250 
THIRTEENTH REPORT. 
MEASURING THE TRANSPIRATION OF EMERSED 
PLANTS. 
WATER 
CHAS. H. OTIS. 
The evaporation taking place from free water surfaces lias been the 
subject of much experimentation during- the past century and the laws 
governing this phenomenon have been quite definitely stated. But, al- 
though the matter is of great economic importance in regions of small 
rainfall and scant water supply, no one has yet determined what is the 
effect of emersed water plants on the evaporation from a water sur- 
face. During the summer of 1010 the writer carried on some experi- 
ments at Portage Lake (Washtenaw Co., Mich.) to determine what 
relations, if any, exist between the evaporation from a free water sur- 
face and the evaporation from a water surface occupied by certain 
plants growing in the water, but extending above the surface to a 
greater or less extent. 
The apparatus used consisted essentially in a battery of nine large 
metal tanks supported in the water by two pontoons, each of these 
tanks being provided with a device for measuring; the evaporation from 
the tank. A recording thermometer was maintained within a few feet 
of the battery, while a barometer, a psvclirometer, thermometers for 
taking water temperature, and graduates for measuring the amount 
of water evaporated were employed. 
The tanks were constructed especially for this work and were 30 
inches in diameter by 32 inches in depth, inside measurements (Fig. 
1). The cylinder and bottom were of heavy galvanized iron riveted 
and soldered. A heavy iron band encircled the rim, serving the double 
purpose of preserving the circular shape of the rim and of a support for 
the tanks. Lighter iron bands were placed at intervals along the height 
of the tank, to which were riveted internal iron braces, one of which 
may be seen at a. Fig. 1. The bottom of the tank was raised 4 inches 
from the edge, allowing two by fours to be fitted and spiked to the 
sides as a support for the bottom (Fig. 2). Holes were drilled in the 
uppermost band, allowing the tank to be supported by spikes driven 
into the cross pieces of the pontoons. The object was to obtain a tank 
so solid that it would not become deformed by the tilting caused by 
wave action, since any deformation of the tank while in position would 
cause a corresponding change in its volume. 
The success of this apparatus was due in large part to a unique 
measuring device which was an integral part of the tank. A hollow 
brass tube extended through the center of the tank. The lower end of 
this tube was screwed into a socket which was riveted and soldered 
to the bottom of the tank in a central position, while its upper end 
terminated in a heavy brass socket into which fitted a solid brass rod 
which was held in place by a set screw in the side of the socket. This 
rod was pointed at its upper end, and, being turned up in a lathe. 
