526 Mr. J. A. Broun on the Directions ^c. of [Feb. 1, 



Table YI. — Directions o£ the Isobars and of the "Wind, 1843-1846. 



Month. 



Green- 

 wich 

 barom. 

 -f29in. 





in. 



+ •091 

 + •005 

 + •042 

 +•037 

 -•040 

 + •051 

 + -043 

 +•017 

 + •009 

 + •071 

 +•112 

 -{-•067 



0. 



Resultant wind-direction. 



e~<p. 



Mat. 



Dubl. 



Green. 



Mean 



January ... 

 February . . . 



May 



August 



September 

 October ... 

 November 

 December 



in. 



0-912 

 0-843 

 0-907 

 0-918 

 0-948 

 0-959 

 0-946 



0- 920 



1- 052 

 0-805 



0- 876 



1- 051 



in. 



+ •185 

 +•068 

 + •084 

 + •064 

 -•026 

 +•091 

 + •106 

 +•086 

 + •037 

 + •143 

 + •136 

 + •104 



267 

 293 

 267 

 260 

 22 

 262 

 274 

 287 

 284 

 267 

 240 

 255 



o 



241 

 287 

 244 

 274 



233 



247 

 244 

 237 

 211 

 239 



o 



255 

 298 

 228 

 266 



255 



JiiO 



300 

 261 

 267 

 251 

 270 



o 



246 

 256 

 245 

 241 



230 



246 

 224 

 232 

 216 

 262 



o 



247 

 280 

 239 

 260 



Q 



240 



264 

 243 

 245 

 226 

 257 



o 



+ 20 

 + 13 

 +28 

 



+22 

 +22 

 +25 

 +23 

 ■H-41 

 +22 

 + 14 

 — 2 





0-928 



+ •090 



+ •042 



269 



243 



261 



243 



249 



+20 



Uelation of the Dii^ection of the Isobars to that of the Wind. — The 

 resultant direction of the wind at Greenwich is the same as that at 

 Makerstoun on the mean of four years, but the monthly directions differ 

 on the average 12° (without reference to sign). The resultant direction 

 at Dublin for the four years differs considerably (18°) from that at the 

 other two stations. The mean of the three resultant directions ((^) is 

 given in Table YI., together with the differences (6 — (f)). 



It appears that the direction of the isobars was positive of that of the 

 wind in ten months of the year, the directions being nearly the same in 

 two months, April and December ; the difference was greatest in Sep- 

 tember. Erom the mean directions for the four years we find 



d-(j)=-\-20° *. 



I believe that the only really exceptional result is that for December, 

 since the determination of B for April and September depends on very 

 small values of (3^ and (3^ ; the mean of the results for these two months 

 is, however, exactly the mean for the year. The most remarkable of the 



^ "The wind in storms neither blows round the centre of least pressure in circles, 

 or as tangents to the concentric isobarics, nor does it blow directly towards that centre. 

 It takes a direction intermediate, approaching, however, more nearly to the direction- 

 and course of the circular curves than of the radii to the centre" ("On the Mean 

 Pressure of the Atmosphere," by Mr. A. Buchan, Edinb. Trans, vol. xxv. p. 581) ; that 

 is, the angle with the isobar is less than 45°. Mr. Buchan adds, " or, according to Dr. 

 Buys-Ballot, the angle is not a right angle, but from 60° to 80° ; " that is, from 30° 

 to 10° with the isobar. The mean of these limits is 20°, exactly what has been found 

 above from the mean direction of the isobars and resultant direction of the wind in 

 all cases. I have not been able to find any memoir by the distinguished Utrecht 

 meteorologist containing the grounds on which his limits are founded. The tendency 

 of the winds inwards (towards the centre) in cyclones is noticed by several early 

 writers on the subject, among others by Eedfield (Silliman's Journal, vol. i. p. 14, 

 1846). 



