A refinement upon this vent-pipe is represented at C, where it is rigidly fastened to the tank of the holder, and has its internal aperture always above the level of the water in the apparatus. Rigidly fixed to the crown of the bell is a tube of wider diameter, _h_, which is closed at its upper end. _h_ is always full of gas, and its mouth is normally beneath the level of the water in the seal; but when the bell rises to its highest permissible position, the mouth of _h_ comes above the water, and communication is opened between the holder and the outer atmosphere. No water enters the vent-pipe from the holder, and therefore no gurgling or irregular pressure is produced. Another excellent arrangement of a vent-pipe, suggested by Klinger of Gumpoldskirchen, is shown at D, a drawing which has already been partly considered as a washer and water-seal. For the present purpose the main vessel and its various pipes are so dimensioned that the vertical height _g_ to _f_ is always appreciably greater than the gas pressure in the service or in the generator or gasholder to which it is connected. 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. For obvious reasons the vent-pipe of a holder should have a diameter equal to that of the gas-inlet tube, and the vent-pipe of a generator be equal in size to the gas-leading tube.