The total volume in the structure is 99-2/7 cubic feet or 3-2/3 cubic yards. Multiplying the quantities in line 7 of Table 1 by 3-2/3, it is found that the following material is needed: 22.14 sacks of cement; 1.65 cubic yards of sand; 3.27 cubic yards of gravel. [1] A practical rule in finding the area of a circle is to multiply one-half the diameter (radius) by itself and the product by 22/7. In finding the volume in the wall of a circular structure, such as a silo or tank, the area of the circle formed by the inside circumference is deducted from the area of the circle formed by the outside circumference and the remainder is multiplied by the height. FORMS. Forms are required to hold the concrete in place until it has attained sufficient strength to sustain itself and the initial loads to which it may be subjected. Concrete is plastic and will assume the shape of the form, thus any imperfection or impression on the face of the forms will be reproduced. Wood is commonly used for forms, as it can be easily worked into different shapes, though various other materials sometimes are better adapted to special conditions. Cast iron, for instance, is suitable for casting small objects that are to be reproduced in quantities, such as concrete block or tile; plaster of Paris, glue, or moist sand are employed for casting ornaments or to produce a fine, smooth surface; sheet metal is suitable when the forms can be used repeatedly or for such circular structures as silos. When the sides of an excavation are not likely to cave in the earth may serve as a form. WOOD FORMS. Wood for forms must be of a kind that is easily worked and that will retain its shape when exposed to the weather. White pine is the best wood, but is seldom used because of its cost. Spruce, yellow pine, and fir are satisfactory woods for forms and are best, used partially green or unseasoned. The edges of boards should be surfaced, tongued and, grooved, or beveled in order to obtain a tight form, so that the soft mortar will not ooze out. A better surface* is secured if the boards are dressed on one side and are free of loose knots or other imperfections. As forms must be removed, they should be so planned that they can be taken down without destroying the lumber, especially if the boards are used for sheathing or again for forms. Therefore the nailing of the boards to the support should be only sufficient to keep them in place until the concrete has hardened. Greasing the surface next to the concrete with crude oil, soap solution, or linseed oil will prevent the concrete from adhering and facilitate removal. METAL FORMS. Metal forms can be used to advantage when the work involved is to be repeated many times. If it is known or if it is probable that the forms may have to be altered, the relative costs of wood and metal forms should be carefully determined.