DYNAMIC METEOROLOGY. 



395 



that is to say 



or for the limitiDg case 



vy 



xllsT 



v=x 



UsT 



(2) 

 (3) 



Each isotherm therefore commences ouly at a certain limiting point, 

 whose abst'issa is given by the equation (3) and whose ordinate is deter- 

 mined by the relation found by combining equations (1) and (3) namely, 



P 



Bx + x-Rs 

 xlis 



(4) 



0.05 



If the quantity x preserves a given constant vahie the isotherm con- 

 tinues to lie in its own plane as the temperature T varies ; the limiting 

 point of tbe isotherm, as just defined, is displaced at the same time, and 

 describes a curve that Bezold calls the line of saturation or the line of the 

 dew-2>oint. (See Fig. 4.) 



This curve has its concavity 

 turned toward the side of positive p. 

 The indicator point for air in the 

 dry stage ought therefore always to 

 be on the concave side of the line of 

 saturation ; if this point passes over 

 to the convex side it indicates that 

 the dry stage has been followed by 

 the rain stage. 



When the quantity x varies while 

 T remains constant the projections 

 upon the plane of coordinates of 

 the isotherms corresponding to the 

 various values of x sensibly agree 



with each otiier, as we have said, at least when one draws a diagram 

 rather than a rigorously exact figure. On the other hand, the limit- 

 ing point in this common ])rojection on the plane of co-ordinates is 

 not the same when we take ditferent values of x. We find, without diffi- 

 culty, that if Vi and v^ are the abscisses of the limiting points belong- 

 ing to the quantities of vapor rj and .r^, respectively, we have '="^^, 



that is to say, that the abscissas of the limiting point vary propor- 

 tionately to X. To each value of x there corresponds a line of satura- 

 tion, i)recisely as to each value of T there corresponds auisot hern. 

 Adiabatics. — The equation of an adiabatic in the dry stage is 



Fig. 4.— Dew-point curve. 



(c,+a-c/) \og^^^-A{B,-\-xRi) log^=0 



-•-1 i?! 



(5) 



