(35) 
§ 4. The compression-cylinders and stopcocks. (Pl. I, fig. 1, 
2, 3.) The manometer-tubes filled with gas and closed by the mer- 
cury in the U-tubes are introduced into the compression-cylinders 
A previously filled to the rim with mercury. In order to prevent 
the air from getting into the U-tube, care is taken that the mercury 
is flowing out of the tube (by heating the manometer reservoir) 
at the moment that its opening is immersed below the mercury 
surface in the compression-cylinders. When the compression-cylinder 
is closed by the flange in which the manometer is cemented, 
the mercury pushed up by that part of the flange which fits 
in the compression-cylinder, drives the air out. The superfluous 
mercury escapes until by firmly screwing on the nut MN we obtain 
a tight fit on the washer p. Then the reservoir of the manometer-tube 
is contained in a space wholly and exclusively filled with mercury. 
By means of this contrivance we can take the manometer-tube 
from the apparatus without its coming in contact with any liquid 
but mercury, and avoid the great number of difficulties which always 
arise when we transfer, as usualy is done, the pressure by another 
fluid on the mercury in the compression-cylinder. 
It is desirable that we should have at hand a greater number of 
manometer-tubes with flanges in order that we may successively 
place several piezometers previously prepared into the compression- 
cylinders. The 4 compression-cylinders of the apparatus on Pl. I 
consist of well-drawn iron tubes, carrying taps welded on at both ends). 
They are mounted together on a stand V, in the notches of which 
they fit in with the parts of the two taps that are filed sexangularly ; 
they are shut up by a counter plate, in which likewise notches 
have been filed. These notches together with those in the stand 
hold the sexangular taps, as clearly shown in fig. 1 Pl. II. 
Thus it is easy to place into and to take out of the apparatus 
each of the compression-cylinders separately, while the tubes with 
the stand form a whole and are kept as it were in a large wrench, 
which can be held firmly so as not to meet with the difficulty, 
which else so often occurs, whenever we want to screw the nut ofa 
compression-cylinder tightly by means of a wrench with a long lever. 
The whole stand is placed in a wooden receptacle for the mercury 
that might flow out. 
The compression-cylinders can be opened on both sides. In 
1) The boring of a bar is very expensive and it is difficult to get smaller pieces 
of tubing with walls sufficiently thick to be provided with serew-thread and a sexan- 
gular tap, 
