286

CHIMNEY.

opening the stop-cock, the junction line rose 2f inches. This experiment was verifiedby repetition upon different days, with fires burning at their average intensity, and con-suming fully 12 lbs. of the best coals hourly for each horse’s power, or nearly one tonand a third in twelve hours. If we divide the number 2j by 8, the quotient 0-28 willrepresent the fractional part of 1 inch of water, supported in the syphon by the un-balanced pressure of the atmosphere in the said chimney; which corresponds to 19J feetof air, and indicates a velocity in the chimney current of 35 feet per second. Theconsumption of fuel was much more considerable in the immense grate under the wortcopper, than it was under the steam-engine boiler.

In my experiments at Messrs. Braithwaite’s factory, the maximum displacement ofthe junction line was 1 inch, when the differential oil and water barometer was placedin direct communication with a chimney 15 inches square, belonging to a steam boiler,and when the fire was made to burn so fiercely, that on opening the safety-valve of theboiler, the excess of steam beyond the consumption of the engine rushed out with suchviolence as to fill the whole premises. The pressure of one eighth of an inch of waterdenotes a velocity of draught of 23-4 feet per second.

In building chimneys, we should be careful to make their area rather too large thantoo small; because we can readily reduce it to any desired size, by means of a slidingregister plate near its bottom, or a damper plate applied to its top, adjustable by wiresor chains, passing over pulleys. Wide chimneys are not so liable as narrow ones tohave their draught affected by strong winds. In a factory, many furnace flues areoften conducted into one vertical chimney stalk, with great economy in the firsterection, and increased power of draught in the several fires.

Vast improvements have been made in this country, of late years, in building stalksfor steam boilers and chemical furnaces. Instead of constructing an expansive, loftyscaffolding of timber round the chimney, for the bricklayers to stand upon, and toplace their materials, pigeon-holes, or recesses, are left at regular intervals, a few feetapart, within the chimney, for receiving the ends of stout wooden bars, which arelaid across, so as to form a species of temporary ladder in the interior of the tunnel.By means of these bars, with the aid of ropes and pulleys, everything may be pro-gressively hoisted, for the building of the highest engine or other stalks. An expertbricklayer, with a handy laborer, can in this way raise, in a few weeks, a considerablechimney, 40 feet high, 5 feet 8 inches square outside, 2 feet 8 inches inside at the base,28 inches outside, and 20 inches inside at the top. To facilitate the erection, and atthe same time increase the solidity of an insulated stalk of this kind, it is builtwith three or more successive plinths, or recedures, as shown in fig. 281. It is neces-sary to make such chimneys thick and Substantial near the base, in order that theymay sustain the first violence of the fire, and prevent the sudden dissipation of theheat. When many flues are conducted into one chimney stalk, the area of the lattershould be nearly equal to the sum of the areas of the former, or at least of as many °*them as shall be going simultaneously. When the products of combustion from anyfurnace must be conducted downwards, in order to enter near the bottom of the mamstalk, they will not flow off until the lowest part of the channel be heated by burningsome wood shavings or straw in it, whereby the air syphon is set agoing. Immed 1 'ately after kindling this transient fire at that spot, the orifice must be shut by whi®it was introduced; otherwise the draught of the furnace would be seriously impeded-But this precaution is seldom necessary in great factories, where a certain degree of he®is always maintained in the flues, or, at least, should be preserved, by shuttingdamper plate of each separate flue, whenever its own furnace ceases to act. Suchchimneys are finished at top with a coping of stone-slabs, to secure their brickwork again^the infiltration of rains, and they should be furnished with metallic conducting rods,protect them from explosions of lightning.

When small domestic stoves are used, with very slow combustion, as has b®recently proposed, upon the score of a misjudged economy, there is great danger otinmates being suffocated or asphyxied, by the regurgitation of the noxious burned n •The smoke doctors who recommend such a vicious plan, from their ignorancechemical science, are not aware that the carbonic acid gas, of coke or coal, must be hca250° F. above the atmospheric air, to acquire the same low specific gravity with it.other words, unless so rarefied by heat, that gaseous poison will descend through the orI ^of the ash-pit, and be replaced by the lighter air of the apartment. Drs. PriestleyDalton have long ago shown the co-existence of these two-fold crossing currents 0 .even through the substance of stone-ware tubes. True economy of heat, and salualike require vivid combustion of the fuel, with a somewhat brisk draught inside «chimney, and a corresponding abstraction of air from the apartment. Whocontinuous ventilation, under the ordinary circumstances of dwelling houses, cbe secured in any other way. Were these mephitic stoves, which have been 0 jjso ridiculously puffed in the public prints, generally introduced, the faculty