BRIEFER ARTICLES 
AN ATMOGRAPH 
(WITH FOUR FIGURES) 
The introduction of the porous cup atmometer by Livincston has 
been followed by great activity in securing evaporation data in relation 
to plant life. Readings have usually been made with reference to 4 
zero point, usually a file-scratch, on the wall of the reservoir containing 
the water. For most workers it has been impossible to visit the field 
more frequently than once a week. The’records secured were of course 
simply the total weekly evaporation. 
It is becoming increasingly clear that the record of total weekly 
evaporation can be evaluated only when we know the rate of evaporation 
at all times, and particularly when we know the maximum and minimum 
rate. This need can be met only by some form of automatic measuring 
and recording device. No such instrument has yet come into general 
use. To meet the conditions of the work such an instrument must 
measure in convenient units and record in graphic form. It must be 
so simple that it may be set up easily in the field and trusted to work 
with only occasional attention. Also the cost of construction must not 
be so great as to prohibit the employment of a sufficient number of shes 
to secure comparative data. In attempting to meet these conditions 
as I have found them in the field, the instrument described herewith 
has been designed (fig. 1). 
The device (fig. 2) consists essentially of an oscillating beam (A), 
which may be caused to tilt by the movement of mercury in . tube 
* (B) and its connected bulbs, advantage being taken of the oscillation 
to open the stopcock (C) and to close it again. At the same time oF 
electric circuit is closed which actuates the pen of the chronograph. 
In practice the whole apparatus is filled with water and the stopcock 
closed. Mercury is poured into the bulb 0’, nearly filling it, the inet . 
is connected with the supply tank of water, and the outlet at d with 
the atmometer. The water which evaporates from the atmometer . 
replaced by a flow through the pipes d, c, and a from the supply ~ 
bulb 6’. The mercury in 6 follows the water, partially filling by. 
Whenever the transfer of weight from b to 6’ has gone far enough 
to shift the center of gravity to the right of the center of suspen 
Botanical Gazette, vol. 50] . ee 
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