DYNAMIC METEOROLOGY. 397 



(2) 111 general, the quantity, a?, of saturated vapor can only diminish. 

 This quantity could increase with v if, in an exceptional case, the tem- 

 perature remained constant, or with T, if the volume remained the 

 same, but it would be necessary that the added vapor should be fur- 

 nished by the evaporation of the water in suspension. jSTow, the quan- 

 tity of water in suspension is very small, and this store would be quickly 

 exhausted. The contact of the air with moist bodies or any circum- 

 stance of the same class would also tend to increase the quantity of 

 vapor X, but still this would be an exceptional case. 



(3) The equations (6) and (7) show that when x is given it suffices to 

 know any one of the quantities e, T, v, oriJ in order that the others may 

 be determined. We see also that if x is given and T varies, the indi- 

 cator of the condition of the air moves along the line of saturation that 

 is peculiar to the special value of x. « 



(4) Let us suppose that at the moment when the vapor becomes sat- 

 urated and that one enters into the rain stage the quantity of vapor is 

 x^, aud that at the end of a certain time this quantity is diminished to 

 .Vj. The indicator was at first on the line of saturation belonging to x^, 

 but it now is found on the line of saturation belonging to a?j. While x 

 has decreased from x„ to .Tj, the indicator has passed from its initial to 

 its final position by cutting across all the lines of dew-point or lines of 

 saturation relative to the intermediate values of a;. 



But the indicator can not retrace its path, because x can not increase 

 in general. Therefore in the rain stage the indicator always describes 

 its path in the same direction. For such paths or trajectories, described 

 in the given direction, one can apply the principles of thermodynamics 

 as if reversible changes were under consideration. Therefore the 

 changes of condition in the case of rain can be described as " partially 

 reversible." 



Isotherms. — As the tension e of saturated vapor is constant so long 

 as the temperature T is constant, and as on the other hand x can only 

 decrease, the equation (7) shows that v can only decrease if T does not 

 change. This being allowed, the equation (6) will be the equation 

 of the isotherm for a given temperature, T, and for decreasing values 

 of V and of x. The isotherm remains entirely throughout its whole ex- 

 tent on the convex side of the line of saturation belonging to x^, the 

 initial value of x. 



We see that the isotherm in the rain stage is still an equilateral 

 hyperbola, and that it varies very little from the isotherm of the dry 

 stage for the same temperature. 



Adiabatics. — Strictly speaking there is no adiabatic, unless we sup- 

 pose that all the condensed water remains in suspension. If all or a 

 part of the water falls to the ground there is an exterior work per- 

 formed, and consequently a loss of internal heat or calorific energy, 

 and the definition of the adiabatic no longer applies, 



