Selection of selection made of the different materials in general use, including stone, brick, timber, and iron, and the respective merits of the particular supplies introduced, affect stability. It is supposed that a brick wall properly constructed is a better basis for a stable structure than stone in the form of rubble, but that the most perfect stability is to be obtained from carefully-dressed and accurately united and bonded blocks of stone, mortar being used to no g-reater extent than is necessary to exclude wind and water. \larious kinds of stone, however, differ widely in their durability and their power of resisting the effects of intense heat. In the selection of stone fitted for general construction, desirable qualities are rela tive freedom from a tendency to absorb moisture and comparative ease in working or dressing into appropriate sizes and shapes.
Thickness of Ii'alls.—A leading consideration in the determination of the thickness of walls is their height. But other conditions affect this subject, such as the extent to which the main wall is strengthened, and the necessity for thickness diminished, by chimney-breasts or other but tress-like projections, and the extent to which the main wall is weakened by spaces devoted to door and window openings or by subjection to -unusual pressures. The thickness of walls required in the erection of dwelling houses and buildings other thau dwellings is prescribed bv laws enforcible in some of the leading cities of the United States. The provisions of the statutes applicable to the city of New York include the following: For a dwelling-house in which the height of walls is not to exceed 55 feet, the thickness of the foundations if of stone must be not less than zo inches, and if of brick not less than 16 inches. The thickness of the external and party walls must be not less than 12 inches. For dwelling-houses exceed ing 55 feet, but not exceeding So feet, stone foundations must be not less than 24 inches in thickness, and brick foundations not less than zo inches; the exterior wall must be not less than 16 inches thick to the top of the second floor, and 12 inches thick in the remaining height, if it is not more than 4o feet, and party-walls must be not less than 16 inches in thickness. So far as the requirements relating to dwelling,-houses more than So feet high, and buildings other than dwellings, differ from those mentioned above, they represent an increase in the thickness of foundations and walls.
Metteria/s: .Brick.—In large cities and towns of considerable size bricks usually form the principal material of the walls of a larg,e proportion of the buildings erected. They vary considerably in size, color, and quality,
some of the differences being caused by variations in the clay nsed, and others by diversified effects in the process of manufacture. The bricks used in the Eastern States are of a smaller size than those mannfactnred in the Middle and Western States. The usual dimensions of Baltimore-front, Philadelphia-front, Wilmington-front, and Trenton-front bricks are sy„ x 4 X 2 inches. Maine bricks arc 7Y? X 3;q x 2 :q inches; Milwaukee bricks are SY, x 4;,g x inches. Bricks of a nominal size, however, are subject to variations in dimensions and weig-ht, and in other qualities. Ordinary London bricks are called 9 x 4 x 2 Y2, inches, but they are fre quently only S •,34 x 41/1, x 2 inches. Approximately, bricks possess the advantage of uniform size and shape, and their power of resisting fire is much greater than that of some of the stones used. Characteristics of good bricks are regularity of shape; the emission of a clear ringing sound when they are struck; the display, when they are broken, of a compact uniform structure, hard, somewhat glassy, and free from air-bubbles and cracks; and inability to absorb more than about ofte-fifteenth of their weight of water. It is considered a useful precaution carefully to inspect bricks sup plied for important buildings, and to reject such as are defective. Com pressed bricks have considerably more strength and weight than ordinary bricks.
Enamelled Bricks, ana' desire to diversify brick fronts so as to produce decorative effects, is, with other causes, leading to an in creased use of various forms of enamelled brick, or brick of different colors, and terra-cotta moulded in ornamental forms (pi. 2, _figs.', 4). In connec tion with such uses, terra-cotta is defined as a superior variety of brick, each piece of which is specially' moulded. Quality depends largely upon the characteristics of the clay used. An advocate of terra-cotta for facing street-fronts says that when properly burned it is impervious to smoke and unaffected by acid fumes of any description, is about half the weight of the lightest building-stones, and is easily moulded into any shape for cornices, window-sills, architraves, or other purposes. Differences in color, includ ing yellow, red, and, when it is glazed, soft browns, greens, reds, or yel lows, are attainable. Terra-cotta sometimes forms the substance of drain pipes, and of porous blocks used in fire-proof buildings in connection with iron girders.