As Fig. 81 is shown it is necessary for the person interpreting the chart to select from the vertical scale some number of feet below the surface and then read the number of bacteria per cubic centimeter by the horizontal distance to the right. It is only after some little puzzling that the reader will notice that the scales for the variables have been reversed and that the chart has been practically turned on its side. How this chart would appear if the horizontal scale were used for the independent variable may be judged by turning the book and looking at Fig. 81 from the left-hand side of the page. Though it is easy to see why the person making Fig. 81 happened to arrange the chart in the manner shown with the variables reversed, the gain due to showing depth below the surface in the vertical direction does not make up for the possibility of misinterpretation which results because of the neglect to follow standard practice.

In Fig. 82 we again have depth plotted downward from the top of the chart. As we wish to determine the velocity of the stream at different depths of water, depth is the independent variable and velocity is the dependent variable. The arrangement of Fig. 82 is not as objectionable as Fig. 81, for the upper half of the illustration sliows quite clearly in pictorial form that the subject under consideration is a stream having a channel shaped as shown, with widths and depths as indicated by the two scales. In the bottom portion of the diagram the scale of depths downward relates very definitely to the upper portion of the illustration so that the reader cannot easily go astray. Notice that the curves for the velocity of the water are each plotted on a separate vertical line which serves as zero line.