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Rh to insure its being pure, and is then pumped up into service-reservoirs, whence it is distributed in the usual manner. The Chicago tunnel is three miles long, that of Milwaukee is shorter.

The Washington Aqueduct leads from a reservoir which impounds the river-water at the Potomac Falls, is 16 miles long, and supplies the cities of Washington and Georgetown. Its capacity is 70,000,000 gallons per day. The water is conveyed in a brick-and-rubble masonry conduit, of circular form, to the service-reservoir five miles from the city, and the rest of the way in three large cast-iron pipes capable of delivering 30,000,000 gallons per day. This aqueduct was constructed at the expense of the United States Government, and cost $3,000,000. It has several fine bridges, of which the most notable is the one across Cabin John Creek. This is a single granite arch, 100 feet high and 220 long. Another remarkable example of the wide, single arch occurs on the Lisbon Aqueduct, finished in 1738. It is 115 feet wide and 250 high.

By far the finest aqueduct in America is the Croton. This was begun in 1837, and finished in 1842, at a cost of $8,575,000, without the means of distribution, which cost $1,800,000 more. The length of conduit from the impounding to the receiving-reservoirs in Central Park is 38¼ miles, for 33 of which the conduit is built of stone, brick, and cement, arched above and below, 8 feet 5 inches high, 6¼ feet wide at the bottom, and 7⅔ at the top. The water crosses Harlem River in two cast-iron pipes 3 feet in diameter, and one wrought-iron pipe 7 feet 6 inches in diameter, laid upon High Bridge, a magnificent granite arcade 1,460 feet long and 114 high. It comprises 15 arches, 7 of which have 50 feet span, and 8, those over the river, 80 feet. The fall is 1.10 foot per mile, the velocity of the water 1½ mile an hour, and the possible discharge 115,000,000 gallons per day.

For the first six years after the completion of the aqueduct, the quantity of water used was only 18,000,000 gallons per day, but it has now increased to over 88,000,000. The supply is drawn from Croton River, a small stream that flows into the Hudson, a short distance above Sing Sing. The river was arrested by a dam 40 feet high, and made to form what is now called Croton Lake. The mouth of the aqueduct is 12 feet below the surface of the lake, whereby it is protected from freezing up in winter, and the water is obtained pure and cool in summer. The lake has an area of 400 acres, and usually affords a daily supply of 50,000,000 gallons; but this fell off, during a severe drought seven or eight years ago, to only 27,000,000, and since then another source has been added by damming up the western branch of Croton River. The receiving-reservoirs, two in number, are located in Central Park: the "old" covers 35 acres, and holds 150,000,000 gallons; the "new" covers 100 acres, and holds 1,030,000,000. The distributing-reservoir is situated 2¼ miles farther down, between Fifth