exhibited at London, some

Give instances. What are fulgurites?

Where have they been discovered?

What is their form?

State their dimensions.