772 CHIMNEY 



of much value.) Chimney is a modern invention for promoting the draught of fires 

 ivnd carrying off the smoko, introduced into England so late as the age of Elizabeth, 

 though it seems to have boon employed in Italy 100 years before. The Romans, 

 with all their luxurious refinement, must have had their epicurean cookery placed in 

 perpetual jeopardy from their kitchen fires, which, having no vent by a vertical tunnel 

 in the walls, discharged their smoke and frequently their flames at their windows, to 

 the no small alarm of their neighbours, and annoyance of oven the street passengers. 



Chimneys in dwelling-houses serve also the valuable purpose of promoting a salu- 

 brious circulation of air in the apartments, when not foolishly sealed with anti-venti- 

 lating stove-chests. 



The first person who sought to investigate the general principles of chimney 

 draughts, in subserviency to manufacturing establishments, was the celebrated Mont- 

 golfier. As the ascent of heated air in a conduit depends upon the diminution of its 

 specific gravity, or, in other words, upon the increase of its volume by the heat, the 

 ascensional force may be deduced from the difference between the density of the 

 elastic fluid in the interior of the chimney, and of the external air; that is, between 

 the different heights of the internal and external columns of elastic fluid supposed to 

 be reduced to the same density. In the latter case, the velocity of the gaseous pro- 

 ducts of combustion in the interior of the chimney is equal to that of a heavy body 

 let fall from a height equal to the difference in height of the two aSrial columns. 



To illustrate this position by an example, let us consider the simple case of a 

 chimney of ventilation for carrying off foul air from a factory of any kind ; and suppose 

 that the tunnel of iron be incased throughout with steam at 212 Fahr. Suppose this 

 tunnel to bo 100 yards high, then the weight of the column of air in it will be to that 

 of a column of external air 100 yards high, assumed at 32 Fahr., inversely as its ex- 

 pansion by 180; that is, as 72727 is to 100. The column of external air at 32 

 being 100 yards, the internal column will be represented by 72'727; and the difference 

 = 27'27, 'will be the amount of unbalanced weight or pressure, which is the effective 

 cause of the ventilation. Calculating the velocity of current due to this difference of 

 weight by the well-known formula for the fall of heavy bodies, that is to say, multi- 

 plying the above difference, which is 27'27, by the constant factor 19*62, and extracting 

 the square root of the product; thus, V19'62 x 27'27 = 23 > 13 will be the velocity in 

 yards per second, which, multiplied by 3, gives 69'39 feet. The quantity of air which 

 passes in a second is obtained of course by multiplying the area or cross section of the 

 tunnel by this velocity. If that section is half a yard, that is = a quadrangle 2 feet 

 by 2, wo shall have 23-13 x 0-5 = ll-fiGo cubic yards, = 312^ cubic feet. 



The problem becomes a little more complicated in calculating the velocity of air 

 which has served for combustion, because it has changed its nature, a variable pro- 

 portion of its oxygen gas of specific gravity Till being converted into carbonic acid 

 gas of specific gravity 1'534. The quantity of air passed through well-constructed 

 furnaces may, in general, be regarded as double of what is rigorously necessary for 

 combustion, and the proportion of carbonic acid generated, therefore, one half of 

 what it would be were all the oxygen so combined. The increase of weight in such 

 burned air of the temperature of 212, over that of pure air equally heated, being taken 

 into account in the preceding calculation, will give us about 19 yards or 57 feet per 

 second for the velocity in a chimney 100 yards high incased in steam. 



In comparing the numbers resulting from the trials made on chimneys of different 

 materials and of different forms, it has been concluded that the obstruction to the 

 draught of air is directly proportional to the length of the chimneys and to the square 

 of the velocity, and inversely to their diameter. 



With an ordinary wrought-iron pipe of from 4 inches to 5 inches diameter, at- 

 tached to an ordinary stove, burning good charcoal, the difference is prodigious be- 

 tween the velocity calculated by the above theoretical rule and that observed by means 

 of a stop watch, and the ascent of a puff of smoke from a little tow dipped in oil of 

 turpentine thrust quickly into the fire. The chimney being 45 feet high, the tem- 

 perature of the atmosphere 68 Fahr. the velocity per second was: 



Mean Tempera- 

 Trials By theory By experiment ture of chimney 



1 . . 26-4 feet . . 5 feet . . . 190 Fahr. 



2 . . 29-4 . . 5-76 . . . 212 



3 . . 34-5 . . 6-3 . . . 270 



To obtain congruity between calculation and experiment, several circumstances 

 must be introduced into our formulae. In the first place, the theoretical velocity must 

 be multiplied by a factor, which is different according as the chimney is made of 

 bricks, pottery, sheet iron, or cast iron. This factor must be multiplied by the 

 square root of the diameter of the chimney (supposed to be round), divjded by its 



