42 Proceedings of the Asiatie Society. [ Fes. 
of density and temperature, which there obtain, and the pressure 
of the dry air alone at the same point exclusive of its contained 
vapour. The first may be calculated from data furnished by ob- 
servation at the supposed point. And in this way conclusions 
have undoubtedly been reached, to the effect that, in general, of 
the two great components of the resultant atmospheric pressure, 
each exhibits a single continuous progression in the twenty-four 
hours, having its points of maximum and minimum non-coincident 
with those of the other, the two progressions being so related as 
not at any point to compensate each other. And thus it has been 
thought that the phenomenon is sufficiently traced to its causes. 
On the other hand, many accurate observers, and notably our 
own distinguished meteorologist, Colonel Strachey, maintain that 
the facts do not bear out this explanation inasmuch as, among 
other things, the barometric oscillations are just as distinctly mark- 
ed at elevations, where scarcely any vapour can be perceived to 
exist as elsewhere. And from curves, exhibiting the diurnal va- 
riations of the calculated vapour tension at Calcutta for every 
month of the year, which Mr. Blanford has been so kind as to 
show me; it is apparent to the eye that the explanation in question, 
to say the least of it, is exceedingly incomplete. 
Colonel Strachey’s own opinion is, I believe, that the single 
rarifying action of the sun is sufficient cause to produce the whole 
of the phenomenon. I have already, somewhat earlier in this ad- 
dress, had occasion to draw your attention to the fact, that that por- 
tion of the earth’s surface, which at a given instant is the more nearly 
so to say under the sun becomes fora time, relatively speaking, 
heated more than the rest, and the consequence is (I do not now 
enter into the process) that the air above it is put into vertical mo- 
tion, and after rising to some elevation makes its escape sideways. 
The proposition that motion of this sort takes place is beyond ques- 
tion. Itis as I have before said the foundation of our theory of 
all winds, and of the trade-winds in particular. The pressure 
which gives rise to the lateral escape must urge in all directions 
alike, but the principal persistent outflow occurs towards the poles. 
Similarly the principal inflow is from the poles, As long as this 
condition of things obtains, the air-material is less than the aver 
