612 
ME. H. F. BLANFOED ON THE WINDS OF NOETHEEN INDIA. 
arises from its displacement of the heavier constituents of the atmosphere is relatively 
small, and in some cases unimportant*. 
With respect to the interior of India, the course of events appears to he as follows : — 
From March onwards the air immediately over the elevated plains is gradually raised 
to a high temperature, but at first only in the lowest stratum. By degrees, convection- 
currents from the sea, of no great vertical thickness, are drawn into this lower stratum, 
introducing vapour, which carries the heat by diffusion and condensation to a greater 
and greater height. Up to the end of May or the beginning of June this diffusion of 
heat, while reducing the pressure, does not lower the temperature of the surface ; and 
it is only when in June a strong steady current of nearly saturated air is drawn from 
the equatorial seas, that the precipitated vapour, partly as cloud and partly as rain re- 
evaporating, absorbs the excess of solar heat and reduces the temperature of the lower 
atmosphere to the extent shown in the temperature Table. 
There is yet another point of importance to be noticed in the Tables on pages 608, 609. 
While the stations at the lower levels, Goalpara and Boorkee, as well as all others 
on the plains, show but one annual maximum and one minimum of pressure, the former 
in December, the latter in June or July, the two hill-stations (Darjeeling and Simla) 
have two maxima and two minima, like places on the Atlantic coast of Europe. The 
epochs of the maxima and minima are, however, very different in the two cases. The 
absolute minimum of the year at these hill-stations coincides with that on the plains, 
and is doubtless due to the same cause ; but the absolute maximum falls in November 
at the former, and is followed by a fall, at first rapid, and gradually decreasing till March. 
A small rise then takes place, which brings the mean pressure of April above that of 
February; it is, however, only temporary, and in May *and June the pressure falls 
rapidly to its minimum. 
* The facts thus indicated relative to the effects of temperature and vapour in affecting pressure seem to 
explain the apparent anomalies that have led some authors, especially Mr. Laughton (Phys. Geog. pp. 120, 
123), to question the soundness of Hadley’s theory of the trade-winds. These are, that winds do not blow 
centripetally toward the Sahara, the Arabian Desert, the interior of Australia, &c., all of them dry regions with 
a day temperature very much above that of the neighbouring seas. In the first place Mr. Laughton has, I 
think, insufficiently considered the fact that a high day temperature in these dry regions generally alternates 
with a low night temperature, the nocturnal radiation being intensified by the same conditions which 
increase the incident solar heat ; so that the mean temperature of the 24 hours, one element of importance 
in determining the general system of the winds in such regions, is frequently below that of places with a much 
lower day temperature. Ex. gr., Lahore in April has a mean maximum diurnal temperature of 98°-4, Calcutta 
one of 93°-5 ; hut the mean temperature of the 24 hours is only 79°-l at Lahore, while at Calcutta it is-84°*5. 
Further, the facts discussed in the text and tabulated at pages 589, 608, 609, show that in a dry atmosphere 
the high temperature prevails only in its lowest stratum, and that the mean temperature of a column of com- 
paratively dry air, say 7000 feet in height, may have an average temperature 16° below that of the surface, 
while another equal column of moist air averages only 9° or 10° less than near the ground. Surface-tempera- 
ture alone, especially that of the daytime, is a very unsafe criterion of the average temperature of the atmo- 
sphere above the place of observation ; but in the equatorial calm belt, to which region Mr. Laughton applies 
conclusions drawn from Arabia, Australia, &c., the atmosphere is highly charged with vapour, and the decrease 
of temperature with elevation therefore probably slow. 
