'Transactions of the Bombay Geographical Society, from September 1850 to June 1852. Edited by the Secretary. Volume X.' [‎76] (97/458)

Ixxvi
PROCEEDINGS OF THE SOCIETY.
It appears therefore, that the total weight of a column of vapour of 19686 feet in height, and
base a square foot, would be 130531 grains if the dew point were 81°35 at the base and the whole
column saturated. At this height the barometer would stand at about 13 inches, and conse
quently only 3-7ths of the atmosphere exists above that height. In the case of this vapour there
must be a much smaller proportion of the whole above this height, because, unlike the air, it is
condensed by cold. If one half be added to the above sum, the whole weight of vapour that can
exist will be equal to 195798 grains. But this weight of 195798 grains is only equal to a column
of mercury of 395 inches, because a column of mercury of the same base and one inch high is
495,833 grains. "When the temperatures are assumed the same as in M. Gay Lussac’s experiment,
the total weight of a column 22,885 feet in height is 181,190 grains ; when the temperature of
the dew point at the base is 87° 35, if one half be added to this for the vapour above this height
the total weight will be 271,195 grains, or equal to 518 inches of mercury.
In the former case the total weight is only equal to the tension of vapour at the temperature
of 53°, although the air was assumed to be saturated at the temperature of 81°5, and in the latter
the weight is only equal to the tension of vapour at the temperature of 62°, the actual tempera
ture being 87°*35.
In the Bombay Observations which we have just received, there is frequently a pressure of
moisture above an inch, and we have seen that the weight of all the vapour that can exist in a
vertical column when the temperature of the lowest point is 87 k 35, would only produce a pres
sure of moisture equal to 588 inches. Of course the temperatures which have been assumed
are not strictly accurate, but from the mode of calculation the error will be on the side of
excess of vapour. Every thing being taken into consideration, I do not think that there is ever
as much vapour in the atmosphere as would produce a pressure of half an inch of mercury. I have
applied a simlar calculation to the theoretical temperatures at various heights given by Sir John
W. Lubbock in his essay on Heat and Vapours, and as the height can be divided into a greater
number of intervals, the result is probably much nearer the truth than those given above. I do
not insert the calculation, because it gives a much smaller amount than has been found from
actual observations, and because the law of temperatures given by him maybe objected to.
I n this calculation we have endeavoured to find what would be the weight of the vapour that
can exist in the atmosphere, and we have found that it by no means corresponds with that indi
cated by the tension at the lowest point, but the same may be proved, perhaps more satisfactorily,
by calculating what quantity of vapour would be required to produce the pressure at the lowest
point. If this be found greater than can exist in the air, it will be an ad-absurdum argument
against the theory we are combating. The following table gives the temperature of dew point,
elastic force of vapour, the number of grains in a cubic foot of vapour, and the height to which
an uniform column of vapour must extend, so that its weight should be equal to the tension at
its lowest point. The second and third columns are taken from Glashier’s tables, and the fourth
is calculated thus—.when the dew point is 30° there are 10.81 grains of vapour in a cubic foot of
air : how many feet high must the column extend so as to produce the tension of 1.001 inches
of mercury, the weight of one inch of mercury on the same base of a square foot being 495.883
grains ? The answer is 45,913 feet.
Temperature of
Dew Point.
10
20
30
40
50
60
70
80
90
Tension of va
pour in inches
mercury.
•039
•129
•186
•264
•373
•523
•727
1*001
1*367
Grains in a cubic
foot.
Ml
1*53
2*21
3 09
4*28
5*87
8*00
10*81
14 49
Height of a ho
mogenous column
whose weight is
equual to the ten.
sion.
39756
40482
41730
42362
43211
44177
45058
' 45913
46777