- THE WATER OF THE HABITAT 31 



in the spring, readings should be taken every week or two until the 

 work closes. 



41. Choice of stations. The stations of a series should be 

 chosen with care, and as a result of considerable previous knowledge 

 of the locality to be studied. The stations must not be too numer- 

 ous nor too far apart. Within these restrictions, however, they 

 should represent several distinct formations, and as many dissimi- 

 lar areas in each as possible. A series can not well be more than 

 two miles long, and one of a mile, or even a half mile, is to be pre- 

 ferred. A good series will contain at least a dry, a wet, and a shadj^ 

 habitat, e.g., a prairie, a swamp, and a forest. In class work of 

 this sort at Lincoln, two series were first established, one in prairie, 

 the other in woodland. For reasons of convenience and time- 

 saving, these are now replaced by a prairie series consisting of the 

 following stations: (1) meadow, (2) crest of ridge, (3) northeast 

 slope, (4) ravine, (5) southwest slope, (6) sandy ridge, (7) willow 

 thicket. (8) high prairie, (9) bog, (10) south slope. 



42. Constant factors. After the stations have been chosen, 

 their location should be permanently indicated in such a way that 

 they can be readily found from time to time. In order to avoid 

 mistakes, the instructor should first take the entire class through 

 all the stations, pointing out the general differences and illustrating 

 the use of instruments not already familiar. If this does not take 

 too long, readings of the more constant factors, water content, 

 slope, exposure, surface, and cover are made at the last station, and 

 a student equipped with thermometer, psychrometer, and pho- 

 tometer is left in charge. At each succeeding station the same 

 plan is followed, so that upon reaching the first station the con- 

 stant factors have all been read, and there is an observer at each 

 station prepared to make readings of the variable factors. When 

 the preliminary survey has occupied all the time available, the 

 same method is employed upon the second visit, but the beginning 

 is naturally made at the first station. The observers are shifted 

 upon successive visits so that each student has an opportunity to 

 become acquainted with every station. When the class is large, 

 two or more students may be left at a station, and the work 

 divided between them. 



43. Simultaneous readings. The task of obtaining readings 

 at the same moment is met by taking ol)scrvatioiis ui)on siu;iial. 

 The instructor places himself at a commanding station, profera- 



