1899] MPPrLIANCES FOR PHYSIOLOGY 267 
whose stem is about the diameter of the inside of the tube into 
which it slips a little. The attachment may be made by putting 
mthe stump close to the glass a short piece of rubber tubing 
about as thick.as the glass of the tube; a 
joint of larger rubber tubing is slipped 
over the glass and the smaller piece, and 
s then wound tightly with tire tape which 
will not yield to pressure from within. 
Such a joint will be pressure tight. A 
litle water is poured into the tube with a 
pipette and brought to some zero mark. 
The stop-cock is then closed. As the 
"ater tises in the tube it will compress 
the air column, and the approximate 
‘mount of pressure exerted may be 
measured by Mariotte’s law, 7. e., under 
“onstant conditions the pressure is in- 
Versely Proportional to the volume of the 
air, 
should 
f course the successive readings 
he oe taken as nearly as possible at 
€mperature, 
the tube is a source’ 
: ‘ppreciable one if the readings are 
‘ken at constant temperatures. 
All Manometer gages used for such 
pur OSES ¢ : 
é as this are liable to an error not 
“Ways reco 
Pantity 
The water vapor 
of error, but not 
Pressure 
K is al y 
Fic, 7.—Root-pressure 
gage. 
If the plant be small 
€, the plant may not 
ough to push the registering fluid up to the 
t should reach in order to register the true pressure 
the water actually is given off. Hence these gages 
tegister pressure more accurately the smaller they 
fd é 
» €t which 
ate likely - 
ate, 
s 
SMITH C 
OL 
LEGE, NortHampron, Mass. 
