584 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



Of the many processes thai occur in moist air we will base our 

 analysis on those onl) r that we consider important for our problem, 

 which is to determine the available kinetic energy of any initial 

 stage; all other considerations we temporarily omit. 



The condensation of vapor into small drops causes the capillary 

 energy to be included in I, and the pressure of the saturated vapor 

 to depend on the size of the drops, and the water floating in the air 

 to contribute slightly by its weight to the pressure of the strata 

 beneath. We free ourselves from these influences which are of 

 minor importance in our present problem, by assuming that in our 

 system the condensed water immediately falls to the ground [or at 

 least separates from the air under consideration]. 



Let a, /?, y be the subscript indices referring to the air, vapor, and 

 water, respectively, so that the mass (m) of moist air is composed 

 of three portions: m a ; mp; m- f respectively; the specific heats are 



C vq ( ',-;■/ c vr '■ C p<* c vl C py '■ moreover C p p - C v p = Rp, e 



L = latent heat of evaporation of a kilogram of water at the 



temperature T; and L' at the temperature T' . 

 L—RpT= internal latent heat of evaporation. 

 C = specific heat of water, assumed to be constant. 

 CT = internal energy of a kilogram of water. 

 CT + L — RpT = internal energy of a kilogram of vapor. 

 Assuming that aqueous vapor follows the laws of ideal gases we 

 have 



L = L — (C - C P s) T 



L - R3 T = L - (C - C v p) T 



L = Constant 



For the temperature T the internal energy c/I of the elementary 

 mass dm which is composed of dm a , dmp and dm r is given by the 

 expression 



(/ 1 = C va T dm a + C T (dm 3 + dm r ) + (L — Ra '/') dm y 



= C va T dm a + C v r T dm j + C T-dm r + L Q dm j 



We will first assume that during any change in the system the con- 

 stituents of any elementary mass remain the same, so that in the 

 final stage all have again a common temperature as in the initial 

 stage; therefore dm a remains unchanged and dm' p + dm! j = dmp 

 + dm r where the superscript primes refer to the final stage. 





