a simple chart. Paper having logarithmic ruling in both directions is the kind most frequently used for mathematical charts. Such paper can be purchased from almost any good store selling engineering supplies. Readers wishing to go further into the preparation of charts of this rather highly specialized character are referred to the excellent work of Mr. John B. Peddle, entitled "The Construction of Graphical Charts", published by the McGraw-Hill Book Company of New York City.
The use of isometric paper for drawing charts representing three dimensions was mentioned for Fig. 167. In Fig. 235 we have another application of this same isometric ruling. Instead of showing the whole sheet of isometrically ruled paper as a background, Fig. 235 shows only enough of the isometric ruling to give the impression of three planes meeting like one corner of a box. The chart illustrating the data is drawn on the isometric ruling so that it appears as though placed in a corner made by the three planes. Parallel ruled lines then permit reading the chart from any one of the three different scales. The dependent variable is made the vertical scale here, just as in most charts where there is only one independent variable instead of two. The use of the isometric ruling is not as common as it would be if it were more widely realized how easily charts may be prepared to give the effect of solid models without the great labor which a solid model necessarily involves.
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Data of E. S. Farwell. Courtesy of the American Machinist
Fig. 236. Tests of a Direct-connected Fan and Engine. The Model Shows the Effect on the Efficiency of the Fan of Different Outlet Openings and of Different Speeds of Operation
Curves are plotted on cardboard by assuming constant values
for one of the three variables, and then plotting the relations
for the other two variables. Each of the curves is
cut from the cardboard and slit halfway up or down the
line for intersection with the cards at right angles to it.
Cards are fitted together to give the effect seen above
Fig. 236 shows another method which may be used instead of the more laborious and costly solid model. Different curves are plotted by assuming at frequent intervals constant values for one of the vari-
