METHOD OF SIMPLE INSTRUMENTS 89 
(3) northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of 
ridge II, (6) bare crest of ridge II, (7) thicket ravine. The other series 
was established in the bur-oak-hickory forest (Quercus-Hicoria-hylium) 
at the following stations: (1) thicket, (2) woodland, (3) knoll in forest, 
(4) depression in forest, (5) level forest floor, (6) nettle thicket, (7) brook 
bank. At Minnehaha the series was primarily one of different formations: 
(1) the pine formation (Pinus xerohyliumt), (2) the gravel-slide formation 
(Pseudocymopterus-Mentzelia-chalicium), (3) east slope of spruce forest 
(Picea-Pseudotsuga-hylinm), (4)-ridge in the spruce forest, (5) north 
slope of spruce forest, (6) brook bank in forest, (7) the thicket formation 
( Quercus-Cercocarpus-lochmodium), (8) the aspen formation (Populus 
hylium). When permanent or temporary quadrats are established, they are 
ordinarily used as regular stations,-since this enables one to refer the 
physical factor readings to a few definite individual plants, as well as to the 
entire formation. The transects in figure 29 illustrate two of the above 
series of stations. 
133. Time of readings. The frequency of simple readings and the 
times at which they are made must be regulated largely by opportunity and 
convenience. In addition to making readings once or twice a week through- 
out the season, the series should be read at least once every day for a rep- 
resentative week or two. It is also very desirable to have a series for each 
hour of a typical day, or of two days, one of which is clear, the other cloudy. 
When a single daily reading is made, it should be taken at or as near me- 
ridian as possible. The usual series is the one obtained by simultaneous 
observations at the same level in different stations. An important series is 
also secured by simultaneous readings at the various levels of the same 
station, though it is not necessary to take this series frequently. 
134. Details of the method. After the stations have been selected by a 
careful preliminary survey of the habitat or series of habitats, their location 
is indicated by a small flag bearing a number, in case there is no danger of 
these being disturbed. Otherwise, less conspicuous stakes are used. The 
ordinary practice is to visit each station of the series, and to take readings 
of water-content, altitude, slope, and exposure. On the first trip these are 
all made by the instructor, but after a short time the determination of each 
factor may be assigned in rotation to each of the students. After these 
constant factors have been read and recorded, one student is equipped with 
photometer, thermometer, and psychrometer, and, if desirable, anemometer, 
and left at the first station. At each succeeding station the same plan is 
followed, so that at the end of the series the constant factors have all been 
