MB. H. F. BLANFOED ON THE WINDS OF NOETHEEN INDIA. 
607 
atmosphere, from opposite quarters, and subject at the same time to cooling, by radia- 
tion, to an extent greater than is compensated by the heat set free from their arrested 
motion. The dynamic pressure which would arise from the contraction and sinking of 
the cooled air may, I think, be sensible to the barometer. In no other way am I able 
to account for the ridge of high pressure in the cold- weather months between Benares 
and Cuttack, where the temperature of the air, though lower than on either side, is 
several degrees higher than over the Gangetic- plain, while the pressure is also higher. 
I shall presently collate the evidence, already given in these pages, of the probable exist- 
ence of those upper currents which I suppose to produce this phenomenon. 
I have now to consider a very important part of the subject of pressure, viz. its changes 
at Shillong and the Himalayan stations Darjeeling and Simla, and the evidence they 
afford of the height to which the density of the atmosphere is affected by the heat of 
the spring and summer months and the accession of vapour. Goalpara, situated about 
midway between Shillong and Darjeeling, affords a convenient standard of reference for 
these two stations ; and Koorkee, at the foot of the hills, 100 miles from Simla, may be 
compared with that station. 
It is apparent, on a glance, that the annual range of the mean monthly pressures 
at all the hill-stations bears a much smaller ratio to that of stations in the plains, than 
do the total pressures at any season of the year ; in other words, that the change of 
mean pressure over the plains between January and July, and vice versa ', is chiefly due 
to a change in the density of the atmosphere below the elevations of these stations. 
The amount of this change in each month is shown in the following Table. In the cases 
of Darjeeling and Shillong the density is computed in two different ways: — first, ( d j) 
from the difference of pressures at the higher and lower stations ; and secondly, ( d 2 ) from 
the mean pressure, temperature, and vapour-elasticity of the intervening atmospheric 
column*. In both columns, dry air at 32° Fahrenheit and 29*921 inches pressure is 
taken as the standard =1. 
By the formulae 
d-Jh-^L 10516, 
1 
\ +K 3(X+« 2 ) 
2 29-921 
1 + -002036 ^ —32^ 
where i 2 , t lf t 2 , e p e 2 are respectively the reduced barometric readings, the temperatures, and vapour-tensions 
at the lower and higher stations, and Ji t and h 2 their elevations in feet. 
It may be objected to the above method of computing the mean density of a column of the atmosphere from 
the observations of pressure, temperature, and vapour-tension of two stations only, that it makes assumptions 
as to the distribution (more especially) of vapour and temperature, the validity of which is extremely doubtful, 
and which, indeed, can be only approximately true, when minor and temporary irregularities have been elimi- 
nated by taking for the values of t v t 2 , e v and e 2 the averages of a very large number of observations. It may 
further be objected that the composition and condition of a very oblique column of the atmosphere (one of 100 
miles in horizontal length) cannot legitimately be assumed to represent those of a vertical column, and that 
