294 Yapp . — On Stratification in the Vegetation of a 
were placed in ‘litter’ vegetation. 1 Cladinm was absent, being largely 
replaced by Carices. Mingled with the Car ices were Phragmites (dwarf), 
Molinia , & c., and a number of species of dicotyledonous herbs (see PI. I, 
Fig. 2, and Text-fig. 6). The general vegetation level was about two feet 
above the ground. 
The instruments were now arranged as follows (see Text-fig. 6) : — 
(1) Set A. 4 feet above the soil. In this position the instruments 
were clear of the vegetation, and quite exposed. 
(2) Set B. 1 ft. 7 in. above the soil level. Here the conditions were 
those of the surface layer of the vegetation. The instruments were fairly 
exposed to the sun, but the wind effect was lessened. 
(3) Set C. 5J inches above the soil, i. e. in the lower layers of the 
vegetation. Instruments lightly shaded, and almost completely sheltered 
from the wind. 
Table II gives the evaporation and other meteorological data during 
the whole period occupied by the 1908 experiments. Cf. also the curves in 
Text-figs. 7 and 8. 
The total evaporation during the whole of the ten days, from the three 
instruments respectively, was as follows: — 
A. 532-85 c.c. 
B. 299-25 c.c. 
C. 78.45 c.c. 
These amounts are in the ratios : — 
A: B: C = IOO : 56-2: 14-7 
as compared with the average relative evaporation during the 1907 experi- 
ments of: — 
A : B : C = 100 : 32-8 : 6-6. 
As was to be expected, by far the greater part of the total evaporation 
occurred during the daytime. Thus in the case of A about 90 per cent, 
(on an average) of the total evaporation for the whole twenty-four hours 
took place during the twelve hours between 8 a.m. and 8 p.m. The corre- 
sponding averages for B and C were 92 per cent, and 76 per cent, respec- 
tively. Further, the day hours themselves are not alike in evaporating 
power. On favourable days evaporation was usually most rapid between 
the hours of 10 a.m. and 4 p.m. 2 Thus, again taking averages, it was found 
that the following percentages of the total evaporation (for twenty-four hours) 
were recorded for the six hours between 10 a.m. and 4 p.m. : — A, 55 per 
cent. ; B, 58 per cent. ; C, 50 per cent. (cf. evaporation curves in Text-figs. 
5, 7 and 8). 
1 Yapp (’08), p. 68. 
2 Houdaille (’92) found that at Montpellier the maximum rate of evaporation occurs between 
2 and 3 p.m. 
