4. The cement employed in laying marble in common or large edifices, is somewhat different from that just described, as it consists of about three fourths of lime and one of sand. The latter substance is obtained, in an unmixed state, on the bays in every part of the world; hence it has received the appellation of bay sand.
5. When it cannot be conveniently had in a pure state, particles of the same kind can be separated in sufficient quantities from their admixture with other substances. This is effected by sifting the compound through a sieve, into a small stream of water, which carries off the lighter particles that are unfit for use, whilst the sand, by its superior specific gravity, sinks to the bottom. The part which may be too coarse, remains in the sieve. This, however, except the rubbish, can be used in the coarser kinds of masonry.
6. The mortar, used in laying bricks and common stone, has a greater proportion of sand, which is generally of an inferior quality. Besides, the materials are incorporated with less care. Lime for the purposes of building is procured chiefly by calcining limestone in a kiln, with wood, coal, or some other combustible substance. It is also obtained by burning chalk, marble, and marine shells. Water poured upon newly-burnt or quick lime, causes it to swell, and fall to pieces into a fine powder. In this state it is said to be slacked.
7. Masonry is often required in situations under water, especially in the construction of bridges and locks of canals. Common mortar resists the action of the water very well, when it has become perfectly dry; yet, if it is immersed before it has had time to harden, it dissolves, and crumbles away.
8. The ancient Romans, who practised building in the water to a great extent, discovered a material, which, when incorporated with lime, either with or without sand, possessed the property of hardening in a few minutes even under water. This was a kind of earth found at Puteoli, to which was given the name of pulvis puteolanus, and which is the same now called puzzolana.
9. A substance denominated tarras, terras, or tras, found near Andernach, in the vicinity of the Rhine, possesses the same quality with puzzolana. It is this material which has been principally employed by the Dutch, whose aquatic structures are superior to those of any other nation in Europe. Various other substances, such as baked clay and calcined greenstone, reduced to powder, afford a tolerable material for water-cements. Several quarries of water lime, which is similar in appearance to common limestone, has been lately discovered in the United States, which, being finely pulverized and mixed with sand, makes very good water-cement.
10. In buildings constructed with marble and other costly stones, the walls are not composed of these materials in their entire thickness; but, for the sake of cheapness, they are formed on the inside with bricks, commonly of a poor quality, so that in reality they can be considered only brick walls faced with stone. These two kinds of materials have no other connexion than what is produced by the mortar which may have been interposed, and the occasional use of clamps of iron. Such walls are said to be liable to become convex outwardly from the difference in the shrinking of the cement employed in laying the two walls.
11. The principal tools employed in cutting and laying stone are the saw, various kinds of steel points, chisels and hammers, the mallet, the square, the compasses, the level, the plumb-rule, the trowel, and the hod, to which may be added, the spade and the hoe. The last three instruments, however, are handled almost exclusively by laborers.
12. Besides these, contrivances are required to raise heavy materials to the various positions which they are to occupy. These consist, for the most part, of one or two shafts, commonly the mast of an old vessel, to which are attached tackle extending in various directions, and also those by which the blocks are to be raised. The rope belonging to the hoisting tackle is pulled by a machine worked with a crank.
13. Masonry is one of the primitive arts, and was carried to great perfection in ancient times. The pyramids of Egypt are supposed to have stood about three thousand years, and they will probably remain for centuries to come, monuments as well of the folly as of the power and industry of man. The temples and other magnificent structures of Greece and Rome, exhibit wonderful skill in masonry, and leave but little, if anything new, to be achieved in modern times.