We have concluded that it would be more advantageous to bind two volumes of this Encyclopædia in one volume. That is, volumes One and Two, Three and Four, Five and Six, Seven and Eight, and Nine and Ten, are each bound together; thus publishing the work in five bound volumes, instead of ten. THE NORMAN W. HENLEY PUBLISHING CO., MAN, NIST. >jects, simple rmulæ. The statement of rience in the AUTHOR OF PRACTICAL METAL TURNING," MODERN MILLING MACHINES," ETC., ETC. H.E. PATTERN MAKING," ASSISTED BY A CORPS OF PRACTICAL MEN, EACH A SPECIALIST OF PRACTICAL ENGINEERING AND ALLIED TRADES A PRACTICAL AND INDISPENSABLE WORK OF REFERENCE Encyclopædic in scope, thorough and practical in its treatment of technical subjects, simple EDITED BY JOSEPH G. HORNER, A.M.I.MECH.E. AUTHOR OF "PRACTICAL METAL TURNING,' MODERN MILLING MACHINES," TOOLS FOR MACHINISTS, AND WOODWORKERS," ETC., ETC. PATTERN MAKING," ASSISTED BY A CORPS OF PRACTICAL MEN, EACH A SPECIALIST 139330 FEB 15 1910 SBH ·438 9-10 The Encyclopædia OF Practical Engineering and Allied Trades. Specific Density. The density of any material is the mass per unit volume, and the specific density of a body is its density compared with that of a standard, (water at 4° Cent.). The number giving the Specific Gravity of a body is identical with that representing its specific density, because mass and weight are proportional to each other. It is important to remember, however, that mass stands for the quantity of matter a body contains, while weight represents the force with which a body is attracted to the earth. Therefore the same mass possesses different weights at different heights above the surface of the earth, or at the poles and equator respectively. The specific density of a body is thus only identical with specific gravity when the force of gravitation is constant. Specific Gravity. The specific gravity (abbreviated, sp. gr.) of a body is its weight as compared with that of an equal bulk of water; it is the relative density of a substance; the amount of stuff it contains in proportion to the volume. In the case of gases, either air, or hydrogen-the lightest of gases—is taken as the standard. Solids and liquids are compared with water, but since this alters in volume with change of temperature, it is necessary to adopt a fixed temperature. The temperature at which water possesses its maximum density, 4° Cent., is adopted, especially on the Continent, but in engineering work a temperature of 60° Fahr. (15.5° Cent.) is more generally selected. Hence specific gravities in Continental text-books are rather lower than those in English text-books, since a higher temperature increases the volume. At 4° Cent. a cubic centimetre of water weighs 1 gramme; therefore the weight of a cubic centimetre of any substance in grammes is its specific gravity. When we say that the sp. gr. of lead is 11.3, it means that the weight of a cubic centimetre of lead is 11.3 grammes, or that the weight of any volume of lead is 11.3 times the weight of an equal volume of water. As the specific gravity of a substance is the ratio of its weight to the weight of an equal volume of water, it may be determined by dividing the absolute weight, W, of a volume of the given substance by the weight, w, of a similar bulk of water: sp. gr. If the W w body is, however, of irregular shape, difficulties arise in estimating this ratio, and various methods are adopted in the case of both solids and liquids. The sp. gr. of a solid may be determined by the principle of Archimedesthat a body wholly or partially immersed in a fluid loses in weight an amount equal to the weight of the displaced fluid. Suppose the body placed in the pan of an ordinary balance weighed 43-4 grammes; suspended from the balance beam and weighed while immersed in water, 37.6 grammes. The loss of weight, 43.4-37.6 5.8 grammes, is the weight of an equal volume of water, and from the formula stated above, the sp. gr. of the substance is 43.45.87·4. Based on the same principle, the sp. gr. of a |