Journal of Agricultural Research 



Vol. V, No. 3 



Fig. 4. — Woods' adaptation of Marvin's weighing rain gage as a trans- 

 piration balance. In this apparatus the loss through transpiration 

 is counterbalanced by a weight controlled by a screw. 



funnel and collected as waste water. When the plant through transpira- 

 tion causes the balance beam to tip sufficiently to close an electric circuit, 

 the funnel F is withdrawn by the solenoid A, and the water drops fall 

 directly into a receiving tube leading to the soil in the pot. Water is thus 



added directly to the 

 l|!l|j|Tgj||^ pot until the balance 



'' tips sufficiently in the 



opposite direction to 

 close a circuit through 

 a second solenoid B, 

 which rest ores the 

 funnel to its inter- 

 cepting position. 

 The time at which 

 the circuit is closed is 

 electrically recorded 

 on a clock drum. 

 The position of the 

 contacts is adjusta- 

 ble, so that the 

 quantity of water 

 added each time — ^i. e., the size of the steps — may be modified to suit the 

 transpiration rate. This method is unique and advantageous in main- 

 taining the moisture content of the soil constant throughout the experi- 

 ment. Under freely 

 exposed conditions, 

 however, the quantity 

 of water added each 

 time would be variable 

 and indeterminate, due 

 to the oscillation of the 

 balance by the wind. 



TRANSPIRATION BAL- 

 ANCES OF THE CON- 

 TINUOUS-RECORD 

 TYPE 



The first continu- 

 ously recording trans- 

 piration apparatus ap- 

 pears to have been de- 

 vised by Krutizky 

 (1878). It is of interest to note that the first step-by-step recording ap- 

 paratus was described by Vesque in the same year. Krutizky's appa- 

 ratus is shown in figure 7. The water lost through transpiration from a 

 potometer is continuously replaced through a siphon from a supply con- 



FiG. 5.— The Marvin register used by Woods for recording trans- 

 piration. 



