42

MODERN STEAM PRACTICE.

chimneys being 80 feet), and the quotient is the area at the top of thechimney, thus for 40 nominal horse-power—

ITfTTi? = ^29, say 26 inches diameter,

or 23 inches square at the top. It is always preferable to make anallowance over and above this for the convenience of leading otherflues into it. For a chimney 80 feet in height the brickwork shouldbe divided into three courses: for 30 feet height from the bottom twobriclcs in thickness, the next course one and a half brick in thick-ness, and the remainder one brick thick. For each 25 feet addedin height the brickwork at the bottom should be increased one-half brick in thickness. The batter or the slope of the side is usually0'3 of an inch to the foot. Thus, with 26 inches inside diameter atthe top, the bottom of the chimney would be 92 inches externaldiameter, while that of the top would be 44 inches. Should theinternal diameter at the top require to be 54 inches and upwards,the top course should be one and a half brick in thickness, and thebottom courses in proportion. The inside at the bottom is linedwith fire-bricks, leaving a space of one inch between the inner liningand the main building, and is carried up to a height of 15 feet fromthe bottom. For a chimney 80 feet in height the foundation shouldbe at least 5 feet in depth, laid on a bed of concrete 2 feet in thick-ness, but this will depend on the soil; on sand or gravel this bed willbe quite sufficient, but of course some soils require the foundationto be carried down to a firm bed. In marsh land, and even for theColonies, wrought-iron chimneys may be used with advantage, butbrick chimneys are to be preferred. The .best temperature for anefficient chimney draught is about 6oo° Fahr.

SMOKE PREVENTION.

Although the distance between the fire-bars varies from y toinch, allowing a good volume of air underneath the grate, soessential for perfect combustion, other means must be taken to con-sume the gaseous constituents thrown off from coal when in the semi-incandescent state; the simplest and most effectual way of doingthis is by admitting a current of air through a series of small holesdrilled through the furnace door, thus supplying the common oxygencontained in the atmosphere, and of which we have an unlimitedcommand. Many schemes have been brought forward from timeto time to consume the gases evolved before a dense mass of smoke