408 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



up in the vortex ; then it expands, grows cool, and condenses its 

 vapour, which spreads out at top like a great mushroom in the 

 air, the liberated heat adding fury to the storm. 



7.94). Such, briefly stated, are the two theories. They appear 

 Erroneous theories, to me, from such observation and study as I have 

 been able to bestow, to be neither of them wholly right or 

 altogether wrong. Both are instructive, and the suggestions of 

 one will, in many instances, throw light upon the facts of the 

 other. That rotary storms do frequently occur at sea we know, 

 for vessels have sometimes, while scudding before the wind in 

 them, sailed round arid round. The United States brig " Perry " 

 did this a few years ago in the West Indies ; and so did the 

 "Charles Heddle" in the East Indies: she went round and 

 round a cyclone five times. 



795. From such observations as I have been able to obtain 

 The wind in a true upou the subject, I am iuduced to believe, with 

 spfrais! ^"^^ ^^ Thom, that the wind in a cyclone does not blow 

 round in a circle, but around in spirals. Nay, I go farther, and 

 conjecture that it is only w^ithin a certain distance of the vortex 

 that the wind gyrates, and that the gyrating column is never 

 liundreds 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 appre- 

 ciable to this extent : The storm having commenced revolving, 

 the flow of air into the vortex is retarded, not prevented, by cen- 

 trifugal tendency ; and this retardation assists in causing the 

 barometer to stand lower than it would if there were no revolu- 

 tion. Any one who has watched the little wliirlwinds 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 



