730 



SCIENCE. 



IVoi.. 11., No. 44. 



It is this deflective force, acting on winds 

 from all sides, as was first shown bj' Tracy ^ 

 (1843), that combines with the centripetal 

 tendencj' of the surface-winds to give rise to 

 the inward spiral blowing of the storm (fig. 

 10) , — a constant feature of all cyclones. 



V^? 



In all hurricanes, the winds greatly increase 

 in strength as they near the centre of the storm, 

 and at the same time their path becomes more 

 nearlj' circular. A cause of this was briefly 

 stated for the whirlwinds : but it now must be 

 more fully analyzed ; aud it will be'best to begin 

 the attempt by resolving the motion of the wind 

 at au3' point of its spiral track into two rec- 

 tangular components (fig. 11), — one, niong a 

 radius toward the centre, 

 P R, the centripetal compo- 

 nent ; the other, circular or ' 

 . tangential, P T. Only the 

 first of these comes directh' 

 from the convectional circu- 

 lation, already described as 

 depending on the central 

 warmth ; and this one would 

 never produce winds of dev- 

 astating strength. The sec- 

 ond, or tangential, arises 

 first from the deflective force 

 of the earth's turning. The 

 higher the latitude, the less 

 the friction at the bottom of 

 the atmosphere, and the 

 greater the distance from which the wind is 

 derived, then the greater its right-handed de- 

 parture from a radial path. Hence in a large 

 storm at sea, where the friction is small, and 

 the indraught has its source several hundred 

 or even a thousand miles awa}- from the centre 

 of low pressure, the deflective tangential com- 

 ponent becomes very considerable, and maj-, 

 near the centre, outrank the centripetal. 



' See Science, i. 98. 



?-.V 



But there is another and even more impor- 

 tant cause of growth in the circular element 

 of the wind's motion ; namely, the increase of 

 its rotary velocity' as the radius of rotation 

 decreases, in accordance with the law of the 

 ' preservation of areas,' already mentioned. 

 Let us suppose, that, when at a distance of five 

 hundred miles from the centre, the inblowing 

 wind has been turned to the right of its radial 

 path by the earth's deflective force so as to 

 have the moderate tangential or rotar}- velocity 

 of one mile an hour; aud, disregarding the 

 further effects of deflection, let us consider the 

 consequences of graduallj- drawing this mass 

 of air towards the centre. The product of its 

 radius and its rotar}' velocity must remain con- 

 stant ; and hence, as the radius is diminished, 

 the velocit}' must increase, one quantity vary- 

 ing inversely' as the other. The -wind lias no 

 visible, material connection with the storm- 

 centre ; but it is slowly moving around that 

 centre, under the control of central forces, de- 

 rived from differences of temperature aud press- 

 ure, that drive it inwards, or, in otlier words, 

 shorten its radius of rotation : and conscquent- 

 h', when, in the case supposed, the radius has 

 been diminished to five miles, the velocity must 

 have been accelerated to one hundred miles an 

 hour, — a violent hurricane-wind. The recog- 

 nition of this important factor of the storm's 

 strength is due to Ferrel (185G). The theo- 

 retical increase of velocity thus provided is 

 never fully realized, for much motion is over- 

 come b}- friction ; but enough is preserved, 

 especially in tropical storms, to give them the 

 greatest share of their destructive strength. 

 The total tangential component of the wind at 

 anj' point must therefore be considered as the 

 sum of the deflective and accelerative forces, 

 minus the loss by friction. Near the storm- 

 centre, where the velocity of tlie wind is very 

 great, this tangential component is much greater 

 than the centripetal, and the spiral path be- 

 comes almost circular ; while the reverse rela- 

 tion holds for the outer part of the storm. 



It will be easily understood, that a consider- 

 able centrifugal force will be developed by the 

 rapid central rotations, as well as by the earth's 

 deflective force; and, as a conseiiucucc, the 

 centripetal force will be partly neutralized, and 

 the winds will be held out from the centre. 

 This must increase the depression already pro- 

 duced there by expansion and overflow ; and, 

 as a matter of fact, the low pressure of a storm- 

 centre, especiallj'in tropical latitudes, is chiefly 

 the effect of this dynamic, and not of the ear- 

 lier named static cause. But so long as the 

 wind maintains its rapid motion, the additional 



