The vent of a furnace has given rise to much difference of opinion as to the size it ought to have. Some make it large, to allow a freer passage for the burnt air into the chimney ; others again, small, that the heat may not be dissipated and carried up into the chimney in waste. It is generally a single opening, but, in por celain furnaces, the manufacturers use a number of snail' openings, instead of a single vent, with the view of assisting in the equal distribution of the heat through out all parts of the chamber, and this practice should be adopted whenever this equal distribution is requisite. These artists are also careful that the sum of the areas of these holes should be exactly equal to that of the throats by which the flame and heated air enters into the cham ber. It seems, therefore, advisable, in all cases, to make the vent or vents equal in area to that of the free space left be tween the bars of the grate. Mr. Losh proposed to remove the vent to the front of the furnace, immediately over the feed ing or stoking-door, and to conduct the burned air, through channels made in the masonry, into the flue of the chim ney. A great advantage attends this construction, that, when either of the en-.
trances into the fire-room are opened, the indraught of air, instead of rushing over the surface of the burning fuel, and striking against the vessels and mate rials, instantly passes up the vent, and does not enter at all into the interior of the furnace, whence this is much less cooler than in the furnaces of the usual construction.
The chimney, or flue, is one of the most important parts of a furnace ; and yet, in general, the least attended unto ; being usually made much too large in its horizontal area. By making it thus large, the draught through it is much diminished, and the soot collects and be comes troublesome. For, when the sides of the flue contain a: larger surface than can be duly heated, the necessary rarefac tion of the nir passing through it is de stroyed. On this principle, alone, the draught of chimneys depends 3 and the cavity being too large proportionably to the current of air, the force of it is so diminished that the soot, instead of being blown out, gathers and rests on the sides till it obstructs the passage, and choking up the draught deadens the fire, espe cially at the first lighting of it, by which means the progress of the operation is sometimes greatly retarded. Instead, therefore, of the large proportion now made use of, if the chimney be intended for the use of one furnace only, an area equal to that of the free space between the bars of the grate is fully sufficient ; and this may be increased in proportion, where it is designed for a greater num ber.
The calculations of Tredgold show that each side of a chimney having a square basis, or the narrowest side, if the basis be rectangular, should be, at the least, one foot in breadth for every 10 feet in height ; and the area of the flue ought not to exceed one-third of the area of the chimney.
The wall of chimneys is usually single, but when the air which passes up the flue is very hot, it has been found pre ferable to have the wall double, with an empty space left between the two, which are tied together from space to space, by bricks passing from one to the other. FURNACE. (Russttx.) A kind of furnace adapted for burning wood, and used much in the villages and rural dis tricts in New England. It consumes less wood than a stove, and requires but little ' care, preserving an agreeable and equal temperature in the room, as it presents a greater amount of heated surface thane, stove, and does not require to be so in tensely heated up.
The annexed cut represents this furnace in a transverse sec tion. A is the fire place, to which an iron door is fixed. C is the brick. work ; D a soap stone cap ; 1 2 8 4 are flues in con nexion with each other, and I being in connexion with A ; to 4, a fun nel connecting the flue with the chimney is fixed, which has a damper attached. The furnace should stand out a few inches from the chimney, so as to save the heat from all sides.
The fire place is filled with wood, and the dampers opened till the wood gets well on fire. The dampers are then closed perfectly tight, though not so suddenly as to make it smoke. It will want no more attention till the wood is nearly gone, when it can be replenished and im mediately shut up if there are plenty of coals. It never need be opened more than three times a day in coldest weather, morning, noon, and night, and in more moderate weather not more than once or twice. The draught is generally good.
A common form is about three feet in length, sixteen inches wide, two and a half feet high—though the size should depend upon the size of the room. They may be built upon the floor by having a sufficient thickness of brick between the floor and fire. The cost of one made all of brick, is not over four dollars, (pressed brick). A new furnace must be dry be fore it is used.