I'sing the horizontal ruled lines, we may read from the curves the average total consumption per day. By reading from the slanting lines, the same curves may be interpreted as the average consumption per capita per day. The scheme of using two sets of co-ordinate rulings is a valuable one. The scale for "million gallons per day" should, however, have been shown only at the left, with the slanting line scale for "gallons per capita" placed in the right-hand margin for the sake of clearness

by referring to the figures near the upper portion of the chart. Note that in 1908, 21 per cent of the services were metered, while in 1911, 45 per cent were metered. The proportion of the services metered in 1912 was not known at the time the chart was made and hence is not recorded.

A httle study of Fig. 90 will show that there is a very striking similarity in the shape of the waves for different calendar years. Water consumption is high in the winter and again high in the summer months, with the lowest point each year usually found in November. The exact amount of resemblance of these waves to each other could be determined in an interesting manner if a separate curve were plotted for each year so that all the curves would be shown one above the other in the manner seen in Fig. 103.

96 GRAPHIC METHODS

Just how much the total consumption in water has been decreased, even though there was an increasing population, may be seen by referring to the dotted line on the chart. This dotted line was plotted from points which represent the average for the whole of each year. Thus, the average in 1905 was about 122 million gallons per day and in 1911 about 113 million gallons per day.