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 very important consideration. In a one-pipe system, such as indicated by Figure 1, the water of condensation flows from the radiators back to the boilers against the current of steam, falling through the steam in the vertical pipes and flowing along the bottom of the horizontal pipes. Such a simple system as this, shown in Figure 1, might be employed, and to a considerable extent, if the pipes are of ample size, and also if there are no valves on the radiators, so that steam can be turned on the entire system at all times. In this case there would be a constant and practically uniform flow of water through the pipes, and, if these were properly laid out, the system might give perfect satisfaction. But it is impracticable to have all the radiators of a system turned on at one time, and the difficulty with such a system is made evident the minute steam is turned into a cold radiator. When the steam comes in contact with a perfectly cold radiator a large amount is condensed at once in heating the cold iron, and as soon as the pressure becomes adjusted this bulk of water flows out of the radiator connection at one time and drops down the vertical pipe. When it reaches the horizontal main in the basement it is picked up by the current of steam and carried to other parts of the system, filling up the pipes in places; and as it is relatively much colder than the steam, the latter, in trying to get by it, is suddenly condensed, disturbing the equilibrium of pressure, as we might say, and producing the disagreeable crackling and pounding noises which are always encountered in poorly constructed heating systems, and which are commonly known under the name of water-hammer. This noise, besides being very annoying to the occupants of the building, interferes with the circulation of steam and also produces undue strains in the piping.

The two-pipe system to a certain extent does away with these difficulties; that is, in using the two-pipe connection it is generally easier to avoid the water-hammer and other annoyances incident to imperfect circulation; but unless the pipes are properly proportioned and properly drained the same difficulties will be encountered. The simple one-pipe system, indicated in Figure 1, is therefore, as before stated, rarely, if ever, used, but there are a number of modifications of it which are used with decided success, and in some of the largest installations.

One-pipe system with separate return main.—The simplest one-pipe system usually employed is represented in Figure 3. In this