METHOD OF SIMPLE INSTRUMENTS gt 
the response is slowest or greatest. He must record the exact time of each 
signal, and note any general changes of sky or wind that produce temporary 
fluctuations at the time of reading. When the readings extend over ‘a whole 
day, the usual plan is to begin at the last station and take a second series of 
water-content samples, noting the exact time in order that the rate of water 
loss may be determined. A check series of physiographic factors may be 
made at this time also, or this may be left for future visits. While it is un- 
necessary to take soil samples oftener than once a day, it is important to 
make at least one series at each visit. Sometimes it becomes desirable to 
know the rate of water loss in different stations during the day, and in this 
event, samples are taken at one or two hour intervals for the entire day. 
In making simultaneous readings at the different levels of one station, the 
observers are grouped in one spot in such a way that they do not interfere 
with the correct reading of each instrument. Readings of this sort are most 
valuable in the case of temperature, which shows greater differences at the 
various levels. Important differences of humidity and wind also are readily 
obtained, and, in layered formations, marked variations in the amount of 
light. In the open, the ordinary levels for temperature are 6 feet, 3 feet, 
surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6 
feet, 3 feet, and surface. In forests the same levels are used for comparison 
with formations in the open, but a more desirable series for light especially 
is secured by making readings at the height of, or better, just below the var- 
ious layers. Series of this sort are likewise made on signal. The best time of 
day is that of a period in which the middle station is read near meridian, 
since the variation due to time is sufficiently small to permit fairly accurate 
comparisons between the readings for the different stations. 
135. Records, The form used for recording the observations made by 
means of simple instruments is shown below. It is hardly necessary to state 
that it may be readily modified to suit the needs of different investigators. 
Ordinarily, each sheet is used for the records of one habitat or series alone, 
but for convenience sake, the records of two different series are here com- 
bined. The figures given are taken from records for the prairie and forest 
formations at Lincoln, 
