Oct. I 



Automatic Transpiration Scale 



121 



Fig. 6. — Schematic diagram of Blackman and Paine's recording tran- 

 spirometer, in which water is automatically added to the pot to offset 

 the transpiration loss, so that the moisture content of the soil is kept 

 uniform. 



tained in a floating cylinder a, which rises as the load decreases and 

 traces its movement on the smoked drum of a clock. Like other appa- 

 ratus involving the 

 principle of flotation, 

 this apparatus is sub- 

 ject to errors arising 

 from changes in buoy- 

 ancy accompanying 

 changes in tempera- 

 ture. 



A transpiration bal- 

 ance devised by Rich- 

 ard Freres (Burger- 

 stein, 1904, p. 8-9) is 

 illustrated in figure 8. 

 The balance is made 

 very insensitive by a 

 heavy bob. The 

 movement of the bal- 

 ance pan from the "down" to the "up" position corresponds to a known 

 loss in weight, depending on the weight and position of the bob. The 



movement of the beam is recorded 

 /r\\ directly on the drum of a clock. 



Copeland (1898) has described an 

 apparatus (fig. 9) for recording tran- 

 spiration in which the weight of the 

 plant is balanced over a pulley by 

 the weight of a partially submerged 

 hydrometer bulb. The pulley shaft 

 rolls on plate-glass supports to re- 

 duce the friction. A tracer sup- 

 ported from a second wheel records 

 the motion on smoked paper on a 

 clock cylinder. With its maximum 

 load (3.5 kgm.) the instrument re- 

 sponds to a change in weight of 

 0.05 gm. 



Corbett (1900) has used a large 

 Nicholson hydrometer for measur- 

 ing transpiration, the plant being 

 placed directly on the pan a of the 

 hydrometer h (fig. 10). The appa- 

 ratus is made self-recording by con- 

 necting the float with the lever of 

 an auxanometer. This apparatus, like that of Copeland, is affected by 

 temperature, which changes the density of the water and consequently 



Fig. 7. — Krutizky's potometer for recording tran- 

 spiration, in which the loss from the potometer is 

 continuously replaced from the supply in the 

 floating cylinder. 



