:28 



Journal of Agricultural Research 



Vol. V. No. 3 



circuit is controlled by the relay contact. The recorder is operated by 

 a third circuit, B3, controlled by the tipping bucket on the ball valve. 

 Each circuit is closed only momentarily, and the dry cells usually need 

 to be renewed but once during the summer. 



AUTOGRAPHIC RECORDS FROM THE AUTOMATIC TRANSPIRATION 



SCALE 



The results of our transpiration measurements will be presented in 

 other papers, but it seems desirable to reproduce here several daily rec- 

 ords illustrating the actual performance of the apparatus. A word of 



explanation in con- 

 nection with the in- 

 terpretation of the 

 records may be help- 

 ful. The clock drum 

 makes four revolu- 

 tions during the day, 

 so that the record is 

 divided into four 6- 

 h o u r periods. The 

 pen is offset at the 

 moment each ball is 

 delivered. There are 

 five such offsets or 

 steps in one direction 

 (up, for instance) and 

 then five steps in the 

 opposite direction. 

 Since each offset cor- 

 responds to a loss of 

 20 gm. of water, the 

 interval from the 

 crest to the trough of 

 the graph is the time 

 required for the transpiration of 100 gm. of water, or from crest to crest, 

 the time interval for 200 gm. loss. 



The wheat records shown in figure 16 were taken from a series ob- 

 tained in 1 91 2 inside the screened inclosure used in the water-requirement 

 experiments. The normal wind velocity was reduced somewhat by the 

 inclosure and by the proximity of other plants. The first record repro- 

 duced (July 2, 1 91 2) was obtained on a clear day. It will be noted that 

 the time interval shortens as midday is approached — that is, the tran- 

 spiration rate increases and attains its maximum value about 3 p. m., 

 after which it falls rapidly. The transpiration at night, represented by 



Fig. 15. — Another view of the spring motor, showing the control mech- 

 anism. When the magnet M is energized, the spring R attached to 

 the armature T is pulled down, releasing the motor. Raising the 

 beam de-energizes M, so that the motor, after making one revolution, 

 is stopped by H again coming in contact with R. 



