The gas meter was invented by Clegg in 1816. Since that epoch no essential modification has been made of its structure. Fig. 1 shows the principle of the apparatus, _mnpq_ is a drum movable around a horizontal axis. This is divided by partitions of peculiar form into four vessels of equal capacity, and dips into a closed water reservoir, RR'. A tube, _t_, near the axis, and the orifice of which is above the level of the water, leads the gas to be measured. This latter enters under the partition, _l'm_, of one of the buckets, and exerts an upward thrust upon it that communicates a rotary motion to the drum. The bucket, _l'mi_, closed hydraulically, rises and fills with gas until the following one comes to occupy its place above the entrance tube and fills with gas in turn. Simultaneously, as soon as the edge of each bucket emerges at _e_, the gas flows out through the opening that the water ceases to close, and escapes from the reservoir through the exit aperture, S. The gas, in continuing to traverse the system, is thus filling one bucket while the preceding one is losing its contents; so that, if the capacity of each bucket is known, the volumes of the gas discharged will likewise be known when the number of revolutions made by the drum shall have been counted. The addition of a revolution counter to the drum, then, will solve the problem. [Illustration: THE GAS METER.] 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.