a live cinder, and put it in the cup, and the electricity of vapour is very admirably exhibited.

A thunder cloud passing over this instrument will cause the gold leaf to strike the sides at every flash of lightning. Ex. VI.-Excite this stick of sealing-wax, and bring it to the cover B. Now observe how often it causes the gold leaf

to strike against the sides of the glass.

Ja. Are the slips of tin-foil intended to carry away the electric fluid communicated by the objects presented to the сар в?

Fa. They are; and by them the equilibrium is restored.

QUESTIONS FOR EXAMINATION.

How is the thumb illuminated by the | is an electrophorus ? -Show me the electrical fluid, and what may be seen during the experiment?

How is

water illuminated? Explain the construction of an electrometer.- What

construction of a greatly improved electrometer, and how is it used? How is electricity shown by evaporation ?

Charles. You said yesterday, that the electrometer was affected by thunder and lightning. Are lightning and electricity similar?

Fa. They are undoubtedly the same fluid, as was discovered by Dr. Franklin more than half a century ago. Ja. How did he ascertain this fact?

Fa. He was led to its theory from observing the power which uninsulated points possess of drawing from bodies their electricity. While waiting for the erection of a spire in Philadelphia, to carry his ideas into execution, it occurred to him that a boy's kite would answer his purpose better than a spire. He therefore prepared a kite, and, having raised it, he tied to the end of the string a silken cord, by which he might make the kite completely insulated. At the junction of the two strings he fastened a key, as a good conductor, in order to take sparks from it.

Ch. Did he obtain any sparks?

Fa. One cloud, which appeared like a thunder-cloud, passed

without any effect. Shortly after, the loose threads of the hempen string stood erect, in the same manner as they would if the string had been hung on an electrified insulated conductor. He then presented his knuckle to the key, and obtained an evident spark. Others succeeded; but when the rain had wetted the string, he collected the electricity very plentifully.

Ja. Could I do so with our large kite?

Fa. I should not like you to raise your kite during a thunder-storm, because, without very great care, it might be attended with considerable danger. A celebrated electrician, Professor Richman, of St. Petersburg, was struck dead by a flash of lightning, which he had collected from the clouds, by a somewhat similar apparatus. Your kite is, however, quite large enough; for it is four feet high, and two feet wide; but everything depends on the string, which, according to Mr. Cavallo, who has made many experiments on the subject, should be made of two thin threads of twine, twisted with a copper thread. If you are desirous of raising kites, for electrical purposes, I must refer you to Mr. Cavallo's work on Electricity, vol. ii., in which you will find ample instruction. Ch. How do those conductors, which I have seen fixed to various buildings, operate in dispersing lightning?

Fa. You know how easy it is to charge a Leyden jar: but when the machine is at work, if a person hold a point of steel, or other metal, near the conductor, the greater part of the fluid will run away by that point instead of proceeding to the jar. Hence it was concluded that pointed rods would draw away from buildings the lightning from clouds that were passing over them.

Ja. Is there not a particular method of fixing them?

Fa. Yes: the metallic rod must reach from the ground, or the nearest piece of water, to a foot or two above the building it is intended to protect, and should terminate in a fine point. Some electricians recommend that the point should be of gold, to prevent rusting.

Ch. What would be the consequence if lightning were to strike a building which was devoid of a conductor?

The

Fa. That may be best explained by telling you what happened, many years ago, to St. Bride's church. lightning first struck the weather-cock: descending thence

the steeple in its progress, it beat out several large stones at different heights; some of which fell upon the roof of the church, and did great damage. The mischief done was so considerable, that it became necessary to take down eightyfive feet of the steeple to repair it.

Ja. The weather-cock was probably made of iron; if so, why did it not act as a conductor?

Fa. Although it was made of iron, yet it was completely insulated by being fixed in stone, which had become dry by continued hot weather. When, therefore, the lightning had struck the weather-cock, by endeavouring to force its way to another conductor, it beat down whatever opposed it.

Ch. The power of lightning must be very great.

Fa. It is irresistible. The following experiment will further illustrate it.

Ex. I.—A is a board representing the gable end of a house: it is fixed on another board B: a b c d is a square hole to which a piece of wood is fitted; ad represents a wire fixed diagonally on the wood a bed; xb, termi- B nated by a knob, x, represents a weather-cock, and the wire cz is fixed to the board a.

Fig. 19.

