'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎74] (95/458)

Ixxiv
PROCEEDINGS OF THE SOCIETY.
reference to the notice of motion given by him at last Meeting, he should not propose anything
to the Society, but leave to the Secretaries to reply to Lieutenant Maury as they considered best.
The Secretary read the following paper on the Correction of the Barometer for Pressure of
Moisture:—
This Society has always taken a great interest in all questions connected with Meteorology,
and I hope they will now bear with me while I make a few remarks on a question that is at
present exciting some attention. I mean the influence of the vapour in the atmosphere on
the barometer. According to the received theory, vapour mingles with the dry air of the
atmosphere as if the dry air were a vacuum to it, and consequently its elastic force is added to
that of the air, and their united force is shewn by the height of the barometer. It is of great
importance that this question should be decided, for observations on the barometer can be of
little use until we have some knowledge of what they really indicate. If the proportion of
vapour to the other constituents of the atmosphere were constant, it would be a matter of
indifference what that proportion is ; but we find that its variations are greater than those due to
any other cause. It is therefore necessary to decide whether the tension of vapour is additive,
or simply takes the place of a portion of dry air. The latter position is what I wish to maintain.
The incorrectness of the present mode of ascertaining the weight of dry air in the atmosphere,
may be established in two ways,—either by shewing that the foundation of the theory is false,
or that the results are inconsistent with the known state of the atmosphere.
1st. The theory that has long held sway among Chemists and Natural Philosophers that one
gas is a vacuum to all others, and that consequently the elastic force of a mixture of gases is
the sum of the elastic force of the separate gases, is the necessary basis of the whole. It is un
necessary to multiply authorities on this point, for it is entwined with the whole modern system
of Meteorology. Mr. Glasher, of the Royal Observatory, has stated it very distinctly. “ As in
an atmosphere of pure steam, the force of it at the earth’s surface would be its weight, so in a
mixture of atmospheres, the elastic force of each at the earth’s surface is the weight of the
whole atmosphere of each kind. Therefore the elastic force of vapour representing the weight
of the entire mass of aqueous vapour diffused throughout the atmosphere, expresses the pressure
on the surface in the cistern of the barometer produced by the vapour present in the air at the
time of observation, and it therefore becomes a correction to be applied subtractively to all
readings of the barometer, to obtain from them the pressure of the atmosphere of dry air.”
Both of these principles we question,—that the elastic force of the separate components at the
earth’s surface is the weight of the atmosphere of each kind, and that the pressure of vapour
becomes a correction to be applied subtractively to obtain the pressure of dry air. Now a
very simple experiment will show this. Place a glass jar over a candle standing on a table, and
depress the jar until it is within about a tenth of an inch from the table on which the candle
is standing. In less than a minute the candle will be extinguished. Now this can only be
explained by supposing that the gases within the jar prevent the ingress of the oxigen to take
the place of that which has been consumed by the light. If the nitrogen within were a vacuum
to the oxygen, there is sufficient place for it to enter. The one gas therefore does act upon
the other. The same thing is seen in many chemical experiments, where one gas, such as
chlorine, being put into a vessel drives the other out. If then one gas presses on another, and
when intermixed removes it from its original position, there is no reason whatever for suppose-
ing that the same does not take place between air and vapour. It is unnecessary here to enter
more at length on this subject, which is more properly a question of Natural Philosophy, my