Now to ascertain the quantity of gas discharged by one revolution of the wheel, we need only to know the capacity of the chambers, and add them together. Let us for example suppose, that each chamber contains 576 cubic inches, then one revolution of the wheel, discharges a cubic foot of gas. To register the total number of revolutions which the wheel makes in a certain time, a train of wheel-work is connected with the axis of the metre, see fig. 8, plate III.; it consists of a pinion impelling a common train of wheel-work, composed of any number of wheels. The pinion on the axis of one wheel, acts into the circumference of the next wheel, and the circumference of the wheel being as ten to one, it is obvious whilst the metre makes 100,000 revolutions, if the series consists of six wheels, the last wheel of the series, will only have made one revolution. Each axis of the wheels is provided with a finger and dial plate, divided into ten parts, therefore any number of revolutions may be read off at any time by inspection betwixt 10,000,000 and one.
The velocity with which the metre acts, is of course in proportion to the quantity of gas passing through it. Thus suppose there is a burner or gas lamp connected with the machine, of one foot capacity lighted, which consumes four cubic feet of gas in an hour, the gas metre performs four revolutions per hour, and so on for every number of burners or lamps, not exceeding the number which the machine is calculated to supply.
To render the construction of the gas metre more obvious, we have at fig. 6, plate III., exhibited a transverse section of the machine; _a_, is the outer case of the machine in which the wheel revolves. B, B, the outer or larger concentric chamber, (marked 1, in fig. 4, plate II.) L, the inner or smaller concentric chamber, (marked 2, in fig. 4, plate II.) _d_, the index on the axis which passes through a stuffing box in front of the machine. 5, 5, 5, 5, are stays or braces for supporting the wheel; they are likewise seen in fig. 4, plate II. A, is the inlet pipe for the gas to enter into the machine. The gas passes through the pipe _h_, and from thence into the curved pipe _i_, into the interior chamber L, of the metre. The pipe _h_, is surrounded by a second pipe K, which has a small aperture at _x_, the office of which is, to act as a siphon, in order to preserve the proper level of the water in the machine. The water is poured into the machine, through the small funnel at the back of the entrance pipe A. _y_, is a float, which stops the performance of the metre altogether, if a fraudulent attempt should be made, to stop the registering of the metre, by drawing off the water with which it is charged. In fig. 1, plate III., _a_, is the inlet pipe; _b_, the outlet pipe of the gas; and fig. 2, shows the interior chamber.