10 



rates of evaporation at the middle and foot of the northwest-facing 

 slope (Fig. 17) during the early part of the season might be attrib- 

 uted to the fact that the instrument at the middle of the slope was 

 almost surrounded by a dense growth of Osmorhiza longistylis (PI. 

 Ill, Fig. 5) ; and the slightly higher rate of evaporation at the middle 

 than at the top of the north-facing slope during the latter part of the 

 season (PI. XIII, Fig. 22) might have been due to the unusual expos- 

 ure of the middle of the slope compared with that at the top (PI. VII, 

 Figs. 10 and 11). Further, the differences in evaporation on different 

 parts of the northwest- and north-facing slopes (Figs. 17 and 22), 

 are less than on corresponding parts on the opposite slopes, and thus a 

 station between two other stations would give less striking differ- 

 ences. In summary, the graphs in figures 17, 22, 19, and 24 show 

 that as the northwest-, southeast-, and south-facing slopes are as- 

 cended there is a graded rise in the evaporative power of the air, the 

 southeast- and south-facing slopes showing this more decidedly than 

 the northwest- and north-facing slopes. 



3. The graphs in Figure 18 (PI. XI) show that the rate of evap- 

 oration was generally greater near the top of the southeast-facing 

 slope than near the top of the opposite slope, the average amount per 

 day for the entire series being nearly 1 c.c, or 13.9%, greater; the 

 graphs in Figure 20 (PI. XII) show that the rate of evaporation was 

 almost uniformly greater at the middle of the southeast-facing slope 

 than at the middle of the northwest-facing one, the average amount 

 of evaporation per day being 1.23 c.c, or 28.7%, greater; the graphs 

 in Figure 23 (PI. XIII) show that the rate of evaporation for every 

 weekly interval was greater near the top of the south-facing slope 

 than near the top of the opposite north-facing one, being 1.93 c.c, or 

 32.1%, greater; while the graphs in Figure 25 (PI. XIV) show re- 

 sults at variance with those in Figure 20 (PI. XII) — a greater rate of 

 evaporation at the middle of the north-facing slope than at the middle 

 of the opposite south-facing one. This was undoubtedly due, as be- 

 fore suggested, to the unusual exposure of the middle of the north- 

 facing slope, which was subject to less shade than any of the other 

 stations in the immediate vicinity excepting the one near the floor of 

 the ravine. The results shown by figures 18, 20, and 23 indicate a 

 greater loss of water and consequently drier conditions on the south- 

 east-facing slope than on the northwest-facing slope, and likewise 

 greater loss of water and drier conditions on the south-facing slope 

 than on the north-facing one. This difference in evaporation is un- 

 doubtedly due to the fact that the incident rays of the sun strike the 

 southeast- and south-facing slopes more nearly at right angles, which 

 means greater absorption and radiation of energy on the southeast- 



