A model of this kind can be made from plaster of Paris by following the methods described in detail in this chapter

known in which portions of the plaster of Paris block the greatest amount must be removed before the wires come in sight. Knowledge of the position of the wires in the block of plaster of Paris permits removing the plaster rapidly without danger of taking off too much. After the wires do come into sight, the model must be carefully scraped so that the surfaces will have uniform curves without any humps or

338 GRAPHIC METHODS

hollows. If all the surfaces are concave, as they are seen to be in Fig. 237, the scraping is rather a simple matter since the surfaces between the two wires are lower than the wires themselves. If a solid model were made for data such as that of Fig. 236, much care would be necessary, for in that case some of the surfaces are convex and it would be essential that enough material should be left between the different wires to permit giving the nicely rounded smooth surfaces which would correctly represent the data. When the surfaces have been scraped as nearly as possible to the proper shape they maj^ be smoothed by rubbing with fine sandpaper. The external flat faces of the model may also be sandpapered advantageously to remove any marks which niay remain from the surface of the box used as a mold. Lines such as are seen in Fig. 237 can be ruled on the surface and the different scales can be lettered by hand. A few coats of boiled linseed oil will harden the surface and give an attractive finish.