The heat obtained from stoves is effected by radiation--the throwing outward of the waves of heat from its source, while the heat obtained from a hot-air furnace is effected by convection--the moving of a body of air to be heated to the source of heat, and then when heated bodily conveyed to the room to be warmed. Hence in stoves and fireplaces only such obstruction is placed between the fire and the room as will serve to convey away the obnoxious smoke and gases, and the greatest facility is offered for radiation, while in hot-air furnaces, although provision is also made to carry away the smoke and impure gases, yet the radiation is confined as closely as possible to chambers around the fire space, which chambers are protected by impervious linings from the outer air, and into which fresh outdoor air is introduced, then heated and conveyed to different apartments by suitable pipes or flues, and admitted or excluded, as desired, by registers operated by hand levers. There are stationary furnaces and portable furnaces; the former class enclose the heating apparatus in walls of brick or other masonry, while in the latter the outer casing and the inner parts are metal structures, separable and removable. In both classes an outer current of pure air is made to course around the fire chamber and around among other flues and chambers through which the products of combustion are carried, so that all heat possible is utilised. Vessels of water are supplied at the most convenient place in one of the hot-air chambers to moisten and temper the air, and dampers are placed in the pipes to regulate and guide the supply of heat to the rooms above. After Watt had invented his improvements on the steam engine the idea occurred to him of using steam for heating purposes. Accordingly, in 1784, he made a hollow sheet-iron box of plates, and supplied it with steam from the boiler of the establishment. It had an air-escape cock, and condensed-water-escape pipe; and in 1799 Boulton and Watt constructed a heating apparatus in Lee’s factory, Manchester, in which the steam was conducted through cast-iron pipes, which also served as supports to the floor. Patents were also taken out by others in England for steam-heating apparatuses during the latter part of the 18th century.