not blow round in a circle, but around in spirals. Nay, I go farther, and conjecture, that it is only within a certain distance of the vortex that the wind gyrates, and that the gyrating column is never hundreds of miles in diameter, as the advocates of this theory make it: I shall allude to this again. The low barometer at the centre is owing, in part, to two causes; one is the condensation of vapour, with its liberated heat, as maintained by Espy; the other is the action of a real centrifugal force, which applies to all revolving bodies. In weighing the effect of this centrifugal force upon the low barometer, care should be taken not to give it an undue weight. It is not sufficient to cause the air to fly off in a tangent. The lateral atmospheric pressure would prevent that, if the centrifugal force were never so great; and the lower the barometer in the centre, the greater would be the pressure of the surrounding air. The proper weight, therefore, due to the centrifugal force I hold to be not very great, though it is appreciable to this extent: The storm having commenced revolving, the flow of air into the vortex is retarded, not prevented by centrifugal tendency and this retardation assists in causing the barometer to stand lower than it would if there were no revolution. Any one who has watched the little whirl-winds so often seen during summer and fall, or who can call to mind the whirls or "sucks" in a mill pond, or at the lock in a canal when the water is drawn off at the bottom, may appreciate the extent to which the centrifugal tendency will help to make a low barometer at the centre of a cyclone.
796. An illustration.—The low barometer, the revolving storm, and the ascending column require for a postulate the approach by spirals of the wind from circumference to centre. The wind (§665) blows towards the place of low barometer; that is admitted by all. It can only reach that place by a direct or by a curvilinear motion. If by the former, then there can be no revolution; but if there be revolution, then the air, while as wind it is revolving around the centre in the gyrations of the storm, is approaching the centre also. Hence we derive the elements of a spiral curve; and the physical necessity for spiral motion is demonstrated from the fact that there is circular motion and an uprising in the centre. This spiral movement and the uprising may be illustrated by familiar examples : The angles and corners of the Observatory, and its wings, are so arranged that at a certain place there is, with westerly winds, always a whirlwind.
