“88 THE HABITAT- 
Mrtuops oF Hazitat INVESTIGATION 
131. The use of the various instruments previously described depends 
largely upon the preponderance of simple instruments or recording ones. 
The former necessitate a number of well-trained assistants; the latter re- 
quire only a part of the time of one investigator. For the most satis- 
factory results, however, an assistant is all but indispensable. Since sim- 
ple instruments are most easily obtained because of their cheapness, and 
are especially adapted to purposes of instruction, the method of using them 
will be described first, and then that of ecograph batteries. 
THE METHOD OF SIMPLE INSTRUMENTS 
132. Choice of stations. This method is based upon simultaneous read- 
ings by means of simple instruments in a series of habitats, or of stations 
\ 
Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation. 
in a single habitat. Such readings are necessary for ‘the variable atmos- 
‘pheric factors, humidity, light, temperature, and wind. . Frequent read- 
ings suffice for water-content and precipitation, while only two or three 
determinations, enough to check out the error, are necessary for the con- 
stant factors, altitude, slope, exposure, and surface. An account of the 
exact procedure employed in class study at Lincoln and Minnehaha will 
best serve to illustrate the use of this method. The series of stations chosen 
at Lincoln’ were primarily within a single formation, for the purpose of 
determining the physical factor variation in different areas. One series 
was located in the prairie-grass formation (Koelera-Andropogon-psilium), 
and consisted of the following stations: (1) low prairie, (2) crest of ridge I, 
