392 RECORD OF SCIENCE FOR 1887 AND 1888. 



airs are being mixed with warm and moist; on the other hand aqueous 

 vapor with hiteut heat is being added to the air by evaporation from 

 the ground, and again being taken from the air, but leaving its heat 

 behind by the process of formation of rain, hail, and snow. Therefore 

 the ascent and descent of atmospheric currents is by no means an adia- 

 batic process, and it is the aim of Bezold to so present graphically the 

 changes that take place in ascending and descending air as that we may 

 at any time calculate its thermal condition. 



(27) Bezold. — The original memoirs of Hertz and Bezold detailing 

 their graphic methods in tbermo-dynamics as applied to our atmosphere 

 will be given in full in the promised collection of translations. The 

 ground covered by these will be easily understood from the following 

 analysis of Bezold's work by Lettry,* with slight additions by myself. 



(A.) DEFINITIONS. — Let ji aud v be respectively the values of the vol- 

 ume and pressure of a unit weight, namely, a unit mass, of gas or gas- 

 eous vapor whose absolute temperature is T. Then according to the 

 law of Boyle Mariotte-Gay-Lussac aud Charles, we have 



pv=IiT (a). 



Let the condition of the unit of gas be graphically represented as to 

 pressure aud volume by the ordinate and abscissa of a point ; when p 

 and V are given, the location of the point is known by the graphic con- 

 struction, but equally is the temperature/! known by the equation {a) ; 

 thus the location of a point in the diagram corresponds to a definite 

 temperature of the gas. If the gaseous mass is maintained at a con- 

 stant temperature then p aud v may vary continuousl3^, only fulfilling the 

 condition that their product p v remains constant. The locus of the 

 continuous series of points thus defined is called an isotherm, and we 

 see that the isotherm occurring for any given temperature must be a 

 hyperbola with the axis^ and v as its asymptotes, as in Fig. 2, where 

 the co-ordinate axes are rectangular aud the hyperbola is equilateral. 



If ou the other hand the tem- 

 perature be allowed to change, but 

 the original amount of heat or 

 thermal energy in a unit mass of 

 gas is forced to remain constant, 

 then a new relation between the 

 pressure aud volume that can co- 

 exist is brought about and is defi- 

 nitely determined by the laws of 

 thermo-dynamics ; this relation is 

 given by the equation. 

 Fig. 2.-isotheim?. " ^^'y^'=coustant 



where k and ¥ are respectively the specific heat of the gas or vapor 

 at constant volume and again at constant pressure. If, as before, the 

 *See Anuuaire Soc. M^t. de France, 1888, *xxvi, p. 236. 



