162 TERRESTRIAL CONDITIONS 
I. EVAPORATION 
“The total effect of air temperature, pressure, relative humidity, 
and average wind velocity upon a free water surface in the shade or in 
the sun is expressed by the amount of water evaporated” (125, p. 72). 
Since temperature in the season without frost is directly due to the sun’s 
rays, light is in part included. In our latitude, clouds in summer slightly 
decrease the air temperature (125, p. 72). In winter, however, the 
temperature of cloudy days is higher. The strongest light is usually 
associated with the greatest evaporation. Yapp (129) found that the 
rate of evaporation was directly correlated with temperature and illumi- 
nation, but most closely correlated with relative humidity. From the 
standpoint of including many factors, the evaporating power of the air is 
by far the most inclusive and is therefore by far the best index of physical 
conditions surrounding animals wholly or partly exposed to the atmos- 
phere. It is not, however, to be expected that it will hold good for all the 
factors under all climatic conditions, and for this reason, records of light, 
temperature, pressure, carbon dioxide, etc., should be made. 
The data are usually obtained by using a porous cup atmometer. 
' Evaporation from the atmometer is more nearly like that from an organ- 
ism than is evaporation from any other device; it was devised by 
Livingston (130). “It consists of a hollow cup of porous clay 12.5 cm. 
high, with an internal diameter of 2.5 cm. anda thickness of wall of about 
3 mm. It is filled with pure water and connected by means of glass 
tubing to a reservoir usually consisting of a wide-mouthed glass bottle of 
one-half liter capacity. The water, passing through the porous walls, 
evaporates from the surface, the loss being constantly replaced from the 
supply within the reservoir. Readings are made by refilling the reservoir 
from a graduated burette to a certain mark scratched upon its neck. 
For convenience in handling, a portion of the base of the cup is coated 
with some impervious substance and, before being used in the field, the 
instrument is standardized by comparing its loss of water with that from 
a free water surface of 45 sq. cm. exposed under uniform conditions. As 
a further check against error this standardization is repeated at intervals 
of six to eight weeks throughout the season” (Fuller, 131). In Fuller’s 
work, the bottles were arranged so that the evaporating surface of the 
instrument was 20-25 cm. above the surface of the soil. 
a) Effect of evaporation upon animals.—In the case of man some 
observations have been made. According to Pettenkoffer and Voit (fide 
125), an adult man eliminates goo gms. of water from his skin and lungs 
