When the fireplace can be inclosed thus, there is little liability to descending cur rents or "smoky chimneys," as they are called, even when the C. is very short, or has a tortuous course. It is chiefly with open fireplaces that this defect occurs, and the means of prevention and cure is a subject of some interest and importance. As with most other evils, the prevention is far easier than the cure; for by properly constructing the C. in accordance with the principles above stated—by placing the of the C. as nearly over the fire, and contracting the open space above the fire, as much as possi ble—downward smoking may in most cases be easily prevented. When a C. is in the neighborhood of a wall or building nearly as high as itself, or—what is still worse— higher, it is apt to smoke on account of the eddies and other complex currents in the air, caused by the interference which such an obstacle presents to the regular movemem of the wind. In towns, such tortuous movements of the atmosphere are very common, and the contrivance for preventing the wind from blowing down the chimneys are very numerous, and often grotesque. Revolving cowls of various forms, but alike in having a nearly horizontal outlet, which is so turned by the wind that the mouth shall always point in the direction opposite to that whence the wind is blowing, are the most com mon, and usually the most effectual. They are generally constructed of sheet-zinc, with au arrow, a flattened pigeon, or other device, as a vane, to determine the rotation of the cowl. The curing of smoky chimneys, in conjunction with the economizing of fuel, was one of the favorite subjects of investigation of that very practical philosopher, count Rumford. He says: "Those who will take the trouble to consider the nature and properties of elastic fluids—of air, smoke, and vapor—and to examine the laws of their motions, and the necessary consequences of their being rarefied by heat, will perceive that it would be as much a miracle if smoke should not rise in a chimney—all hin drances to its ascent being removed—as that water should refuse to run in a siphon, or to descend a river. The whole mystery, therefore, of curing smoky chimneys is com prised in this simple direction: find out and remove those local hindrances which forci bly prevent the smoke from following its natural tendency to go up the chimney; or rather, to speak more accurately, which prevent its being forced up by the pressure of the heavier air of the room."* He then goes on to speak of above 500 smoking chim
neys that he has had under his hands, and which were supposed incurable, and states that he was never obliged, " except in one single instance, to have recourse to any other method of cure than merely reducing the fireplace and throat of the chimney, or that part of it which lies immediately above the fireplace, to a proper form and just dimensions." In this reduction, the section of the fireplace is changed in form and size, whence it is better adapted for radiation into the room than the former square opening; the fire being brought further forward, has also more heating effect; the space of the fireplace being smaller, the air within it will, with a given sized fire, become hotter, and therefore have more ascending power; while in the contracted throat widening downwards, and having its sides strongly heated, there is a rapid rush of heated air, which carries the smoke upwards, and resists the passage of temporary down-draughts. Most modern chimneys and fireplaces are now constructed in accordance with count Ruinford's sue• gestious. Sec GRATE.
One frequent cause of smoky chimneys is the want of sufficient inlet for air to the room. Sand-bags placed under doors, and other devices for preventing ventilation, may cause a well-constructed C. to smoke. Openings must exist somewhere, of sufficient capacity to supply the air which is to ascend the chimney. If the air enters the room on the same side as the fireplace, and sudden gusts of air pass across the front of the fireplace, a temporary descending current is likely to be produced. The openings are best opposite the fire. For the methods of arranging and regulating such openings for the admission of air, see VENTILATION.
Tall factory-chimneys, usually built of brick, are very costly structures, many of them rivaling in height our loftiest cathedral spires. Their construction has been con siderably economized by building from the inside, and thus saving the expensive scaf folding. Their walls are built very thick at the base, and gradually thinner upwards: recesses are left at regular intervals in the inside, and stout wooden or iron bars rest upon these to form a sort of temporary ladder for the workmen to ascend; the materials are hoisted by ropes and pulleys.
Sheet-iron chimneys are largely used in Belgium. They are much cheaper but less durable than brick, and are objectionable on account of their rapid cooling by the action of the external air.