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 an ordinary two-column steam radiator which is full of air and has a tight air valve, after some moments the first few loops will become filled with steam and perfectly hot, the remaining loops being full of air and cold. If the radiator has a two-pipe connection the air may work out to some extent through the return; but if it has only a one-pipe connection it will remain in this condition as long as the air valve is kept closed. If, however, the radiator be of the hot-water type—that is, it has its loops connected by openings through the top as well as at the bottom—the steam will run along the top of the radiator, and as long as the air valve is closed it will remain hot across the top and the lower part of the loops will be cold, with the possible exception of the two end ones. If now the air valves be opened, the air will flow uniformly from the regular steam type, the steam filling one loop after another; but with the hot-water type, as soon as the air valve is opened the steam will flow first across the top, then across the bottom, and the air will gradually work out, first from the loops near the air valve. This will serve to illustrate to some extent the action of air in radiators.

Circulation in direct radiators.—As regards direct radiators, the ordinary two-column steam type gives the most perfect circulation. When steam is turned on it compresses the air to the pressure of the steam, and immediately fills a portion of the bottom and the nearest loops to the inlet. Each loop then acts independently and the air syphons out of each, one after another, until all are full of steam. In the hot-water type, as shown, the steam has a free circulation around the radiator as a whole, which interferes with the air circulation in each loop, so that these radiators will often remain air bound in the center for a considerable length of time. In the same way three and four-column radiators will become air bound in the middle columns, the air syphoning out of the two outside columns and establishing a circulation there, while the contents of the inner columns remain quiet. In this connection it should be stated that a strictly one-column radiator would not allow circulation at all, or but very slowly, but the so-called one-column radiators are practically two columns, as they are cast with a partition running up the loop with an opening at its top. Radiators of the flue pattern always have one or two similar partitions. Direct radiators which are low and wide are almost universally built with the loop opening in the top as well as the