42i PHYSICAL Gi:OGRAriIY OF THE SEA, AND ITS METEOIiOLOGY. 



not blow ruTind in a circle, Lnt around 'in 'spirals. Xa}', T go 

 farther, and conjecture, that it is onl}^ within a certain distance 

 of the vortex that tlie 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 con- 

 densation of vapour,, with its liberated heat, as maintained by 

 Espy ; the other is the action of a real centrifugal force, wliich 

 applies to all revolving bodies. In weighing the effect of tiiis 

 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, there- 

 fore, due to the centrifugal force I hold to be not very great, 

 though it is appreciable to this extent : The storm having com- 

 menced revolving, the flow of air into the vortex is retarded, not 

 p-evented^ 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 

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

 (§665) blows towards the place of low barometer ; that is ad- 

 mitted 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 revo- 

 lution ; 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. 



