Fig. 237 shows a solid model of the type which may be considered the acme of graphic work when there are two independent variables. A model of this kind is ordinarily made of plaster of Paris, as that is a material easily handled and capable of being made into any shape desired. In making such a model the usual procedure would be to rule a flat board with lines at properly spaced intervals for each of the two horizontal scales. Computations made by methods similar to those described for Fig. 236 would give the value, on the vertical scale, for each set of conditions corresponding to the intersection of each two lines ruled for the base and showing the horizontal scales. In Fig. 237 the cost computations can be considered to give points on curves drawn on the surface of the solid model. These curves correspond exactly with the curves drawn on cardboard in Fig. 236. In order to locate the curve points which determine the surface of a solid model like Fig. 237, wires are driven into the bottom board at the intersection of the ruled lines of the two horizontal scales, and these wires are made just the proper length to represent the figures computed for the dependent variable. When all the wires are in position on the board, a box is made the right size for the base of the finished solid model, and with sides as high as the solid model is to be made. This box serves as a mold into which the wet plaster of Paris is poured. Care must be taken to have the bottom edges of the box fit well on the board so that the liquid plaster of Paris may not leak out.