ADIABATIC CHANGES OF MOIST AIR NEUHOFF 



477 



If we neglect the vapor pressure and therefore adopt p' as the 

 total pressure then we can construct this curve in the same net- 

 work as the curves of equal "entropy. " Inversely therefore, from 

 given values of p and t we may compute the entropy and allow the 

 individual crfrves to grow one from the other by the same differences 

 of entropy. In this way the curves of the Hertzian table have 

 been represented. The objection to this arrangement, independent 

 of the error introduced by the neglect of the vapor pressure in the 

 condensation stage, consists in the difficult reproduction of the 

 logarithmic network in comparison with the advantage of having 

 straight lines for the curves of the first or dry stage. 



The important question as to the altitude at which the individual 

 changes of condition occur can also now be easily answered graphi- 

 cally by constructing level curves or isohypsen or lines of equal 

 altitude in the adiabat table of fig. 5; that is to say, we seek the 

 points on the adiabats corresponding to definite altitudes and con- 

 nect these points of equal altitude by curves. 



In doing this we must first consider the two stages separately. 

 We locate the initial level of o meters in the pressure line of 760 

 millimeters. 



In the dry stage we now obtain the difference of the altitudes in 

 the simplest manner by the adiabatic hypsometric formula — dh = 

 CdT. If we take ap- „„„__ 



proximately C = 100 then 

 the temperature diminu- 

 tion of io° C. along any 

 adiabat corresponds to a 

 difference of level of 1000 

 meters. Therefore the 

 intersection of any adia- 

 bat with two tempera- 

 ture lines io° apart gives 

 points in the altitude 

 line corresponding to 

 1000 meters, 2000 me- 

 ters, etc. If now we combine the points having equal altitude we 

 obtain the level lines and these also are represented as straight 

 lines. This construction is carried out in fig. 6. The altitude lines 

 approach each other as the temperatures increase and at the same 

 time are inclined to each other. 



In the condensation stage the altitudes at which certain condi- 

 tions are attained when the initial level "is at 760 millimeters, 



FIG. 6. ALTITUDE LINES FOR THE DRY STAGE 



