528 



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



It has already been stated that the resultant pressures of the wind 

 are only rough approximations, yet these indicate very distinctly the same 

 annual law of A ariation as the diabars ; not only so, the results for each 

 month show no great divergences from the rado stated above derived 

 from the mean for the four years. 



When we remember that these resultant forces are obtained in some 

 cases from winds proceeding from opposite directions, which destroy 

 each other to a great extent, and that the mean differences of barometric 

 pressure are obtained similarly from pressures which increase in opposite 

 directions, it seems probable that the relation found always holds, 

 though the ratio may vary somewhat with winds of different forces and 

 from different directions. The variations including the greater diver- 

 gences from the mean must also be the object of special investigations, 

 in which winds from the same direction, and with nearly the same mean 

 forces, must be considered alone. 



Ratio of the mean pressure of the luind (surface current), independent of 

 direction, to the resultant pressure. — It has been stated that the pressure 

 of the. wind was always noted at Makers toun whenever the maximum 

 pressure within an interval of 10 miuutes at the hour of observation was 

 at least one tenth of a pound on a square foot of surface, and that 

 although the absolute mean pressures cannot be derived directly from 

 these observations, jQt that their relative values are probably determined 

 with considerable accuracy. We may then obtain a measure' of the 

 variability of the wind, that is of the degree of opposedness of the masses 

 of air in motion at the earth's surface, from the ratios of the mean (M) 

 to the resultant (E) pressures. Those at Makerstoun f or the four years, 

 1843 to 1846, were as follow*:— 





M. 



M 



I. 





M. 



M 



I. 





lb. 



R' 



miles. 





lb. 



R' 



miles. 



January . 



.0-72 



1-5 



135 





0-40 



1-7 



227 



February . 



.0-63 



3-1 



286 



August . . . 



.0-32 



2-5 



250- 



March . . . 



.0-63 



2-3 



286 



September 



. 0-27 



3-4 



588 



April . . . 



.0-52 



4-6 



357 



October . 



.0-58 



2-2 



175 



May . . . 



.0-51 



7-5 



625 



November . 



.0-59 



2-0 



185 



June . . . 



.0-bX 



2-0 



278 



December . 



.0-60 



1-6 



250 



"We see that the greatest proportion of the surface current proceeded 

 from one quadrant in the winter months of December and January, and 

 in the summer months of June and July; that the wind was most 

 variable in May, April, and September. We have already found (p. 522) 



atmospheric surfaces with bills and valleys, into the latter of which the former are 

 supposed to flow. On this account I should much prefer some other word (such as 

 diabar or harode), which would serre to indicate the relation of the difference of 

 pressure to the unit of distance. 



* "Eesults of Makerstoun ObserTations," Ti-ans. Eoy. Soc. Edinb. vol.xix. pt. 2, art, 

 209, p. xcvi (multiplied by 1'13, as before). 



