312 Yapp. — On Stratification in the Vegetation of a 
The 1907 laboratory experiments may be taken first. The total 
evaporation, as obtained from a number of consecutive readings, was as 
follows : — 
Evaporimeter, 1907, 
33 » 
53 3 3 
I 
II 
III 
Crystallizing dish, 1907 
37*75 c.c. 
35-2 c.c. 
40-15 c.c. 
I 3*9 grams. 
The individual readings were not always precisely in the same ratios as 
the above totals ; but the maximum variation did not exceed 10 per cent, 
on either side of the mean. 
We may first compare the evaporation from the porous cylinders with 
that from the open surface of water in the dish. For this purpose 1907, III, 
may be taken as a standard. Then 
(1) Comparing the areas of the two evaporating surfaces, we have the 
ratio : — • 
T907, III _ 149-40 sq. cm. __ 2-57 
3907 cryst. dish 58-09 sq.cm. i-oo 
(2) From the above readings we find that the total evaporation for 
a given time, from the standard instrument and the crystallizing 
dish respectively, is in the ratio : — 
From 1907, III _ 40-15 _ 2-89 
From 1907 cryst. dish 13*90 i-oo 
(3) Therefore, the different rates of evaporation, per unit area, from the 
respective evaporating surfaces are according to the ratio : — 
Porous earthenware _ 2-89 __ 1-12 
Open water surface 2-57 i-oo 
For the 1908 experiments new porous cylinders, made of earthen- 
ware of a finer grain, were used. Leaving, for the moment, the comparison 
between the evaporimeters themselves; the 1908 calibration data, and the 
corresponding comparison between the evaporation from open water and 
earthenware, may be given. 
Taking combined readings as before, we have the following totals : — 
Evaporimeter, 1908, I 
33 
33 
33 
33 
II 
III 
5°-4 c.c. 
52-2 c.c. 
5045 c.c. 
Crystallizing dish, 1908 .... 15-51 grams. 
Here the maximum variation for individual readings was less than 
7 per cent, on either side of the mean. 
