284 Yapp.— On Stratification in the Vegetation of a 
Houdaille 1 also refers to the injurious effect of high evaporation on the 
development of vegetation in the south of France. 
Meteorologists seem to have been much impressed by the difficulties of 
obtaining reliable comparative records, and of devising satisfactory evapori- 
meters. According to Hann 2 the Wild evaporimeter is the best. 3 In 
most evaporimeters used up to the present time evaporation is allowed to 
take place from a free, open surface of water. 4 The instrument is either 
sheltered from rain (and therefore, of course, from sun and wind), or else 
a rain gauge is used to indicate the amount of replenishing which occurs 
naturally. But the results obtained have varied greatly, according to the 
size of the vessel used, the degree of exposure, the depth of the water, the 
distance of the water-level below the edge of the vessel, and so on. 
British meteorologists, who aim at measuring the natural loss, by 
evaporation, from large surfaces of water, have now adopted a standard 
evaporation tank. This is 6 feet square, and 1 ft. 6 in. deep, with a rim 
rising 3 inches above the ground. 5 They regard the results obtained from 
small exposed evaporimeters as quite worthless, owing to the greatly 
increased rate of evaporation induced by the heating of the water in direct 
sunshine. 6 
But it must be strongly emphasized that the object of the ecologist is 
a totally different one from that of the meteorologist. The former requires 
an instrument which can be placed under practically the same conditions as 
those which the plant itself is called upon to endure. For this purpose 
a small evaporimeter is not only unobjectionable, but in all respects 
desirable. 
As there were obvious objections to the use of any small evaporimeter 
with an open water surface, I had already, in 1906, conducted some prelimi- 
nary experiments with the view of devising a suitable instrument, when Dr. 
F. F. Blackman called my attention to the then recent paper of Livingston. 
The instrument described by this author 7 seemed distinctly superior, for 
biological purposes, to anything previously tried, and its main principle was 
1 Houdaille (’92), pp. 59 et seq. 2 Hann (’03), p. 73. 
3 This is a small instrument in which the loss of water is determined by weighing. It is 
described in Wild (’74), pp. 440-5, and a much more elaborate self-recording instrument in Wild (’90). 
4 Many forms of open-water gauge are described in Symons’s British Rainfall, 1869-70. In the 
former year Leslie’s atmometer is also described, pp. 169-71. This instrument has an evaporating 
surface of porous earthenware, but the principle on which it was constructed is quite different from that 
of the evaporimeter about to be described. In France the Piche gauge, and an improved form of 
this devised by Houdaille (’90, pp. 31 et seq.) have been much used. In both of these the evaporating 
surface consists of a circular disk of blotting paper placed in a horizontal position. 
5 Mill (’06), p. 35. 
6 Field and Symons (’69), p. 153 ; also Mill (’07), p. 47. On the other hand, the Wild evapori- 
meter, which has been widely used in Russia, the Piche gauge, which has been used at most French 
meteorological stations, as well as Houdaille’s evaporimeter, are all small instruments, and contain 
no large body of water. 
7 Livingston (’06), pp. 24 et seq., and Fig. 4, p> 26. 
