268 THE MECHANIC'S OF THE EARTh'.S ATMOSPHERE. 



the maximnin value of I or the value of t correspoudiug thereto. Then 

 the value of a is given by the formuhi 



, tan a 



a=l . 



tan a + tan p 



whose deduction may here be omitted. 



Thus both a numerical and a graphic metliod are at our disposal. 

 If we follow the former, we can easily perceive that an extreuiely 

 accurate knowledge of the quantity of vai)or contained in a kilogram 

 when in the condition of saturation is presupposed for an even moder- 

 ately accurate determination of the value of a and t, as well as of the 



ratio '. 



Because of the unreliability of the data at hand the values obtained 

 by computation have in themselves a ratlier high degree of uncertainty, 

 so that one can equally well make use of the far more convenient 

 graphic method without thereby in fact losing anything in accuracy. 



lu this latter way the following small tables have been computed, 

 which give the limiting cases above treated as especially interesting for 

 the pressures 700 and 400 mm. and for temperatures that proceed by 

 steps of 10^ and thereby makes possible a quick review of the various 

 questions relative to mixtures of an\ 



The hrst horizontal line of each of these twelve tables relates to the 

 case where both component masses are completely saturated, and gives 

 in the column a the greatest precipitation that can occur* under these 



circumstances aud under the most favorable mixing ratio ' . 



Therefore tlie a on the first line of each table, gives the maximum 

 possible precipitation that can be brought about by mixture at the 

 given temperatures. 



The second line of each table gives the value of the relative humidity 

 wliich must (at least for one of tlie componeuts) be exceeded if precipi- 

 tation by mixture is to be any wa3' possible. We also find on this line 



under the headings t aud ' the mixing temperature and the mixing 



ratio for which the point of saturation will be just attained when in 

 both components the relative humidity has the minimum values, given 

 under Ri and Rz. 



The third line shows the value of R:. that must be exceeded by the 

 relative humidity of the warmer component, if the cooler couiponent is 

 completely saturated and if precipitation is to become jiossible by 

 mixture. 



The fourth line gives the mixing ratio which must be exceeded if 

 precipitation is to become possible by means of any proper mixing 



[Expressed in grains of water per kilogram of uioist air.] 



