46 THE HABITAT 
upon moisture by mixing the air of different habitats and layers. So far 
as the plant is concerned, it has practically the effect of incteasing or de- 
creasing the humidity by the removal of the air above it. Thus, while the 
anemometer can furnish no direct evidence as to the amount of variation, 
it is of aid in explaining the reason for it. Likewise, the rate of evaporation 
as indicated by a series of atmometers, affords a ready method of estimat- 
ing the comparative effect of humidity in different habitats. Potometers 
and other instruments for measuring transpiration throw much light upon 
humidity values. Since they are concerned with the response of the plant 
to humidity, they are considered in the following chapter. 
73. Humidity tables. To ascertain the relative humidity, the difference 
between the wet and dry bulb readings is obtained. This, with the dry bulb 
temperature, is referred to the tables, where the corresponding humidity is 
found. A variation in temperature has less effect than a variation in the 
difference ; in consequence, the dry bulb reading is expressed in the nearest 
unit, and the difference reckoned to the nearest .5. The humidity varies with 
the air pressure. Hence, the altitude must be determined for the base station, 
and for all others that show much change in elevation. Within the ordinary 
range of growing-period temperatures, the effect of pressure is not great. 
For all ordinary cases, it suffices to compute tables for pressures of 30, 20, 
27, 25, and 23 inches. The following table indicates the decrease in pres- 
sure which is due to altitude. 
ALTITUDE PRESSURE 
Feet Meters Inches Centimeters 
0 0 30 76 
910 277 29 73.5 
1850 574 28 71 
2820 860 aT 68.5 
3820 1165 26 66 
4850 1477 25 63.5 
5910 1792 24 61 
7010 2138 23 58.5 
8150 2485 22 56 
9330 2845 21 53.5 
10550 3217 20 51 
13170 4016 18 46 
16000 4880 16 41 
The fluctuations of pressure due to weather are usually so slight that: 
their influence may be disregarded. An excellent series of tables of relative 
humidity is found in Marvin’s Psychrometric Tables, published by the U. S. 
Weather Bureau, and to be obtained from the Division of Publications, 
Washington, D: C., for 10 cents. A convenient field form is made by remov- 
ing the portion containing the tables of relative humidity, and binding it in 
stiff oilcloth. 
