SMOKE; SMOKE REVELATION. Smoke is the vapour arising from substances in a state of combustion. In its more extended sense the word is applied to all the volatile products of combustion, which consist of gaseous exhalations charged with minute portions of carbon aceous matter or soot ; but the term is frequently employed to express the carbonaceous matter only. It is important to bear thie distinction in mind, as it involves a fact which appears to have been sometimes overlooked : namely, that however completely the soot may be destroyed, and the smoke be thereby rendered invisible, it still remains necessary to provide means of free exit for the deleterious gases. The action of an ordinary chimney in conveying the smoke from a fire situated at its lower extremity is very simple. The air in the chimney, being rarefied by the heat, becomes lighter in proportion to its bulk than the surrounding atmosphere, and therefore rises: its place being supplied by fresh air forced in at the lower end by the pressure of the comparatively heavy cold air outside the chimney. A constant rising current is thus created, the force of which is sufficient to carry up with it any light bodies, such as the particles of soot which escape from the fire. The intensity of this current depends much upon the height of the chimney ; for it is evident that the higher a chimney is, within reasonable limits, the greater must be the difference between the weight of the column of hot air which it contains, and that of a column of cold air of equal elevation. It is also evident that the hotter the air in the chimney is kept, the more rapidly it will rise. Hence chimneys act better when built in stacks, or when in the interior of a house, than when single, or when outside the walls.
The circumstances which impede the proper action of chimneys, and occasion the annoyance of smoke in houses warmed by common open stoves, have excited the attention of many individuals, and formed the subject of several treatises. Franklin analysed the subject very judiciously, and published a pamphlet pointing out nine causes, or kinds of cause, for the evil. I. The want of a free supply of air to the bottom of the chimney. It is of little consequence bow spacious the room may be into which the chimney opens, if the access of fresh air to the room be cut oft As the hot air escapes from the top of the chimney, its place must be filled by fresh air taken from the room. But if the entrance of the external atmosphere be insufficient, every chimney-full of air abstracted from the room lessens the density of what remains, so that the draught will decrease until tho air in the chimney and that in the room are of equal density ; after which it will cease altogether, and the smoke will no longer ascend. This inconvenience can only be remedied by providing openings for air commensurate to the demands of the fire. Whenever it is practicable, the beet situation for them is near the top of the room, and over the fire-place; because the entering air is then warmed by the warm air which rises to the top of the room, and becomes pretty generally diffused. The object may be attained by leaving a window a little open at the top, or by pro viding long narrow openings above the window or immediately beneath the cornice. Another plan is the use of a pane of glass in the window, hinged to the frame at its lower edge, and capable of being opened more or lees! as required ; side-pieces Of glass being added to prevent the, air from entering laterally. Sometimes a number of strips of plate-glass, so arranged as to resemble a Venetian. blind, are used ; these (being so placed as to throw the air upwards. The common ventilator, or sohirligig, answers the same purpose, and diffuses the air in some degree by its rapid revolution, occasioned by the action of the current upon its inclined vanes. 2. Many chimneys smoke because the opening at the lower end vs too large. While a small oPening to a tall chimney increases the draught to an improper degree, and causes a wasteful consumption of fuel, a large opening to a short funnel will allow the smoke to escape into the room ; because all the air required by the chimney may enter at one side of the opening, leaving the other side free from current, and therefore allowing the smoke to puff out. In such a case the draught is weakened by the coldness of the air which enters the chimney at such a distance from the fire as to be very little affected by it. This defect must be remedied by contracting
the opening. 3. A third cause of defective action is the funnel or chimney being too short. The same effect is produced when the flue from an tipper story is turned into one from a lower room. The inconvenience of such an arrangement may be somewhat diminished by a contrivance for closing the collateral flue when not in use. The shortness of a chimney may sometimes be rendered harmless by con tracting the entrance, so that all the air entering it shall be highly heated, by passing immediately over the fire. 4. Different chimneys in the same house occasionally overpower each other. If we suppose two stoves, each having a distinct chimney, in a room without a sufficient supply of air from without, we may conceive that one fire becoming stronger than the other, may overpower it, and obtain a supply of air down the chimney of weakest draught ; the descending current of course blowing the smoke of the weaker flue into the room. Precisely the same effect will talte place if the stoves are in different rooms ; provided there be, owing to the opening of the doors, a com munication between them. This will account for the common case of a parlour chimney smoking whenever the room-door is opened, although it may act properly when the room is closed, and thereby cut off from the effect of the kitchen chimney. The proper cure is clear; if every room have a free supply of air from without, there will cease to be any probability of the chimney of one apartment over powering that of another. 5. Another cause arises from the situation of the house. 1f a house stand under the brow of a hill, or in the vicinity of a much higher building, the wind, passing over the higher obstacle, beats down into the chimneys of the sheltered house, and so prevents the exit of smoke. This may sometimes be remedied by raising the chimney, and in other cases by means of a cowl, or turning cap, the opening of which always turns from the wind. The ordinary cowl is turned by means of a vane attached to its upper part; but one patented by Mr. Pollard is turned by wheels set in motion by the action of the wind upon the oblique vanes of a rotatory flyer. Another contrivance consists of a square box placed on the top of the chimney, each side of which is a door, hinged at one edge, and connected, by means of an iron rod, with the door on the opposite side of the box : in such a way that when one door is closed by the force of the wind, the opposite one opens, and allows the smoke to escape. 6. Chimneys occasionally smoke from a cause just the reverse of that last described. This occurs when the chimneys are low, and stand between the wind and a high building, or neighbouring edifice, so that the air is dammed up, as it were, round about them. Raising the chimneys appears to be almost the only alternative. 7. Another cause of smoking is the injudicious arrangement of the door or doors of room. If the deer be on the same side as the fire-place, and occasion it to smoke by sending a current across the front of the stove, either it must be altered so as to open in the opposite direction, or a screen must be used between it and the stove. S. Smoke is sometimes blown down a chimney which is out of use. This arises from the circum stance that a stack of chimneys usually maintains a more uniform temperature than the surrounding air. When in the middle of the day, the air generally becomes warmer than that in the chimneys, the current moves downward, carrying with it smoke that may happen to be passing over, or escaping from a neighbouring flue. A -phato or register, closing the bottom of the chimney when out of use, obviates this inconvenience. 9. Chimneys, which under ordinary circumstances perform very well, occasionally smoke In consequence of the passage of a strong wind over them, which the force of the tieing vapour is insufficient to cope with. Among various modes of cure adopted, one is to make the top of the flue funnel-shaped, with a view to facilitate the escape of the smoke under such circumstances ; another, to reduce tho opening at the top to a long narrow slit. Probably a cowl might be of use in such a case.