Sometimes an indirect path is better than a direct course; or, as the sailors say: “The longest way round is the shortest way there.” We can readily measure the contents of solids which are regular or fairly regular of outline. It is easy to compute or estimate the contents of a stone as hewn by a mason to form part of a wall, but to find the volume of a rough boulder by direct measurement is too difficult a task to be worth while. Let us have recourse, then, to an indirect plan which goes back to Archimedes: it will remind us of how the casting process evades the toil of chipping or hammering a mass of metal into a desired form. We take a vessel of regular shape, preferably a cylinder, duly graduated, and partly fill it with water. Any solid, however irregular, immersed therein, will at once have its contents declared by the height to which the water rises in its container, the water-levels before and after the immersion being compared. Incidentally we here have a means of ascertaining specific gravities. Weigh this body before and during immersion; comparison of the two quantities will tell the specific gravity of the body, that is its density as compared with that of water. For example a mass of iron which in air weighs 7.75 pounds will in water weigh 6.75 pounds, so that the specific gravity of iron is 7.75, the difference between the two weights being unity. Sometimes we wish to know the solid contents of a body which will not bear immersion in water; a mass of gum, for instance. In such a case we immerse the body in a graduated vessel filled with fine dry sand, carefully sifted free of hollow spaces. Both before and after immersion the sand is brought to a level which is carefully noted. The difference between these levels, measured in the graduations of the container, gives the solid contents of the immersed body. Measuring Refraction. The degree in which a crystal, or a particular kind of glass, bends a beam of light is usually measured by giving the crystal or glass the form of a prism, through which rays are sent. Sometimes a crystal is so small and irregular that this method is not feasible. Then the inquirer resorts to an indirect plan. He immerses the crystal in liquids which he mixes until the crystal disappears through ceasing to bend light differently from the surrounding bath. He then fills a hollow glass prism with this liquid, and in noting its refraction he learns that of the immersed crystal. [Illustration: Blenkinsop’s locomotive, 1811. Middleton Colliery, near Leeds, England.] Omission of Needless Elements.