ME. H. F. BLANFORD ON THE WINDS OF NOETHEEN INDIA. 
611 
Thus, then, it would appear that the increased temperature of the column itself reduces 
its density to an extent six times as great as is effected either by the decrease of pressure 
above or by the large increase of its vapour constituent ; and if we may further assume 
that the second of these elements results from the variation of the other two in the like 
proportion, and that the condition of the oblique column of air between Darjeeling and 
Goalpara fairly represents, for our present purpose, the general state of the atmosphere 
over Lower Bengal, we must conclude that of the reduction of the atmospheric pressure 
between January and July on the plains of Lower Bengal , six sevenths are due to the 
increased temperature of the atmosphere , and only one seventh to the displacement of dry 
air by aqueous vapour. 
I have not at present the data for a similar analysis of the density of the atmospheric 
column between Simla and Koorkee, or indeed any pair of stations in the drier climate 
of the North-western Provinces ; but it can hardly be doubted that such an analysis 
would show results more or less similar to the above ; and it may be expected that the 
lower mean pressure of the atmosphere from May to September would be found to 
depend on its higher mean temperature, up to a height of about 9000 or 10,000 feet. 
That the mean temperature of the column below 7000 feet is actually higher in the 
neighbourhood of the hills of the North-western Provinces than that of the similar 
column below the Sikkim Himalaya, is shown by the following comparison of mean 
temperatures of the atmosphere below Chuckrata and Darjeeling, situated at nearly 
equal elevations above sea-level. 
Table of the mean temperature of the air-columns below Darjeeling (6941 feet) and 
Chuckrata (6884 feet). 
Darjeeling. 
Chuckrata. 
Darjeeling. 
Chuckrata. 
January 
53-6 
50*1 
July 
72-4 
75*1 
February 
55-9 
53-4 
August 
72-6 
74-6 
March 
61-7 
59-9 
September 
71 
72-6 
April 
66*6 
69-9 
October 
68-1 
67-6 
May 
69-2 
78-6 
November 
60-6 
58*6 
June 
71 
79-2 
December 
54-4 
50-8 
Year 
64*6 
65-9 
Range 
19 
29-1 
Changes of temperature are then the principal cause of the variations in the weight 
of the atmosphere ; but the part played by vapour is not the lfess important, though its 
action is chiefly indirect. This action is evidently to equalize the temperature of the 
air-column, to carry heat from the lower and more highly heated strata to those at 
greater elevations, and also, as Dr. Tyndall has shown, to arrest and absorb both solar 
and terrestrial radiated heat in its passage through the atmosphere. Indirectly, there- 
fore, water-vapour greatly influences the pressure, though the change of density that 
MDCCCLXXIV. 4 N 
