522 



Mr. J. A. Broun on the Directions <Sfc. of [Feb. 



January 0'064 



February 0*054 



March 0*039 



April 0-012 



May 0-006 



June 0-035 



July 0-045 



August 0-045 



September- 0-018 



October 0-042 



Koyember 0-054 



December 0-056 



The year 0-038 inch. 



It thus appears that the difference of pressure for 100 miles is greatest 

 in the coldest months, is a secondary maximum in the warmest months, 

 and is a minimum in April, May, and September*. The only laws 

 resembling this are those for magnetic disturbance, frequency of the 

 aurora borealis, and for the horizontal force of the earth's magnetism. 



Annual Variation of the direction of the Isobars. — The values of 6 given 

 in Table lY. can be considered as only rough approximations in some 

 months. The probable errors of the mean result (Table Y.) have been 

 calculated from Table lY., giving each determination equal weight. I 

 have found that the probable errors thus obtained differ little from the 

 mean deviation of ^, deduced from equation (4) with the limiting values 

 of (+ p. e.) and of /S^ (+ P- ^■), excepting for the months of May 

 and April, for which the mean deviations are much greater. This is due 

 to the very smaU mean values of and /S^, and their comparatively 

 large probable errors for these months. 



From the seventh column of Table Y. it appears that the isobars run 

 most from south of west in December (nearly from S.W.) and in 

 May (S.W. by S.), and most from north of west in September. Begin- 

 ning with October, the isobars run from a little south of west ; they are 

 from further south in November, and from furthest south of west in 

 December. They again approach the west point in January, remain 

 nearly west and east (within a few degrees) from February to July (ex- 

 cepting May, to which I shall refer presently) ; move further north in 

 August, to the maximum north of west (25°) in September. 



The lines from May run from 54° south of west (to 54° north of east) ; 

 but this result cannot receive much value, as the probable errors are 

 greater than the mean values of jj^ and f3^. This remark applies also, to 

 a great extent, to the result for April, which, however, agrees nearly with 



^ I think there is some reason to believe that this law, as regards the months, 

 with some slight modification, is general. Mr. Biiehan, in his valuable " first approxi- 

 mation ' to charts of isobars for the whole earth, has stated that April is the month in 

 which " pressure is more equally distributed over the globe than in any other month" 

 (Edinb. Trans, vol. xxv. p. 578, 1869). This conclusion depends on the value of the 

 approximation to the isobars for the southern hemisphere, for which the data are not 

 only least numerous, but probably least exact. Mr. Buchan's charts show May to be 

 the month of widest distribution for the northern hemisphere, as has been found here 

 for England. 



