158 BULLETIN 105&, IT. s. DEPARTMENT OF' 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 

 Shive. 



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

 a paraffined 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 milli- 

 meter inside diameter. Each is bent into a U and continued upward as A' 

 and B'. One (A') extends through the paraffined cork 

 CB') extends upward 6 centimeters to 8 centimeters 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' is expanded into a similar bulb 1 centimeter to 2 

 centimeters 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 under- 

 surface of the stopper in filling the reservoir, to the exterior. 

 The tube extends 5 centimeters 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', B, and E have been properly fitted, 

 is tightly pressed into the mouth of the reservoir F. 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 centi- 

 meters 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', and gentle 

 suction is applied. Water rises from the reservoir into 

 the tube B', 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 

 cap is filled, water passes into the tube A A', tbe mercury 

 in this tube having been drawn into the bulb D, 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 is placed over the end of this tube. A suitable vial is also placed 

 over the end of the tube A' 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 suc- 

 tion is applied to the tube A', as in installing. 



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

 A and falls in tube A', coming to rest with the mercury level in A, slightly 



B 



Fig. 4. — Shive's 

 n o n a b sorbent 

 porous cup at- 

 mometer. 



