In order to get the shape of the model as seen in Fig. 237, the plaster of Paris is very carefully scraped away with a piece of tin or some other simple tool until the ends of the vertical wires just show through. Before the plaster of Paris is poured in, care should be taken to mark the different sides of the board so that it will be

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Fig. 237. Three-dimensional Model Showing Cost of Light in Cents per 1,000 Candle-hours with 40-watt "Mazda" Lamps for Any Practicable Combination of Efficiency and Smashing Point. Price of the Lamp is 50 Cents and Cost of Electric Current is Assumed at 10 Cents per Kilowatt Hour

Three variables are considered here. The two independent variables are represented at the base of the model and the dependent variable, "cost", is read from the vertical scale. 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