+----------+-----------+------------+------------+------------+ | Number | F = 30. | F = 40. | F = 60. | F = 100. | | of Years | | | | | | whose | | | | | | Supply | | | | | |is to be +-----+-----+-----+------+-----+------+-----+------+ |Equalised.| R. | S. | R. | S. | R. | S. | R. | S. | +----------+-----+-----+-----+------+-----+------+-----+------+ | 1 | 35 | 300 | 79 | 695 | 166 | 1475 | 345 | 3040 | | 2 | 85 | 470 | 140 | 900 | 258 | 1815 | 495 | 3600 | | 3 | 120 | 560 | 190 | 1050 | 329 | 1987 | 610 | 3900 | | 4 | 150 | 620 | 230 | 1110 | 390 | 2103 | 710 | 4100 | | 5 | 175 | 650 | 260 | 1170 | 441 | 2187 | 800 | 4230 | | 6 | 195 | 680 | 290 | 1220 | 487 | 2255 | 887 | 4320 | +----------+-----+-----+-----+------+-----+------+-----+------+ In all cases the loss is supposed to be 14 inches annually. If it is 15 or 13 inches, the reservoir capacity is less or more by about five, ten, or fifteen million gallons, according as the number of years in column 1 is 1, 3, or 6. And the daily yield is less or more by about 50,000 gallons. With a low rainfall the advantage of a large reservoir is somewhat increased. The capacity of the six-year reservoir for a fall of 30 inches is 63 per cent. more than that of the three-year reservoir, but the supply is 22 per cent. greater. The figures given above for reservoir capacities are suitable for the British Isles. They assume that the distribution of the rainfall is the least favourable that is at all likely to occur. Deacon states that the figures do not relieve the engineer of the exercise of judgment. As regards the British Isles, the chief questions on which judgment has to be exercised are whether to equalise the flow of three years or of another number, and how much to allow for loss. As already stated, three years is the period usually taken. The figures are suitable for most places in Europe, but in some places, _e.g._ on the Mediterranean coast, the distribution of the rainfall is somewhat less favourable than in the British Isles. In other parts of the world, and notably in or near the tropics, the distribution of the rainfall must be specially studied, and a diagram be prepared on the same principle as in the case of fig. 58. The diagram should be extended to cover the desired number of years. In hot countries loss by evaporation from the surface of the reservoir should be allowed for. In India during the hot dry months this loss may be half an inch in twenty-four hours.