CHIMNEY. An aperture or passage for the escape of the smoke and heated air from a furnace or fire-place, and for producing a more perfect com bustion by determining a rapid current of air through the fuel. The principle upon which the action of a chimney depends, is, that the air in the chimney becoming rarefied, its specific gravity is diminished, and the weight of the column of air within the chimney becoming less than the weight of a column of the external air of the same altitude, the heated air in consequence escapes at the top of the chimney, and is replaced by the colder and denser air, which enters at the bottom. The greater, therefore, the height of the chimney, the greater will be the effect ; for theater will be the difference in the weight of the two columns pf air. In Mr. Tre4old's work on warming and ventilating apart ments, the following rule is given for the orifices of chimneys according to the height and magnitude of the fire-places. Multiply by 17 the length of the fire place in inches, and divide by the square root of the height of the chimney (above the grate) in feet, and the quotient is the area in inches for the aperture of the chimney. Mr. Hiort, of the Office of Works, Whitehall, has taken out a patent for a new method of building chimney flues and tunnels, which has been adopted in the new palace, Pimlico, and in several public buildings, and has given great satisfaction. The plan consists in building within the usual walls, and incorporated with the common brick-work, circular smoke flues or tunnels, as seen in the annexed plan or horizontal section at b. Each flue or tunnel is surrounded in every direction, from top to bottom, by cavities or hot-air chambers c c, commencing at the back of each fire-place, and connected with each other. The air confined within these chambers is said to be rendered sufficiently warm by the heat of any one fire, to prevent condensation in all the flues con tained in the same stack of chimneys. The figure of each brick composing these circular flues is wedge-like, or inclined, as respects its upper and lower surfaces : the external face is composed of two planes, forming a very obtuse angle with each other; and the internal, of the arc of a circle, to the centre of which the two ends of the brick tend or radiate ; and the whole circle is completed by placing four bricks together, end to end, as shown in the fore going plan. Their inclined figure is best shown in the engraving above, from
which it will likewise be seen that the flues or tunnels may be carried in any direction without producing any internal angles, the bricks being readily adapted to any required curvature. To make the flue straieht. it will be observed that the thick ends of one course of bricks are placed alternately upon the thin ends of the next course; and in order to make curves, the thick ends are placed together on one side, and the thin ends on the opposite sida The circular flue commences at the throat of the chimney below the usual line of the chimney bar, and imme diately over the fire. From below the chimney-bar the flue is continued, downward to the hearth in a half circle, forming the centre of the back of the fire-place. From the construc tion of these chimneys, and the nature of the materials of which they con sist, no danger need be apprehended should the soot ignite (an accident not likely to happen), for such an accu mulation of soot as common chimneys are liable to cannot take place within these flues, there being no angles within which it can lodge, - the draught of air being much stronger through them, and the necessity for cleansing them may be rendered less frequent by vitrifying the inside of the bricks to prevent adhesion ; never theless, the operation of cleansing may with facility be performed without the aid of climbing boys, all sharp angular turns, and other impediments which have hitherto opposed the use of ma chinery for this purpose being totally avoided.