Whirlwinds and spouts, are not always, though most commonly, in the day time. The terrible whirlwind which damaged a great part of Rome, June 11, 1749, happened in the night of that day. The same was supposed to have been first a spout, for it is said to be beyond doubt, that it gathered in the neighbouring sea, as it could be tracked from Ostia to Rome. I find this in Pere Boschovich's account of it, as abridged in the Monthly Review for December 1750. In that account, the whirlwind is said to have appeared as a very black, long, and lofty cloud, discoverable, notwithstanding the darkness of the night, by its continually lightning or emitting flashes on all sides, pushing along with a surprising swiftness, and within three or four feet of the ground. Its general effects on houses, were stripping off the roofs, blowing away chimneys, breaking doors and windows, _forcing up the floors, and unpaving the rooms_ (some of these effects seem to agree well with a supposed vacuum in the centre of the whirlwind) and the very rafters of the houses were broken and dispersed, and even hurled against houses at a considerable distance, &c.
It seems, by an expression of Pere Boschovich's, as if the wind blew from all sides towards the whirlwind; for, having carefully observed its effects, he concludes of all whirlwinds, "that their motion is circular, and their action attractive."
He observes, on a number of histories of whirlwinds, &c. "that a common effect of them is, to carry up into the air, tiles, stones, and animals themselves, which happen to be in their course, and all kinds of bodies unexceptionably, throwing them to a considerable distance, with great impetuosity."
Such effects seem to shew a rising current of air.
I will endeavour to explain my conceptions of this matter by figures, representing a plan and an elevation of a spout or whirlwind.
I would only first beg to be allowed two or three positions, mentioned in my former paper.
1. That the lower region of air is often more heated, and so more rarefied, than the upper; consequently, specifically lighter. 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 diffus'd 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.