se between the
meter and the burners.

THE GAS-METER

commonly used in houses acts on the principle shown in Fig. 200. The
air-tight casing is divided by horizontal and vertical divisions into
three gas-chambers, B, C, and D. Gas enters at A, and passes to the
valve chamber B. The slide-valves of this allow it to pass into C and D,
and also into the two circular leather bellows E, F, which are attached
to the central division G, but are quite independent of one another.

[Illustration: FIG. 200.--Sketch of the bellows and chambers of a "dry"
gas meter.]

We will suppose that in the illustration the valves are admitting gas to
chamber C and bellows F. The pressure in C presses the circular head of
E towards the division G, expelling the contents of the bellows through
an outlet pipe (not shown) to the burners in operation within the house.
Simultaneously the inflation of F forces the gas in chamber D also
through the outlet. The head-plates of the bellows are attached to rods
and levers (not shown) working the slide-valves in B. As soon as E is
fully in, and F fully expanded, the valves begin to open and put the
inlet pipe in communication with D and E, and allow the contents of F
and C to escape to the outlet. The movements of the valve mechanism
operate a train of counting wheels, visible through a glass window in
the side of the case. As the bellows have a definite capacity, every
stroke that they give means that a certain volume of gas has been
ejected either from them or from the chambers in which they move: this
is registered by the counter. The apparatus practically has two
double-action cylinders (of which the bellows ends are the pistons)
working on the same principle as the steam-cylinder (Fig. 21). The
valves have three ports--the central, or exhaust, leading to the outlet,
the outer ones from the inlet. The bellows are fed through channels in
the division G.

INCANDESCENT GAS LIGHTING.

The introduction of the electric arc lamp and the incandescent glow-lamp
seemed at one time to spell the doom of gas as an illuminating agent.
But the appearance in 1886 of the Welsbach _incandescent mantle_ for
gas-burners opened a prosperous era in the history of gas lighting.

The luminosity of a gas flame depends on the number of carbon particles
liberated within it, and the temperature to which these particles can be
heated as they pass through the intensely hot outside zone of the flame.
By enriching the gas