SMOKE. The action of an ordinary chim ney in conveying the smoke from a fire situ ated 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 there fore 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 tho particles of soot which escape from the fire. The higher the chimney, and the warmer the air within it, the better will the smoke ascend. Franklin enumerated vino causes for the smoking (as it is termed) of ordinary chimneys 1—the want of a free supply of air to the bottom of the chimney ; the opening at the lower end of the chimney being too large; the chimney being too short ; different chimneys in the same house having different degrees of draught, whereby one overpowers another; the situation of the house near a higher house or a hill ; the lower level of a chimney than those ;which surround it; the injudicious arrangement of the doors of a room; the descent of smoke in a chimney out of use; and the occasional effeets of high or contrary winds. He treated all these causes in succession, and proposed such measures as he thought likely to meet the exigencies of each.
The nuisance occasioned by the smoke of coal fires has formed a subject of complaint from the earliest times in which mineral fuel was extensively used ; and the great increase of steam-engine • and other furnaces, conse quent on the extension of manufactures, has afforded, of late years, additional grounds for attempts to abate the nuisance. Such attempts are important, not only for the purification of the air but also for the economy of fuel ; since the matter which gives smoke objectionable density and colour is unconsumed fuel in a finely-divided state. It appears therefore that if a supply of air could be thrown into a fire in such a way as to occasion the combustion of the carbonaceous matter, the result would be that a greater amount of heat would be obtained from a given quantity of fuel, at the same time that the nuisance of smoke would be abated.
The quantity of smoke emitted from furnace chimneys varies much with the state of the fire ; being greatest when a mass of fresh fuel is thrown on, and least when the fire has burned clear, or the fuel is fully ignited. At tention to this circumstance, on the part of the stoker, will greatly diminish the nuisance; because if he throw on the fresh fuel in a thin layer, it will the sooner become perfectly ignited ; and, by laying it in the fore part of the furnace, the dense smoke arising from it has to pass over that part of the fire which is in a state of more perfect combustion, and is thereby in a great measure consumed. Many
of the contrivances introduced or suggested as smoke-consuming furnaces act on the Principles ; arrangements being adopted to insure the right feeding of the fire without much attention on the part of the firemen. The first important attempt made in this country for the combustion of smoke was that of Watt, who obtained a patent in 1785 for a method of constructing furnaces in such a way as to cause the smoke or flame of the fresh fuel, in its way to the flues, or chimney, to pass, together with a current of fresh air, through, over, or among fuel which has al ready ceased to smoke, or which is converted into coke, charcoal, or cinders, and which is intensely hot ; by which means the smoke and grosser parts of the flame, by coming into close- contact with, or by being brought near unto, the said intensely hot fuel, and by being mixed with the current of fresh or unburned air, are consumed or converted into heat, or into pure flame, free from smoke.' Since that time innumerable plans have been brought forward for introducing the necessary supply of air to the furnace ; hut while many of them accom plish the purification of the smoke as com pletely as could be desired, they are generally found either to increase the consumption of fuel, or to weaken the draught of the furnace. A singular plan has been introduced by.Mr. Iveson, of injecting steam into the furnace ; the steam being thrown into the fire in several minute jets, from a fan-shaped distributor in the fore part of the furnace. The steam not only destroys the smoke, but also greatly in creases the intensity of the fire ; so much so, indeed, as to sanction the supposition that the steam is decomposed, and that its component gases are consumed. One drawback to the scheme is the great consumption of steam. A patent was obtained in 1838, by Mr. Chappe, for the use of a stream or shower of hot water thrown into the furnace in the same way.
Besides the numerous plans for the com bustion of smoke, various methods have been tried on a limited scale for conducting it to a distance from the buildings in. which it is formed, by means of subterraneous channels ; and for condensing it by means of a shower of water, so that tho sooty matter might be conveyed away by the sewers.