ME. H. F. BLANFOED ON THE WINDS OF NOETHEEN INDIA. 
613 
The fall of pressure in December and January is, I think, evidently due to the rapid 
condensation of the lower stratum of the atmosphere by the radiation of its heat, and, 
while this cooled air flows away to the south as the source of the north-east monsoon, 
by the consequent setting in of a compensating anti-monsoon current of higher humidity 
and comparatively equable temperature at (or, in the case of Darjeeling, chiefly above) 
the level of the hill-stations. From January to March, as well as from April to October, 
the pressure at Darjeeling and Simla must, on this assumption, be lower than at equal 
elevations to the southward in the course of this upper current. 
The temporary rise in April is, I think, to be attributed to the expansion of the lower 
atmosphere, by which, for a time, a larger proportion of the atmosphere is lifted above 
the level of 7000 and 8000 feet. This requires further investigation. 
Certain effects of the Winds. — I have now discussed most of the more important facts 
relating to temperature, vapour-diflusion, and atmospheric pressure, to be gathered from 
the registers of the past few years, in so far as they bear on the causes of the winds. It 
remains to notice certain effects of the winds, especially on temperature and rainfall ; 
and I shall then briefly sum up the results of the whole discussion, and add a few 
remarks, in an Appendix, on the storms of the Bay of Bengal, to the explanation of 
which a knowledge of the normal wind-system is indispensable. 
In respect of temperature, it is obvious that, except in the case of dynamic heating 
and cooling, a wind cannot 'per se raise the temperature of a place above, nor depress 
it below, that of the region from which it immediately comes. Any change of tempe- 
rature that it may undergo along its course must therefore be due to local causes, such 
as evaporation, radiation, or the absorption of solar or terrestrial heat in transitu. The 
effect of a wind is to tend to equalize the temperature of places along its path. Before 
applying this postulate to the meteorology of the region in question, it is, then, necessary 
to consider those changes of temperature that may arise from dynamic causes ; and of 
these one class of cases only need be noticed, viz. that of a current which is cooled by 
a rapid ascent to a higher level, or heated by descent to a lower level. The latter, if 
recognizable, will chiefly affect the temperature of stations on the plains, the former 
that of hill-stations. 
Both these actions doubtless take place on a great scale over the plains of Northern 
India, since we have seen that at one season this region is the terminus a quo , at another 
the terminus ad quern air-currents are set in motion, and during the continuance of these 
currents there must be a constant passage of air from the higher to the lower strata, or 
vice versd. If, then, dynamic heating be appreciable, it should be detected in a relatively 
higher temperature of the air on the plains of Upper India in the cold-weather months ; 
and for evidence of cooling by the ascent of the air, we should look for a relatively lower 
temperature of the hill-stations in the months of the rains — the effect in either case 
being shown by a difference of temperature between hill- and plain-stations greater than 
at those times when the interchange between the different strata is at a minimum. But 
the evidence tabulated at page 589 shows that it is precisely at these former seasons 
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