Metal forms of various types and designs may be purchased. Although the first cost may be high, yet their use may lower the total cost when the work is such as to warrant it. Circular forms may be built as shown in Figure 3. The sheathing is generally of wood 4 to 6 inches wide, or sheet metal, and, if of wood, is laid perpendicular to the battens. In forms of small diameter, sheet metal sheathing is necessary if a smooth surface is desired, as the 4-inch boards can not be made to conform to a true circle. The radius used for cutting the battens of the inner circle should be the thickness of the sheathing less than the inside radius of the structure and the same amount greater than the outer radius for the outside battens. REMOVAL OF FORMS. The period of time after which forms may be removed varies according to conditions. Rich and dry mixtures set quickly, and warm weather tends to hasten the setting of concrete. The character of the structural member and the loadings also must be considered. Thus, an unloaded wall 12 inches or more thick may be stripped of forms in from 1 to 3 days, while the forms of thinner walls should remain in place from 2 to 5 days. Slab forms and the sides of beam and girder forms may be removed in from 6 to 14 days if the span is not over 7 feet. The bottoms of beam and girder forms, even though of a span less than 7 feet, should remain in place and braced form 10 to 14 days and even longer. Experience is the best guide to the time of removal, but if there is any doubt ample time should be allowed, especially in cold weather. [Illustration: Fig. 3.--Suggestion for circular form.] BUILDING AND SETTING FORMS. Concrete, while plastic, exerts a great pressure on the confining walls, necessitating rigid tying and bracing of the forms to keep them from bulging out of alignment. The effect of the bulging of a form is corrected only at a considerable expense; hence it is advisable to pour the concrete to a depth of not more than 2-1/2 or 3 feet, allowing it to set or harden before pouring more. The form most used in concrete construction is that for a straight wall. The methods of building such a form apply in general to the forms for most structural work, though modifications may be necessary to meet particular conditions. [Illustration: Fig. 4.--Form for basement or cellar wall. The earth may be used as an outside form if it is sufficiently firm.]