462 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



For instance, in the snow stage for/ = — o°C.;/> = -j6o mu \))i = 3.48 

 (for 6 = 5 grams) and for a diminution of temperature in conse- 

 quence of expansion down to — 20 C, we find the value p' = 527™'", 

 whereas if we had used m = 3.44 we should have had p f = 528. 

 The departure therefore in this extreme case amounts to only i mm . 



Still less important is the influence of the neglect of the weight of 

 the moisture with respect to that of the air in the factor m in the 

 dry stage, where m has the value 3.46 even for a moisture content 

 of 15 grams. In the case of a large moisture content the saturation 

 point is very quickly attained as the cooling proceeds; if the tem- 

 perature is much re luced before cooling produces saturation, then 

 the moisture content must be small. For the dry stage we are per- 

 fectly justified in adopting in our computations the value m = 3.44, 

 such as holdsgoodfor absolutely dry air if we do not desire to 

 obtain the values of pressure accurate to within o.i mm . 



§9. ADIABATIC EXPAXSIOX OF ASCENDING AIR 



The passage of a mass of , air from a given initial condition p 

 and t by adiabatic expansion or compression into another condition 

 p and / occurs in the atmosphere principally and on the largest scale 

 through a change in the altitude of the mass. 



Assuming that the air is dry and that we have a uniform dis- 

 tribution of temperature at o° C. through the whole column of air 

 we arrive, by integration of the equation 



- dh = vdp (14) 



at the well-known formula 



h = 18401 log^- 2 = K log ?* 

 A A 



which latter enables us to determine the difference of altitude of 

 two atmospheric layers from their difference of pressure, approxi- 

 mately, it is true, but in a very simple way. 



K is ordinarily designated the barometric constant and the sig- 

 nification of the remaining letters in this formula may from the 

 preceding paragraph be considered as well known. In order to 

 determine the altitudes Hertz has made use of this formula in the 

 construction of the scale of altitudes given in his adiabatic diagram. 20 



20 Hertz: Met. Zeit., 1884. 



