3H 
Yapp, — On Stratification in the Vegetation of a 
be reduced to a common standard. Using evaporimeter 1908, I, as the 
standard instrument, these factors were as follows : — - 
1907, I 
33 
33 
II 
III 
1908, I 
33 
33 - 
II 
III 
multiply by 1-32 
3, * !*39 
» » !* 2 7 
„ „ i*o (standard) 
», -965 
„ „ i-o (equivalent to 
standard). 
All the evaporation results given in the main paper have been corrected 
according to these factors. 
Porosity of the Earthenware Cylinders. 
As earthenware is practically a new (and apparently a highly satisfac- 
tory) material, so far as evaporation experiments are concerned, 1 it may be 
well to consider the relative porosity of fine and coarse-grained earthenware 
somewhat in detail. 
An important question is whether, when the rate of evaporation 
is high, water can percolate through the pores at a sufficient rate to keep 
pace with evaporation from the surface. Apparently in all the field experi- 
ments, the instruments were equal to the demands made upon them ; 
as the cylinders were always cool and moist to the touch, even when 
evaporation was greatest. One cylinder alone proved inadequate when 
evaporation was high, but as this was discovered during the calibration 
experiments, it did not affect the results obtained in the field. The excep- 
tion was 1907, II. When exposed on the laboratory roof this instrument 
exhibited a very marked incapacity to keep pace with the other two 
evaporimeters, if evaporation was great. The following actual readings will 
show the lag of this cylinder : — 
1907, 1. 
1907, II. 
1907, III. 
(a) 70-7 c.c. 
40-9 c.c. 
71*2 c.c, 
\b) 7<5-9 
2 9*7 » 
79*9 33 
C) 9 6> 4 „ 
3 1 * 2 » 
100.4 „ 
(d) 17-05 » 
x 5*5 ,3 
17*3 >3 
The periods for which these readings were recorded varied in length from 
twenty-one to twenty- eight hours. The first three were windy and sunny 
(during the daytime). During the fourth, however, the weather was dull 
and damp. Careful comparison showed that 1907, II, seemed to be incapable 
1 cf. references given in the main paper. 
