§'{'§ ECONOMICAL rnOCESS FOR EVAPORATION; 



combustible ia other cases*. We should thus obbin th'id 

 same advantage as by employing the atmospheric air at its 

 proper temperature. With the air heated by the fire cold 

 air might be mixed, so as to make the temperature of the 

 mixture 30", 40<', or 50° [86°, 104'', or 122° F.] dt most. 

 Beyond this we are not certain^ that the juices of fruits 

 would not be altered : at least Montgolfier stopped at this 

 point in his experiments, 

 withateTing Though in this case fuel is employed to transmit to the 



of fuel. water the latent heat of the air, there is still a saving of 



(Expense. Here the caloric evolved by combustion is well 

 mixed with the air, which carries oft' almost the whole of 

 it, and easily parts with it to the water, from the vast sur^ 

 face this presents in the apparatus to be described. Besides, 

 if the air contained a little free caloric before the combus.. 

 tion, it would still present it to the water as in the evapo- 

 ration without artificial heat ; and if it escape at a lower 

 temperature, than it had when it entered, we may be sure 

 we have etnployed the fuel to good purpose. In the ordi- 

 ivary mode of evaporation we are far from enjoying tKese 

 advantages : the caloric is obliged to traverse substances of 

 considerable thickness and little surface, to combine with 

 the water; and the air escapes rapidly from its contact with 

 the surfaces, and carries with it a considerable quantity of 

 heat. 

 Calculations of ^c have here no new idea, but the result of well known 

 'ihe effect. truths; the advantages of which have been overlooked. 

 To form a proper judgment of it, we should know with 

 Montgolfier, that, in the common state of the air in au- 

 tumn, a cubic foot of air, by its contact With water, will 

 evaporate from one to four grains, or at a medium 2^ grs. 

 We will suppose it only 2 grains, the proportion adopted 

 by Montgolfier from his experiments in Dauphiny. TO put 

 the air in motion, like any other body, requires more 

 power for a greater velocity; and this power is proportional 

 to the square of the velocity. That Is, If a man by a diay's 

 labour, as is the fact, can give a velocity of 7] feet [8 feet 

 Eng.] per second to 8100000 cubic feet [9800000 Kng.], 



* The air to be heated should tiot be passed through the ttakcd 

 fire, but through a tube or tubes heated by the fire. C. 



it 



