28 BULLETIN 509, U. S. DEPARTMENT OP AGRICULTURE. 



2. The following equation gives the value of the density (grams 

 per c. c.) of the mixture of air and vapor: 

 ._ B-0.378e .00129305 ^ 

 — 760 ^ l+.003670t' 

 B=total barometric pressure in millimeters of mercury. 

 e=pressure of the vapor in the mixture. 

 t=temperature Centigrade of the mixture. 

 .00129305 is the weight in grams of 1 c. c. of dry air at 0° 

 C. pressure 760 mm. under gravity at 45° latitude and 

 sea level. The figure .003670 is the coefficient of ther- 

 mal expansion of air at 760 mm. 

 The first fractional expression may be explained as follows : 

 Let di=density of dry air at B-e mm. pressure. 



dv=^ density of vapor at e mm. pressure. , 

 Then d^d^^-fdv The air pressure alone is B-e and 



dv=.622 X do X ;^. 



when .622 is the density of vapor compared to air at 760 pressure. 



yjrr, -, -, [B^ , .622 Xe] -, fB-378e] 



Whence d=d, j^^^ +^f^\=^o {^eo-j 



Knowing the values t^ and tg and the vapor pressures at these two 

 points (pressures at the dew points) the values of d^ and dg are 

 obtained from the above equation. 



It will be noted that in every case chosen in Table 3 the density 

 increases due to the evaporation, hence the tendency of the air is to 

 descend as it passes through the pile of lumber. 



1 See Smithsonian Meteorological Tables, Tables 83 to 86. 



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