On the place of this spattering, arises the appearance of a bush, into the centre of which the spout comes down. This bush I take to be formed by a spray, made by the force of these drops, which being uncommonly large and descending with unusual force by a stream of wind descending from the cloud with them, increases the height of the spray:

Dr. Perkins to Dr. Franklin.-Read at the which wind being repulsed by the surface of Royal Society, June 24, 1756.

truth between us, I will send you my present thoughts, with some observations on your reasons on the accounts in the Transactions, and on other relations I have met with. Perhaps, while I am writing, some new light may strike me, for I shall now be obliged to consider the subject with a little more attention.

I agree with you, that, by means of a vacuum in a whirlwind, water cannot be supposed to rise in large masses to the region of the clouds; for the pressure of the surrounding atmosphere could not force it up in a continued body, or column, to a much greater height, than thirty feet. But if their really is a vacuum in the centre, or near the axis of whirlwinds, then, I think, water may rise in such vacuum to that height, or to a less height, as the vacuum may be less perfect.

that happened in cold weather, in the Downs, described by Mr. Gordon in the Transactions, was, for that reason, thought extraordinary; but he remarks withal, that the weather, though cold when the spout appeared, was soon after much colder: as we find it, commonly, less warm after a whirlwind.

You agree, that the wind blows every way towards a whirlwind, from a large space round. An intelligent whale-man of Nantucket, informed me that three of their ves sels, which were out in search of whales, happening to be becalmed, lay in sight of each other, at about a league distance, if I remember right, nearly forming a triangle: after some time, a water-spout appeared near the middle of the triangle, when a brisk breeze of wind sprung up, and every vessel made sail; and then it appeared to them all, by the setting of the sails, and the course each vessel stood, that the spout was to the leeward of every one of them; and they all declared it to have been so, when they happened afterwards in company, and came to confer about it. So that in this particular likewise, whirlwinds and water-spouts agree.

I had not read Stuart's account, in the Transactions, for many years, before the receipt of your letter, and had quite forgot it; but now, on viewing his draughts, and considering his descriptions, I think they seem to favour my hypothesis; for he describes and draws columns of water, of various heights, terminating abruptly at the top, exactly as But, if that which appears a water-spout at water would do, when forced up by the pres- sea, does sometimes, in its progressive motion, sure of the atmosphere into an exhausted tube. meet with and pass over land, and there proI must, however, no longer call it my hy-duce all the phenomena and effects of a whirlpothesis, since I find Stuart had the same wind, it should thence seem still more evident, thought, though somewhat obscurely express that a whirlwind and a spout are the same. ed, where he says, "he imagines this phe- I send you, herewith, a letter from an ingenomenon may be solved by suction (impro- nious physician of my acquaintance, which perly so called) or rather pulsion, as in the gives one instance of this, that fell within his application of a cupping glass to the flesh, observation. the air being first voided by the kindled flax." In my paper, I supposed a whirlwind and a spout to be the same thing, and to proceed from the same cause; the only difference between them being, that the one passes over land, the other over water. I find, also, in the Transactions, that M. de la Pryme was of the same opinion; for he there describes two spouts, as he calls them, which were seen at different times, at Hatfield, in Yorkshire, whose appearances in the air were the same with those of the spouts at sea, and effects the same with those of real whirlwinds.

Whirlwinds have generally a progressive, as well as a circular motion; so had what is called the spout, at Topsham, as described in the Philosophical Transactions, which also appears, by its effects described, to have been a real whirlwind. Water-spouts have, also, a progressive motion; this is sometimes greater, and sometimes less; in some violent, in others barely perceivable. The whirlwind at Warrington continued long in AcrementClose.

Whirlwinds generally arise after calms and great heats: the same is observed of water-spouts, which are, therefore, most frequent in the warm latitudes. The spout

A fluid, moving from all points horizontally, towards a centre, must, at that centre, either ascend or descend. Water being in a tub, if a hole be opened in the middle of the bottom, will flow from all sides to the centre, and there descend in a whirl. But, air flowing on and near the surface of land or water, from all sides, towards a centre, must at that centre ascend; the land or water hindering its descent.

If these concentring currents of air be in the upper region, they may, indeed, descend in the spout or whirlwind; but then, when the united current reached the earth or water, it would spread, and, probably, blow every way from the centre. There may be whirlwinds of both kinds, but from the commonly observed effects, I suspect the rising one to be the most common: when the upper air descends, it is, perhaps, in a greater body, extending wider, as in our thunder-gusts, and without much whirling; and, when air descends in a spout, or whirlwind, I should rather expect it would press the roof of a house inwards, or force in the tiles, shingles, or thatch, force a boat down into the water, or a piece of timber into the earth, than that it would lift them up, and carry them away.

per.

