And it seems more eligible, that the lightning should fall on the point of the conductor (provided to convey it into the earth), than on any other part of the building, thence to proceed to such conductor. Which end is also more likely to be obtained by the length and loftiness of the rod; as protecting more extensively the building under it.

It has been objected, that erecting pointed rods upon edifices, is to invite and draw the lightning into them; and therefore dangerous. Were such rods to be erected on buildings, without continuing the communication quite down into the moist earth, this objection might then have weight; but when such complete conductors are made, the lightning is invited not into the building, but into the earth, the situation it aims at, and which it always seizes every help to obtain, even from broken partial metalline conductors.

It has also been suggested, that from such electric experiments nothing certain can be concluded as to the great operations of nature; since it is often seen that experiments, which have succeeded in small, in large have failed. It is true that in mechanics this has sometimes happened. But when it is considered that we 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 points must have some effect. And if the quantity 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 20 years 1)

About the year 1752.

five of them have been struck by lightning; viz. 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 case but past facts; and we know of no instance where a complete conductor to the moist earth has been insufficient, if half an inch 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 undercharged (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, or 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 moveableness 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 acquainted with, is scarce to be expected.'

B. F.

ON THE SPOTS IN THE SUN: A NEW HYPOTHESIS.
To MR. HUMPHRY MARSHALL.

SIR,

London, Feb. 14, 1773. A considerable time after its arrival I received the box of seeds you sent me the beginning of last year, with your observations on spots of the sun. The seeds I distributed among some of my friends who are curious: accept my thankful acknowledgments for them. The observations I communicated to our

I 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, 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,

astronomers of the Royal Society, who are much pleased with them, and hand them about from one to another; so that I have had little opportunity of examining them myself, they not being yet returned to me. Here are various opinions about the solar spots. Some think them vast clouds of smoke and soot arising from the consuming fuel on the surface, which at length take fire again on their edges, consuming and daily diminishing till they totally disappear. Others think them spots of the surface, in which the fire has been extinguished, and which by degrees is rekindled. It is however remarkable, that though large spots are seen gradually to become small ones, no one has observed a small spot gradually become a large one; at least I do not remember to have met with such an observation. If this be so, it should seem they are suddenly formed of their full size; and perhaps if there were more such constant and diligent observers as you, some might happen to be observing at the instant such a spot was formed, when the appearances might give some ground of conjecture by what means they were formed. The professor of astronomy at. Glasgow, Dr. Wilson, has a new hypothesis. It is this: that the sun is a globe of solid matter, all combustible perhaps, but whose surface only is actually on fire to a certain depth, and all below that depth unkindled, like a log of wood, whose surface to half an inch deep may be burning coal, while all within remains wood. Then he supposes, by some explosion similar to our earthquakes, the burning part may be blown away from a particular district, leaving bare the unkindled part below, which then appears a spot, and only lessens as the fluid burning matter by degrees flows in upon it on all sides, and at last covers or rekindles it. He founds this opinion on certain appearances of the. edges of the spots as they turn under the sun's disk, or emerge again on the other side: for if there are such hollows in the sun's face as he supposes, and. the bright border round their edges be the fluid burning matter flowing down the banks into the hollow, it will follow, that while a spot is in the middle of the sun's disk, the eye looking directly upon the whole may discern that border all round; but when the hollow is moved round to near the edge of the disk, then, though the eye which now views it aslant can see full the farthest bank, yet that which is nearest is hidden, and not to be distinguished; and when the same spot comes to emerge again on the other side of the sun, the bank which before was visible is now concealed, and that concealed which before was visible, gradually changing, however, till the spot reaches the middle of the disk, when the bank all round may be seen as before. Perhaps your telescope

may be scarce strong enough to observe this. If it is, I wish to know whether you find the same appearances. When your observations are returned to me, and I have considered them, I shall lodge them among the papers of the society, and let you know their sentiments. With great esteem and regard, I am, &c. B. FRANKLIN.

ON THE ANALOGY BETWEEN MAGNETISM AND

SIR,

ELECTRICITY.

To MONSIEUR DUBOURG.

London, March 10, 1773.

As to the magnetism which seems produced by electricity, my real opinion is, that these two powers of nature have no affinity whatever with each other, and that the apparent production of magnetism is purely accidental. The matter may be explained thus :

1st. The earth is a great magnet.

2dly. There is a subtile fluid, called the magnetic fluid, which exists in all ferruginous bodies, equally attracted by all their parts, and equally diffused through their whole substance; at least where the equilibrium is not disturbed by a power superior to the attraction of the iron.

3dly. This natural quantity of the magnetic fluid, which is contained in a given piece of iron, may be put in motion, so as to be more rarefied in one part and more condensed in another; but it cannot be withdrawn by any force that we are yet made acquainted with, so as to leave the whole in a negative state, at least relatively to its natural quantity; neither can it be introduced so as to put the iron into a positive state, or render it plus. In this respect therefore magnetism differs from electricity.

4thly. A piece of soft iron allows the magnetic fluid which it contains to be put in motion by a moderate force, so that being placed in a line with the magnetic pole of the earth, it immediately acquires the properties of a magnet; its magnetic fluid being drawn or forced from one extremity to the other; and this effect continues as long as it remains in the same position, one of its extremities becoming positively magnetised, and the other negatively. This temporary magnetism ceases as soon as the iron is turned east and west, the fluid immediately diffusing itself equally through the whole iron, as in its natural state,