ATMOMETRY AND THE ATMOMETER 111 



to do work in promoting the vaporization of water from the 

 atmometer surface. In so far as the surface exposed by the 

 instrument is like that exposed by plants, and in so far as the ex- 

 posure is the same in the two cases, the readings are indices of 

 the power of the air to remove water from unchanging plant 

 surfaces. From the work that has so far been done in this new 

 line of study, it appears that the porous cup instrument does, 

 indeed, rather closely approximate the exposure, etc., of plants 

 in general, so that the atmospheric evaporating powers to which 

 plants in different habitats or at different times are exposed can 

 be approximately measured and compared by this means. No 

 doubt special problems will require special forms of atmometer, 

 as has been indicated. 



(To be continued.) 



