Lastly, as the lower air, and nearest the surface, is most rarefied by the heat of the sun, that air is most acted on by the pressure of the surrounding cold 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 trump, strongest, and the centrifugal force of its particles greatest; and hence the vacuum round the axis of the whirl should be greatest near the earth or sea, and be gradually diminished as it approaches the region of the clouds, till it ends in a point, as at P in Fig. II. [Plate V.] forming a long and sharp cone.
In Fig. I. which is a plan or ground-plat of a whirlwind, 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 moving towards it, from all sides without, and by its centrifugal force from within, moving round with prodigious swiftness, (having, as it were, the momenta of all the currents ——> ——> ——> ——> united in itself) and with a power equal to its swiftness and density.
Plate V.
Vol. II. page 26.
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Published as the Act directs, April 1, 1806, by Longman, Hurst, Rees & Orme, Paternoster Row.