1863.] 385 



IV. " A Development of the Theory of Cyclones." By FRANCIS 

 GALTON, F.R.S. Received December 25, 1862. 



Most meteorologists are agreed that a circumscribed area of baro- 

 metric depression is usually a locus of light ascending currents, and 

 therefore of an indraught of surface winds which create a retrograde 

 whirl (in our hemisphere), because they bring to their destination a 

 lateral impulse, partly due to the greater easterly speed of the earth's 

 surface whence the southern portion of the indraught took its de- 

 parture, and partly due to the less easterly, or we may say greater 

 westerly, speed of its northern portion. 



Conversely, we ought to admit that a similar area of barometric 

 elevation is usually a locus of dense descending currents, and there- 

 fore of a dispersion of a cold dry atmosphere, plunging from the 

 higher regions upon the surface of the earth, which, flowing away 

 radially on all sides, becomes at length imbued with a lateral motion 

 due to the above-mentioned cause, though acting in a different manner 

 and in opposite directions. The currents necessarily travel with 

 diminished radial speed as they widen out from their central area of 

 dispersion, and the eastward tendency of the northern portion of the 

 system and the westward tendency of the southern become more 

 overpowering. It may be presumed, on consideration of the extreme 

 mobility of the air, that a continuous dispersion of currents would 

 result in the yielding of the east and west winds, which had no 

 tangential movement of their own, to the curvature of the others, 

 and that we should witness a disposition of currents like those in the 

 annexed diagram, which is copied from an actual 

 occurrence on December 2, 1 86 1 . The appearance 

 is that of a centre of calms whence currents flow 

 in radial lines, rapidly curving to the right and 

 forming a sort of " anticyclone." 



Dove's law of gyration is so fertile in result, that 

 it accounts for the same direct rotation of a cold Scale 1C 

 wind by a wholly different process. As an antithesis to his theory 

 of cyclones being due to an equatorial current pressing against 

 quiescent air, he adds (Law of Storms), with a view of illustra- 

 ting his position, and not of meeting cases that practically occur, 

 polar cyclones, "if they exist," would have a direct rotation. 



