, 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
capacity of the buckets will be increased, and the counter will indicate a
discharge less than is really the case, and _vice versa_. If the level
descends as far as to the orifice in the partition, _hh'_, the gas will
flow out without causing the apparatus to move. The water is introduced
into the counter through _f_, which is closed with a screw cap, and
passes through the opening shown by dotted lines into the reservoir, RR',
whence it flows to the box, BB', When it has reached the desired level, it
gains the orifice, _r_, of a waste pipe, escapes through the siphon,
_ruv_, and makes its exit through the aperture, _b'_, when the screw cap
of the latter is removed. If, by accident, the level of the water should
fall below a certain limit, a float, _f_, which follows its every
movement, would close the valve, _s_, and stop the flow of the gas.
Finally a tube, _tt'_ soldered to the lower part of the tube, _lnl'_, and
dipping into the water of a compartment, P, serves to allow the surplus
water to flow out at _b'_. To prevent the apparatus from being disarranged
upon the drum being revolved in the opposite direction, there is fixed to
the axle, _aa'_, a cam which lifts a click, _z_, when the rotation is
regular, but which is arrested by it when the contrary is the
case.--_Science et Nature_.

*