* The word boiler is a generic term covering a great variety of kettles, saucepans, clothes boilers, evaporators, coppers, retorts, etc.

* The earliest steam boilers were usually spheres or sections of spheres, heated wholly from the outside. Watt used the wagon boiler (shaped like the top of a covered wagon) which is still used with low pressures. Most of the boilers in present use may be classified as plain cylinder boilers, flue boilers, sectional and tubular boilers.

Barrel of a boiler, the cylindrical part containing the flues. -- Boiler plate, Boiler iron, plate or rolled iron of about a quarter to a half inch in thickness, used for making boilers and tanks, for covering ships, etc. -- Cylinder boiler, one which consists of a single iron cylinder. -- Flue boilers are usually single shells containing a small number of large flues, through which the heat either passes from the fire or returns to the chimney, and sometimes containing a fire box inclosed by water. -- Locomotive boiler, a boiler which contains an inclosed fire box and a large number of small flues leading to the chimney. -- Multiflue boiler. Same as Tubular boiler, below. -- Sectional boiler, a boiler composed of a number of sections, which are usually of small capacity and similar to, and connected with, each other. By multiplication of the sections a boiler of any desired capacity can be built up. -- Tubular boiler, a boiler containing tubes which form flues, and are surrounded by the water contained in the boiler. See Illust. of Steam boiler, under Steam. -- Tubulous boiler. See under Tubulous. See Tube, n., 6, and 1st Flue.