278 THE MECHANICS OF THE EARTH'S ATMOSPHERE. 



mixture in this ratio before the subsequent dissolution. From this 

 value of t a this mixing ratio itself can be determined. 



We find by a very simple consideration that for this special value of 

 t 3 the following equation liolds good : 



But since 



and 

 therefore 



and consequently 





K 



F- S F 3 = 



F\t\=y,-y x 



h—h h—h 



yi—Th g-ts—ti^nh 



U—U U- h «»i 



Whenever /K/< 8 , that is to say, when the cooler " mechanically super- 

 saturated air," or at least the saturated component enters into the mix- 

 ture with greater weight, then will t<U and of course t<t 2 , or in other 

 words, the finally resulting temperature will be lower than that of 

 either component. 



These considerations lead to the following apparently very paradox- 

 ical result: " If icarmer air is mixed with mechanically saturated or me- 

 chanically super-saturated air then a part of the suspended water can be 

 evaporated and thereby a lower temperature produced." 



" If the given mechanically saturated air is hygroscopically unsaturated, 

 that is to say, if the vapor is unsaturated, then this lowering of tempera 

 ture will occur even by the mixing of saturated warmer air (of course in 

 the proper ratio) ; if the air is saturated as to its vapor and the corre- 

 sponding mechanical mixture is present as pure super-saturation, then the 

 warmer air must possess a certain degree of dryness that is not difficult to 

 determined 



The latter of these two propositions is evident as soon as we allow 

 F x to coincide with F as in Fig. 46, and then with F 2 still on the ordi- 

 nate T 2 F' 2 push so far down wards that F F 2 shall come to lie below FF 2 , 

 a condition which, however, is only to be satisfied so long as T 2 does 

 not have too high a value. 



The very paradoxical sentences just set forth lose their extraordinary 

 appearance as soon as we recognize that a mixture of water and un- 

 saturated moist air is not in a condition of equilibrium, but that in such 

 a mixture evaporation must always occur unless by some special proc- 

 ess the condition is kept stationary. Such mixtures are exemplified 

 in the fogs and clouds, and during rain. The behavior of such mix- 

 tures has been implicitly investigated in the previous paragraphs and 

 now only a few worls further need be said. 



