1871. | Proceedings of the Asiatic Society. 61 
total atmospheric pressure, (2) the temperature, (3) the vapour pres- 
sure, (4) the curve of saturated vapour pressure corresponding to 
that of temperature, and (5) the curve of humidity. The first shews 
the variation of the diurnal barometric tides for each month of the 
year, and I will draw attention to the great regularity of the wave 
curve which is one of double curvature, having an absolute maxi- 
mum about 10 A. M. and an absolute minimum about 4 Pp. mM. with a 
secondary maximum and minimum at 10 p.m. and 4 A. mM. The 
hours of absolute maximum and minimum vary a little during the 
year, the former being about an hour earlier and the latter about 
an hour later in the hot months then in the cold. The difference 
of the morning and evening maximum is greatest in the driest 
months and least in the rains. In the latter this difference is com- 
paratively small, the two crests having nearly the same height; but 
the afternoon minimum is always considerably lower then that of 4 
A.M. The explanation of the double tide is a subject on which great 
diversity of opinion exists. The explanation found in most of our 
treatises is that originally suggested, I believe by Dove, and adopted 
by General Sabine and Sir John Herschell, vz. that the curve is com- 
pounded of two distinct elements, one of the dry air pressure which 
taken by itself has a maximum at about 4 a. m. and a minimum 
at 4p. m., the other a curve of vapour pressure which has two maxima 
and two minima at or about the periods of the maxima and minima 
of the total pressure curve. Col. Strachey* who, as you are aware has 
given much attention to this subject, has long since pointed out the 
insufficiency of this explanation in the case of the barometric curves 
in India, or rather the failure of verification when the curve of ob- 
served vapour pressure is superimposed on a supposed dry air curve 
of a single periodic variation. And you will see that no composition 
of the vapour pressure curve of Calcutta with any such supposed 
curve will give a curve approaching in form or regularity to that of 
our tidal curve of total pressure. 
The curves of vapour pressure exhibit great variations in the 
different months of the year. In the months of the rains, the pres- 
sure is almost unvarying, or there is but a slight increase during 
* Similar objections have been raised by Mr. Broun, Professor Lamont of 
Munich and others. 
