384 Mr. Herapath on True Temperature, and the Nov. 



concern of its kind in England, can yet find time to attend to the 

 advancement of science, and to promote with unlimited zeal 

 every thing which has the prospect of benefit or utility to 

 science and the community. 



We see by this corollary why a current of air in apparently 

 increasing the evaporation likewise diminishes the temperature, 

 and, consequently, the reason of the common method of cooling- 

 bodies, as meat, wine, &c. by wrapping them in wet cloths, and 

 placing them id a current of air. Were the superincumbent air 

 fraught with sufficient vapour to make the condensation equal to 

 the evaporation, the momentary diminution of temperature result- 

 ing from the evaporation would be balanced by the contempora- 

 neous increase of temperature arising from the equal condensa- 

 tion, and the fluid would remain of the same temperature. But 

 were the condensation greater than the evaporation, the temper- 

 ature of the fluid, as long as the excess continued, would rise ; 

 and if, on the contrary, the condensation was less, the tempera- 

 ture would sink. Now a current of air, we have shown, increases 

 the apparent evaporation ; it, therefore, diminishes the tempera- 

 ture. 



Dr.Wistar, in the Transactions of the American Philosophical 

 Society, informs us, that apparent evaporation takes place when 

 the moist body is warmer than the medium in which it is 

 enclosed ; and, vice versa, apparent condensation when the 

 enclosed air is warmer than the body. This, under the circum- 

 stance of enclosing the air and a sufficient quantity of vapour 

 being already raised, agrees with our theory ; but if the air be 

 unconfined, it may not hold good. Apparent evaporation in 

 this case will depend on the quantity of vapour in the air imme- 

 diately over the fluid. 



A current of air is not from this theory indispensably necessary 

 to increase the apparent evaporation. Simple agitation of the 

 air, if it equally disperse the vapour and diminish the reconden- 

 sation, will do as well, which agrees with experiments. 



Because the apparent evaporation is increased by a current of 

 air, it may happen that water of a lower temperature in a current 

 or an agitated air may lose more by evaporation in a given time 

 than water of a much superior temperature in a still atmosphere. 

 It may likewise even happen, that ice itself will lose more weight 

 by evaporation when placed in a strong current than water of a 

 considerably higher temperature in a quiet air, particularly also 

 if there be much vapour in the air. This indeed has been 

 found to be the case. Calculations on this subject may easily be 

 made, from the theorems I have given to show the force and 

 extent of this view by any one who likes to amuse himself 

 with them. 



Considering that loss of temperature must always accompany 

 evaporation when it is not balanced by recondensation, and 

 knowing that the vapour in the air is scarcely ever sufficient to 



