PAPER BY PROF. BEZOLD. 



237 



Fig. 33. 



This is represented by a diagram, as shown in Fig. 33. Let a be the 

 initial condition, the corresponding dew-point curve S a , then the air ex- 

 pands according to the adiabaticfor 

 the dry stadium until it cuts the 

 curve 8 a in a point b, the curve ab 

 thus lies in a plane parallel to that 

 of PV distant therefrom by x a . A 

 glance at the course of the isotherms 

 (of which only the one correspond- 

 ing to the initial temperature is 

 drawn and designated by T a ) shows 

 that in this passage from a over to 

 U the temperature sinks rapidly. 

 As soon as the condition b is reached 

 the representative point [the indi- 

 cator] slides down on the dew-point 

 surface, the adiabatic of the dry stage goes over into be, or that of the rain 

 stage, and forms at b an obtuse angle with the former. The tempera- 

 ture, with continued uniform progressive expansion, sinks much more 

 slowly than before, water is condensed, since the curve be prolonged 

 cuts the dew-point lines of lower quantities of vapor. The condensed 

 water is deposited first as rain, afterwards *as snow, and therefore be is 

 the projection of the pseudo adiabatic. 



In this case the hail stage is entirely wanting, and although the cool- 

 ing due to the continued expansion goes on beyond the freezing point, 

 still this does not make itself so strongly felt in the course of the pseudo- 

 adiabatic as that this transition should be perceptible in a drawing like 

 the present diagram. 



Let expansion continue up to a condition c, and now let compres. 

 sion occur, that is to say, the air reaches the summit or ridge of the 

 divide and the ascent now becomes a descent on the other side. Now, 

 all depends upon whether the condensed water was really completely 

 precipitated or not. If not precipitated then during the compression 

 there will be a retrogression of the indicator along the curve be in the 

 direction from c to b, and so much the farther along in proportion as 

 more water has been carried with the air. If all the condensed water 

 has remained suspended, then the change of condition in the retrograde 

 direction continues back to b, and thence beyond to a, and we find on 

 reaching the same level on the other side of the mountain again the 

 same relations as in the beginning. This is always the case when- 

 ever the curve of saturation is not reached in the expansion, that is 

 to say, when the whole process is entirely transacted in the dry stage 

 in which case also the characteristic peculiarities of the foehn are 

 wanting. 



If however the rain siage is attained, and if in it the condensed 

 water is actually precipitated then the process can not be reversed, and 



