287 
Marsh , and its Relations to Evaporation , drV. 
side of the stand, the point midway between a pair of thermometers being 
at the same level as the centre of the porous cylinder of the corresponding 
evaporimeter (Text-figs. 4 and 6). The thermometers were always read at 
the same times as the evaporimeters. Soil temperatures were also taken. 
The thermometers were fully exposed and not shaded as is usual in meteoro- 
logical observations. On account of the impossibility of comparing the 
actual readings obtained, when thermometers by different makers are 
employed, meteorologists regard ‘ temperatures in the sun ’ as quite unreli- 
able, at all events unless the black bulb thermometer in vacuo is used. 1 
But here again the objects of the biologist and the meteorologist are 
different. Plants are in nature exposed to the effects of direct insolation and 
radiation, both of which are practically cut off if a screen is used. So that 
whether the object is to trace the connexion between temperature and the 
loss of water from an exposed evaporimeter, or to ascertain the relative 
temperatures in different layers of the vegetation, it is as well to use exposed 
thermometers. 2 So long as a set of quite similar thermometers is employed 
for any given experiments, the relative results obtained will be comparable ; 
the absolute temperatures are of little importance. 3 
Experiments in the Field. 
Field observations on evaporation and temperature were carried on 
during the summers of 1907 and 1908. The 1907 experiments were as 
follows :~— 
1st series July 9th to 1 5th. 4 
2nd „ July 18th to 21st. 
3rd „ August 13th to 1 6th. 
4th „ September 3rd to 8th. 
During 1908 continuous observations were made from July 30th to 
August 9th. 
( a ) Evaporation in 1907. 
During this year the experiments were confined to the so-called 
£ Sedge ’ vegetation. 5 In this Cladium Mariscus is dominant, though it is 
everywhere mixed with other species. In the part selected the herbage 
had not been cut for about four years, and in consequence formed a dense 
growth, with the ‘ general vegetation level ’ about 4 ft. to 4 ft. 6 in. above the 
soil (see PI. I, Fig. 1, and Text-fig. 4), 
1 Hann (’03), p. 39 . 
2 Clements (’05, p. 66 ) says that he uses unsheltered thermometers for ecological work. 
3 The maximum were mercury, and the minimum alcohol thermometers. They were supplied 
by Messrs. Gallenkamp & Co. of Finsbury Square, London. 
4 Of all the months of the year, evaporation is usually greatest in July. June and August are 
also, as a rule, months of high evaporation ; cf. Mill (’08), p. 47 . 
5 Cf. Yapp (’08), p. 68 . 
