1877.] 



the Lines of equal Barometric Pressure. 



523 



the directions for February and March. I shall return to the results 

 for April and May in the next part of this paper. 



The directions here obtained from eight years' observations, made 

 under the most unexceptionable circumstances in three observatories, 

 will probably not be improved for some time*. On this account I have 

 thought it desirable to project the isobaric lines on small charts, so as to 

 show at a glance their general direction and distribution (see Plate 12). 

 The results for the whole year have so small probable errors that I have 

 projected them on a larger scale t (see Plate 11). 



It should be here remarked that there is no ground as yet for affirm- 

 ing that the mean direction and distribution of the isobars for any series 

 of years will be the same as for any following series ; and the best 

 determination that can be obtained for any given time may be of the 

 greatest importance for comparison with another of equal value at a 

 later period for the answer to this question. 



2. Relation of the Direction and Interval of the Isobars to the Direction 

 and Force of the Wind. 

 These relations have been hitherto studied chiefly with reference to 

 cases of strong winds or storms. The conclusions have been obtained 

 directly from the projections on charts of the isobaric lines and wind- 

 directions on particular days. In the following part of this paper, as in 

 \ the preceding, the results are sought from the observations made on 



every day throughout the series of years employed ; they may therefore 

 serve as a base for the study of particular cases. 



^ I have sought to obtain some confirmation for the directions of the isobars in 

 Great Eritain from the numerous observations made in 1857 to 1867, employed by Mr. 

 Buchan in the formation of his charts. Mr. Buchan's object in the construction of 

 these charts appears to have been to obtain the best possible approximation to the 

 broad features of the distribution of the isobars for the whole earth ; and he has 

 exercised his judgment in employing the mass of obser-rations vrhich he was able to 

 collect (Edinb. Trans, vol. xxv. pp. 575, 576). My examination of these for the 

 months of April and May, in which the mean change of pressure for a distance of 100 

 miles is only a few thousandtlis of an inch of mercury, has shown the difficulty of 

 deducing from them any accurate determination, such as is here attempted, for a small 

 space like England. 



t These charts are not projections, and were devised by me for another investiga- 

 tion, for which the true areas on the sphere were required ; they are, however, probably 

 not new. The following will give the most simple idea of their construction. If we 

 suppose the surface of a globe to be built up of series of rings of cylindrical wires laid 

 on the parallels of latitude, and cut out the wires covering any part of the surface, then 

 if these be laid out touching each other on a plane surface, in straight lines perpendi- 

 cular to a central or principal meridian, the external meridians will be curves of sines 

 (straight lines for 20° squares on the scale of the chart for the year). The distances of 

 any point perpendicularly to a circle of latitude and to the principal meridian are 

 the true lengths of the ai-cs in latitude and longitude from these circles. For small 

 spaces the distortion is inconsiderable. In the charts here given the principal meridian, 

 that of Greenwich, is not in the middle, the direction of the isobaric lines having been 

 referred to that meridian. 



