This circle is of various diameters, sometimes very large.
If the vacuum passes over water, the water may rise in a body or column therein to the height of about thirty-two feet. This whirl of air may 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 it will soon become visible at its extremities. The agitation of the water under the whirling of the circle, and the swelling and rising of the water in the commencement of the vacuum, renders it visible below. It is perceived above by the warm air being brought up to the cooler region, where its moisture begins to be condensed by the cold into thick vapour; and is then first discovered at the highest part; which being now cooled, condenses what rises behind it, and this latter acts in the same manner on the succeeding body; where, by the contact of the vapours, the cold operates 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 by consequence increase their centrifugal force, and being risen above the concentrating currents that compose the whirl, they fly off, 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, although 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: and the friction of the whirling air on the sides of the column may detach great quantities of its water, disperse them into drops, and carry them up in the spiral whirl mixed with the air. The heavier drops may indeed fly off, and fall into a shower about the spout; but much of it will be broken into vapour, and yet remain visible.
As the whirl weakens, the tube may apparently separate in the middle; the column of water subsiding, the superior condensed part drawing up to the cloud. The tube or whirl of air may nevertheless remain entire, the middle only becoming invisible, as not containing any visible matter.
Dr. Stuart, in the Philosophical Transactions, says, “It was observable of all the spouts he saw, but more perceptible of a large one, that towards the end it began to appear like a hollow canal, only black in the borders, but white in the middle; and though it was at first altogether black and opaque, yet the sea-water could very soon after be perceived to fly up along the middle of this canal like smoke in a chimney.”
When Dr. Stuart’s spouts were full charged, that is, when the whirling pipe of air was filled with quantities of drops and vapour torn off from the column, the whole was rendered so dark that it could not be seen through, nor the spiral ascending motion discovered; but when the quantity ascending lessened, the pipe became more transparent, and the ascending motion visible. The spiral motion of the vapours, whose lines intersect each other on the nearest and farthest side of this transparent part, appeared therefore to Stuart like smoke ascending in a chimney; for the quantity being still too great in the line of sight through the sides of the tube, the motion could not be discovered there, and so they represented the solid sides of the chimney.
Dr. Franklin concludes by supposing a whirlwind or spout to be stationary, when the concurring winds are equal but if unequal, the whirl acquires a progressive motion in the direction of the strongest pressure. When the wind that communicates this progression becomes stronger above than below, or below than above, the spout will be bent or inclined. Hence the horizontal process and obliquity of water-spouts are derived.
Water-way, gouttiere, a long piece of timber serving to connect the sides of a ship to her decks, and form a sort of channel to carry off the water from the latter by means of scuppers. See that article.
The convexity of the decks, represented by N, M, N, in the Midship-frame, plate [VII]. necessarily carries the water towards the sides, where this piece is fixed, which is principally designed to prevent the water from lodging in the seams, so as to rot the wood and oakum contained therein. The water-ways N N are therefore hollowed in the middle lengthways, so as to form a kind of gutter or channel, one side of which lies almost horizontally, making part of the deck, whilst the other rises upwards, and corresponds with the side, of which it likewise makes a part. They are scored down about an inch and a half, or two inches, upon the beams, and rest upon lodging-knees or carlings. They are secured by bolts driven from without through the planks, timbers, and water-ways, and clinched upon rings on the inside of the latter.
The scuppers, which are holes by which the water escapes from off the deck, are accordingly cut through the water-ways.