WATER-CONTENT 37 
the mean, 18 per cent, while the !atter gives little or no.clue to the extremes. 
It is hardly necessary to state that means and extremes should be deter- 
mined for a certain habitat, or particular area of it, and that the results may 
be expressed with reference to holard and chresard. ._ ; 
58. Curves. The value of graphic methods and the details of plotting 
curves are reserved for a particular section. It will suffice in this place to 
indicate the water-content curves that are of especial value. Simple curves 
are made with regard to time, place, or depth. The day curve shows the 
fluctuations of the water-content of one station from day to day or from 
time to time. The station curve indicates the variation in water from sta- 
tion to station, while the depth curve represents the different values at var- 
ious depths in the same station. These may be combined on the same sheet 
in such a way that the station curves of each day may be compared directly. 
Similar combinations may be used for comparing the day curves, or the 
depth curves of different stations, but these are of less importance. A 
combination of curves which is of the greatest value is one which admits of 
direct comparison between the station curves of saturation, holard, 
chresard, and echard. 
HUMIDITY 
59. Instruments. As a direct. factor, humidity is intimately connected 
with water-content in determining the structure and distribution of plants. 
The one is in control of water loss; the other regulates water supply. 
Humidity as a climatic factor undergoes greater fluctuation in the same 
habitat, and the efficient difference is correspondingly greater. Accordingly, 
simple instruments are less valuable than automatic ones, since a continuous 
record is essential to a proper understanding of the real influence of 
humidity. As is the rule, however, the use of simple instruments, when they 
can be referred to an ecographic hasis, greatly extends the field which can 
be studied. In investigation, both psychrometer and psychrograph have 
their proper place. In the consideration of simple instruments for obtaining 
humidity values, an arbitrary distinction is made between psychrometers 
and hygrometers. The former consist of a wet and a dry bulb thermometer, 
whiie the latter make use of a hygroscopic awn, hair, or other object. 
Psychrometers 
60. Kinds. There are three kinds of psychrometer, the sling, the cog, 
_ and the stationary. All consist of a wet bulb and a dry bulb thermometer 
set in a case; the first two are designed to be moved or whirled in the air. 
The same principle is applied in each, viz., that evaporation produces a 
