1877.] On the Lines of equal Barometric Pressure. 515 



February 1, 1877. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The follomng Papers were read : — 



1. On the Mean Directions and Distribution of the Lines 

 of equal Barometric Pressure, and their Relations to the 

 Mean Direction and Force of the Wind over the British 

 Isles, &c.'' By J. A. Broun, F.R.S. Received December 30, 

 1876. 



[Plates 11 & 12.] 



1. Mean Directions and Distribution of the Isoharic Lines. 



In considering atmospheric variations, it is always desirable to know, 

 if possible, the mean values about which the others fluctuate : this ap- 

 pears to be especially the case with reference to the direction of the 

 lines of mean barometric pressure and of the atmospheric currents. If 

 any common law exist connecting the statical and dynamical pressure of 

 the air, this will probably show itself with some precision by an investi- 

 gation in which, all the cases (the observations of every day) being included, 

 deviations from the law may be expected to neutrahze each other, and 

 the final results give absolute measures directly comparable with each 

 other. 



For any exact determination of the lines of equal barometric pressure 

 it is essential to possess observations from stations whose heights above 

 the mean sea-level are accurately known, and made with good instruments 

 which have been compared directly or indhectly with each other. These 

 conditions are well satisfied by the observations made at the Greenwich, 

 Dublin, and Makerstoun Observatories in the eight years 1842 to 1849 

 (both inclusive). The barometers were all by the same maker, each 

 having a tube of nearly 0-6 inch internal diameter; they were all com- 

 pared directly or indirectly with the Royal Society's standard ; and the 

 heights of the cisterns were determined by levelling from the sea in each 

 case. Under such circumstances the directions and intervals of the 

 isobaric lines may be found with much more precision than from obser- 

 vations made at any number of stations where these conditions are not 

 fulfilled. 



In the following investigation I shall assume that, within the limits 

 of the three stations, the mean directions of the isobars for each month, 



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