610 
MR. H. F. BLANFORD ON THE WINDS OF NORTHERN INDIA. 
since the annual oscillation of pressure is absolutely greater at Simla at an elevation of 
7071 feet than at Darjeeling at 6941 feet. Calling the annual mean density of the 
atmospheric column below the hill-station in each case =1, the range between January 
and July is 0-048 in Bengal, and 0-058 in the North-western Provinces. 
If the method of computation which gives the values d 2 be accepted as legitimate, by 
varying successively the values of each of the terms (#.-{-# 2)5 (^ 1+^)5 and ( 0 i + 0 2 ) in the 
formula for d 2 while the others remain constant, we may ascertain another fact of high 
importance, viz. the degree in which the density of this column is affected respectively 
by the variation of the top pressure, the rise of temperature, and the introduction of 
additional vapour in the place of dry air of the same pressure. Between January and 
July the mean density of the air below Darjeeling falls from 0-837 to 0 ‘7 98. This total 
difference is made up in the manner following*: — 
(1) By decrease of the top pressure from 23-344 inches to 23-224 inches, the mean 
density of 0-837 would be reduced to 0-832. 
(2) By increase of the mean temperature from 53°"6 to 72°-4, the mean density of 
0-837 would be reduced to 0-807. 
(3) By the infusion of water-vapour, which, in conjunction with the rise of tempe- 
rature, raises the mean vapour-tension from 0-325 inch to 0-732 inch, the mean density 
of 0*837 would be reduced to 0*832, supposing the temperature to remain constant. 
And summing up these differences, we find — 
Density of air in January 0-837 
Reduction due to decreased pressure above .... 0-005 
Reduction due to increased temperature of column . . 0-030 
Reduction due to introduction of water-vapour .. . . 0-005 
Total reduction 0-040 
Density of air in July 0-797 
which accords almost exactly with that obtained from the difference of observed pressures, 
viz. 0-798. 
* These values are obtained by the following formulae, by which, from the figures in the Table on pages 608, 
609, the change of density arising from the variation of any element between two given months may be found. 
Calling and B 2 the initial pressures at the lower and higher stations, b 2 the final pressure at the latter 
T and t, E and e the initial and final mean temperatures and vapour-tensions respectively, and D, d/,, dt, and d e 
the inital observed and final computed densities respectively, we have 
d b = D 
Bi + B 2 
_ r 1 + -002036(T — 32) 
Ct l + -002036(* — 32)’ 
d e = D 
8 l + -002036(T-32) 
4 l + -002036(£ — 32) _ 
(I^ + Bj) — 
