In the article above quoted it is shown that if, as commonly happens, the consumption of water is, in summer, greater than the mean, and in winter less, the conditions are still more trying for the reservoir; and that in the case where the summer consumption is 13 per cent. greater than the mean, the capacity of the reservoir which impounds the water of the driest year must be 33 per cent., instead of 30 per cent., of the total supply impounded during the year. It would then contain 121 days’ instead of 110 days’ mean supply. In the table on page 170 the number of days’ supply is 113. From this it appears that the tables from which extracts have been given are calculated on the basis of a constant consumption. This, however, in the case where the number of years whose supply is equalised is greater than one, makes, owing to the increased size of the reservoir, no practical difference. The calculations for the great reservoirs in Radnorshire for the supply of the city of Birmingham are as follows (_Min. Proc. Inst. C.E._, vol. clxxx.). The ratio of the mean fall in the three driest years to the mean annual fall was taken as ·80 instead of ·77. There is some difference of opinion as to the best figure:-- Mean annual fall determined from readings of various gauges 65 inches Mean fall of three driest years 52 ” Deduct loss from evaporation and absorption and losses during floods 15 ” Available rainfall 37 ” This multiplied by 44,000 acres, the area of the catchment, gives 102 million gallons per day. Of this, 27,000,000 gallons is compensation water, leaving 75,000,000 gallons for Birmingham. Capacity of reservoirs, 17,250,000,000 gallons, or 169 days’ supply. [Illustration: FIG. 59.] 3. =Earthen Dams.=--Before an earthen dam is made, any soft soil on the site should be removed and the ground downstream of the site should be drained. A few trenches, running parallel to the axis of the dam, can be dug so as to give the dam a hold, though there is never any danger of its being moved horizontally by the thrust of the water. If the ground has a side-long slope it should be benched as shown in fig. 59. The front slope of an earthen dam is generally about 3 to 1, and the rear slope about 2 to 1. The top has a width of ⅓ to ½ the greatest depth of water held up, and is 5 to 10 feet above the highest water-level. The borrow pits from which the earth for the dam is got should not be near enough to it to in any way affect its stability. [Illustration: FIG. 60.]