ATMOMETRY AND THE ATMOMETER 53 



danger of removal of water by animals, especially small birds, 

 which can rest on the edge of the tank while drinking. More than 

 that, small animals, especially insects, fall into the water and 

 often remain floating, thus seriously altering the surface. To 

 avoid such disturbance wire screens must be introduced. These 

 modify the wind action seriously and thus complicate the instru- 

 ment, for all instruments with screens must have similar screens. 

 It is scarcely practicable to screen tanks from small insects, the 

 wind effect is so greatly modified by fine-meshed wire cloth. 



In dealing with the evaporating power of the air as an en- 

 vironmental condition for organisms, the free, water surface is 

 practically useless beyond a rough approximation; it is next to 

 impossible to give the evaporating surface any but a horizontal 

 position (barring wave motion) and it cannot conveniently have 

 any other form than that of a plane. Since it is essential, in 

 such environmental studies, to give to the instrumental evaporat- 

 ing surface approximately the same exposure as that presented 

 by the animal or plant surfaces to be dealt with, it is clear that 

 only water-imbibing surfaces (e.g., paper, porous clay) can thus 

 be conveniently employed. 



Finally, even fair accuracy of reading for short periods has 

 never been possible with free surfaces; only small pans can be 

 carefully weighed, the instrument must be protected from wind 

 during the process of weighing, which occupies considerable time, 

 and the errors thus introduced become relatively very large when 

 periods of minutes or hours are employed. 



The Piche Atmometer {Fig. 1) 



The Piche instrument, 5 having a disk of saturated blotting 

 paper held horizontally against the lower (otherwise open) end 

 of a graduated glass tube, which is closed above, possesses a num- 

 ber of advantages over the free water surface. The evaporating 



5 Piche, A., Note sur l'atmismometre, instrument destinee a mesurer l'cvapo- 

 ration. Bull. Assoc. Sci. France 10:166-167, 1872. In this connection and in 

 that of the Piche-Cantoni atmometer, see: Livingston, B. E., Paper atmometers 

 for studies in evaporation. Plant World 14: 281-289, 1911. Diagrams are there 

 presented, and are reproduced in the present paper. 



