283 



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 a common 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 9i 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 9^ 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 9^- 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, 

 /./, J., 48ad ^ p 



in which e is the caloric of elasticity of vapour, at the temperature t' of the hygro- 

 meter ; p is the atmospheric pressure ; d is the difference between the tempera- 

 tures of the dry and wet thermometers :/' and/" 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 a is 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/":= o, and 



ef w 30 

 a=zJ— X — ; 

 4M p 



in which, as also in the other formula, it would be a little more exact to write 



p—f instead of;?. 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. When a is divided by 



