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it: then take from the fire 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. 6. I will excite this stick of sealing-wax and bring it to the cover B: you see how often it causes the gold leaf to strike against the sides of the glass.

James. Are the slips of tin foil intended to carry away the electric fluid communicated by the objects presented to the cap B?

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

CONVERSATION XXXVIII

Of Atmospherical Electricity.

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

Tutor. They are, undoubtedly, the same fluid;

and that they are the same, was discovered by Dr. Franklin more than half a century ago. James. How did he ascertain this fact?

Tutor. He was led to the theory from observing the power which uninsulated points have in drawing off the electricity from bodies. And having formed his system, he was waiting for the erection of a spire, in Philadelphia, to carry his views into execution, when 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 the kite was completely insulated. At the junction of the two strings he fastened a key as a good conductor, in order to take sparks from it.

Charles. Did he obtain any sparks?

Tutor. 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 before the string was wet, but when the rain had wetted the string, he collected the electricity very plentifully:

-Led by the phosphor light, with daring tread,
Immortal Franklin sought the fiery bed;

Where, nurs'd in night, incumbent tempest shrouds
The seeds of thunder in circumfluent clouds;

Besieg'd with iron points his airy cell,

And pierc'd the monster slumb'ring in the shell,

DARWIN.

James. Could I do so with our large kite? Tutor. I hope you will not try to raise your kite during a thunder storm, because, without very great care, it may be attended with the most serious danger. Your kite is, however, quite large enough, being four feet high, and two feet wide: every thing 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. And to Mr. Cavallo's work on electricity, Vol. 11. such persons as are desirous of raising kites, for electrical purposes, should be referred, in which they will find ample instruction.

Charles. How do the conductors, which I have seen fixed to various buildings, act in dispersing lightning?

Tutor. You know how easy it is to charge a Leyden jar: but if, when the machine is at work, 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 silently draw away the lightning from clouds passing over any building.

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

Tutor. 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 the iron rod should come to a fine point. Some electricians recommend that the point should be of gold, to prevent its rusting.

Charles. What effects would be produced, if lightning should strike a building without a conductor?

Tutor. That may be best explained, by informing you of what happened, many years ago, to St. Bride's church. The lightning first struck the weathercock, from thence descending in its progress, it beat out a number of large stones at different heights, some of which fell upon the roof of the church, and did great damage to it. The mischief done to the steeple was so considerable, that eighty-five feet of it was obliged to be taken down.

James. The weathercock was probably made of iron, why did not that act as a conductor?

Tutor. Though that was made of iron, yet it was completely insulated by being fixed in stone, that had become dry by much hot and dry weather. When therefore the lightning had taken possession of the weathercock, by endeavouring to force its way to another conductor, it beat down whatever stood in its way.

Charles. The power of lightning must be very great.

Tutor. It is irresistible in its effects; the following experiment will illustrate what I have been saying.

Ex. 1. A is a board (Plate VIII. Fig. 19.) 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 b c d; x b terminated by a knob x, represents a weathercock, and the wire cz is fixed to the board a.

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 then 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.

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

Tutor. Ex. 2. Just so; 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 b c to %, and now the phial may be dis

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