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it ends in a point, as at P, Fig. II. in the plate, low, broad cone, whose top gradually rises forming a long and sharp cone.
and sharpens, as the force of the whirl inIn Fig. I. which is a plan or ground-plat of creases. At its upper end it becomes visible, a whirlwind, the circle V. represents the cen- by the warm air brought up to the cooler retral vacuum.
gion, where its moisture begins to be conBetween a a a a and 6.6 6 6 I suppose a densed into thick vapour, by the cold, and is body of air, condensed strongly by the pres- seen first at A, the highest part, which being sure of the currents moving towards it, from now cooled, condenses what rises next at B, all sides without, and by its centrifugal force which condenses that at C, and that confrom within, moving round with prodigious denses what is rising at D, the cold operating swiftness, (having, as it were, the entire mo- by the contact of the vapours faster in a right menta of all the currents
line downwards than the vapours can climb united in itself) and with a power equal to in a spiral line upwards; they climb, however, its swiftness and density.
and as by continual addition they grow denser, It is this whirling body of air between and, consequently, their centrifugal force a a a a and b b b b that rises spirally; by its greater, and being risen above the concen. force it tears buildings to pieces, twists up trating currents that compose the whirl, fly great trees by the roots, &c. and, by its spiral off, spread, and form a cloud. motion, raises the fragments so high, till the It seems easy to conceive, how, by this sucpressure of the surrounding and approaching cessive condensation from above, the spout apcurrents diminishing, can no longer confine pears to drop or descend from the cloud, them to the circle, or their own centrifugalthough the materials of which it is composed force increasing, grows too strong for such are all the while ascending. pressure, when they fly off in tangent lines, The condensation of the moisture, containas stones out of a sling, and fall on all sides, ed in so great a quantity of warm air as may and at great distances.
be supposed to rise in a short time in this proIf it happens at sea, the water under and digiously rapid whirl, is perhaps, sufficient to between a a a a and 6 b b b will be violently form a great extent of cloud, though the spout agitated and driven about, and parts of it should be over land, as those at Hatfield; and raised with the spiral current, and thrown if the land happens not to be very dusty, perabout so as to form a bush-like appearance. haps the lower part of the spout will scarce
This circle is of various diameters, some become visible at all; though the upper, or times very lage. If the vacuum passes over what is commonly called the descending part water, the water may rise in it in a body, or be very distinctly seen. column, to near the height of thirty-two feet. The same may happen at sea, in case the If it passes over houses, it may burst their whirl is not violent enough to make a high windows or walls outwards, pluck off the vacuum, and raise the column, &c. In such roofs, and pluck up the floors, by the sudden case, the upper part A, B, C, D only will be rarefaction of the air contained within such visible, and the bush, perhaps, below. buildings; the outward pressure of the at But if the whirl be strong, and there be mosphere being suddenly taken off; so the much dust on the land, and the column WW stopped bottle of air bursts under the exhausted be raised from the water, then the lower part receiver of the air pump.
becomes visible, and sometimes even united Fig. II. is to represent the elevation of a to the upper part. For the dust may be carwater-spout, wherein I suppose P P P to be ried up in the spiral whirl, till it reach the rethe cone, at first a vacuum, till W W, the gion where the vapour is condensed, and rise rising column of water, has filled so much of with that even to the clouds : and the friction it 8 SSS, the spiral whirl of air, surround of the whirling air, on the sides of the column ing the vacuum, and continued higher in a W W, may detach great quantities of its close column after the vacuum ends in the water, break it into drops, and carry them up point P, till it reaches the cool region of the in the spiral whirl mixed with the air; the air. B B, the bush described by Stuart, sur- heavier drops may, indeed, ily off, and fall, in rounding the foot of the column of water. a shower, round the spout; but much of it
Now, I suppose this whirl of air will, at will be broken into vapour, yet visible ; and first be as invisible as the air itself, though thus, in both cases, by dust at land, and by reaching, in reality, from the water, to the water at sea, the whole tube may be darkened region of cool air, in which our low summer and rendered visible. thunder-clouds commonly float: but presently As the whirl weakens, the tube may (in it will become visible at its extremities. At appearance) separate in the middle; the coits lower end, by the agitation of the water, lumn of water subsiding, and the superior under the whirling part of the circle, between condensed part drawing up to the cloud. Yet P and S forming Stuart's bush, and by the still the tube, or whirl of air, may remain enswelling and rising of the water, in the be- tire, the middle only becoming invisible, as ginning vacuum, which is, at first, a small, not containing visible matter. VOL. II. ... 2U