inoperative and free from gas pressure before opening the residue drain of
the carbide filling opening on top of the hopper. Some machines are made so
that they automatically cease to generate should there be a sudden and
abnormal withdrawal of gas such as would be caused by a bad leak. This
method of adding safety by automatic means and interlocking parts may be
carried to any extent that seems desirable or necessary to the maker.

All generators should be provided with escape or relief pipes of large size
which lead to the open air. These pipes are carried so that condensation
will drain back toward the generator and after being led out of the
building to a point at least twelve feet above ground, they end in a
protecting hood so that no rain or solid matter can find its way into them.
Any escape of gas which might ordinarily pass into the generator room is
led into these escape pipes, all parts of the system being connected with
the pipe so that the gas will find this way out.

Safety blow off valves are provided so that any excess gas which cannot be
contained by the gas holder may be allowed to escape without causing an
undue rise in pressure. This valve also allows the escape of pressure above
that for which the generator was designed. Gas released in this way passes
into the escape pipe just described.

Inasmuch as the pressure of the oxygen is much greater than that of the
acetylene when used in the torch, it will be seen that anything that caused
the torch outlet to become closed would allow the oxygen to force the
acetylene back into the generator and the oxygen would follow it, making a
very explosive mixture. This return of the gas is prevented by a hydraulic
safety valve or back pressure valve, as it is often called.

Mechanical check valves have been found unsuitable for this use and those