Volume II Part 25 (1/2)

Such effects see current of air

I will endeavour to explaina plan and an elevation of a spout or ind

I would only first beg to be alloo or three positions ion of air is often more heated, and so hter The coldness of the upper region is manifested by the hail which sometimes falls from it in a hot day

2 That heated air may be very moist, and yet the moisture so equally diffused and rarefied as not to be visible till colder air mixes with it, when it condenses and becomes visible Thus our breath, invisible in summer, becomes visible in winter

Now let us suppose a tract of land or sea, of perhaps sixty miles square, unscreened by clouds and unfanned by winds during great part of a summer's day, or, it may be, for several days successively, till it is violently heated, together with the lower region of air in contact with it, so that the said lower air becoion of the atmosphere in which the clouds co this tract has not been sothose days, and, therefore, remains heavier The consequence of this should be, as I conceive, that the heated lighter air, being pressed on all sides,cannot be in all parts, or the whole area of the tract at once, for that would leave too extensive a vacuuin precisely in that coluhtest or most rarefied; and the warm air will flow horizontally from all points to this colu to rise, a whirl is naturally formed, in the same manner as a whirl is for from all sides of the tub to the hole in the centre

And as the several currents arrive at this central rising coluree of horizontal e it to a verticalthe whirl, decline fro joined the whirl, they _ascend_ by a spiral motion, in the sah the hole in the tub before mentioned

Lastly, as the lower air, and nearest the surface, is most rarefied by the heat of the sun, that air iscold 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 trureatest; and hence the vacuureatest near the earth or sea, and be gradually diion of the clouds, till it ends in a point, as at P, _Fig 2 in the plate_, forure 1, which is a plan or groundplat of a ind, the circle V represents the central vacuum

Between _a a a a_ and _b b b b_ I suppose a body of air, condensed strongly by the pressure of the currents al force fro, as it were, the entire 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 _a a a a_ and _b b b b_ that rises spirally; by its force it tears buildings to pieces, twists up great trees by the roots, &c, and, by its spiral h, till the pressure of the surrounding and approaching currents dier confine therows 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 under and between _a a a 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 diae If the vacuum passes over water, the water ht of thirty-two feet If it passes over houses, it may burst their s 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 at suddenly taken off; so the stopped bottle of air bursts under the exhausted receiver of the airpu 2 is to represent the elevation of a water-spout, wherein I suppose P P P to be the cone, at first a vacuu column of water, has filled sothe vacuuher in a close column after the vacuuion 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, froion of cool air, in which our low summer thunder-clouds commonly float: but presently it will becoitation of the water under the whirling part of the circle, between P and S, for of the water in the beginning vacuuradually rises and sharpens, as the force of the whirl increases _At its upper end_ it becoion, where its ins 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 doard than the vapours can climb in a spiral line upward; they clirow 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 froh the

The condensation of the reat a quantity of warm air as iously rapid whirl, is perhaps sufficient to forh 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 becoh the upper, or what is co part, be very distinctly seen

The sah to h 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 ion 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 colureat 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 , and the superior condensed part drawing up to the cloud Yet still the tube or whirl of airinvisible, as not containing visible matter

Dr Stuart says, ”It was observable of all the spouts he saw, but reat one, that, towards the end, it began to appear like a hollow canal, only black in the borders, but white in the ether black and opaque, yet now one could very distinctly perceive the seawater to fly up along the middle of this canal, as s a ind, says, ”A thick dark, sht or ten feet diaround in a tract not wider than a street, horribly tearing up trees by the roots, blowing thereat weight to a considerable height in the air,” &c

These accounts, the one of water-spouts, the other of a ind, seeentleman describes as a tube, black in the borders and white in the ht in it; the latter expression has only a littleis 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 b b_, fig 1, with quantities of drops, and vapour torn off fro 2, the whole was rendered so dark as that it could not be seen through, nor the spiral ascendinglessened, the pipe beca iven 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 adarker than the middle Dr Mather's whirl was probably filled with dust, the sides were very dark, but the vacuu the ht

[Illustration:

Fig 1 Fig 2 Fig 3]