Marsh , and its Relations to Evaporation , < 3 v. 289 
The vegetation was disturbed as little as possible when taking the 
readings, but it was judged advisable to place the instrument-stand in a 
fresh, though similar, position, for each series of observations. 
For the first and second series the instruments were disposed of as 
follows : — 
(1) Set A, 1 4ft. bin. above the soil-level. This was just above the 
general vegetation level ; and on about the same level as the stray projecting 
shoots of Phragmites , Cladium^ &c. The instruments were exposed to both 
sun and wind. 
(2) Set B, 2 ft. 2 in. above the soil, i. e. at about the middle of the 
vegetation. In this position the instruments were lightly shaded, and 
sheltered from wind. 
(3) Set C, five inches above the soil, i. e. at the bottom of the vegetation. 
Here the instruments were almost completely in shade, and quite sheltered 
from wind. 
(4) Set D (thermometers only), six inches below the surface of 
the soil. 
For the third and fourth series of observations set A was placed at 
a height of 5 ft 6 in., i. e. quite clear of the vegetation. The positions of sets 
B and C remained practically unchanged (see Text-fig. 4). 
It seemed desirable to know not only the aggregate differences of 
evaporation in the various positions, but also something of the extremes 
which might occur. Readings were therefore made at first twice a day, at 
about 10 a.m. and 6 p.m. The c day ’ period of eight hours would of course 
include the hours of maximum evaporation. Subsequently an additional 
reading was made at 4 p.m., as the rate of evaporation was often observed 
to fall rapidly after about that time. On certain days readings were taken 
at more frequent intervals. It is unnecessary to give all the actual readings 
in detail. Table I gives for a number of days (twenty-four hours) the total 
evaporation from each instrument, and also various other meteorological data. 
The records of relative humidity, sunshine, and rainfall were made at the 
Cambridge Botanic Garden, distant some eleven miles from Wicken Fen. 
These probably represent fairly nearly the conditions at Wicken on the 
corresponding days. I am indebted to Mr. R. I. Lynch for kindly furnishing 
me with these Cambridge readings. 
The effect of the various meteorological factors on evaporation can be 
seen to some extent from the following table ; but this question will be 
more fully discussed after the 1908 results have been given. 
1 Each set (A, B, and C) included one evaporimeter and a pair of thermometers. The 
measurements refer in each case to the height above the soil, of the middle point of the porous 
cylinder of the evaporimeter, and of the point midway between the two thermometers. In the 
second series the height of set A was actually 4 ft. 8 in., but the relative position was the same, the 
vegetation being slightly taller. 
