1871.] Proceedings of the Asiatic Society. 41 
India as an area of meteorological observation. The height at 
which the mercury of the barometer stands, indicates to us 
something more than the mere amount of atmospheric pressure 
on the surface of the mercury in the bowl of the instrument. 
It gives us the weight of the whole superincumbent column of air 
and this necessarily varies with the composition of that column. 
Now with us in India during a great part at least, if not the whole 
of the year, the vertical atmospheric column is made up of two 
radically distinct portions, a lower and an upper, the one polar, 
comparatively cool, dry and dense, the other equatorial warm, va- 
pour-bearing and specifically light. The barometer is immediate- 
ly affected by any alteration of the relations between these portions. 
Also the heating of the surface soil has the effect, in an interval of 
time more or less short, of diminishing the air-material in the 
column which is vertically above it, and of contemporaneously in- 
creasing the vapour therein as long as a source of vapour remains 
at the base. These two changes are, it is manifest, diametrically 
opposite in character, and it would be impossible to say, @ priort 
which would at any given hour prevail over the other. As a mat- 
. ter of fact, almost universally over all zones of the earth’s surface, 
the barometric column exhibits regular diurnal oscillations in its 
height. In the temperate zones these oscillations are comparative- 
ly speaking, small. But in the tropical and subtropical regions 
the case is different. There, the barometer discloses considerable 
and, well marked periodic changes of atmospheric pressure during 
the twenty four-hours, constituting in the whole two distinct oscil- 
lations. At different times, various theories have been put forward 
to account for this phenomenon. I need not now endeavour to 
specify them in detail. I will, however, very shortly refer to two, 
in order to indicate the veil of uncertainty which still obscures the 
subject, and which we cannot hope to pierce except by the force of 
exhaustive observation. One explanation, which has been very 
extensively accepted, is based on the particular solar agency of 
which I have just spoken. The pressure at a given point in the 
atmosphere (in accordance with a well known law of pressure in 
elastic fluids) is taken to be the sum of two separate pressures, 
namely, the tension of the vapour at that point under the conditions 
