WHIRLWIND, a wind that rises suddenly, is exceedingly rapid and impetuous when risen, but is soon spent. See Wind.
The origin of whirlwinds has been already given Whirlwind.
under that of Water-Spouts. It is, however, highly probable that all these violent motions of the air are ultimately owing to the emission of electric matter from the earth. It is mentioned as a certain sign of an approaching hurricane in the West Indies, that the sea-water becomes exceedingly clear and transparent of a sudden; after which it seems to boil, and the waves begin to rise, though there is not the least wind. Both these phenomena can be accounted for on no other principle than that of a quantity of electric matter attempting to get out from the earth. Now though hurricanes are not precisely of the nature of whirlwinds, as not having a circular motion, it is highly probable that the latter also are occasioned somehow or other by a similar cause. A stream of electric matter issuing with violence into the atmosphere from any part of the earth, will cause a kind of vacuum in the atmosphere directly above that place, and of consequence a whirlwind or water-spout by the air rushing in on all sides to supply the vacuum*. If this is the true principle on which these phenomena depend, then we are sure that there never was any hurricane or whirlwind so violent as to remove an obstacle of the size of only one cubic inch, provided that was supported by a power equivalent to 15 pounds; for this is the utmost force of the atmosphere when rushing into a perfect vacuum, which never could take place in the centre of a whirlwind or water-spout. Indeed, notwithstanding the dreadful effects sometimes observed from hurricanes and whirlwinds, we shall easily perceive that the utmost of their power always falls very far short of this. The diminution of the specific gravity of the air by only th in the middle of the column, would produce such an afflux of air from all quarters, that an obstacle presenting a surface of one foot square would require a force of 504 pounds to prevent it from being carried away; which scarce the strongest walls that can be built by human art could resist. Nay, even the tenth part of this, or the diminution of the gravity of the atmosphere by th part, would produce a pressure of upwards of 50 pounds on every square foot of surface, which it is to be doubted whether any of our common houses could resist.
As this degree of rarefaction in the atmosphere would only produce a descent of about ths of an inch in the barometer, it may seem wonderful why the most violent whirlwinds do not take place at those times when the mercury descends two or more inches, which descents are by no means unfrequent. But here it is to be remembered, that by whatever means this descent of the barometer is occasioned, by a certain expansive power in the rarefied place of the atmosphere, the air is prevented from rushing in eddies, and forming the whirlwind. Now, in the case of electric matter escaping from the earth, ascending into the high regions of the atmosphere, and carrying part of the air along with it, we see that there could be no expansion, but on the contrary a continued suction; and if the electric stream should happen to be stopped in one place and break out in another, the spout or whirlwind would immediately vanish in the one place and appear in the other, as is often observed in whirlwinds and water-spouts. Its progressive motion might be owing to the electric current perpetually shifting its
Whirlwind. place; in which case the whirlwind would constantly follow it, for the reasons already given.
Some kinds of whirlwinds move with a slow motion, and are injurious only by their vortex; while others seem to do mischief as well by their progressive as their whirling motion. Of this kind are those called typhoons; which, by their frequently following the course of rivers, seem thus also to discover their electrical origin. Of the destructive effects of these, we have an instance in what happened at Charlestown in South Carolina on the 1st of June 1761. It was first observed about noon, on land, upwards of 50 miles west-by-south of Charlestown, and destroyed several houses, &c. as it passed along, in many places making wide avenues through the woods; from whence every tree and shrub was torn up, and great branches of trees were driven about in the column as it passed along. It directed its course to Ashley river, down which it came with surprising velocity; in its appearance resembling a column of smoke or vapour, whose motion was very irregular and tumultuous. Its momentum was so great, that Ashley river was ploughed to the bottom, and the channel laid bare. As it came down this river it made a constant noise like thunder; its diameter being computed about 300 fathoms. It was met at White Point by another of the same kind which came down Cooper's river, but with inferior strength; however, on their meeting together, the agitation of the air was much greater, while the clouds, which were driving in all directions to the place, seemed to be precipitated and whirled round with incredible velocity. It then fell upon the shipping in the road; entirely destroying some, and damaging others: being scarce three minutes in its passage, though the distance was near two leagues. In that short time it did damage to the amount of 20,000 l.; and had not its direction been altered by that gulf which came down Cooper's river, it must have totally destroyed Charlestown, as no obstacle whatever seemed capable of resisting its fury.
It is observable, that whirlwinds do not arise with all their fury at once, but increase in strength as they go along. Dr Franklin relates, that he once rode for a considerable way by the side of one which had but newly arisen, and could scarce lift up any thing but dust, its vortex being only a few inches in diameter; but as it went along the strength and diameter constantly increased, till at last, as it began to take up and whirl round large branches of trees, it became dangerous for him to accompany it any longer. See Water-Spout and Wind.