156 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



It is unnecessary to calibrate these instruments, because of* the fre- 

 quent changing of the filter papers and the fact that papers of one 

 grade may be quite uniform in their capillary properties. Beyond 

 tin-, the evaporation rate is somewhat controlled by the adjustment 

 and the degree to which the paper is wetted. 



The Piche evaporimeter is not now considered so desirable an in- 

 strument as some of the other types. Though it is simple and fairly 

 easy to operate under most circumstances, it is fragile and hardly 

 suited to severe weather conditions; the adjustment of the feeding for 

 changes in the weather is always vexatious and sometimes beyond 

 one's power; a correction for rainfall is out of the question: and the 

 technical point may be raised that the moist surface is somewhat too 

 freelv exposed to wind, while the white filter paper absorbs only a 

 -mall proportion of incident radiation. The conditions for evapora- 

 tion are therefore very different from those within the leaf. 



Porous-cup atmomeier. 



The name "atmometer," while describing any instrument for the 

 measurement of evaporation from a moist surface, is usually asso- 

 ciated with evaporimeters of the porous cup type. A very satisfac- 

 tory field instrument of this type has been described in several 

 papers by Livingston (159), and may be obtained on the market 

 either with or without standardization. Only standardized instru- 

 ments should be used in comparative studies. The instrument con- 

 sists of a closed cup with porous walls, into which the water is fed 

 at any desired pressure by regulating the beight of the reservoir. 

 The reservoir, connected with the cup by rubber tubing, may be a 

 flask of any size or a graduated tube. In the former case, the 

 amount of evaporation in any specified period may not be deter- 

 mined directly, but rather by measuring the amount of water re- 

 quired to fill the flask to its original level. This feature is incon- 

 venient, but the use of a large flask so increases the possibilities of 

 the instrument for long-period observations that it is. in this re- 

 ject, far superior to the Piche evaporimeter. 



The moisture of the cup reaches the outer surface through the 

 porous walls, its rate of movement being determined by the internal 

 pressure and also by the difference in capillary tension caused by 

 the loss at the outer ends of the pores. Presumablv capillary move- 

 ment is sufficiently rapid to maintain the supply at the outer sur- 

 face at any reasonable rate of evaporation. Yet there must be a 

 limit to this capillary action in both directions, and for this rea- 

 son the movement must, under extreme conditions, be governed 



7 H 



