90 THE HABITAT 
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 oper- 
ation 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 success- 
ful in practice, since the signal fixes the attention at the exact moment. 
Fig. 30. A denuded station in the aspen formation, 
The best plan is for the instructor’ to occupy a commanding position some- 
where near the middie of the series, and to give the signals by shout or 
whistle at the proper interval. Considerable care and experience are neces- 
sary 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 instru- 
ment 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 
