Another cause is too short a flue, and the remedy is to lengthen it. As a general rule, the longer the flue the stronger the draught. But in calculating the length of a flue, reference must be had to side-flues, if any open into it. Where this is the case, the length of the main flue is to be considered as extending only from the bottom to the point where the upper flue joins it, and where the lower will receive air from the upper flue. If a smoky flue can not be increased in length, either by closing an upper flue or lengthening the chimney, the fireplace must be contracted so that all the air near the fire will be heated and thus pressed upward. If a flue has more than one opening, in some cases it is impossible to secure a good draught. Sometimes it will work well and sometimes it will not. The only safe rule is to have a separate flue to each fire. Another cause of poor draughts is too tight a room, so that the cold air from without can not enter to press the warm air up the chimney. The remedy is to admit a small current of air from without. Another cause is two chimneys in one room, or in rooms opening together, in which the draught in one is much stronger than in the other. In this case, the stronger draught will draw away from the weaker. The remedy is, for each room to have a proper supply of outside air; or, in a single room, to stop one of the chimneys. Another cause is the too close vicinity of a hill or buildings higher than the top of the chimney, and the remedy for this is to raise the chimney. Another cause is the descent, into unused fireplaces, of smoke from other chimneys near. The remedy is to close the throat of the unused chimney. Another cause is a door opening toward the fireplace, on the same side of the room, so that its draught passes along the wall and makes a current that draws out the smoke. The remedy is to change the hanging of the door so as to open another way. Another cause is strong winds. The remedy is a turn-cap on top of the chimney. Another cause is the roughness of the inside of a chimney, or projections which impede the passage of the smoke. Every chimney should be built of equal dimensions from bottom to top, with no projections into it, with as few bends as possible, and with the surface of the inside as smooth as possible. Another cause of poor draughts is openings into the chimney of chambers for stove-pipes. The remedy is to close them, or insert stove-pipes that are in use. Another cause is the falling out of brick in some part of the chimney so that outer air is admitted. The remedy is to close the opening. The draught of a stove may be affected by most of these causes. It also demands that the fireplace have a tight fire-board, or that the throat he carefully filled. For neglecting this, many a good stove has been thrown aside and a poor one taken in its place.