258 THE MECHANICS OF THE EARTH'.S ATMOSPHERE. 



that the production of a moderately heavy precipitation in this waj' is 

 impossible. 



At the same time he showed that the adiabatic expansion in this re- 

 spect i)layed an entirely different and much more important role, and 

 that, in it we have to recognize the source of all considerable precipi- 

 tations. 



In this paper, so far as it concerned mixture Hann confined himself 

 to the computation of an example from whicli it appeared that even 

 under verj^ improbable assumptions there could in this way only be 

 lealized very slight quantities of i)recipitations. 



Peruter many years later* contributed to the solution of the prob- 

 lem in that he brought it into an exact mathematical form and at the 

 same time also computed small numerical tables in order to facilitate 

 the comprehension of the quantities that enter into the question. 



But since the empiric formula for the tension of aqueous vapor en- 

 ters into the expression given by Peruter, therefore the latter is rather 

 comj)lex and is not especially clear. 



It seems therefore to me not only desirable but really necessary to 

 take up the question anew and if possible prosecute it to a definite 

 conclusion. This is the object of the following lines. 



It will be shown how graphic methods give with extraordinary ease 

 an insight into the whole theory of the mixture of air and how in such 

 methods we possess at the same time the simjjlest means for the nu- 

 merical evaluation of the quantities that enter into the question. 



Various tables — some of which may also be welcome for other investi- 

 gations — will also facilitate a general survey as well as the exact com- 

 l)utations. After these preparatory sections there will be considered 

 the various causes of the fortuatiou of precipitation, namely, direct 

 cooling, adiabatic expansion, and mixture, in their relative importance 

 and it will be shown how that only by the consideration of all these 

 causes is it jjossible to obtain a deeper insight into the methods of the 

 formation of clouds. 



(a.) THE MIXTURE OF QUANTITIES OF AIR OF UNEQUAL TEMPERATURE 



AND MOISTURE. 



Before we proceed to the mathematical treatment of this problem 

 we must first come to a clear understanding as to whether definite 

 masses or definite volumes shall be made the basis of the computation. 



At the first view it would seem appropriate to adopt the volume, 

 since we can from well known tables obtain directly the quantity of 

 water which corresponds to the saturation of one unit of volume. 



This is doubtless the reason why in the older investigations of this 

 subject based on Button's theory, one always started with the con- 

 sideration of the unit of volume, and why Hann — when he w^uld 



" Zeitschft. Uesterr. Gesell. Met., lrfH2, Vol. xvii, pp. 4^1-426. 



