_To find the solid content of a rectangular parallelopiped._ Multiply together three sides which meet: the result is the number of cubic units required. If the figure be not rectangular, multiply the area of one of its planes by the perpendicular distance between it and its opposite plane. _To find the solid content of a pyramid._ Multiply the area of the base by the perpendicular let fall from the vertex upon the base, and divide by 3. _To find the solid content of a prism._ Multiply the area of the base by the perpendicular distance between the opposite bases. _To find the surface of a sphere._ Multiply 4 times the square of the radius by 3·1415927. _To find the solid content of a sphere._ Multiply the cube of the radius by 3·1415927 × ⁴/₃, or 4·18879. _To find the surface of a right cone._ Take half the product of the circumference of the base and slanting side. _To find the solid content_, take one-third of the product of the base and the altitude. _To find the surface of a right cylinder._ Multiply the circumference of the base by the altitude. _To find the solid content_, multiply the area of the base by the altitude. The weight of a body may be found, when its solid content is known, if the weight of one cubic inch or foot of the body be known. But it is usual to form tables, not of the weights of a cubic unit of different bodies, but of the proportion which these weights bear to some one amongst them. The one chosen is usually distilled water, and the proportion just mentioned is called the _specific gravity_. Thus, the specific gravity of gold is 19·362, or a cubic foot of gold is 19·362 times as heavy as a cubic foot of distilled water. Suppose now the weight of a sphere of gold is required, whose radius is 4 inches. The content of this sphere is 4 × 4 × 4 × 4·1888, or 268·0832 cubic inches; and since, by (217), each cubic inch of water weighs 252·458 grains, each cubic inch of gold weighs 252·458 × 19·362, or 4888·091 grains; so that 268·0832 cubic inches of gold weigh 268·0832 × 4888·091 grains, or 227½ pounds troy nearly. Tables of specific gravities may be found in most works of chemistry and practical mechanics. The cubic foot of water is 908·8488 troy ounces, 75·7374 troy pounds, 997·1369691 averdupois ounces, and 62·3210606 averdupois pounds. For all rough purposes it will do to consider the cubic foot of water as being 1000 common ounces, which reduces tables of specific gravities to common terms in an obvious way. Thus, when we read of a substance which has the specific gravity 4·1172, we may take it that a cubic foot of the substance weighs 4117 ounces. For greater correctness, diminish this result by 3 parts out of a thousand. THE END. =WALTON AND MABERLY’S= CATALOGUE OF EDUCATIONAL WORKS, AND WORKS IN SCIENCE AND GENERAL LITERATURE. =ENGLISH=.