in which _w_ = weight of stack, in pounds; _h_ = height of stack, in feet; _d_ = mean diameter of stack, in feet; _c_ = constant; _b_ = width of base. The constant _c_ varies with the shape of the stack. For a square stack, when the wind is blowing at hurricane violence, 56 is used; for an octagonal stack, 35; and for a round stack, 28. To demonstrate this formula, consider a square chimney having an average breadth of 8 feet and a width at base of 10 feet, the stack being 100 feet high. The problem is, therefore, to find what the weight of the stack must be in order to resist the greatest wind pressure likely to occur. By substitution, in the formula, 100 × 100 × 8 × 56 _w_ = ------------------ = 448,000 pounds 10 With brickwork weighing about 120 pounds per cubic foot, the chimney in question must therefore have an average thickness of somewhat more than 13 inches. [Illustration: FIG. 32] =66.= A good rule to follow in designing brick stacks is to make the base at least one-tenth of the height. For stacks under 5 feet in diameter, the walls for the first 25 feet from the top may be 8 inches, increased 4½ inches for each additional 25 feet from the top. If the stack is more than 5 feet in diameter, the thickness at the top should be 1½ bricks, or 12 inches, with a 4½-inch increase for each 25 feet. If the stack is less than 3 feet in diameter, the brickwork for the first 10 feet from the top may be as little as 4½ inches; this thickness, however, is not recommended, as the weather is likely to penetrate such a thin wall, and sooner or later, together with the exposure to the gases, destroy the brickwork. =67. Construction of Brick Chimneys.=—All brick stacks must be provided with a cast-iron or stone coping at the top, and it is usually well to tie them in toward the base with good heavy stone band courses. In constructing brick stacks, the brickwork should be laid up in lime-and-cement mortar, and the bricks well covered and slid in place, not just tapped or hit with the handle of a trowel. All chimneys should also be provided, for a distance of at least one-third of their height from the base, with a fire-brick lining, laid up in fireclay, and at the bottom of this lining, where the flues from the boiler enter the stack, cast-iron cleaning doors and frame should be provided for removing soot that will accumulate and drop down. A good example of a brick stack is given in Fig. 32; this stack has a capacity of 500 horsepower, and is sufficiently stable to resist any wind pressure. FIRE-PROTECTION OF MILL BUILDINGS SPRINKLER SYSTEM