gas flows in the normal direction as described there will be no escape to
the air. Should the gas in the torch line return into the hydraulic valve
its pressure will lower the level of water in the cylinder by forcing some
of the liquid up into the two pipes. As the level of the water lowers, the
shorter pipe will be uncovered first, and as this is the pipe leading to
the open air the gas will be allowed to escape, while the pipe leading back
to the generator is still closed by the water seal. As soon as this reverse
flow ceases, the water will again resume its level and the action will
continue. Because of the small amount of water blown out of the escape pipe
each time the valve is called upon to perform this duty, it is necessary to
see that the correct water level is always maintained.

While there are modifications of this construction, the same principle is
used in all types. The pressure escape valve is often attached to this
hydraulic valve body.

_Construction Details._--Flexible tubing (except at torches), swing
pipe joints, springs, mechanical check valves, chains, pulleys and lead or
fusible piping should never be used on acetylene apparatus except where the
failure of those parts will not affect the safety of the machine or permit,
either directly or indirectly, the escape of gas into a room. Floats should
not be used except where failure will only render the machine inoperative.

It should be said that the National Board of Fire Underwriters have
established an inspection service for acetylene generators and any
apparatus which bears their label, stating that that particular model and
type has been passed, is safe to use. This service is for the best
interests of all concerned and looks toward the prevention of accidents.
Such inspection is a very important and desirable feature of any outfit and
should be insisted upon.