=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. [Illustration: 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