158 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 



instrument is shown in the following detailed diagram (fig. 4). The 

 description of the self-contained instrument, which is particularly 

 adapted to general field work, is quoted from that prepared by 

 Strive. 



From the reservoir (F) two glass tubes (A and B) extend upward through a par- 

 affined cork stopper, and then through a two-perforated rubber stopper into the porous 

 cup, one passing to the tip of the cup, the other just to the upper surface of the rubber 

 stopper. These tubes are of small bore, about 0.8 millimeter inside diameter. Each 

 is bent into a U and continued upward as A 7 and B 7 . One (A 7 ) extends through the 



paraffined cork (B 7 ) extends upward 6 centimeters to 8 centi- 

 meters and is stopper and ends about 5 centimeters above it. 

 The other again bent near the bottom of the reservoir. The tube 

 B is expanded into a small bulb C at its lower extremity, and the 

 tube A 7 is expanded into a similar bulb 1 centimeter to 2 centi- 

 meters from its lower end. The tube E is about 1.2 centimeters 

 in diameter, and forms a shallow, inverted funnel with the lower 

 surface of the paraffined cork stopper. This serves to conduct 

 air bubbles, which may catch on the undersurface of the stopper 

 in filling the reservoir, to the exterior. The tube extends 5 cen- 

 timeters above the cork stopper and is graduated to tenths of 

 cubic [centimeters. The zero point on the tube serves as a zero 

 point in filling the reservoir. 



To install the instrument the paraffined cork stopper, into 

 which the tubes A A 7 , B, and E have been properly lifted, is 

 tightly pressed into the mouth of the reservoir I". A sufficient 

 amount of clean mercury is allowed to fall from a pipette into the 

 openings in the upper end of each of the tubes A' and B to 

 form a column 5 centimeters to 6 centimeters high in tube A', 

 and slightly more than this in tube B. After the porous cup has 

 been placed in position and the reservoir filled with distilled 

 water, a rubber tube is attached to the free end of the filling 

 tube A 7 , and gentle suction is applied. Water rises from the 

 reservoir into the tube B 7 , at the same time that the mercury 

 in this tube is drawn into the bulb C, where water passes freely 

 and rises in the tube B, filling the porous cup. When the cup 

 is filled, water passes into the tube A A', the mercury in this 

 tube having been drawn into the bulb L), where the water is 

 allowed to pass freely and escape into the rubber tube, which is 

 then removed. The mercury in the bulbs C and D drops back 

 into the tubes below. To prevent water loss from the reservoir 

 by evaporation through the tube E and to prevent the entrance 

 of water through this tube from without in times of rain, a vial 

 i plac< ver the end of this tube. A suitable vial is also placed over the end of the 

 tube A 7 to exclude dirt. The instrument is now ready for operation. 



To replace the cup with a new one, it is only necessary to remove the old cup from 

 its support and to place the new cup into position, after which suction is applied 

 to the tube A 7 , as in installing. 



As water evaporates from the surface of the cup, the mercury rises in tube A and 

 falls in tube A 7 , coming to rest with the mercury level in A, slightly higher than in 



B 



1 



B' 



Fig. 4.— Shivc's non- 

 absorbent porous 

 cup atmometer. 



