Now let us suppose a Tract of Land or Sea of perhaps 60 Miles square unscreen’d by Clouds and unfann’d by Winds during great Part of a Summer’s Day, or it may be for several Days successively till ’tis 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 imagine that the heated lighter Air being press’d on all Sides must ascend, and the heavier descend; and as this Rising cannot be in all Parts or the whole Area of the Tract at once, for that would leave too extensive a Vacuum, the Rising will begin precisely in that Column that happens to be the lightest or most rarified; and the warm Air will flow horizontally from all Points to this Column, where the several Currents meeting and joining to rise, a Whirl is naturally 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 Center.
And as the several Currents arrive at this central rising Column with a considerable Degree of horizontal Motion, they cannot suddenly change it to a vertical Motion, therefore as they gradually in approaching the Whirl decline from right to curve or circular Lines, so having join’d the Whirl they ascend by a spiral Motion; in the same Manner as the Water descends spirally thro’ the Hole in the Tub before-mentioned.
Lastly, as the lower Air and nearest the Surface, is most rarified 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 diminish’d as it approaches the Region of the Clouds, till it ends in a Point, as at A in Fig II. forming a long and sharp Cone.
In Fig I. which is a Plan or Ground Plot of a Whirlwind, the Circle V represents the central Vacuum.
Between aaaa and bbbb I suppose a Body of Air condens’d 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 Momenta of all the Currents — — — — united in itself) and with a Power equal to its Swiftness and Density.
It is this whirling Body of Air between aaaa and bbbb 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 encreasing 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 between aaaa and bbbb will be violently agitated and driven about, and parts of it raised with the spiral Current, and thrown about so as to form a Bushlike Appearance.
This Circle is of various Diameters, sometimes very large.
If the Vacuum passes over Water the Water may rise in it in a Body or Column to near the Height of 32 feet. If it passes over Houses, it may burst their Windows or Walls outwards, pluck off the Roofs and blow up the Floors, by the Sudden Rarefaction of the Air contain’d within such Buildings, the outward Pressure of the Atmosphere being suddenly taken off; So the stop’d Bottle of Air bursts under the exhausted Receiver of the Air Pump.
Fig II. is to represent the Elevation of a Water Spout; wherein I suppose PPP to be the Cone, at first a Vacuum till WW the rising Column of Water has fill’d so much of it. SSSS the Spiral Whirl of Air surrounding the Vacuum and continu’d higher in a close Column after the Vacuum ends in the Point P. till it reach the cool Region of the Air. B.B. the Bush describ’d by Stuart, surrounding the Foot of the Column of Water.
Now I suppose this Whirl of Air will at first be as invisible as the Air itself tho’ 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 low broad Cone whose Top gradually rises 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 condens’d into thick Vapour by the Cold, and is seen first at A. the highest Parts, which being now cool’d, 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 themselves 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, they flie off, spread and form a Cloud.
It seems easy to conceive, how by this successive Condensation from above the Spout appears to drop or descend from the Cloud, tho’ the Materials of which it is composed are all the while ascending.
The Condensation of the Moisture contain’d in so great a Quantity of warm Air as may be suppos’d to rise in a short Time in this prodigiously rapid Whirl, is perhaps sufficient to form a great Extent of Cloud, tho’ 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; Tho’ the upper or what is commonly call’d 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 ABCD only will be visible, and the Bush perhaps below.
But if the Whirl be strong, and there be much Dust on the Land, or the Column WW be rais’d from the Water; then the lower Part becomes visible, and sometimes even united to the upper Part. For the Dust may be carried up in the Spiral Whirl till it reach the Region where the Vapour is condens’d, and rise with that even to the Clouds. And the Friction of the Whirling Air on the Sides of the Column WW may detach great Quantities of its Water, break it into Drops and carry them up in the Spiral Whirl mix’d with the Air; the heavier Drops may indeed fly off, and fall in 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 darkned and render’d visible.
As the Whirl weakens, the Tube may (in Appearance) separate in the Middle; the Column of Water subsiding, and the superior condens’d 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.
Dr. Stuart says, “it was observable of all the Spouts he saw, but more perceptible of the great One; that towards the End it began to appear like a hollow Canal, only black in the Borders but white in the Middle, and tho’ at first it was altogether black and opaque, yet now one could very distinctly perceive the Sea Water to fly up along the Middle of this Canal, as Smoak up a Chimney.” And Dr. Mather describing a Whirlwind says, “a thick dark small Cloud arose, with a Pillar of Light in it, of about 8 or 10 foot Diameter and passed along the Ground in a Tract not wider than a Street, horribly tearing up Trees by the Roots, blowing them up in the Air like Feathers, and throwing up Stones of great Weight to a considerable Height in the Air, &c.”
