24 THE HABITAT 
more desirable for a long trip or reconnaissance than-a battery of automatic 
ones. It is practically impossible to carry the latter into the field, and in any 
event a continuous record is out of the question. As there are other tasks 
at such times also, it becomes evident that the taking of single readings 
which can be compared with a continuous record offers the most satisfactory 
solution. 
ConstTRUCTION AND Use oF INSTRUMENTS 
39. The selection of instruments. In selecting and devising instruments 
for the investigation of physical factors, emphasis has first been laid upon 
accuracy. This is the result of a feeling that it is better to have instruments 
that read too minutely than those which do not make distinctions that are 
sufficiently close, particularly until more has been learned about efficient 
differences. On the other hand, no hesitation has been felt in employing in- 
struments which are not absolutely accurate, when it was clear that the 
error was less than the efficient difference. Similarly, the margin of error 
practically eliminates itself in the case of simultaneous comparative readings, 
when the instruments have been checked to the same standard. Simplicity 
of construction and operation are of great importance, especially in saving 
time where a large number of instruments are in operation. Expense is 
likewise to be carefully considered. It is impossible to have too many in- 
struments, but cost practically determines the number that can be obtained. 
It is further necessary to secure or invent both simple and automatic instru- 
ments for all factors, except such invariable ones as altitude, slope, etc. 
Simple instruments must be of a kind that can be easily carried, and so con- 
structed that they can be used at a minimum of risk. The sling psychro- 
meter, for example, is very readily broken in field use, and it has been 
replaced by a protected modification, the rotating form. 
In describing the construction and operation of the many factor instru- 
ments, there has been no attempt to make the treatment exhaustive. Those 
instruments which the author has found of greatest value in his own work 
are given precedence, and the manner of using them is described in detail. 
Other instruments of value are also considered, though with greater brevity. 
Some of the most complex and expensive ones have been ignored, as it is 
altogether improbable that they can come into general use in their present 
form. While the conviction is felt that the methods described below will 
enable the most advanced investigators to carry on thorough work, it is 
hoped that they will be seen to be so fundamental, and so attractive, that they 
will appeal to all who are planning serious ecological study. 
