1871. ] Proceedings of the Asiatic Society. 43 
age in the column of ascending air, and probably greater than the 
average in all azimuthal directions at some point where the two 
currents overlap each other. In the general case, then, it would 
seem that we should thus be presented at any instant with a locus 
of maximum pressure, forming a closed curve round the place of the 
sun, and surrounding an area of pressure less than the average. 
In fact this is very nearly the converse of the tidal action of the 
sun, and accordingly accounts very well for the periodic minimum 
pressure, which occurs in the day. But it would also seem to fol- 
low from this exposition that in general each of the diurnal maxi- 
ma would take place at an hour which would vary from point to 
point along a given meridian ; the antemeridional and postmeridional 
times approaching each other as you went polarwards until a point 
was reached at which they coalesced ; after which point no diurnal 
period would be apparent. I need hardly say that the actual facts 
are very different from this. It may be, however, that aclose enquiry 
into the conditions of the problem would shew that the maxima do 
not under this explanation lie along a closed curve, as I have assumed 
to be the general case, but that on the contrary the accumulation of 
material must be east and west of the sun in meridional lines. I 
am ashamed to say that I am not acquainted with Col. Strachey’s 
papers on this topic, and I do not know precisely how he works out 
the explanation. But I think I may nevertheless venture to affirm 
that whatever may be his confidence in its completeness, he would 
be exceedingly glad to get extensively collected data, wherewith 
to test and to fortify it. 
And even if we assume, as we rightly may, that in this matter a 
false importance has been attached to the element of vapour ten- 
sion calculated at the point of observation, because it appears now 
to be certain that that element is not a simple function, and does 
not afford a measure, of the total vapour material which may exist 
in the superincumbent atmospheric column; still for that very rea- 
son the question remains open, how far does the local atmospheric 
pressure depend upon the existence of local sources of vapour. 
If I have succeeded thus far in bringing you to my own point of 
view, I am sure you will see at once what immense value a com- 
plete, and connected system of barometric observations made 
