number of gallons used per day has been divided by the number of inhabitants in the district so as to obtain a figure for the average daily consumption of water per capita. As the population figure used depends upon census records it may be necessary to get the rate of growth in the population from records as much as ten years apart. In Fig. 90 it can be observed that the slanting lines showing the rate of growth of the city are straight lines, indicating probably that the census figures were used in the drawing of these lines because j-early figures could not be obtained. If yearly figures were obtainable the slanting lines could be extended vear bv vear until thev reached completely across the chart. The rate of growth in population determines the angle of the slanting lines, the more rapid the growth the greater the angle of the lines.

The slanting lines are located on the page so that a curve can l)e read from either the horizontal lines or from the slanting lines. The method of locating the slanting lines can be worked out by anyone who will experiment a little in making a chart of this type. When the population is known and the total consumption is known, it is only a matter of division to determine the consumption per capita. After the slanting lines are once placed upon the chart, the curve can be read either from the horizontal lines showing the total consumption or from the slanting lines showing gallons per capita.