years ago, by Dr. Fiedler, of Germany. One, discovered at Paderborn, in Westphalia, was forty feet long.

360. That these tubes are really produced by lightning, has been proved by actual observation. A number of sailors, being upon the isle of Amrum, in Denmark, saw a flash of lightning fall upon the sand; upon examining the spot, they found a fulgurite: a similar circumstance happened on the borders of Holland. Savart and others have obtained artificial fulgurites, by passing powerful electric sparks through powdered glass, and a mixture of sand and salt; tubes were thus formed an inch in length, and the tenth of an inch in thickness, the inner diameter being the twenty-fifth of an inch.

361. VOLCANIC-LIGHTNING. The clouds of smoke, ashes, and vapor, that issue from volcanoes during their eruption, are the scene of terrific lightning and thunder. Pliny the younger, in his letters to Tacitus, mentions the lightning that was seen above Vesuvius, during its eruption, in the year 79, A. D. In that which occurred in 1767, the inhabitants at the foot of the mountain assured Sir William Hamilton, that they were more terrified at the lightning which flashed around them, than by the burning lava, and all the other attendant dangers.

During the eruptions of the same mountain in 1779 and 1794, there appeared, in the midst of the dark volcanic clouds, globes of fire, which, bursting like bombshells, darted on every side vivid flashes of zigzag-lightning. In the latter eruption were heard loud and continued peals of thunder.

362. The cause of volcanic-lightning is found, in the rapid condensation of the vast volumes of heated vapor, which are carried up from the crater of the volcano into the higher and colder regions of the atmosphere.

In like manner, in the midst of water-spouts and

How is it known that they are actually caused by lightning?
In what manner have they been artificially made?

Relate the instances given of volcanic lightning and th:inder
How are volcanic lightnings caused?

whirlwinds, an abundant condensation of vapor suddenly occurs, which frequently develops such an amount of electricity, that the lightning here displays itself in all its fearful energy.

363. THUNDER. In consequence of the lightning passing through the atmosphere with an amazing velocity, it leaves a void space behind it, into which the surrounding air instantly rushes, with a loud report. This noise is thunder.

When the lightning is near the observer, the report is sharp and quick, but when at a distance, it is long and rolling.

364. The rolling of thunder is frequently occasioned by the reverberations of the sound, from clouds and adjacent mountains; but this is by no means always the case. When the lightning-flash darts to a great distance, such is its velocity, that the thunder may be considered as occurring at every point of the flash at the same time. But sound has a progressive motion of 1142 feet per second, and all the thunder will not reach the ear at the same instant. It will be first heard from the nearest point, in the path of the flash, and later and later from points more remote; and the combined effect will be a continued peal.

The zigzag form of the flash, and its division into several streams, is regarded by Herschel as affording an adequate explanation for all the changes that occur in the sound of the thunder-peal.

365. The time that elapses between the lightning and the thunder, enables us to form an estimate of the distance of the former, which is a little more than a mile

for every five seconds. This interval usually varies from three to sixteen seconds; but cases have occurred, where it has amounted to fifty, and even seventy-two seconds.

366. IDENTITY OF LIGHTNING AND ELECTRICITY.

What is the cause of thunder?

How is its rolling occasioned ?

How can we estimate the distance of lightning?

How great an interval of time sometimes occurs?

The resemblance between lightning and electricity was noticed by the earlier electricians, Wall, Grey, and Nollet; but their identity was first established by Dr. Franklin. The strong points of similarity which convinced him of this fact, were the following.

1st. Lightning and the electric spark are both zigzag in form.

2d. Lightning strikes trees, chimneys, spires, masts of vessels, mountains and elevated points upon the surface of the earth. Electricity is likewise attracted by pointed bodies.

3d. Both choose the best conductors.

4th. Both ignite combustibles.

5th. Both fuse metals.

6th. By the action of each, a bad conductor is shivered when struck.

