518 



4 



Cannon : A method of measuring transpiration 



ature of the dew-point may be approximately determined. The 

 maximum vapor-pressure for any temperature is written on the 

 thermometer, which unfortunately is graduated according to the 

 Fahrenheit scale. This instrument provides a means of telling 



Figure 2. CoviUea iridenlata^ showing base made of cement which was made 

 water-proof by being covered by a mixture of vaseline and was. 



directly, therefore, the temperature of the air, the maximum vapor- 

 pressure, the relative humidity, and the dew-point. From what 

 has been said regarding the polymeter it will appear at once that 

 it can be used to demonstrate the fact of transpiration, and if the 

 cubic contents of the bell-glass are known, from data given by it 

 the amount of transpiration can also be estimated. 