It is evident that in the state in which it is drawn in the figure, there is an interruption in the conducting rod; accordingly, if the chain m is connected with the outside of a Leyden phial, and that phial is discharged through x, by bringing one part of the discharging rod to the knob of the Leyden phial, and the other to within an inch or two of x, the piece of wood, a b c d, will be thrown out with violence.

Ja. Are we to understand by this experiment that if the wire ab had been continued to the chain, the electric fluid would have run along it without disturbing the loose board?

Fa. Ex. II.-Yes: for if the piece of wood be taken out, and the part a be put to the place b, then d will come to c, and the conducting rod will be complete, and continued from x through a and d to z; now the phial may be discharged as often as you please: but the wood will remain in its place, because the electric fluid runs over the wire to z, and makes its way by the chain to the outside of the phial.

Ch. Then if a represent the weather-cock of the church,

the lightning having overcharged it, by its endeavours to reach another conductor, as cz, has forced away the stone or stones represented by a b c d?

Fa. That is what I meant to convey to your minds by the first experiment; and the second illustration shows very clearly, that if an iron rod had gone from the weather-cock to the ground, without interruption, it would have conducted the electricity safely to the earth without doing any injury to the church.

Ja. How was it that all the stones were not beaten down? Fa. Because, in its passage downwards, it met with many other conductors. I will read part of what Dr. Watson says on this fact, who examined it very attentively:

"The lightning," says he, "first took a weather-cock, which was fixed at the top of the steeple, and was conducted without injuring the metal or anything else, as low as where the large iron bar or spindle which supported it terminated: there the metallic communication ceasing, part of the lightning exploded, cracked, and shattered the obelisk, which terminated the spire of the steeple, in its whole diameter, and threw off, at that place, several large pieces of Portland stone. Here it likewise removed a stone from its place, but not far enough to be thrown down. Thence the lightning seemed to have rushed upon two horizontal iron bars, which were placed within the building across each other. At the end of one of these iron bars, it exploded again, and threw off a considerable quantity of stone. Almost all the damage was done where the ends of the iron bars had been inserted into the stone, or placed under it; and, in some places, its passage might be traced from one iron bar to another."

QUESTIONS FOR EXAMINATION.

Who discovered that electricity and lightning were the same? How was this ascertained? Can lightning be obtained by a kite? — In what way do conductors save buildings from danger? How are they formed? What

church has been injured by lightning? -Explain the structure of the thunderhouse. What do the experiments on it teach? Give me some account of Dr. Watson's description of the injury done to St. Bride's church.

CONVERSATION XIII.

ON ATMOSPHERIC ELECTRICITY-OF THE AURORA BOREALISY OF WATERSPOUTS AND WHIRLWINDS-OF EARTHQUAKES.

Charles. Does the air always contain electricity?

Fa. Yes; and it is owing to the electricity of the atmo sphere that we observe a number of curious and interesting phenomena, such as the Aurora Borealis, or Northern Lights; Water-spouts; Hail; the Ignis Fatuus, or Will-o'-the-wisp.

Ja. Since lofty objects are most exposed to the effects of lightning, or, as it is scientifically called, the electric fluid, do not the tall masts of ships run considerable risk of being struck by it?

Fa. Certainly: we have many instances recorded of the mischief done to ships by lightning: one of which is related in the Philosophical Transactions; it happened on board the Montague, on the 4th of November, 1748, in lat. 42° 48′, and 9° 3' west longitude, about noon. The master of the vessel looked to windward, and observed a large ball of blue fire, rolling apparently on the surface of the water, at the distance of three miles from them: it rose almost perpendicularly when it was within forty or fifty yards of the ship, and then burst with an explosion, as if a hundred cannons had been fired at one time; it left so strong a smell of sulphur, that the ship seemed to contain nothing else. After the noise had subsided, the main-top-mast was found shattered to pieces, and the mast itself was split down to the keel. Five men were also knocked down, and one of them greatly burnt.

Ch. Must it not have been a very large ball to produce such effects?

Fa. Yes: the person who noticed it said it appeared to him the size of a millstone.

The aurora borealis is another electrical phenomenon: this is admitted without any hesitation, because electricians can readily imitate its appearance with their experiments.

Ja. It must be, I should think, on a very small scale. Fa. True: there is a glass tube about thirty inches long, and the diameter of it is about two inches: it is nearly exhausted of air, and capped on both ends with brass. Connect