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in fire control, a means of protecting a building against fire by causing an automatic discharge of water, usually from pipes near the ceiling. The prototype, developed in England about 1800, consisted of a pipe with a number of valves held closed by counterweights on strings; when a fire burned the strings, the valves were opened. Many manually operated systems were installed in 19th-century buildings; in these a number of perforated pipes were fed by a main riser that could be turned on in an adjoining area. Because this system resulted in frequent water damage in parts of a room or building untouched by fire, an improvement was sought and found in the Parmelee sprinkler head, introduced in the United States in the 1870s. In this, the normally closed orifice is opened by heat from a fire. Modern versions use a fusible link or a bulb containing chemicals, which breaks at about 160 F (70 C) to open the orifice. Modern sprinkler heads are designed to direct a spray downward. Most sprinkler systems are wet-head-i.e., they use pipes filled with water. Where there is danger of freezing, however, dry-head sprinklers are used in which the pipes are filled with air under moderate pressure; when the system is activated, the air escapes, opening the water-feeder valves. An improved version has air under only atmospheric pressure and is activated by heat-sensing devices. Another special type, used in high-hazard locations, is the deluge system, which delivers a large volume of water quickly.