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 readily 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 



