DYNAMIC METEOROLOGY. 



393 



Fig. 3. — Adiabatic curve. 



co-existing jp and v are made tlie co-ordinates of a pojnt, the locus of the 

 points representing the pressure and volume for a given constant quan- 

 tity of heat is called the adiabatic curve for that special quantity, as is 

 shown in Fig. 3. 



An isotherm and an adiabatic line 

 may be imagined passing through any 

 point whatever if the latter be con- 

 sidered as an indicator of a given 

 initial state of the gas ; that is to say, 

 a point whose position is determined 

 by co-ordinates having the initial 

 values ^o and Vo. The indicator point 

 will follow the isotherm if we make 

 the condition of the gas vary while 

 maintaining a constant temperature ; 

 it will follow the adiabatic if the con- 

 dition of the gas varies without increase or diminution of the quantity 

 of heat contained within it. 



Hitherto in the application of the mechanical theory of heat to me- 

 teorology the adiabatic changes only have been considered, but Hertz 

 has shown how to approximately consider non-adiabatic changes, and 

 especially has Bezold freed himself from the adiabatic hypothesis, which 

 is in fact not generally realized in nature. 



Dynamic cooling.— Bezohl first considers the preliminary question, 

 Why does air cool on ascending to higher elevations ? Most meteorol- 

 ogists exi)lain this cooling as the transformation of molecular energy 

 into external work done in the expansion of the gaseous mass as it 

 comes under and acts against the diminished pressure of the upper re- 

 gions. This is correct, and it is necessary to be on our guard against 

 an erroneous explanation adopted by Guldberg and Mohn,to the effect 

 that the work done is the elevation of the gas to the higher level ; this 

 latter explanation is not allowable, since the work of raising the gas is 

 really done by gravity, namely, the heavier descending air pushes up 

 the lighter rising gas. 



(B.) THE DRY STAGE.— Let x be the mass of aqueous vapor (namely, 

 not the weight nor the tension, but strictly the mass of vapor) that is as- 

 sociated with a unit mass, i. e., a kilogram of dry air, and m the mass of 

 the mixture; then m = l-\-x. Kote that a; differs slightly from the quan- 

 tity of vapor in a kilogram of the mixture, which latter is the quantity 

 generally given in meteorological tables. 



LetpK and ps bo the partial pressures of the dry air and the vapor, 

 respectively ; the total barometric pressure will hep=px+2h. 



Let "Ra and Kj be the constants in equation (a) for the air and vapor; 

 that equation then gives for the mixed gas and vapor 



p^v = Ji^T psV=x-RsT p = ^ {lii,-\-xlh)T. ... (1) 



