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{ 
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ratio of 2976 to 35, or in the ratio nearly of 85 to 1, if the weight of 
a cubic inch of hydrogen gas were as great as that of watery vapour, 
under a common pressure, and at acommon temperature, But under 
such circumstances, a cubic inch of vapour weighs about nine times as 
much as a cubic inch of hydrogen; we must therefore divide the 
number 85 by 9, and we find that in the present case the mixture con- 
tained only about 9} grains of hydrogen for every grain of vapour ; 
and thus we learn, from this experiment, that the heat required for 
the evaporation of a grain of water at the temperature of 48° might 
be (and was in fact) supplied by the cooling of about 94 grains of 
hydrogen from 68° to 48°. But in order to produce the same 
amount of evaporation by the heat which water would give out, in 
cooling through the same range of temperature, it is known from 
other experiments that it would be necessary to employ about 56 
grains; therefore 91 grains of hydrogen have nearly as much heating 
power as 56 grains of water, or one grain of the former contains 
almost as much specific heat as six grains of the latter. All this is 
stated in round numbers, and with the omission of all lesser correc- 
tions, for the sake merely of such members as may not have attended 
to the subject, and yet may wish to have a clear, though general no- 
tion of it. Those who desire a more exact account will, of course, 
turn to the Essay itself.* 
With respect to those independent, but analogous researches of 
* The formula given by Dr. Apjohn for the general solution of the problem of the 
moist bulb hygrometer, in any gaseous atmosphere, is, 
11 gr __ 48ad P. 
f= cnt hart 
in which e is the caloric of elasticity of vapour, at the temperature ?¢’ of the hygro- 
meter; p is the atmospheric pressure ; d is the difference between the tempera- 
tures of the dry and wet thermometers: f’ and f” are the elastic forces of the vapour 
of water, at the temperature of the hygrometer, and at that other temperature at 
which dew would begin to be deposited ; and ais the specific heat of the gas, com- 
pared with that of an equal weight of water, and multiplied by the specific gravity 
_ of the same gas, compared with that of atmospheric air. For the case of a current 
of dry gas f’ = 0, and 
48d °° pt 
in which, as also in the other formula, it would be a little more exact to write 
p—f' instead of p. A correction is given for the case of a mixture of gas with air ; and 
the influence of other corrections also is taken into account. Whena is divided by 
