=36. Method of simple instruments.= By simple instruments are denoted those that do not record, but must be read by the observer at the time. They are standard instruments of precision, but possess the disadvantage of requiring an observer for each one. They are well illustrated by the thermometers and psychrometers used by the Weather Bureau. In the hands of trained observers the results obtained are unimpeachable; in fact, standard simple instruments must be constantly employed to check automatic 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 compared should be made at the same instant. This requires a number of observers; 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 accurately 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 trustworthy 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. Ecographs 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 disadvantage, as the scattered observations possible on such a journey can best be made by simple instruments.