7th. Lightning reverses the poles of a magnet, and renders iron magnetic. Electricity does the same. 8th. Animal life is destroyed by each.

9th. Blindness is produced by both.

368. Franklin, however, did not stop here. He resolved to test the truth of his reasoning, by drawing lightning from the clouds, and in June, 1752, made the hazardous experiment in the vicinity of Philadelphia.

369. FRANKLIN'S EXPERIMENT. Having made a kite, by tying the corners of a large silk handkerchief to the ends of two light strips of cedar that crossed each other, and placed upon it a pointed iron wire connected with the string, Franklin went out into a field upon the approach of a thunder-storm, accompanied by his son. When the kite was raised, he attached a key to the lower end of the hempen string; to the key one end of a silk ribbon was now tied, the other being fastened to a post. The kite was thus insulated, and the experimenter, for a considerable time, awaited the result with intense solicitude. A dense cloud passed over, but no indica

By whom was the identity of lightning and electricity first established? What points of similarity did Franklin observe?

Relate Franklin's experiment.

tions of electricity appeared upon the string; when, just as Franklin began to despair of success, he beheld the loose fibres of the cord starting asunder, and immediately presenting his knuckle to the key he received an electric spark. The rain now descending, increased the conducting power of the string, and vivid electric sparks issued from the key in great abundance. By means of the lightning thus obtained, all the common electrical experiments were performed, and the identity of lightning and electricity thus indubitably proved.

370. ROMAS' EXPERIMENT. No sooner was this wonderful discovery made know, than men of science were eager to repeat the experiment.

With a kite eleven feet high and three feet wide, Romas obtained in France the most brilliant and astonishing results. In one instance, when the kite was raised during a storm, such an accumulation of electricity occurred, that streams of electric fire nine or ten feet long, and an inch in thickness, flashed spontaneously from the string, with reports, like those of a pistol. Thirty streams of this magnitude burst forth in the space of an hour, without counting a multitude of others, seven feet in length.

371. RICHMAN'S DEATH. That such experiments are, however, attended with great danger, unless every precaution is strictly observed, is proved by the unfortunate death of Prof. Richman, of St. Petersburg, who was killed by lightning, on the 6th of August, 1753. He had erected, upon the top of his house, an iron rod from which proceeded a chain that entered his study. The whole apparatus was entirely insulated. On the day in question while examining the electrometer, as a thunder-storm was approaching, a large globe of blue fire flashed from the conductor to his head, instantly depriving him of life.

Relate Romas' experiment.

What error did Richman commit in the construction of his apparatus ?

LIGHTNING-ROD,

372. The invention of the lightning-rod for the protection of buildings was the fruit of the brilliant discovery of Franklin. Even before his decisive experiment he had been led to suppose, from the analogies existing between lightning and electricity, that pointed metallic rods might possibly disarm the thunder-cloud of its terrific power.

373. In order that the lightning-rod, or conductor, may afford an effectual protection, regard must be had to the material of which it is made, its size, and the mode of erection.

374. MATERIAL. Wrought iron is usually employed, and forms a good conductor; but copper is preferable, inasmuch as it is less liable to be corroded or fused, and possesses a greater conducting power.

375. SIZE. The rod, if made of iron, should be threequarters of an inch in diameter, and its upper extremity should terminate in one or more points. Each of these points (which are usually three in number) ought to be capped with some metal which does not rust, as silver, gold, or platina; for the conducting power of the points, if made of iron, would be weakened by the

rust.

376. MODE OF ERECTION. The rod should be continuous from the top to the bottom; an entire metallic communication existing throughout its whole length. This law is violated, when the joints of the several parts that form the conductor are imperfect, and the whole is loosely put together. The parts may be screwed one into the other; or the rod may be formed of wires twisted together.

377. The conductor should be fastened to the building by wooden supports, but if masses of metal, as

By whom was the lightning-rod invented?

To what particulars must attention be directed, that the lightning-red may afford an effectual protection?

What is said in regard to the material?

To the size? To the mode of erection?