106. The rain gauge, as the illustration shows, is a cylindrical vessel with a funnel-shaped receiver at the top, which is 8 inches in diameter. The receiver fits closely upon a narrower brass vessel or measuring tube in which the rain collects. The ratio of surface between receiver and tube is 10 to 1. For readings covering a general area, the rain-gauge is placed in the open, away from buildings or other obstructions, and is sunken in the ground sufficiently to keep it upright. In localities where winds are strong, it is usually braced at the sides also or supported by a wooden frame. In measuring the amount of rain in the measuring tube, the depth is divided by ten in order to ascertain the actual rainfall. The depth is measured by inserting the measuring-rod through the hole in the funnel until it touches the bottom. It is left for a second or so, quickly withdrawn, and the limit of the wetted portion noted. In the case of standard rods, the actual rainfall is read directly in hundredths, so that the division by ten is unnecessary. After each reading, the measuring tube is carefully drained, replaced, and the receiver put in position. No regular time for making readings is necessary. During a rainy period, it is customary to make a measurement each day, but it has been found more satisfactory for ecological purposes to measure each shower, and to record its duration. These two facts furnish a ready clue to the relative amount of run-off in each fall of rain. The measurement of snowfall is often made merely by determining its depth. For comparison with rainfall, the rain gauge with receiver and tube withdrawn is used. The snow which falls is melted, poured into the measuring tube, and measured in the ordinary way. The U. S. Weather Bureau standard rain gauge, with measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25.
107. Precipitation records. From the periodic character of precipitation, rainfall sums, means, and curves have little importance in the careful study of the habitat. The rainfall curve for the growing season is an aid in explaining the curve of water-content, and the mean rainfall of a region gives some idea of its vegetation, though even here the matter of its distribution is of primary importance. The rain and snow charts published by the U. S. Weather Bureau furnish data of some importance for the general study of vegetation, but it is evident that they can play little part in a system which is founded upon the habitat. Precipitation records, for reasons of brevity and convenience, are united with wind records, and the form will be found under the discussion of this factor.
WIND
Fig. 23. Simple anemometer.
108. Value of readings. On account of its direct effect upon humidity, and its consequent influence upon water-content, the part which wind plays in a habitat can not be ignored in a thorough investigation. It is an important element in exposure, and accordingly has a marked mechanical effect upon the vegetation of exposed habitats, alpine slopes, seacoasts, plains, etc. Owing to its inconstancy and its extreme variation in velocity, single wind readings are absolutely without value. When read in series, anemometers give some information upon the comparative air movement in different habitats, but the chance of error is great, except when the breeze is steady. Anemographs alone give real satisfaction. Accurate results, however, are not obtainable without a series of two or more in different habitats, and it is still an open question whether the results obtained justify the expense. For a completely equipped base station, anemometer, anemograph, and wind vane are desirable instruments, but the study of the habitat has by no means reached the stage of precision in which their general use is necessary.
Fig. 24. Standard anemometer.
109. The anemometer in its simplest form is adapted only to readings made under direct observation, as a sudden change in the direction of the wind reverses the movement of the indicator needle. This simple wind gauge, shown in figure 23, has been used for instructional purposes, and to a slight extent, also, in ascertaining the effect of cover. In constant winds, successive single readings are found to have value, but, ordinarily, the observations must be simultaneous. Careful tests of this simple instrument show that it is essentially accurate. It may be obtained from the C.H. Stoelting Company, 31 W. Randolph St., Chicago, for $25. The standard anemometer (Fig. 24) is practically a recording instrument up to 1,000 miles, but as the dials run on without any indication of the total number of revolutions, it must be visited and read each day. This renders its use difficult for habitats which are some distance apart. When exact determinations of wind values become necessary, the most successful method is to establish a series of three standard anemometers. One of these should be placed upon the most exposed part of a typically open habitat, the second in the most protected part of the same habitat, while the third is located in the midst of a representative forest formation. If the two habitats are close together, the daily visits can be made without serious inconvenience. The reading of the registering dials requires detailed explanation, and for this the reader is referred to the printed directions which accompany the instrument. In setting up the anemometer it must be borne in mind that the ecologist desires the wind velocity for a particular habitat. In consequence, the precautions which the meteorologist takes to place the instrument at a certain height and well away from surrounding obstructions do not hold here. Standard anemometers are furnished by H. J. Green, and J. P. Friez for $25 each.
The anemograph is an anemometer electrically connected with an automatic register. It is the only instrument adapted to continuous weekly records in different habitats, but the price, $75 ($25 for the anemometer and $50 for the register) is practically prohibitive, at least until a complete series of ecographs for other factors has been obtained.