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 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 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 dis tances. 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 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 thirty-two feet. If it passes over houses, it may burst their windows or walls outward, 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 airpump. Fig. 2 is to represent the elevation of a waterspou, wherein I suppose P P P to be the cone, at first a vacuum, till W W, the rising column of water, has filled so much of it. SSSS, the spiral whirl of air, surrounding the vacuum, and continued higher in a close column after the vacuum ends in the point P, till it reaches the cool region of the air. B B, the bush described 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, 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, 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 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 downward than the vapours can climb in a spiral line upward; 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. 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 to the upper part. For the dust may be carried up in the spiral whirl till it reach the region where the vapour is condensed, and rise with that even to the clouds: and the friction of the whirling air on the sides of the column W W, may detach great quantities of its water, break it into drops, and carry them up in the spiral whirl, mixed 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 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. 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 though at first it was altogether black and opaque, yet now one could very distinctly perceive the seawater to fly up along the middle of this canal, as smoke 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 eight or ten feet 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 life 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. Stuart's spouts were full charged, that is, when the whirling pipe of air was filled between a a a a and b b bb, fig. 1, with quantities of drops, and vapour torn off from the column W W, fig. 2, the whole was rendered so dark as 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. For, by inspection of the figure given in the opposite page, respecting 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 opaque particles to be equally mixed 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 showing darker than the middle. Dr. Mather's whirl was probably filled 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 |