1912] GLEASON & GATES—RATES OF EVAPORATION 479 
Province, but the climatic dominance of the associations of the 
Deciduous Forest Province is now gradually asserting itself 
wherever conditions are not interfered with by man. The climax 
type of vegetation for this part of Illinois, the Acer saccharum 
association, does not occur in the immediate vicinity, although it 
is present on clay and loess bluffs on the opposite side of the river. 
The principal object of the investigation was the determination 
of the relative amounts of evaporation in certain well marked 
associations whose successional relations were clearly evident, in 
order to correlate the phenomena of succession and evaporation. 
A minor object was the comparison of evaporation in two areas 
with the same dominant vegetation, but with different secondary 
species. The results obtained were even more striking than had 
been anticipated. 
The atmometers used were porous clay tubes, of the model 
recommended by TRANSEAU, about 30 cm. long and 2 cm. in 
diameter. The tubes were inserted into bottles through holes in 
the corks, and were then sealed with red sealing wax to prevent 
the entrance of rainwater and the loss of water by evaporation. A 
small $-shaped groove on the side of the cork permitted the equaliza- 
tion of the atmospheric pressure within the bottle. Reading the 
instruments immediately before and after heavy rains indicated that 
the arrangement was water-tight. ‘The instruments were filled with 
distilled water, and to each 250 cc. was added one drop of formalin 
to prevent the growth of organisms. According to LivincsTon® 
the addition of formalin is not recommended in studies of absolute 
values of evaporation. Since this study was concerned with rela- 
tive values only, and since all the instruments were treated alike, 
the slight effect of formalin is without significance. The instru- 
ments were set up and allowed to run for three weeks before the 
field records were taken, to insure the thorough saturation of the 
clay tubes. Each bottle was marked just below the cork, and was 
refilled by pouring in water from a graduate up to the mark. The 
readings were estimated to be correct within 0.5 cc. All the in- 
struments were exposed together in the open air for 93 hours for 
standardization. Eleven instruments were then exposed in the 
association selected, while a twelfth, arbitrarily chosen as the stand- 
3 Plant World 13:118. 1910. 
