Fifty-four barometric observations, taken at the level of the water
on the voyage between Dacca and the Soormah, and compared with
Calcutta, showed a gradual rise of the mercury in proceeding
eastwards; for though the pressure at Calcutta was .055 of an inch
higher than at Dacca, it was .034 lower than on the Soormah: the mean
difference between all these observations and the cotemporaneous ones
at Calcutta was + .003 in favour of Calcutta, and the temperature
half a degree lower; the dew-point and humidity were nearly the same
at both places. This being the driest season of the year, it is very
probable that the mean level of the water at this part of the delta
is not higher than that of the Bay of Bengal; but as we advanced
northwards towards the Khasia, and entered the Soormah itself, the
atmospheric pressure increased further, thus appearing to give the
bed of that stream a depression of thirty-five feet below the Bay of
Bengal, into which it flows! This was no doubt the result of unequal
atmospheric pressure at the two localities, caused by the disturbance
of the column of atmosphere by the Khasia mountains; for in December
of the same year, thirty-eight observations on the surface of the
Soormah made its bed forty-six feet _above_ the Bay of Bengal,
whilst, from twenty-three observations on the Megna, the pressure
only differed + 0.020 of an inch from that of the barometer at
Calcutta, which is eighteen feet above the sea-level.

These barometric levellings, though far from satisfactory as compared
with trigonometric, are extremely interesting in the absence of the
latter. In a scientific point of view nothing has been done towards
determining the levels of the land and waters of the great Gangetic
delta, since Rennell's time, yet no geodetical operation promises
more valuable results in geography and physical geology than running
three lines of level across its area; from Chittagong to Calcutta,