TABLE Of the Ratios of the Successive Hardnesses of Bodies. Ductility is the property of being drawn out in length without breaking. This property is possessed in a pre-eminent degree by gold and silver, as also by many other metals, by glass in the liquid state, and by many semi-fluid resinous and gummy substances. The spider and the silkworm exhibit the finest natural exercise of ductility, upon the peculiar viscid secretions from which they spin their threads. When a body can be readily extended in all directions under the hammer it is said to be malleable; and when into fillets, under the rolling press, it is said to be laminable. There appears, therefore, to be a real difference between ductility and malleability; for the metals which draw into the finest wire are not those which afford the thinnest leaves under the hammer, or in the rolling press. Of this fact iron affords a good illustration. Among the metals permanent in the air seventeen are ductile and sixteen are brittle. But the most ductile cannot be wire-drawn or laminated to any considerable extent without being annealed from time to time during the progress of the extension, or rather the sliding of the particles alongside of each other, so as to loosen their lateral cohesion. TABLE Of the Ratio of the Ductility and Malleability of Metals. Conducting Powers of Various Substances. The conducting power of wood is very low; the softer woods being lower in this respect than those which are harder. Of metals, and some other substances, the following is the order, according to Despretz: Radiating Power of Various Substances. Bodies that have polished surfaces radiate heat less than those that are roughened, and metallic surfaces less than those of more imperfect conductors. The following are the proportions of some of each, according to Leslie : Lamp-black, Glass, Ice, A Tissue-paper, Rough lead, 100 45 100 20 98 Polished lead, 19 Reflecting Powers of Various Substances. Heat is reflected from the surface on which its rays fall, in the same manner as light; the angle of reflection being opposite and equal to that of incidence. The metals are the strongest reflectors of heat, in the following order, according to Leslie: Lead, 60 90 Tinfoil rubbed with mer., 10 85 85 70 Power of Various Substances to Transmit Heat. All bodies capable of transmitting heat are, more or less, transparent, though their powers of transmitting heat and light are not in the same relative proportions; as the following list of the relative powers of equal masses, determined by Melloni, will show: Showing the Scale of Proofs for Chain Rigging close-linked, &c.; the extreme Length of Links not to exceed five diameters of their size in Iron. MASONRY. Of the different kinds of Masonry. Masonry, in the general acceptation of the term, is the art of cutting or squaring stones, to be applied to the purposes of build ing; or, in a more limited sense, it is the art of joining stones toge ther with mortar, or otherwise. The ancients enumerate seven different methods in which they arranged the stones of their buildings. Vitruvius thus classes them: three of hewn or squared stones, threw of unhewn, and one a mixture of both methods. B 1. Net masonry. E This is represented in fig. 33, within the area DEFG, where the stones are squared and placed upon one of the angles, their joints thus forming a net-like appearance. This method, though very neat, is wanting in firmness and strength; for the oblique position of the stones, in regard to each other, gives them a tendency to separate rather than to form a compact assemblage of parts that unite in supporting each other. Whenever this form of masonry is employed, it is consequently necessary to keep the work together by a border of stones, having some other arrangement, one that is not only capable of supporting itself, but of overcoming the resistance of the net-like form. This is shown in the same figure at ABC; and where the network is merely a casing of stone to the brickwork of a wall, it will be found to answer tolerably well, and looks very neat. Fig. 1. 2. Bound masonry is that represented in fig. 2, and is remarkably strong. The perpendicular joints in each course fall directly in the middle of the stones composing the course below and above it; and while it has every requisite of solidity, the joints have, at the same time, a regular and pleasing appearance. 1 3. Greek masonry is that represented in fig. 3, where every alternate stone, as shown at AD, EF, and BC, is made of the whole thickness of the wall, and serves to bind together the stones which compose the external and internal faces of the building; and this may be called double binding, as from the perpendicular joints being somewhat similarly situated to that in bound masonry, it has also an additional binding, by extending to the courses above and below it, thus forming a compact and durable wall, which resists every effort to separate in any direction: 4. Masonry by equal courses. This method of uniting stones is shown in fig. 4, and only differs from the bound masonry in its being composed of unhewn stones, or rather in being formed of stones that are not so accurately cut, nor the edges so perfectly squared; it being only necessary that the external face should be level, and the horizontal joints at equal distances from each other, care being taken at the same time that the perpendiculars are so situated as to bind the courses above and below them. Fig. 4. D 5. Masonry by unequal courses. This is represented in fig. 5, and is, like the last, formed of unhewn stones, without any regularity as to their size, it being sufficient that each course is made to bind with the preceding, and the only regularity observed is in the joining which separates each course, the courses themselves being of unequal thickness, as shown at ABCD. 6. Masonry filled up in the middle, as shown in fig. 6, is formed of un B Fig. 5. hewn stones of unequal courses, and the middle, as at D, is filled up with stones thrown in at random among the mortar. 7. Compound masonry is, as its name imports, a mixture of the other kinds. It is represented in fig. 7, where the external course AB is formed of bound masonry, and the corresponding internal course is at some distance from it, but held to the former by means of iron cramps, as shown at a, b, c, d, e, f, the space between being filled in with small stones or flints thrown into the mortar. |