286 Yapp. — On Stratification in the Vegetation of a 
The top of tube (D) is bent to allow air to enter, but not rain. The 
rubber cork closing tube (C) is fitted with a bent glass tube for the same 
purpose. 
The open ends of the evaporating cylinder are closed by rubber corks. 
Through the upper cork passes a glass tube with a stop cock. 1 This facili- 
tates the preliminary filling of the cylinder, by suction, and would also serve 
to indicate the presence of any air bubbles which might find their way into 
the cylinder. A metal hood (E) prevents rain collecting on the upper part 
of the apparatus. Through the lower cork a brass tube passes to the bottom 
of the reservoir, the tube being pierced with holes just above its base. 
Water can thus pass readily from the reservoir to the cylinder, while at the 
same time no air can enter the latter, unless the supply of water in the 
reservoir is practically exhausted. 
The capacity of the reservoir is 200 c.c. It is made of copper, enamelled 
white outside to reflect as much heat as possible. 2 Its roof slopes upwards 
from all parts to the point of insertion of the filling tube (C) ; this facilitates 
the exit of air during filling. The reservoir is fixed to a massive base of 
lead, so that the evaporimeter is quite stable, even when exposed to strong 
winds. 3 
Such an instrument is portable, and can be readily used in the open in 
all weathers except during severe frosts. It will be seen later that the 
evaporimeter is, like the similar one of Livingston, 4 very sensitive to even 
slight changes in the atmospheric conditions. Being rainproof, it may be 
used during rain, wind, sunshine, &c., and, in short, exposed to precisely the 
same conditions as the plants themselves in nature. But while the evapori- 
meter can be subjected to the same external conditions as the plant, 
comparison between the two must not be carried too far. The evaporimeter 
lacks the controlling mechanisms and the osmotic cell-sap of the plant. It 
is, in fact, merely a physical instrument for recording the cumulative effect 
of the constant or frequent local changes in the relative humidity of the air. 
For use in the field, a light but rigid iron stand, fitted with wire guys, 
was made (PI. I, Fig. 2). The stand was provided with movable brackets, 
on which three evaporimeters were placed. Two of these were at different 
levels in the vegetation, and the third above it (Text-figs. 4 and 6). Maxi- 
mum and minimum thermometers (Centigrade) were fixed on the opposite 
employed a ‘ pint jar ’. He states that ‘ because of the large water-area in the jar, the error in these 
readings is estimated to be more or less 1 c.c/ 
1 A thermometer might with advantage be also inserted through the upper cork, with its bulb 
in the water. This has not been done hitherto, but it would be useful to know the temperature 
reached by the water under different conditions. 
2 For the effect of different surface colours on temperature, see Hann (’03), p. 42. 
3 The evaporimeters were made for me by Mr. Bellingham, of the Physics Department, University 
College of Wales, Aberystwyth. 
4 Livingston (’06), pp. 30-33. 
