ATMOMETRY AND THE ATMOMETER 55 



surface is here that of imbibed water, held in place by the cohe- 

 sion of the surface films themselves (surface tension) and by their 

 adhesion to the supporting paper fibers. Distilled water is sup- 

 plied to the disk from the tube and air enters at the center of the 

 disk, through a very small opening in the paper, as water is with- 

 drawn. Waves and splashing, considerable removal of water by 

 animals, and serious obstruction of the surface by the bodies of 

 the latter, are here not encountered. Also, the entire instrument 

 may be readily weighed or it may be read in volume units. 



The hydrostatic pressure upon the paper is rather irregularly 

 variable, however; the entrance of air through the pin-hole is 

 spasmodic, so that a considerable error is introduced into small 

 readings. Also, the paper disk is apt to be deformed by strong 

 wind, a feature comparable to that of ripples and waves on 

 free water. 



The horizontally placed disk, with its central obstruction to 

 free wind action, while furnishing an exposure to atmospheric 

 conditions much more suitable to biological needs than that 

 offered by the open pan, nevertheless leaves much to be desired. 

 Especially noteworthy is the fact that the Piche instrument must 

 always operate as a unit; it is practically impossible to place the 

 evaporating portion at a distance from the graduated reservoir, 

 an arrangement often desirable in physiological studies. 



Since all the water evaporated must pass longitudinally 

 through the paper disk, from the central point of supply to the 

 place of final vaporization, the size of the disk must be suited 

 to the atmospheric evaporating power to be dealt with. In a 

 region of low evaporation intensity the disk may be large, but 

 must be smaller (to prevent the edges becoming dry at times) 

 in an arid region. 6 



6 This phenomenon of dried disk edges has been described, in another con- 

 nection, in the following: Livingston, B. E., and Brown, W. H., Relation of the 

 daily march of transpiration to variations in the water content of foliage leaves. 

 Bot. Gaz. 53: 309-330, 1912. See page 313. 



