168 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54 
proper; the two rear slides for the "compensator" manometer fo 
adjustable screens, etc. The four uprights, firmly screwed to a bas 
below and to a cap plate above, possess all the stability desirable, with I 
out any braces at all. The cap piece carries threaded holes, into which 
vertical or horizontal rods can be screwed, to give further length or 
height to the apparatus whenever necessary. In figure 43 this form oi 
stand is used to support the pneumatic apparatus in the viscosity 
measurements. 
Fig. 30 gives a more succinct account of the method of adjusting the 
parts of the air thermometer, the parts being drawn about one-sixth of 
actual size. 
The manometer proper, A B C D E F G, has the usual terminal glass 
tube A B about 2 cm or more internal diameter, which is joined to a length 
of about 2 meters of rubber hose, doubly wrapped by a steel three-way 
stop-cock, B. This carries a lateral tubulure, through which superfluous 
mercury in the tube A B may be tapped off, an operation often desirable 
when the constant-pressure method is used, in which case the tube A B 
may be expediently chosen wider. B may also be shut off to keep the 
mercury column at a fixed height. The metallic sleeve of the manom- I 
eter stand, which clutches the tube just below the faucet B, is provided j 
with a micrometer screw (thread 0.1 cm apart) by which the height of the i 
meniscus may be adjusted with nicety. 
The other end, i/, of the rubber tubing is joined to the lateral branch i 
of this glass barometer tube H E F G, held by the second slide of the i 
stand. This widens above between F and G to allow freer play of the 
upper meniscus. Inasmuch as it is frequently necessary to employ air* 
thermometers, glazed on the outside only, it is desirable to work so far 
as possible with pressures less than an atmosphere. The result is a ten- 
dency to force the viscous glaze into the pores of the porcelain. Satis- 
factory work with low pressures is possible only when the upright branch 
E F is impervious to air. Hence it is made of glass, like an ordinary; 
barometer tube, and all rubber connections and tubing here are dis- 
carded. It is convenient, however, to insert a glass stop-cock at F, (Fig. 
30), but not essential. A stop-cock is always an opportunity for leakage. 
The top G of the tube E G may either be drawn to a fine aperture, 
just large enough to admit the end of the capillary platinum tubej 
G I K M N 0, or it may be cut off straight and admit a rubber stopper 
sealed in with resinous cement, through which stopper the platinum tube 
passes. In either case this end of the platinum tube, sharpened to a' 
needle-like point below the mouth of the capillary canal, subserves the 
purpose of a fiducial mark excellently. In the constant-volume method 
the volume of the space above the meniscus must be small and accu- 
rately known. Hence it should be nearly cylindrical in figure, and the 
part F G of the barometer tube not chosen too wide. 
The extreme end of the capillary tube passes symmetrically through 
a metallic cap, 0, the top of which is soldered hermetically to the tube. 
(822) 
