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heated by the sun: and such heated surface, not be ing changed, heats the air that moves over it.
Seas, lakes, and great bodies of water, agitated by the winds, continually change surfaces; the cold surface in winter is turned under by the rolling of the waves, and a warmer turned up; in summer the warm is turned under, and colder turned up. Hence the more equal temper of seawater, and the air over it. Hence, in'winter, winds from the sea seem warm, winds from the land cold. In summer the contrary.
Therefore the lakes northwest of us,* as they are not so much frozen, nor so apt to freeze as the earth, rather moderate than increase the coldness of our winter winds.
The air over the sea being warmer, and, therefore, lighter in winter than the air over the frozen land, may be another cause of our general northwest winds, which blow off to sea at right angles from our North American coast. The warm, light seaair rising, the heavy, cold land-air pressing into its place.
Heavy fluids, descending, frequently form eddies or whirlpools, as is seen in a funnel, where the water acquires a circular motion, receding every way from a centre, and leaving a vacancy in the middle, greatest above, and lessening downward, like a speaking-trumpet, its big end upward.
Air, descending or ascending, may form the same kind of eddies or whirlings, the parts of air acquiring a circular motion, and receding from the middle of the circle by a centrifugal force, and leaving there a vacancy; if descending, greatest above and lessening downward; if ascending, greatest below and lessening upward; like a speaking-trumpet standing its big end on the ground.
When the air descends with a violence in some
* In Pennsylvania.
places, it may rise with equal violence in others, and form both kinds of whirlwinds.
The air, in its whirling motion, receding every way from the centre or axis of the trumpet, leaves there a vacuum, which cannot be filled through the sides, the whirling air, as an arch, preventing ; it must then press in at the open ends.
· The greatest pressure inward must be at the lower end, the greatest weight of the surrounding atmosphere being there. The air, entering, rises within, and carries up dust, leaves, and even heavier bodies that happen in its way, as the eddy or whirl passes over land.
If it passes over water, the weight of the surrounding atmosphere forces up the water into the vacuity, part of which, by degrees, joins with the whirling air, and, adding weight and receiving accelerated motion, recedes farther from the centre or axis of the trump as the pressure lessens; and at last, as the trump widens, is broken into small particles, and so united with air as to be supported by it, and become black clouds at the top of the trump.
Thus these eddies may be whirlwinds at land, water-spouts at sea. A body of water so raised may be suddenly let fall, when the motion, &c., has not strength to support it, or the whirling arch is broken so as to admit the air: falling in the sea, it is harmless unless ships happen under it; and if in the progressive motion of the whirl it has moved from the sea over the land, and then breaks, sudden, violent, and mischievous torrents are the consequences.
To Dr. Perkins. Water-spouts and Whirlwinds compared.-Read at the Royal
Society, June 24, 1753.
Philadelphia, Feb. 4, 1753. I ought to have written to you long since, in answer to yours of October 16, concerning the water-spout; but business partly, and partly a desire of procuring farther information by inquiry among my seafaring acquaintance, induced me to postpone writing, from time to time, till I am almost ashamed to resume the subject, not knowing but you may have forgot whạt has been said upon it.
Nothing certainly can be more improving to a searcher into nature than objections judiciously made to his opinion, taken up, perhaps, too hastily : for such objections oblige him to restudy the point, consider every circumstance carefully, compare facts, make experiments, weigh arguments, and be slow in drawing conclusions. And hence a sure advantage results; for he either confirms a truth before too slightly supported, or discovers an error, and receives instruction from the objector.
In this view I consider the objections and remarks you sent me, and thank you for them sincerely; but, how much soever my inclinations lead me to philosophical inquiries, I am so engaged in business, public and private, that those more pleasing pursuits are frequently interrupted, and the chain of thought necessary to be closely continued in such disquisitions is só broken and disjointed, that it is with difficulty I satisfy myself in any of them; and I am now not much nearer a conclusion in this matter of the spout than when I first read your letter.
Yet, hoping we may, in time, sift out the 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 there 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.
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 water would do when forced up by the pressure of the atmosphere into an exhausted tube.
I must, however, no longer call it my hypothesis, since I find Stuart had the same thought, though somewhat obscurely expressed, where he says "he imagines this phenomenon may be solved by suction (improperly so called) or rather pulsion, as in the application of a cupping-glass to the flesh, 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 the 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. Waterspouts 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 Acrement Close.
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 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 whaleman of Nantucket informed me that three of their vessels, 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 afterward 'in company,
and came to confer about it. So that in this particular, likewise, whirlwinds and waterspouts agree.
But if that which appears a water-spout at sea does sometimes, in its progressive motion, meet