Acetylene service-pipes should, wherever possible, be laid with a fall, which may be very slight, towards a small closed vessel adjoining the gasholder or purifier, in order that any water deposited from the gas owing to condensation of aqueous vapour may run out of the pipe into that apparatus. Where it is impossible to secure an uninterrupted fall in that direction, there should be inserted in the service-pipe, at the lowest point of each dip it makes, a short length of pipe turned downwards and terminating in a plug or sound tap. Water condensing in this section of the service-pipe will then run down and collect in this drainage-pipe, from which it can be withdrawn at intervals by opening the plug or tap for a moment. The condensed water is thus removed from the service-pipe, and does not obstruct its through-way. Similar drainage devices may be used at the lowest points of all dips in mains, though there are special seal-pots which take the place of the cock or plug used to seal the end of the drainage-pipe. Such seal-pots or "syphons" are commonly used on ordinary gas-distributing systems, and might be applied in the case of large acetylene installations, as they offer facilities for removing the condensed water from time to time in a convenient and expeditious manner. EXPULSION OF AIR FROM MAINS.--After a service-pipe system has been proved to be sound, it is necessary to expel the air from it before acetylene can be admitted to it with a view to consumption. Unless the system is a very large one, the expulsion of air is most conveniently effected by forcing from the gasholder preliminary batches of acetylene through the pipes, while lights are kept away from the vicinity. This precaution is necessary because, while the acetylene is displacing the air in the pipes, they will for some time contain a mixture of air and acetylene in proportions which fall within the explosive limits of such a mixture. If the escaping acetylene caught fire from any adjacent light under these conditions, a most disastrous explosion would ensue and extend through all the ramifications of the system of pipes. Therefore the first step when a new system of pipes has to be cleared of air is to see that there are no lights in or about the house--either fires, lamps, cigars or pipes, candles or other flames. Obviously this work must be done in the daytime and finished before nightfall. Burners are removed from two or more brackets at the farthest points in the system from the gasholder, and flexible connexions are temporarily attached to them, and led through a window or door into the open air well clear of the house. One of the brackets selected should as a rule be the lowest point supplied in the house. The gasholder having been previously filled with acetylene, the tap or taps on the pipe leading to the house are turned on, and the acetylene is passed under slight pressure into the system of pipes, and