But Air flowing
on and near the Surface of Land or Water from all Sides toward a
Center, must at that Center ascend; the Land or Water hindering its
Descent.
If these concentring Currents of Air be in the upper Region,
they may indeed descend in the Spout or Whirlwind; but then when the
united Current reach'd the Earth or Water it would spread and
probably blow every way _from_ the Center: There may be Whirlwinds of
both kinds, but from the common observ'd Effects, I suspect the
Rising one to be the most common; and that when the upper Air
descends, tis perhaps in a greater Body, extending wider and without
much whirling as in our Thunder Gusts. When Air descends in a Spout
or Whirlwind, I should rather expect it would press the Roof of a
House inwards, or force in the Tiles, Shingles or Thatch; force a
Boat down into the Water, or a Piece of Timber into the Earth than
that it would lift them up and carry them away.
It has so happen'd that I have not met with any Accounts of
Spouts, that certainly descended. I suspect they are not frequent.
Please to communicate those you mention. The apparent dropping of a
Pipe from the Clouds towards the Earth or Sea, I will endeavour to
explain hereafter.
The Augmentation of the Cloud, which, as I am inform'd is
generally if not always the case during a Spout, seems to show an
Ascent rather than a Descent of the Matter of which such Cloud is
composed.
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