It has so happened, that I have not met with I would only first beg to be allowed two or any accounts of spouts, that certainly descend- three positions, mentioned in my former paed; I suspect they are not frequent. Please to communicate those you mention. The apparent dropping of a pipe from the clouds towards the earth or sea, I will endeavour to explain hereafter.

1. That the lower region of air is often more heated, and so more rarefied, than the upper; consequently, specifically lighter. The coldness of the upper region is manifested by the hail which sometimes falls from it in a hot day.

2. That heated air may be very moist, and yet the moisture so equally diffused and rarefied, as not to be visible, till colder air mixes with it, when it condenses, and becomes visible. Thus our breath, invisible in summer, becomes visible in winter.

The augmentation of the cloud, which, as I am informed, is generally, if not always the case, during a spout, seems to show an ascent, rather than a descent of the matter of which such cloud is composed; for a descending spout, one would expect, should diminish a cloud. I own, however, that cold air descending, may, by condensing the vapours in a lower region, Now let us suppose a tract of land, or sea, form and increase clouds; which, I think, is of perhaps sixty miles square, unscreened by generally the case in our common thunder-clouds, and unfanned by winds, during great gusts, and, therefore, do not lay great stress on this argument.

Whirlwinds and spouts, are not always, though most commonly, in the day time. The terrible whirlwind, which damaged a great part of Rome, June 11, 1749, happened in the night of that day. The same was supposed to have been first a spout, for it is said to be beyond doubt, that it gathered in the neighbouring sea, as it could be tracked from Ostia to Rome. I find this is in Père Boschovich's account of it, as abridged in the Monthly Review for December, 1750.

part of a summer's day, or, it may be, for several days successively, till it is violently heated, together with the lower region of air in contact with it, so that the said lower air becomes specifically lighter than the superincumbent higher region of the atmosphere, in which the clouds commonly float: let us suppose, also, that the air surrounding this tract has not been so much heated during those days, and therefore remains heavier. The consequence of this should be, as I conceive, that the heated lighter air, being pressed on all sides, must ascend, and the heavier descend; and, as this In that account, the whirlwind is said to rising cannot be in all parts, or the whole area have appeared as a very black, long, and lofty of the tract at once, for that would leave too cloud, discoverable, notwithstanding the dark- extensive a vacuum, the rising will begin ness of the night, by its continually lightning precisely in that column that happens to be or emitting flashes on all sides, pushing along the lightest, or most rarefied; and the warm with a surprising swiftness, and within three air will flow horizontally from all points to or four feet of the ground. Its general effects this column, where the several currents meeton houses, were stripping off the roofs, blow-ing, and joining to rise, a whirl is naturally ing away chimneys, breaking doors and windows, forcing up the floors, and unpaving the rooms (some of these effects seem to agree well with a supposed vacuum in the centre of the whirlwind) and the very rafters of the houses were broken and dispersed, and even hurled against houses at a considerable distance, &c.

formed, in the same manner as a whirl is formed in the tub of water, by the descending fluid flowing from all sides of the tub, to the hole in the centre.

And, as the several currents arrive at this central rising column, with a considerable de gree of horizontal motion, they cannot suddenly change it to a vertical motion; there It seems, by an expression of Père Boscho- fore as they gradually, in approaching the vich's, as if the wind blew from all sides to- whirl, decline from right curved or circular wards the whirlwind; for, having carefully lines, so, having joined the whirl, they asobserved its effects, he concludes of all whirl-cend by a spiral motion, in the same manner winds, "that their motion is circular, and their 'action attractive."

He observes, on a number of histories of whirlwinds, &c. "that a common effect of them is, to carry up into the air, tiles, stones, and animals themselves, which happen to be in their course, and all kinds of bodies unexceptionably, throwing them to a considerable distance, with great impetuosity."

Such effects seem to show a rising current of air.

I will endeavour to explain my conceptions of this matter by figures, representing a plan and an elevation of a spout or whirlwind.

as the water descends spirally through the hole in the tub before-mentioned.

Lastly, as the lower air, and nearest the surface, is most rarefied by the heat of the sun, that air is most acted on by the pressure of the surrounding cold and heavy air, which is to take its place; consequently, its motion towards the whirl is swiftest, and so the force of the lower part of the whirl, or trump, strongest, and the centrifugal force of its particles greatest; and hence the vacuum round the axis of the whirl should be greatest near the earth or sea, and be gradually diminished as it approaches the region of the clouds, till

In Fig. I. which is a plan or ground-plat of a whirlwind, the circle V. represents the central vacuum.

