210 MEASUREMENT OF HIGH TEMPERATURES. IbullJ 
cury may be withdrawn either from the tube B G or from the reservo 
K L L. The latter and the cock H may be unscrewed from the ste 
piece inserted into the hose at G in the manner shown in Fig. 30, abov 
This system is practically a U-tube, one of the arms of which may I 
varied in height or length at pleasure. For this purpose it is attache 
to the manometer stand, already described. (See frontispiece under E 
B G is fixed to one of the uprights with its weight mainly resting on tl 
foot at G. The reservoir K L L and the cock H are fixed to the slid 
which may be moved at pleasure up or down and clamped in any pos 
tion on the second parallel upright of the manometer stand. The slic 
is conveniently provided with an ordinary micrometer screw, by whic 
a finer adjustment is obtainable. Enough mercury is introduced to jui 
fill the tube B G and leave a well-defined meniscus in the reservo 
K L L. Moreover, the dimensions ofKLL are such that when it o 
cupies its lowest position, and the mercury is almost completely out < 
the tube B (7, the reservoir may be about filled. Measurements ai 
made with the cathetometer, which in this case, however, has no oth< 
purpose than to indicate identity in the level of the menisci in B G an 
KLL. The pressure is then that of the barometer, which may be coi 
veuiently suspended from the same stand. (See frontispiece.) Volume 
are read off' directly on the glass tube B G. Two sensitive thermom 
ters at the lower and upper parts of this tube, respectively, show ii 
temperature. 
This is the form in which the apparatus was used. It will be remen 
bered that the chief purpose of the present memoir is to test tj 
availability of methods. Whenever it becomes desirable to investigat 
data of extreme accuracy, it is, of course, necessary to surround B 
with a jacket of water, so that the temperature throughout its lengt 
may be kept rigorously constant. Again, the increment of volurm 
corresponding to a given increment of temperature, decreases in propo: 
tion as temperature itself increases. Hence, if it be desirable to tneai 
ure high temperatures, a volume tube may expediently be chosen, < 
which the lower part is of smaller diameter than the upper part. Dat 
for the construction of such tubes for any special purposes will be full 
given in the tables below. Long capillary metallic tubes cool down tb 
gas to atmospheric pressure before it enters tbe volumenometer tube B ( 
CONSTANT-PRESSURE THERMOMETRY— METHOD OF COMPUTATION. 
The general equation. — Equation (I), on page 180, 
<"(^S-4SXH <■ 
applies here as it did in the case above. The former simplification cot 
sisted in so conducting the measurements that A=0. The presen 
(864) 
