improvement consists in diminishing the quantity of metal in immedi ate contact with the fuel, and forming the back and sides of the grate of fire-bricks. The bricks act like clothing, and keep in the heat of the coals, thus rendering the com bustion more complete, and the fire far hotter; while iron, being a good conductor, runs away with the heat as fast as it is generated, and passes it into the wall, making the coals that touch it dull and black. The same quantity of fuel, therefore, burned in a brick-lined grate, not only produces more heat, but throws a greater proportion of that beat out into the room, and less up the flue and through the wall, than when it is surrounded by a mass of iron; for radiation depends more upon the intensity of heat than upon its quantity.
Another point deserving attention is the shape given to the chimney-mouth, or recess above the grate. When the sides are square with the back none of the heat falling on them is given out again into the room. With a view, therefore, to throw out the heat better, the sides, or conies, as they are called, are inclined to the back at an angle of about 130'; and sometimes they are made curved and of polished metal, in order that they may reflect the heat without absorbing it. It is questionable if simple brick slabs, placed at the proper angle, do not throw out more heat than the most splendid polished metal plates; for though the bricks do not reflect the rays of the fire, they become heated themselves, and then radiate their heat into the room. Plates of rough metal absorb the heat that falls upon them as the brick does; but being good conductors, the heat passes through them into the wall, and thus they never become hot enough to radiate sensibly.
Much also depends upot shape of the fire-box, or grate itself. To see the impor tance of this, it is necessary -Atend carefully to the exact way in which an open fire heats a room. It does so almost entirely by the rays of heat that it throws out; and thes,e rays do not warm the air directly; they pass through it like light through glass, just as the hottestyays of the sun pass through the upper atmosphere, leaving it cold enough to freeze mercury. It is only when the rays of the fire fall on the floor, furniture, and walls of the room that they gave out their heat; and it is by coming in contact with these solid heated bodies that the air is gradually warmed. We may thus see the neces sity of having a fire lighted and burning brightly for a considerable time before the hour when the apartment is expected to be comfortable.
The law that radiant heat neither affects nor is affected by the surrounding air, also explains the fact that an apartment may feel very cold, though the air in it be at high summer heat. A church or other massive stone building in frosty weather may be filled with artificially-heated air and yet retain its chilling effect for many hours. The warmth of the living body is lost in two ways: the film of colder air that touches it receives part of its heat by conduction, and, rising up, makes room for another film to do the same; a moderately heated body in cooling is robbed of about half its heat in this way. The other half is given off in rays, which pass through the air, and impinge upon the objects around. These objects are radiating back heat in return; but their temperature being low, the return is small, and the warmer body is colder by the difference. Hence we are chilled by a cold wall or a cold window without touching it, and though the air between us and it may be at 70°.—To return to the shape of the grate:
The chief object is to present as large a surface as possible of glowingfire to the front. With this view, the grate is made long and deep, in proportion to its width from front to back. This principle, however, is carried too far in many grates. The stratum of fuel is too thin to burn perfectly, especially in the narrow angles at the sides, where the coals seldom get to a red heat, and are only warm enough to distil away in smoke. Such fires are constantly going out, and are further from being economical than a square box.
The practice recently come into vogue of placing grates almost on a level with the floor, is also a mistake. The floor and the lower part of the person receive no share of the radiant heat.
The chimney-throat, instead of a gulf drawing in a constant wide current of the warm air. of the room, and causing draughts from windows and doors toward tha fire place, 'should just be sufficient to admit the burned gases and smoke that come directly from the fire, and no more. Sec CITIMNEY. This is the object of the movable plate in what are called It would be endless to attempt to enumerate the various forms of grate constructed with more or less success on the above principles. We shall content oursqves with a notice of the recent invention of the late Dr. Arnott, to whom the subject of warming apartments is more indebted than to any individual since the days of count It comes nearer to the idea of perfection in an open fire-place than any previous contriv ance. Its peculiar advantages will be understood from the following description: Arnott's Smokeless ef (fig. 1), represent the front bars of a grate in s chimney of the usual construction, man. The grate has no bottom, and below it is an iron box, open only at top, into which the charge of coal for the day—from 20 to 30 lbs.—is put. Any kind of coke or coal may be used To light the fire the usual quantity of wood is laid on the surface of the fresh coal at ef, and a thickness of 3 or 4 in. of cinders or coked coal, left from the fire of the preceding day, is laid over all. " The wood being then lighted, very rapidly ignites the cinder above, and at the same time the pitchy vapor from the fresh coal below rises through the wood flame and cinders, and becomes heated sufficiently itself to become flame and so to augment the blaze. When the cinder is once fairly ignited, all the bitu men rising through it afterwareburns, and the fire remains smokeless." As there is no supply of air but through the bars in front, the box being close underneath, the fire must be gradually raised up as the combustion goes on; and this is effected by having a false bot tom, ss, in the box, which can be moved like a piston by means of a rod. The rod has notches in it, and by means of the poker used as a lever can be raised up and then retained at any height by a ratchet-catch. When the piston comes level with the bottom bar of the grate, the coals may be replenished while the fire is burning by pushing in a fiat shovel over the piston, so as to form a tem porary bottom to the grate and support the fire, while the piston is allowed to descend to the bot tom. The shovel is then raised up a little in front, or a part of the upper edge of the box is made 01 fold down, and fresh coals are shot into the box; on which the shovel is withdrawn and the combustion goes on as before.