=134. Details of the method.= After the stations have been selected by a careful preliminary survey of the habitat or series of habitats, their location is indicated by a small flag bearing a number, in case there is no danger of these being disturbed. Otherwise, less conspicuous stakes are used. The ordinary practice is to visit each station of the series, and to take readings of water-content, altitude, slope, and exposure. On the first trip these are all made by the instructor, but after a short time the determination of each factor may be assigned in rotation to each of the students. After these constant factors have been read and recorded, one student is equipped with photometer, thermometer, and psychrometer, and, if desirable, anemometer, and left at the first station. At each succeeding station the same plan is followed, so that at the end of the series the constant factors have all been read, and there is an observer at each station prepared to make readings of the variable ones. The task of acquainting the students with the operation of photometer, psychrometer, etc., can best be done in class or at a previous field period, as it is evident that they must be familiar with the instruments before they can use them accurately in the field series. The details of operation have already been given and need not be repeated here. The task of obtaining readings at the same moment may be met by supplying each observer with a watch, which runs exactly with all the others, or by making observations upon signal. The second means has been found most successful in practice, since the signal fixes the attention at the exact moment. The best plan is for the instructor to occupy a commanding position somewhere near the middle of the series, and to give the signals by shout or whistle at the proper interval. Considerable care and experience are necessary to do the last satisfactorily. Sufficient time must be given for the operation of the instrument and the making of the record. In addition, a period must be permitted to elapse which is long enough for every instrument to reach the proper reading. For example, in a series which contains a gravel slide and a forest, the thermometer which has just been used for an air reading will require four or five times as long an interval to respond to the temperature of the gravel as to that of the cool forest floor. In such series, the instructor should regularly take his place in the station where the response is slowest or greatest. He must record the exact time of each signal, and note any general changes of sky or wind that produce temporary fluctuations at the time of reading. When the readings extend over a whole day, the usual plan is to begin at the last station and take a second series of water-content samples, noting the exact time in order that the rate of water loss may be determined. A check series of physiographic factors may be made at this time also, or this may be left for future visits.