In
these circumstances the gas entering at _a_ depresses the water in
the pipe below the level _f_ to an extent equal to the pressure at
which it enters that pipe--an extent normally less than the distance
_f_ to _g_; and therefore gas never passes into the body of the
vessel, but travels away by the side tube _b_ (which in former
references to this drawing was supposed to be absent). If, however, the
pressure at _a_ exceeds that of the vertical height _f_ to
_g_, gas escapes at _g_ through the water, and is then free to
reach the atmosphere by means of the vent _c_. As before, _d_
serves to charge the apparatus with water, and _e_ to ensure a
proper amount being added. Clearly no liquid can enter the vent-pipe in
this device. Safety-valves such as are added to steam-boilers and the
like, which consist of a weighted lever holding a conical valve down
against its seat, are not required in acetylene apparatus, for the
simpler hydraulic seals discussed above can always be fitted wherever
they may be needed. It should be noticed that these vent-pipes only come
into operation in emergencies, when they are required to act promptly. No
economy is to be effected by making them small in diameter.
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