WATER-CONTENT 37 



the mean, i8 per cent, while the latter 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. rnstruments. 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 basis, greatly extends the field which can 

 be studied. In investigation, both psych rometer 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, 

 while 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 



