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



of the under side of a leaf. All vapor formed must escape through 

 these perforations. The disk is firmly attached to, and flush with, 

 the upper end of tin 1 stem. 



(her the wick is a cover only slightly larger than the disk, whose 

 flanged edge extends down over the edge of the disk. This is 

 held down by two screws, which engage the flanged edge. The 

 cover is flat, but seamless, and completely excludes rain or snow. It- 

 surface is finished with nickel and an instrument having this polished 

 surface absorbs practically no radiant energy and is called a " shade' 

 instrument. To obtain the effects of radiation, an instrument whose 

 cover has been coated dead-black is used. The "shade" instrument 

 is dow entirely abandoned, since it has been seen that the difference 

 between the two is not a measure of sunlight intensity, but a measure 

 of the additional effect of sunlight in producing evaporation. This 

 effect can not under an}' circumstances be ignored in ecological 

 studies. 



The tests which have been made show that the looses from the 

 blackened instrument of this type follow more closely those from 

 potted trees, under a great variety of atmospheric and solar condi- 

 tions, than do the losses from any other type of instrument at 

 present available. The instrument has also shown itself remarkably 

 free from annoying characters, and responsive t<> all degrees of 

 evaporation stress. It may be considered, however, something of a 

 disadvantage that the amount evaporated i- relatively small. The 

 losses for short periods may, therefore, only he determined by very 

 precise weighing. 



OBSERVATIONS. 



With the Forest Service evaporimeter the daily observations con- 

 sist in weighing the instrument, usually on an inexpensive balance 

 such as the Harvard trip scale. Refilling is undertaken as -ft en as 

 necessary to maintain between 100 and 200 cubic centimeters of water 

 in the tank. The "closing" weight and "refilled" weight are en- 

 tered on the field observation form, together with the number of 

 the instrument. Computations of losses are usually made when 

 entering the data on Form 8, and the correction in accordance with 

 the calibration of the instrument is usually applied only to the total 

 losses for 10-day periods. 



With porous cups the daily or periodic observations will usually 

 ist m an entry of the amount of water required to fill the reser- 

 voir to datum level. A graduate is taken into tin- field to measure 

 this amount. 



In the case of the Piche evaporimeter, losses are calculated from 

 the readings of the graduated reservoir. Note should always be 

 made of observed overflowing or drying-up of the filter paper or 



