Research methods in ecology

by Frederic E. Clements

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 station to station, while the depth curve represents the different values at various 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 basis, 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, 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 decrease in temperature proportional to the amount of moisture in the air. The dry bulb thermometer is an ordinary thermometer, while the wet bulb is covered with a cloth that can be moistened. The former indicates the normal temperature of the air, the latter gives the reduced temperature due to evaporation. The relative humidity of the air is ascertained by means of the proper tables, from two terms, i. e., the air temperature and the amount of reduction shown by the wet bulb. The sling and the cog psychrometers alone are in general use. The stationary form has been found to be unreliable, because the moisture, as it evaporates from the wet bulb, is not removed, and, in consequence, hinders evaporation to the proper degree.

Fig. 5. Sling psychrometer.

61. The sling psychrometer. The standard form of this is shown in the illustration, and is the one used by the Weather Bureau. This instrument can be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, 107 E. German St., Baltimore, at a cost of $5. It consists of a metal frame to which are firmly attached two accurately standardized thermometers, reading usually from –30° to 130°. The frame is attached at the uppermost end to a handle in such fashion that it swings freely. The wet bulb thermometer is placed lower, chiefly to aid in wetting the cloth more readily. The cloth for the wet bulb should be always of the same texture and quality; the standard used by the Weather Bureau can be obtained from the instrument makers. A slight difference in texture makes no appreciable error, but the results obtained with different instruments and by different observers will be more trustworthy and comparable if the same cloth be used in all cases. The jacket for the wet bulb may be sewed in the form of a close-fitting bag, which soon shrinks and clings tightly. It may be made in the field by wrapping the cloth so that the edges just overlap, and tying it tightly above and below the bulb. In either case, a single layer of cloth alone must be used. The cloth becomes soiled or thin after a few months’ constant use and should be replaced. It is a wise precaution to carry a small piece of psychrometer cloth in the field outfit.

Fig. 6. Cog psychrometer.