§ 795, 796. STORMS, HURRICANES, AND TYPHOONS. 425 



Ileddle" 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 uDon the subjcct, I am iuduccd to believe, with 



cyclone blows in spi- ^ , . , . , , 



rais. inom, that the wmd m a cyclone does 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 vapor, 

 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 cen- 

 trifugal force were never so great ; and the lower the barometer 

 in the centre, the greater would be the pressure of the surround- 

 ing air. The proper weight, therefore, due the centrifugal force 

 I hold to be not very great, though it is appreciable to this ex- 

 tent : 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 low- 

 er than it would if there were no revolution. Any one who has 

 watched the little whirlwinds 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 bot- 

 tom, may appreciate the extent to which the centrifugal tendency 

 will help to make a low barometer at the centre of a cyclone. 



796. The low barometer, the revolving storm, and the ascend - 

 An illustration, ing column rcquirc for a postulate the approach 



by spirals of the wind from circumference to centre. The wind 

 blows toward the place of low barometer ; that is admitted by all. 

 It can only reach that place by a direct or by a curvilinear mo- 

 tion. 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 ; 