Between a aa a and b b b b I suppose a body of air, condensed strongly by the pressure of the currents moving towards it, from all sides without, and by its centrifugal force from within, moving round with prodigious swiftness, (having, as it were, the entire momenta of all the currents

it ends in a point, as at P, Fig. II. in the plate, | low, broad cone, whose top gradually rises forming a long and sharp cone. and sharpens, as the force of the whirl increases. At its upper end it becomes visible, by the warm air brought up to the cooler region, where its moisture begins to be condensed into thick vapour, by the cold, and is seen first at A, the highest part, which being now cooled, condenses what rises next at B, which condenses that at C, and that condenses what is rising at D, the cold operating by the contact of the vapours faster in a right line downwards than the vapours can climb in a spiral line upwards; they climb, however, and as by continual addition they grow denser, and, consequently, their centrifugal force greater, and being risen above the concentrating currents that compose the whirl, fly off, spread, and form a cloud.

E united in itself) and with a power equal to its swiftness and density.

It is this whirling body of air between a a a a and b b b b that rises spirally; by its force it tears buildings to pieces, twists up great trees by the roots, &c. and, by its spiral motion, raises the fragments so high, till the pressure of the surrounding and approaching currents diminishing, can no longer confine them to the circle, or their own centrifugal force increasing, grows too strong for such pressure, when they fly off in tangent lines, as stones out of a sling, and fall on all sides, and at great distances.

If it happens at sea, the water under and between a aa a and b b b b will be violently agitated and driven about, and parts of it raised with the spiral current, and thrown about so as to form a bush-like appearance. This circle is of various diameters, sometimes very lage. If the vacuum passes over water, the water may rise in it in a body, or column, to near the height of thirty-two feet. If it passes over houses, it may burst their windows or walls outwards, pluck off the roofs, and pluck up the floors, by the sudden rarefaction of the air contained within such buildings; the outward pressure of the atmosphere being suddenly taken off; so the stopped bottle of air bursts under the exhausted receiver of the air pump.

It seems easy to conceive, how, by this successive condensation from above, the spout appears to drop or descend from the cloud, though the materials of which it is composed are all the while ascending.

The condensation of the moisture, contained in so great a quantity of warm air as may be supposed to rise in a short time in this prodigiously rapid whirl, is perhaps, sufficient to form a great extent of cloud, though the spout should be over land, as those at Hatfield; and if the land happens not to be very dusty, perhaps the lower part of the spout will scarce become visible at all; though the upper, or what is commonly called the descending part be very distinctly seen.

The same may happen at sea, in case the whirl is not violent enough to make a high vacuum, and raise the column, &c. In such case, the upper part A, B, C, D only will be visible, and the bush, perhaps, below.

But if the whirl be strong, and there be much dust on the land, and the column W W be raised from the water, then the lower part becomes visible, and sometimes even united Fig. II. is to represent the elevation of a to the upper part. For the dust may be carwater-spout, wherein I suppose P P P to be ried up in the spiral whirl, till it reach the rethe cone, at first a vacuum, till W W, the gion where the vapour is condensed, and rise rising column of water, has filled so much of with that even to the clouds: and the friction it. SSSS, the spiral whirl of air, surround- of the whirling air, on the sides of the column ing the vacuum, and continued higher in a W W, may detach great quantities of its close column after the vacuum ends in the water, break it into drops, and carry them up point P, till it reaches the cool region of the in the spiral whirl mixed with the air; the air. B B, the bush described by Stuart, sur-heavier drops may, indeed, fly off, and fall, in rounding the foot of the column of water.

Now, I suppose this whirl of air will, at first be as invisible as the air itself, though reaching, in reality, from the water, to the region of cool air, in which our low summer thunder-clouds commonly float: but presently it will become visible at its extremities. At its lower end, by the agitation of the water, under the whirling part of the circle, between P and S forming Stuart's bush, and by the swelling and rising of the water, in the beginning vacuum, which is, at first, a small, VOL. II. ... 2 U

29

a shower, round the spout; but much of it will be broken into vapour, yet visible; and thus, in both cases, by dust at land, and by water at sea, the whole tube may be darkened and rendered visible.

As the whirl weakens, the tube may (in appearance) separate in the middle; the column of water subsiding, and the superior condensed part drawing up to the cloud. Yet still the tube, or whirl of air, may remain entire, the middle only becoming invisible, as not containing visible matter.