(33°) 
fiew capillary to the upper part of the reservoirs. If this happens 
to be the case we can no longer calculate the new volume of the 
reservoir from the old one by means of an insignificant and perfectly 
sure correction, which is possible as long as we preserve the same 
capillary tube. 
The graduated stem ¢ is made from a carefully selected perfectly 
straight and almost cylindrical tube. The graduation extends from 
0 to 50 cm., continued on either side over some cm. in order to 
make sure that the diameter of the tube in the neighbourhood of 
O and 50 does not show any particular change. It did not seem 
desirable to make the graduation extend over more than 50 cm. as 
it is necessary to keep the whole length at a constant temperature. 
The divisions are at distances of 1 mm. and the readings are taken 
by means of a kathetometer. | 
It is of great importance that the whole tube should be perfectly 
vertical. Therefore care is taken that the stem and the cylinders of 
the manometer-tubes are well centred, and that the tube is truly 
centred in the steel tube with flange O (fig. 3. Pl. I) the whole 
apparatus being placed vertical by means of the plummet (compare 
also § 5). 
The reading tube is connected with the lower reservoir a by 
means of a wider tube, b fig. 3. By means of this wider part the 
manometer-tuke is cemented!) in the flange 0, which for this pur- 
pose must be made so that it can be pushed over the upper reservoir. 
The outer diameter and the thickness of the wider part are taken 
a little larger than those of the upper reservoir, and the bore of 
the flange belonging to it so much larger that between the glass 
and the flange space remains for a thin layer of cement (about 
0.5 mm.). 
The lower cylinder a is thin, as in the manometers of the type 
used by CaiLLeTer and Amacat. At its lower end however the 
manometer-tube terminates differently; it is provided with a U-shaped 
tube placed under the lower reservoir, of which the branch f con- 
nected with the reservoir is graduated. The purpose of this tube is 
to enable us to determine accurately the normal volume or to test it 
(as has been mentioned in § 1) at any time we should wish. 
Before we proceed to the filling with gas we, in the manner in- 
1) In some cases, when for instance we should want to heat the piezometer-tube, 
it may be desirable not to cement the piezometer but to enclose it in the flange 
by packing. But as we have principally in view its use as gasmanometer we need 
not dwell on this particular, 
