Page:Graphic methods for presenting facts (1914).djvu/114

 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 yearly figures could not be obtained. If yearly figures were obtainable the slanting lines could be extended year by year until they 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 be 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.

Taking the peak for February, 1912, we see that the total consumption averages during the month 142,000,000 gallons per day. Reading this same month from the slanting lines we observe that the average consumption per capita daily was 127 gallons. Notice, that while the total consumption in gallons was much larger in February, 1912, than in January, 1909, as seen by considering the horizontal lines, the consumption per capita in February, 1912, read from the slanting lines, was somewhat less than in January, 1909.

If we consider the growth in the population of the city of Boston, it is permissible that the total water consumption in 1912 should be greater than in 1908. In spite of the large growth of the city from 1908 to 1912, there has been a general decrease in the total quantity of water consumed. The decrease in total consumption is chiefly due to the metering of water to individual users, eliminating a large part of the water waste which formerly occurred because of carelessness on the part of consumers. The actual percentages of the services which were metered in each one of the years considered may be seen