402 RECORD OF SCIENCE FOR 1887 AND 1888. 



Moreover, the curves of condition are in winter nearer the i^lane^ov 

 than in sammer, because in winter the absolute quantity of aqueous 

 vapor contained in the air is always smaller. 



When the air rises in the cyclone, starting from the initial condition «, 

 the indicator-point very nearly follows the adiabatic until it attains the 

 upper limit of the mass of clouds; in fact, below this limit the insolation 

 and the radiation can produce only inappreciable effects. As for the 

 rest, in so far as the curve departs from the adiabatic, it approaches the 

 axes, contrary to what happens in the summer time. 



In the accompanying diagram, Fig. 

 j,^ 7, ahc is the curve of conditions from 



1 the initial point a up to the moment 



\ when the compression begins. We 



have supposed that the initial mass 

 passes immediately from the dry stage 

 ab to the snow stage be. 



It is probable that at high altitudes 

 the compression of the descending air 



-— Ta proceeds adiabatically, according to 



the adiabatic of the dry stage; but 

 V nearer the ground the radiation causes 

 , iG- •— tia>aics. ^ deviation toward the co-ordinate 



axis ov. Thus one obtains a curve somewhat analogous to cd in Fig. 

 7. The curve cd is only a graphic representation of the well-known 

 fact that there is an inversion in the vertical distribution of tempera- 

 ture during clear days in winter. 



Near d the curve approaches the line of saturation, so that it may 

 even intersect it ; this case corresponds to the formation of fog at the 

 surface of the ground. 



Numerical data are wanting to determine whether the passage from 

 c to d can be made in any other manner, as when the cooling exerts its 

 action near the point c. The curve of condition in the plane pov would 

 then possess a double point. 



These examples suffice to enable us to judge of the usefulness of this 

 graphic method, devised by Bezold, and of which, as he says, when per- 

 fected from a mathematical point of view, this method will give an ex- 

 cellent means of discussing the numerical data furnished by observa- 

 tion ; it will at the same time make known in what direction other ob- 

 servations are to be sought to the greater profit of dynamic meteorology. 

 In his second memoir of November, 1888, Bezold adopts the term, 

 "potential temperatures" as equivalent to Helmholtz's expression 

 " thermal contents," and as the term has been applied by the latter it 

 will, we hope, obtain general use, although it is perhaps objectionable, 

 as involving a new modification of the much used word potential. The 

 "potential temperature" is simply the absolute temperature that a 

 body would assume if it were brought to a normal pressure without 

 loss or gain of heat. * 



