=14.= The ordinary rules of mensuration are all that are needed to compute the =volume= of any excavation. The work is very simple when the area to be removed is regular; but when the outlines are very irregular and broken, the easiest method to employ in calculating the excavation is to divide the plan into geometrical figures that are easy to compute, and then calculate the area of each one separately. Adding these areas and multiplying their sum by the depth of the cellar will give the volume of the excavation. This method will be made clear by referring to Fig. 1, which represents the plan of an irregular foundation. To compute the area of the excavation, the plan is divided into the rectangles _a d c b_, _l k b m, j i h g, g f e c_, and the polygons _n q p o, t u r s_, and _a x w v_. By scaling on the drawing the dimensions of these figures, the area of each may then be readily determined by calculation. [Illustration: FIG. 2] =15.= It is sometimes necessary to find the volume of an excavation, the surface of which is very irregular, as in Fig. 2. In such a case, the following method may be used: Divide the surface of the excavation into a number of squares, or rectangles, as at _d e f c_; these represent the ends of prisms, the other ends of which are the bottom of the excavation, as at _a h g b_. Then calculate the volume of each prism by ascertaining the height of the four corners above the bottom; add these measurements together, divide the sum by 4 (the number of corners), and multiply the result by the end area, as _a h g b_; the product will be the volume of the prism. The sum of these partial volumes will be an accurate estimate of the contents of the excavation. CONCRETE WORK =16. Plain concrete work= is usually paid for by the cubic yard. The contractor furnishes all material, including the lumber, to make the forms; he also erects the forms and removes them after the concrete has been placed. There is no fixed practice regarding openings in walls. Usually, small openings under, say, 100 square feet in superficial area are considered as solid. All larger openings are deducted from the work when measuring for payment. In some localities, the actual volume of the concrete work is taken by the contractor as a basis of the cost. In either case, it is of prime importance that the architect and the contractor make some distinct agreement _beforehand_ as to exactly how the concrete work is to be measured and paid for.