Automatic Transpiration Scale 



119 



that by offsetting the cyUnder daily a weekly record could be obtained 

 on one sheet. 



Transeau (191 1), in working with xerophytes, employed hollow brass 

 balls standardized to 0.4 gin. in place of X-irich steel balls of i gm. weight, 

 but states that the hollow balls are not as light as could be desired. The 

 writers have found that >^-inch steel balls weighing 0.13 gm. can be 

 readily used, provided the valve ^ is constructed to fit them. 



Woods (1895) used the automatic weighing rain gage of Marvin (1903) 

 as a transpiration balance, the apparatus being modified to record loss 

 instead of gain in weight (fig. 4). In this apparatus the counterpoise is 



Fig. 3.— Ganong's automatic transpirometer in which steel balls are employed 

 as weights. 



moved along the beam in -^-gm. steps by a screw actuated by an electro- 

 magnet carried on the balance itself. The recorder (fig. 5) is independent 

 of the balance. 



Blackman and Paine (1914) have recently described a recording 

 transpirometer operating on the step-by-step principle, in which "water 

 drops are used in place of steel balls, the water being added directly to 

 the soil." Their apparatus has been represented schematically in figure 6. 

 Water is allowed to drip continuously from a Mariotte system. During 

 the greater part of the time the drops are intercepted by a movable 



For description of valve, see imder " Ball-dropping device," p. 



5772° 



