that houses near the gasometer end
of the main will receive their gas at a higher pressure than those at
the other end. The gas stored in the holders may be wanted for use in
the street lamps a few yards away, or for other lamps several miles
distant. It is therefore evident that if there be just enough pressure
to give a good supply to the nearest lamp, there will be too little a
short distance beyond it, and none at all at the extreme point; so that
it is necessary to put on enough pressure to overcome the friction on
all these miles of pipe, and give just enough gas at the extreme end. It
follows that at all intermediate points the pressure is excessive. Gas
of the average quality is burned to the greatest advantage, as regards
its light-giving properties, when its pressure is equal to that of a
column of water half an inch high, or about 1/50 lb. to the square inch.
With less it gives a smoky, flickering light, and with more the
combustion is also imperfect.

[Illustration: FIG. 199.]

Every house supply should therefore be fitted with a gas governor, to
keep the pressure constant. A governor frequently used, the Stott, is
shown in section in Fig. 199. Gas enters from the main on the right, and
passes into a circular elbow, D, which has top and bottom apertures
closed by the valves V V. Attached to the valve shaft is a large
inverted cup of metal, the tip of which is immersed in mercury. The
pressure at which the governor is to act is determined by the weights W,
with which the valve spindle is loaded at the top. As soon as this
pressure is exceeded, the gas in C C lifts the metal cup, and V V are
pressed against their seats, so cutting off the supply. Gas cannot
escape from C C, as it has not sufficient pressure to force its way
through the mercury under the lip of the cup. Immediately the pressure
in C C falls, owing to some of the gas being used up, the valves open
and admit more gas. When the fluctuations of pressure are slight, the
valves never close completely, but merely throttle the supply until the
pressure beyond them falls to its proper level--that is, they pass just
as much gas as the burners in use can consume at the pressure arranged
for.

Governors of much larger size, but working on much the same principle,
are fitted to the mains at the point where they leave the gasometers.
They are not, however, sensitive to local fluctuations in the pipes,
hence the necessity for separate governors in the hou