28 BULLETIN 509, U. S. DEPARTMENT OE 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 1 
760 X 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. 
d v = density of vapor at e mm. pressure. 
Then d=d 1 +d v . The air pressure alone is B-e and 
a -a B Z e . 
di-do 760 - 
d v =.622 X d. 
e 
760" 
when .622 is the density of vapor compared to air at 760 pressure. 
Whence d-d [ B ^ I - 622 Xe ] d | B-378e | 
wnence a-a | 760 -f- 76Q j_a j ?6() j 
Knowing the values t 2 and t 3 and the vapor pressures at these two 
points (pressures at the dew points) the values of d 2 and d 3 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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