The sewers are designed on a very liberal basis, namely, on the assumption that when flowing half full the quantity to be dealt with will be 380 liters per capita per day, with a maximum rate of flow of 200 per cent. It was assumed that each house would be occupied by 7 persons and have a frontage of 12-1/2 m. The minimum velocities in the sewers, when running full, vary between 0.91 and 1.5 m. per sec., with the exception of a few blocks. The minimum size adopted was 24.3 cm. (8 in.) in internal diameter. The sewers of diameters between 24.3 and 50 cm., are 0.91 m. (36 in.) long, and are of salt-glazed vitrified clay, imported from San Antonio, Tex. Table 12 gives the details of the length of the various sewers laid. TABLE 12.--LENGTH OF SEWERS. +----------+------------------------------------------+-----------+ |DIAMETER: | | | +-----+----+ Kind. | Length, | | cm | in.| | in meters.| +-----+----+------------------------------------------+-----------+ |24.3 | 8 | Fire-clay | 38,332.85 | |25.4 | 10 | " | 16,400.69 | |30.5 | 12 | " | 7,953.15 | |38.1 | 15 | " | 4,850.56 | |45.7 | 18 | " | 2,023.40 | |50.8 | 20 | " | 1,450.53 | |55.9 | 22 | Reinforced concrete tubes, 6.9 cm. thick | 3,134.20 | |61.0 | 25 | " " " 7.6 " " | 357.40 | |68.6 | 27 | Brick and concrete | 484.05 | |76.2 | 30 | " " " | 662.69 | | | | | | | | | Total | 75,649.15 | +-----+----+------------------------------------------+-----------+ The greater number of the manholes are of brickwork, 23 cm. thick, and have concrete inverts. They have a diameter of 1.2 m., which is reduced to 0.61 m. at the top, and each is provided with a heavy cast-iron frame and closed cover weighing about 190 kg. There are 521 manholes, and they are placed at every block and on long lines about 80 m. apart. [Illustration: FIG. 16.--STANDARD 300-GAL. FLUSH TANKS.] The sewers are flushed with 15-cm. (6-in.) automatic flushing siphons of the Miller pattern with 20-cm. (8-in.) discharge pipes. There are 278 of these siphons, and they are placed in flush-tanks (Fig. 16) built of brickwork and plastered with 1:1 cement mortar. Their capacity varies from 800 to 1,200 liters, and they discharge from 22-1/2 to 28-1/2 liters per sec. They are timed to flush once in 24 hours.