from the Cold induced by the Evaporation of' Water. 105 



for the smaller specific heat of rarer air. Hence radiation, be- 

 ing unaided by mere circulation, probably furnishes almost as 

 small a proportion of heat to the wet bulb, in rare air as in 

 dense *, compared with what the air' imparts to it by contact ; 

 so that there is little reason to think that radiation makes any 

 material alteration on the rate of the depression -|*. 



Since, then, something farther seems necessary to explain why, 

 in comparing cases in which the temperature of the air is the 

 same, the depression should increase in a slower ratio than the 

 reciprocal of tire pressure. It occurs to me, that it is principally 

 owing to the following, which, as seems conform to observation, 

 would cease to have that effect on the comparison of cases where 

 the temperature, not of the air, but of the moist bulb, is the 

 same, namely, the smaller capacity which a given volume of air 

 is known to have for moisture, at a lower temperature : so that 

 less heat must be expended in forming vapour sufficient to satu- 

 rate that volume at a lower temperature, and therefore a less 

 cooling of the air, or a depression less than in the ratio of the 

 reciprocal of the pressure will suffice for furnishing that heat; 

 especially if, as is extremely probable, the specific heat of air is 

 greater at a lower temperature than a higher. For if a given 

 mass of air, under a constant pressure, expand in geometrical 

 progression, for equal increments of heat, the increments or de- 

 crements of heat for the degrees of Fahrenheit, and which are 

 usually called the specific heats, will be. inversely as their re- 

 spective distances from — 448° ; but when the volume and pres- 

 sure are given with a variable mass, the increment, decrement, 

 or specific heat for one degree of Fahrenheit will be inversely as 

 the square of the distance of that degree from — 448°, viz. in 

 the ratio compounded of the value of a degree of Fahrenheit, 

 and the density of the air under a given pressure, each of which 

 varies inversely as the temperature reckoned from — 448°. 



Our views likewise differ a little respecting the correction for 



• From the experiments of Dulong and Petit, it appears that the quan- 

 tity of heat propagated by radiation, is, cceteris paribus, independent of the 

 presence, density, or movements of any gaseous medium ; such being, of 

 course, free from opaque or gross particles floating in it. 



f That any heat supplied by radiation to the moist bulb, must, in ordi- 

 nary cases, be very inconsiderable, appears from the dei)ression being so little 

 diflerent in the wind and in the calm. 



