ELECTRICITY.

owe our first knowledge of the nature and operations of lightning to observations on such small experiments; and that, on carefully comparing the most accurate accounts of former facts, and the exactest relations of those that have occurred since, the effects have surprisingly agreed with the theory; it is humbly conceived, that in natural philosophy, in this branch of it at least, the suggestion has not so much weight; and that the farther new experiments, now adduced in recommendation of long, sharp-pointed rods, may have some claim to credit and consideration.

It has been urged, too, that, though points may have considerable effects on a small prime conductor at small distances; yet, on great clouds and at great distances, nothing is to be expected from them. To this it is answered, that in those small experiments it is evident the points act at a greater than the striking distance; and, in the large way, their service is only expected where there is such nearness of the cloud as to endanger a stroke; and there, it cannot be doubted, the And, if the quantity points must have some effect. discharged by a single pointed rod may be so considerable as I have shown it, the quantity discharged by a number will be proportionably greater.

But this part of the theory does not depend alone on small experiments. Since the practice of erecting pointed rods in America (now near twenty years), five of them have been struck by lightning, namely, Mr. Raven's and Mr. Maine's in South Carolina, Mr. Tucker's in Virginia, Mr. West's and Mr. Moulder's in Philadelphia. Possibly there may have been more, that have not come to my knowledge. But, in every one of these, the lightning did not fall upon the body of the house, but precisely on the several points of the rods; and, though the conductors were sometimes not sufficiently

large and complete, was conveyed into the earth, without any material damage to the buildings. Facts then in great, as far as we have them authenticated, justify the opinion that is drawn from the experiments in small, as above related.

It has also been objected, that, unless we knew the quantity that might possibly be discharged at one stroke from the clouds, we cannot be sure we have provided sufficient conductors; and therefore cannot depend on their conveying away all that may fall on their points. Indeed we have nothing to form a judgment by in this, but past facts; and we know of no instance, where a complete conductor to the moist earth has been insufficient, if half an inch in diameter. It is probable, that many strokes of lightning have been conveyed through the common leaden pipes affixed to houses to carry down the water from the roof to the ground; and there is no account of such pipes being melted and destroyed, as must sometimes have happened, if they had been insufficient. We can, then, only judge of the dimensions proper for a conductor of lightning, as we do of those proper for a conductor of rain, by past observation. And, as we think a pipe of three inches bore sufficient to carry off the rain that falls on a square of twenty feet, because we never saw such a pipe glutted by any shower; so we may judge a conductor of an inch diameter more than sufficient for any stroke of lightning that will fall on its point. It is true, that, if another deluge should happen wherein the windows of heaven are to be opened, such pipes may be unequal to the falling quantity; and, if God for our sins should think fit to rain fire upon us, as upon some cities of old, it is not expected that our conductors, of whatever size, should secure our houses against a miracle. Probably, as water drawn up into the air and there forming

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, LL

VOL. V.

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.

ELECTRICITY.

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 Proceed in the same be dipped once in salt water.* manner with regard to the other two balls, and the other two rods.

the

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

up

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.