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 increasing- 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, 29, 

 27, 25, and 23 inches. The following table indicates the decrease in pres- 

 sure which is due to altitude, 



PRESSURE 



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, pubhshed 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. 



