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as invisible as the air itself, though reacliing, 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 ils 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 as. cending.

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.

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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

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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 h, and that between the arrows c and d, will appear much darker than that between 6 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 6 and c, that Stuart could see the spiral motion of the vapours, whose lines on the nearest and farthest

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side of the transparent part crossing each other, represented 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.

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 of Rome.

But you object, if water may be thus carried into the clouds, why have we not 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, though they strongly attract water.

Hence, though all metals, even gold, may be united with air and rendered volatile, salt remains fixed 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 repelled and forced off from the water by some power in the air; or, as some metals, dissolved 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

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