(34) 
dicated by CAILLETET, introduce in the lower reservoir, held in 
a sloping position, a quantity of mercury sufficient to fill this 
U-tube. After the manometer has been filled with gas in the said 
position, we cause the mereury to enter the U-tube by turning 
the manometer into the vertical position. Then by reading the posi- 
tion of the surface of the “mercury in the divided and calibrated 
branch f, after the manometer-tube is detached from the filling appa- 
ratus, we can determine the volume of the enclosed gas, while the 
difference in level with the other branch g indicates the excess ot 
pressure, above the pressure indicated by the barometer. And this 
determination can be made with great accuracy because we could 
allow the diameter of both the branches of the small manometer 
to be 8 mm., so that the correction for the capillary depression 
can be determined with sufficient accuracy from the form of the 
menisci. 
The length of the U-tube warrants that the gas remains shut off 
even When changes of temperature and atmospheric pressure occur. 
The peculiar position of the U-tube with regard to the mano- 
meter-tube leaves room under the lower reservoir for the tube A, 
which acts a very important part in different operations, viz. the 
exhausting with the mereury-pump, the filling with pure gas and 
the shutting off of a definite quantity of gas. This tube A is bent 
downwards slantingly and backwards and carries at the end a 
ground tap which fits in a ground cap, welded on to the glass 
conduit of the mercury-pump and the gas-generating apparatus 
(Comm. N°. 27 p. 15). 
After the piezometer-tube is cleaned and dried, the capillary end 
at the top of its upper reservoir has been sealed off, and the wider 
part of its stem has been cemented in the flange, it can be suc- 
cessively exhausted and filled with gas by means of this tube 
h. Then by revolving the piezometer-tube round the axis of the 
joint we can adrnit the mercury from the lower reservoir into the 
U-tube and so shut off the gas perfectly sure, after which the tube 
may be separated from the gas-generating apparatus. It needs no 
comment that the operation deseribed must be done with great care 
in order to prevent that the tube 4 breaks off, as the manometer 
is already burdened with the heavy flange. 
In order to facilitate any repairs to be made in the U-tube, its 
branches are connected by a narrower tube that can easily be 
straightened in the flame and after the repair is finished can be 
bent again into the original form without any damage to the 
calibrated tube. 
