158 BULLETIN 1059, U. S. DEPARTMENT OE AGRICULTURE. 
instrument is shown in the following detailed diagram (fig. -1) . 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 
(B') extends upward 6 centimeters to S 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 nasses 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 i>, 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 r'ses in tube 
A and falls in tube A', coming to rest with the mercury level in A. slightly 
B' 
Fig. 4. — Shire's 
n on absorbent 
porous cup at- 
mometer. 
