186 
IOWA ACADEMY OP SCIENCE 
air; thus the amount of evaporation is increased especially during the night 
time. 
In order to measure the evaporation from the surface of streams and lakes, 
pans are floated in them and the evaporation from these pans is measured. In 
order that comparisons might be made upon the relative rates of evaporation 
from pans similar in every respect except that one is surrounded by land and 
the other by water, two pans were installed. 
One was floated upon the river and the other imbedded in the ground upon 
the bank of the river; the pans were three feet square and eighteen inches 
deep, filled with water to within about one and one-half inches of the top. A 
sharp point set in the middle of the pan is just kept covered; the amount of 
evaporation being measured by the number of cups of water required at time 
of observation to just cover this point. 
By using a rain-gage to allow for rainfall it is possible to measure with 
accuracy to the one-hundredth of an inch the amount of evaporation. 
The surfaces of the water in the pans were adjusted morning and evening at 
8 A. M. and 6 P. M.; the temperature of the water being taken both in the pan 
and in the river outisde. By means of a sling psychrometer the relative humid- 
ity was determined at seven o’clock both morning and evening. An estimate 
was also made of the maximum velocity of the wind in the course of the 
twenty-four hours. 
The amount of data collected, to the present time, is not sufficient to war- 
rant more than the noting of certain facts; one can hardly say conclusions, for 
conclusions resting on insufficient statistical evidence becomes nothing but 
weighted guesses. To note briefly some of the results obtained; The total 
amount of evaporation, from the river pan during the summer of 1906, was in 
excess of that from the land pan in the ratio of 100 to 90; during the six 
months from April 1st to October 31st, 1907, while reading were made, the 
evaporation from the land pan exceeded that from the river in the ratio of 
100 to 92, and the month of April, 1908, just past, has shown an evaporation 
from the land pan one-fourth greater than from the river pan. While at first 
thought it might be expected that the evaporation from the land pan would be 
the greater, it should be borne in mind that the river pan, especially in the late 
summer and fall, has a temperature markedly above that of the land, and for 
this reason the rate tends to rise higher for the river pan at this time of the 
year. It is the reader’s opinion that on the whole the evaporation from the 
river pan will be found to be above that from the land pan. This is of course 
for a pan upon a stream as small as the Iowa river. 
The following table gives the evaporation from the two pans for the six 
months of 1907, named above and also April, 1908. 
Relative amounts for 
Land pan 
River pan 
night or day 
1907 
Inches 
Inches 
Land 
River 
April 
2.95 
2.14 
57% 
56% 
May 
3.38 
3.18 
61 
80 
June 
3.03 
2.60 
66 
74 
July 
4.75 
66 
68 
August 
4.73 
4.34 
90 
86 
September 
2.98 
71 
75 
October 
1.91 
1908 
April 
2.40 
51 
71 
