clouds, is disposed to fall again in rain by its natural gravity, as soon as a number of particles sufficient to make a drop can get together; so, when the clouds are (by whatever means) over or under charged with the electric fluid to a degree sufficient to attract them towards the earth, the equilibrium is restored, before the difference becomes great beyond that degree. Mr. Lane's electrometer, for limiting precisely the quantity of a shock that is to be administered in a medical view, may serve to make this more easily intelligible. The discharging knob does by a screw approach the conductor to the distance intended, but there remains fixed. Whatever power there may be in the glass globe to collect the fulminating fluid, and whatever capacity of receiving and accumulating it there may be in the bottle or glass jar, yet neither the accumulation nor the discharge ever exceeds the destined quantity. Thus, were the clouds always at a certain fixed distance from the earth, all discharges would be made when the quantity accumulated was equal to the distance. But there is a circumstance, which, by occasionally lessening the distance, lessens the discharge; to wit, the movableness of the clouds, and their being drawn nearer to the earth by attraction when electrified; so that discharges are thereby rendered more frequent and of course less violent. Hence, whatever the quantity may be in nature, and whatever the power in the clouds of collecting it, yet an accumulation and force beyond. what mankind has hitherto been acquanted with is scarce to be expected.*

August 27th, 1772.

B. F.

It may be fit to mention here, that the immediate occasion of the dispute concerning the preference between pointed and blunt conductors of lightning arose as follows. A powder-mill having blown up at Brescia,

Description of a Portable Apparatus, invented by Mr. John Canton, for the Purpose of easily demonstrating Dr. Franklin's fundamental Principles of Electricity.*

THIS apparatus is very simple, and takes up but little room in the pocket.

ITS CONSTRUCTION.

Take a rod of common wood, two feet long, an inch wide and three lines in thickness, planed by a joiner. Divide it in four pieces, or four rods of equal length, each being six inches long.

Place two of these rods end to end on a table, and unite the contiguous ends, by pasting over them a small strip of fine linen, which, when it has become dry, will answer the purpose of a hinge, to double or fold the two pieces together at pleasure.

Unite the other two pieces in the same manner. Have two pairs of little balls, made of cork or the pith of the elder, and of nearly the size of a pea.

Attach one pair of these to the two ends of a very fine

in consequence of its being struck with lightning, the English Board of Ordnance applied to their painter, Mr. Wilson, then of some note as an electrician, for a method to prevent the like accident to their magazines at Purfleet. Mr. Wilson having advised a blunt conductor, and it being understood that Dr. Franklin's opinion, formed upon the spot, was for a pointed one, the matter was referred in 1772, to the Royal Society, and by them as usual to a committee, who, after consultation, prescribed a method conformable to Dr. Franklin's theory. But a harmless stroke of lightning having, under particular circumstances, fallen upon one of the buildings and its apparatus in May, 1777, the subject came again into violent agitation, and was again referred to the Society, and by the Society again referred to a new committee, which committee confirmed the decision of the first committee. B. V.

* Translated from M. Dubourg's edition of Franklin's works. — EDITOR.

linen thread, about twelve inches long; then, doubling the thread in the middle, these two balls, suspended at the ends of the thread, will be in contact.

Then fasten the middle of the thread to one end of one of these pairs of rods. The threads should be very smooth, and free from any little fibres; and must be dipped once in salt water.* Proceed in the same manner with regard to the other two balls, and the other two rods.

Make little hemispherical holes in each pair of rods, to place the balls in, when you wish to shut up the apparatus.

MODE OF USING IT.

To make use of this apparatus, take three large drinking-glasses, and warm and dry them well before the fire.

On one of the glasses, place one pair of the rods. open, the hinge being on the upper side, and the little balls suspended at the end. Place the other pair in like manner on the second glass.

Place these two glasses near a corner of the table, in such a position that the ends of the rods from which the balls are suspended may extend beyond the table, and thus the balls may hang entirely free of the table, on either side of the same corner, while the two ends without balls are an inch apart, and the rods in a line with each other.

Rub the third glass with a silk handkerchief, of any color; a piece of black silk, however, is better than any thing else.

The reason why the threads should be dipped once in salt water is, that they may always continue to be good conductors; for, unless this is done, in dry weather they would sometimes become too dry to conduct freely. For this ingenious method, we are indebted to Mr. Cavendish

1. Bring the rubbed glass near one pair of balls; they will be attracted by it, and receive electricity from it; and, when you withdraw the glass, the balls will hang apart, because they will repel each other.

2. To show that this electricity is a subtile fluid, which penetrates wood, and passes readily through it from end to end, being at the same time susceptible of division and of communication;

Without touching the wood, bring the glasses nearer each other, so that the ends of both pairs of rods will meet; and you will immediately perceive that the two separated balls will approach each other by half the distance that divides them, and that the two balls which were hanging in contact will recede from each other, to an equal distance.

3. To show that electricity does not pass into wax, although it passes into wood;

Touch the wood with a stick of sealing-wax; and you will perceive no change in the respective positions of the balls.

4. A proof that it does not enter glass is, that it is retained as long as the rods are supported on the glass.

5. But, to prove that it enters metals and animal bodies, touch the rods either with a key, or with the finger, and the electricity will instantly pass off into the ground, whence it was attracted by rubbing the glass, and each pair of balls will come together again.

6. To show that the particles of the fluid mutually repel each other, and that the natural quantity of it contained in any substance whatever can be put in motion by repulsion;

Rub the glass well, and, having separated the two pairs of rods, hold the glass over that end of one pair from which the balls are not suspended; on the approach of the glass, you will see the balls separate and

recede from each other. Remove the glass, and they will come together again; which shows that the divergence of the balls was not occasioned by any electricity communicated, for none remains; but solely by the motion of the quantity naturally contained in the wood, which the repellent power of that of the rubbed glass has driven from one end of the wood to the other, so that it is accumulated at the end where the balls are suspended, the end next the glass being deprived of it in proportion. By withdrawing the glass it is made to resume its place, because the equilibrium is restored, and the balls reunite.

7. Again hold the glass over the end of one pair of rods; and, when the natural quantity of electricity is driven to the end from which the balls are suspended, and has separated them, touch this end with the finger, which will carry off the accumulated electricity, leaving in this end and the balls only their natural quantity, and the balls will consequently come together again; then withdraw at the same moment the glass and the finger, and you will see the balls again recede from each other; but they are now, as well as the wood, in a negative state; for, on removing the glass, the natural quantity of electricity, which the finger had left at the other end, returns and is diffused equally throughout the wood; and, as this wood has lost a portion of its natural quantity, which was carried off by the finger, what remains is, in reference to the whole substance, less than the natural quantity.

8. To prove that these balls are now in a negative state, present the rubbed glass to them, and it will attract them, whereas it would repel them if they were in a positive state. On the other hand, they would be repelled by a stick of sealing-wax that had been rubbed, whose electricity is negative; instead of which

VOL. V.

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LL*