﻿and Fluctuations. 



437 



Modifying the formula in accordance with the customary unit, 

 let d denote the depression of the aerial strata of equal pres- 

 sure at the vortex, in feet ; 

 v } the velocity of the wind in miles per hour ; 

 a, the velocity in feet per second ; 

 then 



a — 



and 



6000 _10 

 3600 V ~ 6 v ' 



d=^ = 



100 



,2. 



2g 36 x 64 ' 

 so that if 8 denote the barometric depression in inches, we have 



d 



or 



8= 



8= 



1000 



nearly, 



36 x 640 



and arranging the results so obtained in a tabular form, we 

 have : — 



V. 



d. 



v. 



S. 



v. 

 190 



S. 

 1-566 



60 



156 



120 



•625 



70 



•212 



130 



•733 



200 



1-736 



80 



•278 



140 



•850 



210 



1-914 



90 



•351 



150 



•976 



220 



2-100 



100 



•434 



160 



1111 



230 



2-296 



110 



•525 



170 

 180 



1-254 

 1-406 



240 



2500 



The depressions here tabled agree very closely with those which 

 have been observed, not only in cyclones, but in comparatively 

 moderate storms — as, for instance, in that which will be so 

 sadly remembered as " the Captain's gale," off Cape Finisterre, 

 on the 7th of last September, when, about l h 30 m a.m., the 

 wind, having a force 9 on the Beaufort scale, corresponding to 

 a velocity of about 60 miles, suddenly shifted from S.S.W. to 

 N.N.W., the barometer at the time standing at 29*32, thus 

 showing a depression of '68 of an inch. But though such an 

 agreement is far closer than could have been anticipated, and 

 seems to leave little room for doubt that it indicates this centri- 

 fugal tendency as the first cause of the depression, I would not 

 be understood to imply that the whole depression is due to it. 

 Twice the greatest observed velocity is probably an exaggerated 

 estimate ; and a considerable reduction may readily be allowed, 

 since to whatever depression is produced by the centrifugal mo- 



Phil. Mag. S. 4. Vol. 41. No. 275. June 1871. 2 G 



