104 Mr H. Meikle onjinding the T)ew^Puint, ^c. 



yet a cubic inch of rare air is, ccet. par. just as capable of con- 

 taining vapour as an inch of it ever so dense. Hence, if neither 

 more nor less air be cooled down by its touching the wet ball 

 than is to be saturated with vapour, it should follow, that when 

 one cubic inch of air has only half the density of another, and 

 consequently only half the specific heat *, it must be cooled 

 down twice as far to give out heat sufficient to form as much va- 

 pour ; and therefore the depression, if affected by no other cir- 

 cumstance, should vary inversely as the density or barometric 

 pressure. 



Such I presume to be what chiefly regulates the depression 

 in air of different densities, supposing the temperature of the 

 moist bulb constant ; but, to render the explanation a little more 

 complete, some reason should be given, why, in comparing cases 

 in which the temperature of the air is the same, the depression 

 should increase so much more slowly than the reciprocal of the 

 pressure, as we shall afterwards see from observation it does. It 

 is obvious that the greater the depression or excess of the tem- 

 perature of surrounding bodies over that of the wet bulb, the 

 greater propensity will these bodies have to radiate or throw in 

 heat upon it ; and, consequently, the greater will be the supply 

 of heat from radiation. At first sight, this might be supposed 

 to afford the reason of which we are in quest ; but evaporation, 

 as is well known, goes on more quickly in rarer air, which im- 

 plies a corresponding acceleration in the circulation of that air 

 over ihe evaporating surface ; and this acceleration again, which 

 no doubt is owing to the greater difference of temperature, and 

 increased force of evaporation, implies a correspondingly more 

 rapid influx of heat from the greater volume of air which, in a 

 given time, passes close over the cold moist bulb; for, as was 

 already noticed, the greater difference of temperature makes up 



• In this Journal for September 1 82C, page 339, in the Annals of Philoso- 

 phy for November 1826, p. 368, and in Brande's Journal of Science for March 

 1829, page 65, it is shewn necessarily to follow from admitted data, that the 

 specific heat of a given mass of air, under a constant pressure, is the same 

 whether that pressure be great or small. The proof of this is the more satis- 

 factory, as it neither depends upon a particular scale of temperature, nor re- 

 quires the true scale to be previously known. Hence, at the same tempera- 

 ture, the specific heat of a cubic inch of air under a constant pressure, is as 

 its density. The same thing has been more recently advanced, and as quite 

 new too, by MM. De la Rive and Marcet. 



