(5) The supply of air may be deficient, as when all doors and windows are tightly closed. (6) The chimney may be obstructed by soot or some foreign material. (7) The wind above the house may be so strong that its pressure will cause the smoke from the chimney to be forced back. (8) If two chimneys rise together from the same house, and one is shorter than the other, the draught of the longer chimney may cause an inversion of the current of air in the lower chimney. (9) Wet fuel when used will cause smoke by its incomplete combustion. (10) A chimney without a fire may suck down the smoke from a neighboring chimney; or, if two fireplaces in different rooms are connected with the same chimney, the smoke from one room may be drawn into the other. =Methods of Heating.= =Open Fireplaces and Grates.=--Open fireplaces and fires in grates connected with chimneys, and using coal, wood, or gas, are very comfortable; nevertheless there are weighty objections to them. Firstly, but a very small part of the heat of the material burning is utilized, only about twelve per cent being radiated into the room, the rest going up the chimney. Secondly, the heat of grates and fireplaces is only local, being near the fires and warming only that part of the person exposed to it, leaving the other parts of the room and person cold. Thirdly, the burning of open fires necessitates a great supply of air, and causes powerful draughts. The open fireplace can, however, be greatly improved by surrounding its back and sides by an air space, in which air can be warmed and conveyed into the upper part of the room; and if a special air inlet is provided for supplying the fire with fresh air to be warmed, we get a very valuable means of heating. These principles are embodied in the Franklin and Galton grates. A great many other grates have been suggested, and put on the market, but the principal objection to them is their complexity and expense, making their use a luxury not attainable by the masses. [Illustration: FIG. 13. A GALTON GRATE. (TRACY.)] =Stoves.=--Stoves are closed receptacles in which fuel is burned, and the heat produced is radiated toward the persons, etc., near them, and also conducted, through the iron or other materials of which the stoves are made, to surrounding objects. In stoves seventy-five per cent of the fuel burned is utilized. They are made of brick, tile, and cast or wrought iron. Brick stoves, and stoves made of tile, are extensively used in some European countries, as Russia, Germany, Sweden, etc.; they are made of slow-conducting material, and give a very equable, efficient, and cheap heat, although their ventilating power is very small.