TEMPERATURE OF ASCENDING CURRENTS VON BEZOLD 337 



and descending branches this leads of course to the conclusion that 

 either the sections of the two branches have a definite ratio to each 

 other or else the velocities at different places must be different. 



The former of these assumptions is the simpler, whereas changes 

 of velocity would materially burden our course of reasoning unless 

 indeed they are neglected entirely. In consideration of this fact 

 the scheme assumes that there is a variation in sectional areas 

 of the two branches. But under all conditions the existing surplus 

 of pressure after initiating the motion or after attaining a station- 

 ary condition serves only to overcome the friction, and if that did 

 not exist it would, after equilibration of the temperatures, maintain 

 the movement forever. 



Exactly the same conditions exist in the interchange of air between 

 ascending and descending currents, as soon as the movements are 

 started and the steady condition is established. Here also the rise 

 in the ascending branch takes place at the expense of the mass that 

 is sinking in the descending branch and there can be no thought of a 

 work of elevation performed by other forces such as heat introduced 

 from without or by the loss of internal energy, i. e., cooling. 



But these preceding remarks obtain only for the steady condition. 

 If the motion is to be first initiated then either the center of gravity 

 must be raised on one side by warming or on the other hand it must 

 be lowered by cooling. In the first case actual work must be per- 

 formed to raise the mass and this is to be added to the work done by 

 expansion ; in the second case the energy that is necessary to set in 

 motion the whole mass that enters into circulation must be obtained 

 from the descent of the center of gravity of the cooling side of the 

 whole system. 



In this process, however, it is absolutely necessary that the heat 

 be "taken in" by the warming side or else "given up" by the cooling 

 side unless mechanical acceleration be given to the mass by other 

 masses of air not belonging to this system, as, for instance, by fric- 

 tion or pressure or suction. 



If we ignore these last mentioned influences and confine attention 

 to those processes in which only heat comes in play, we get an excel- 

 lent insight into the behavior of the phenomena by the presentation 

 given at pp. 107-110 of Sprung's Lehrbuch der Meteorologie and the 

 experiment there described. 



If now we apply similar considerations to the rise and fall of the 

 surfaces of equal pressure, such as occur in the production of land 

 winds and sea-breezes or mountain and valley winds, then we at 

 once see that here also in case of warming, work is actually done by 