These Accounts, the one of Water Spouts, the other of a Whirlwind, seem in this particular to agree; what one Gentleman describes as a Tube black in the Borders, and white in the middle; the other calls a black Cloud with a Pillar of Light in it; the latter Expression has only a little more of the marvellous, but the Thing is the same. And it seems not very difficult to understand. When Dr. Stuarts Spouts were full charg’d; that is, when the whirling Pipe of Air was filled, between aaaa and bbbb [Fig. I], with Quantities of Drops and Vapour torn off from the Column WW [Fig. II], the whole was render’d so dark as that it could not be seen thro’, nor the spiral ascending Motion discover’d; but when the Quantity ascending lessen’d, the Pipe became more transparent, and the ascending Motion visible. For by Inspection of this Figure in the Margin representing a Section of our Spout with the Vacuum in the Middle, it is plain, that if we look at such a hollow Pipe in the Direction of the Arrows, and suppose opacous Particles to be equally mix’d in the Space between the two circular Lines, both the Part between the Arrows a and b and that between the Arrows c and d, will appear much darker than that between b and c; as there must be many more of those opaque Particles in the Line of Vision across the Sides than across the Middle. It is thus, that a Hair in a Microscope evidently appears to be a Pipe, the Sides shewing darker than the Middle. Dr. Mather’s Whirl was probably fill’d with Dust; the Sides were very dark, but the Vacuum within rendering the Middle more transparent he
calls it a Pillar of Light. It was in this more transparent Part between b and c that Stuart could see the spiral Motion of the Vapours, whose Lines on the nearest and farthest Side of this transparent Part crossing each other, represented Smoke ascending in a Chimney; for the Quantity being still too great in the Line of Sight thro’ the Sides of the Tube, the Motion could not be discover’d there, and so they represented the solid Sides of the Chimney.
When the Vapours reach in the Pipe from the Clouds near to the Earth, it is no Wonder now to those who understand Electricity, that Flashes of Lightning should descend by the Spout, as in that at Rome.
But you object, If Water may be thus carried into the Clouds, why have we no salt Rains? The Objection is strong and reasonable; and I know not whether I can answer it to your Satisfaction. I never heard but of one Salt Rain, and that was where a Spout passed pretty near a Ship, so I suppose it to be only the Drops thrown off from the Spout by the centrifugal Force, (as the Birds were at Hatfield) when they had been carried so high as to be above or to be too strongly centrifugal for the Pressure of the concurring Winds surrounding it. And indeed I believe there can be no other kind of Salt Rain; for it has pleased the Goodness of God so to order it, that the Particles of Air will not attract the Particles of Salt; tho’ they strongly attract Water. Hence tho’ all Metals, even Gold, may be united with Air and render’d volatile, Salt remains fix’d in the Fire, and no Heat can force it up to any considerable Height or oblige the Air to hold it; Hence when Salt rises as it will a little Way into Air with Water, there is instantly a Separation made; the Particles of Water adhere to the Air, and the Particles of Salt fall down again, as if repell’d and forc’d off from the Water by some Power in the Air: Or as some Metals dissolv’d in a proper Menstruum will quit the Solvent when other matter approaches, and adhere to that, so the Water quits the Salt and embraces the Air but Air will not embrace the Salt and quit the Water. Otherwise, our Rains would indeed be salt, and every Tree and Plant on the Face of the Earth be destroy’d, with all the Animals that depend on them for Subsistence. He who hath proportioned and given proper Qualities to all Things, was not unmindful of this. Let us adore him with Praise and Thanksgiving!
By some Accounts of Seamen, it seems the Column of Water WW sometimes falls suddenly, and if it be as some say 15 or 20 Yards Diameter it must fall with great Force, and they may well fear for their Ships. By one Account in the Transactions of a Spout that fell at Coln in Lancashire one would think the Column is sometimes lifted off from the Water, and carried over Land, and there let fall in a Body; but this I suppose happens rarely.
Stuart describes his Spouts as appearing no bigger than a Mast! and sometimes less: but they were at a League and half Distance.
I think I formerly read in Dampier, or some other Voyager, that a Spout in its progressive Motion went over a Ship becalmed on the Coast of Guinea: and first threw her down on one Side, carrying away her Foremast; then suddenly, whipt her up, and threw her down on the other Side, carrying away her Mizen Mast; and the whole was over in an Instant. I suppose the first Mischief was done by the foreside of the Whirl, the latter by the hinder Side, their Motion being contrary.
I suppose a Whirlwind or Spout may be stationary when the concurring Winds are equal; but if unequal, the Whirl acquires a progressive Motion, in the direction of the Strongest Pressure.
Where the Wind that gives the progressive Motion becomes stronger below than above, or above than below, the Spout will be bent, and the Cause ceasing, straiten again.
Your Queries towards the End of your Paper, appear judicious and worth considering. At present I am not furnish’d with Facts sufficient to make any pertinent Answer to them. And this Paper has already a sufficient Quantity of Conjecture.
Your manner of accommodating the Accounts to your Hypothesis, of descending Spouts, is I own ingenious; and perhaps that Hypothesis may be true: I will consider it farther; but as yet I am not satisfy’d with it, tho’ hereafter I may be. Here you have my Method of Accounting for the principal Phaenomena, which I submit to your candid Examination. If my Hypothesis is not the Truth itself, it is least as naked: For I have not with some of our learned Moderns disguis’d my Nonsense in Greek, cloth’d it in Algebra, or adorn’d it with Fluxions. And as I now seem to have almost written a Book instead of a Letter, you will think it high time I should conclude, which I beg Leave to do with assuring you that I am most sincerely, Dear Sir Your obliged Friend and humble Servant.