2145. Burbidge and Healy's conical ribbed boiler, with Sylvester's patent fire-doors, is now considered one of the best methods of heating by hot water. The boiler is round, with a double set of supply and return pipes, as shown in fig. 638.; and it is hollow in the centre, as shown in the section (fig. 639.); so that the flame from the furnace not only goes up each side of the boiler, but up the middle, and thus the water is kept so continually hot, that the circulation, as indicated by the course of the arrows, is extremely rapid. Sylvester's patent fire-doors are very superior to the ordinary hinged doors used for furnaces, as they are fixed in an iron frame, and slide backwards and forwards on rollers. It is obvious that doors of this kind, which can be easily opened and closed by pushing them with the shovel used for putting on the coal, are much less liable to go out of repair than the old doors, which were hung on hinges, and fastened with a hasp, and were ever in danger of being broken or strained by being opened with the shovel.