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



Annual Variation of Barometric Pressure in England. — In the discus- 

 sion o£ the Makerstoun observations of barometric height for the years 

 1842-49 I sought to determine this law, and found that the greatest 

 pressures occurred in the months from May to September (both inclusive), 

 while the least occurred in October and November ; the probable errors 

 of the monthly means were, however, found to be too considerable to 

 give much value to the secondary variations which were shown in the 

 monthly mean values. The mean for the six months April to Sep- 

 tember was one tenth of an inch greater than that for the six winter 

 months*. Very similar conclusions were arrived at by Dr. Lloyd, in 

 his discussion of the Dublin observations for 1840 to 1850, both as 

 regards the epochs of maximum and minimum pressure, as well as of 

 the probable error (or divergence) of the monthly meansf. Somewhat 

 similar conclusions may be deduced for Greenwich from the second 

 column of Table Y., the maximum in December being slightly more 

 marked than at Makerstoun and Dublin. 



If the isobars made the same angle with the meridian in all the months 

 of the year, and kept at the same interval, the annual variations would 

 be the same at all the stations. This, however, it will be seen, is not 

 the case, and the annual variation of barometric pressure for any place 

 is a compound result. 



I have sought to determine the annual law for the centre of gravity 

 of the three stations (a point a little to the east of Liverpool) as derived 

 from the monthly means, reduced to the sea-level, for the three places ; 

 these are given below. 







Probable error. 





Probable error 





in. 



in. 





in. 



in. 



January . 



. 29-864 



0'027 



July 



29-926 



0-015 



February . 



•859 



•049 



August . . . 



•917 



•019 



March . . . 



. -877 



•035 



September . 



. -993 



•022 





•867 



•039 



October . . . 



•815 



•033 



May 



•956 



•027 



November . 



•826 



•027 





•926 



•022 



December . 



•939 



•049 



The probable errors are least for the four months of June to Sep- 

 tember, and greatest in the months of December and February. The 

 probable errors are much diminished if we determine the means for 

 groups of months in each year. Thus we find : — 



in. 



October and November 29-820 +-01 6 



December to April -881 ±^017 



May to August •931±-010 



September •993+-022 



* Trans. Eoy. Soc, Ediub. xix. part 2. p. xci, 1850. 



t Obseryations made at the Magnetical and Meteorological Observatory at Trinity 

 College, Dublin, under the direction of Humphrey Lloyd, D.D., D.O.L., vol. ii. p. 347, 

 1869. 



