516 



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



as deduced from the eight years' observations, may be represented 

 approximately by parallel straight lines drawn on a plane chart. 

 The positions of the three barometers are as follow : — 



Height aboTe 

 Lat. Long. onean sea-leyeL 



ft. 



Greenwich 51 29 159 



Dubliai 53 21 6 16 W. 18* 



Makerstonn 55 35 2 16 W. 213 



Taking the meridian of Greenwich for the axis of y, and the circle of 

 latitude of Greenwich for that of x, represented by a straight line on the 

 chart perpendicular to y (both on the same plane), we obtain the follow- 

 ing equations for the isobars passing through Maherstoun (M) and 

 Dubhn (D) :— 



M ... y = ax^\, 

 D . . . . y = cLV-\-h,^; 



and if y, x, 6^, and h.^ be expressed in geographical miles, then from the 

 preceding coordinates we have for the points M and D, 



M .... 246=- 77a + (1) 



D . . . . 112=-224« + 5„ ....... (2) 



where the usual trigonometrical directions are employed. 



If jj^ be the difference of barometric pressures (always for the mean 

 sea-level) for Greenwich minus Makerstonn, and /3, for Greenwich ?Hf;n(s 

 Dublin, these will be proportional to the perpendiculars from G to the 

 isobaric Imes through M and D ; hence 



jr\ • • • 



From these three equations w^e find 



„_ tan e_ 112ft-246fe _. ^,-2-2ft . 



a- tan 6- ■ 0-344^-^/ • • ' • W 



so that, if be counted from the north through the east, south, and 

 west (as for the directions of the wind), we find for the isobar passing- 

 through G and M, 



+ 2-20 



/3,=0, tan0=^^, = 343° or 163^ 

 for the isobar passing through G and D, 



* The height given by Dr. Lloyd is 244 feet above \ov^ vrater of si^ring-tides, from 

 which I have subtracted 6 feet to reduce to mean sea-level, as at the other stations 

 (Dublin Observations, vol. i. p 386) 



