INSTRUMENTS 23 
ones. As physical factors vary greatly through the day and through the 
year, it is all-important that the readings in habitats which are being com- 
pared should be made at the same instant. This requires a number of ob- 
servers ; as many as twelve stations have been read at one time, and there is 
of course no limit to the number. It is very important, also, that observers be 
carefully trained in the handling of instruments, and in reading them ac- 
curately and intelligently at the proper moment. In practice it has been 
found impossible to do such work in elementary classes, and, even in using 
small advanced classes, prolonged drill has been necessary before trust- 
worthy results could be obtained. When a class has once been thoroughly 
trained in making accurate simultaneous readings, there is practically no 
limit, other than that set by time, to the valuable work that can be done, 
both in instruction and investigation. 
37. Method of automatic instruments. The solitary investigator must 
replace trained helpers by automatic instruments or ecographs. These have 
the very great advantages of giving continuous simultaneous records for 
long periods, and of having no personal equation. They must be regulated 
and checked, to be sure, but as this is all done by the same person, the error 
is negligible. There is nothing more satisfactory in resident investigation 
than a series of accurate recording instruments in various habitats. Eco- 
graphs have two disadvantages. The chief perhaps is cost. The expense 
of a single “battery” which will record light, water-content, humidity, and 
temperature is about $250. Another difficulty is that they can be used 
only within a few miles of the base, since they require attention every week 
for regulation, change of record, etc. While this means that ecographs in 
their present form are not adapted to reconnaissance, this is not a real dis- 
advantage, as the scattered observations possible on such a journey can best 
be made by simple instruments. 
38. Combined methods. The best results by far are to be obtained by the 
combined use of simple and automatic instruments. This is particularly true 
in research, but it applies also to class instruction. The ecographs afford a 
continuous, accurate basal record, to which a single reading made at any 
time or place can be reddily referred for comparison. On the other hand, 
it is an easy matter to carry a full complement of simple instruments on the 
daily field trips, and to make accurate readings in a score or more of forma- 
tions in a single day. An isolated reading, especially of a climatic factor, 
has little or no value in itself, but when it can be compared with a reading 
made at the same time in the base station by an ecograph, it is the equivalent 
of an automatic reading. This method renders a set of simple instruments 
