METHOD OF SIMPLE INSTRUMENTS 9^1 



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 

 lluctuations 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 

 w"ater-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 dift"erent 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, 



