(_c_) Must, when constructed on the gasometer principle, be so arranged that when the gas-bell is filled to its maximum with gas at normal pressure its lip or lower edge will extend at least 9 inches below the inner water-level. (_d_) Must, when constructed on the gasometer principle, have the dimensions of the tank portion so related to those of the bell that a pressure of at least 11 inches will be necessary before gas can be forced from the holder. (_e_) The bell portion of a gasholder constructed on the gasometer principle must be provided with a substantial guide to its upward movement, preferably in the centre of the holder, carrying a stop acting to chock the bell 1 inch above the normal blow-off point. This tends to insure the proper action of the bell and decreases the liability of escaping gas. (_f_) A space of at least three-quarters of an inch must be allowed between the sides of the tank and the bell. (_g_) All water-seals must be so arranged that the water-level may be readily seen and maintained. 19. WATER-SUPPLY.--(_a_) The supply of water to the generator for generating purposes must not be taken from the water-seal of any gasholder constructed on the gasometer principle, unless the feed mechanism is so arranged that the water-seals provided for in Rules 18, (_c_), (_d_), and (_e_) may be retained under all conditions. This provides for the proper level of water in the gasholder. (_b_) In cases where machines of the carbide-feed type are supplied with water from city water-mains or house-pipes, the pipe connexion must discharge into the regularly provided filling trap on the generator and not through a separate continuous connexion leading into the generating chamber. This is to prevent the expulsion of explosive mixtures through the filling trap in refilling. 20. RELIEFS OR SAFETY BLOW-OFFS.--(_a_) Must in all cases be provided, and must afford free vent to the outer air for any over- production of gas, and also afford relief in case of abnormal pressure in the machine. Both the above-mentioned vents may be connected, with the same escape- pipe. (_b_) Must be of at least 3/4-inch internal diameter and be provided with suitable means for connecting to the pipe loading outside of the building. (_c_) Must be constructed without valves or other mechanical working parts. (_d_) Apparatus requiring pressure regulators must be provided with an additional approved safety blow-off attachment located between the pressure regulator and the service-pipes and discharging to the outer air. This is intended to prevent the possibility of undue pressure in the service-pipes due to failure of the pressure regulator. 21. PRESSURES.--(_a_) The working pressure at the generator must not vary more than ten-tenths (1) inch water column under all conditions of carbide charge and feed, and between the limits of no load and 50 per cent. overload.