The instrument, as usually constructed, is shown in Figs. 2 and 3.

The reservoir, RR' contains the measuring drum, _mmmm_, movable around the
horizontal axis, _aa'_. The gas enters at E, passes at S into an opening
that may be closed by a valve, and is distributed through the box, BB',
which communicates with the reservoir through an orifice in the partition,
_hh'_. This orifice is traversed by the axle, _aa'_. The box, like the
reservoir, contains water up to a certain level, _r_. Through a U-shaped
tube, _lnl'_, the gas passes from the box, BB', into the movable drum,
sets the latter in motion, and makes its exit at S. In order to count the
volume discharged, that is to say, the number of revolutions of the drum,
the axle terminates at a in an endless screw which, by means of a cog
wheel, moves a vertical rod that traverses the tube, _gg_, and projects
from the box. As the tube, _gg_, dips into the water, it does not allow
the gas to escape, and this permits of the revolution counter that the rod
actuates being placed in an external case, CC'.

The counter consists of toothed wheels and pinions so arranged that if the
first wheel makes one complete revolution corresponding to a discharge of
1,000 liters, the following wheel, which indicates cubic meters, shall
advance one division, and that if this second wheel makes one complete
revolution marked 10 cubic meters, the third, which indicates tenths,
shall advance one division, and so on. Hands fixed to the axles of the
wheels, and movable over dials, permit the volume of gas to be read that
has traversed the counter.

The object of the other parts of the instrument are to secure regularity
in its operation by keeping the level of the liquid constant. It is
evident, in fact, that if the level of the water gets below _r_, the