tunnel is 8½ miles in length, and the longest inverted siphon, of iron pipe, is about the same length. Another inverted siphon is under a head of 480 feet. The masonry aqueduct is 7 feet in diameter. In the United States notable aqueducts were completed by New York in 1842 and a second in 1890 (old and new Croton); Boston in 1848 and a second in 1878; Brooklyn in 1859); Baltimore in 1862 and a second in 1880; Washington in 1863, with a second one begun in 1883, abandoned before fully completed and nearing completion in 1901; St. Louis, Mo., about 1893: the Metropolitan Water Board (Boston and surrounding towns) in 1897; Jersey City, in progress in 1901, but on this work masonry aqueducts and tunnels are used only where steel pipe lines are not available. Cast-iron, steel, or wood pipe is used in place of masonry aqueducts for nearly all American water-works, especially in recent years, and with the introduction of riveted steel pipes, the likelihood of using masonry is still further decreased. The one exception to this is in the case of tunnels, and particularly the intake tunnels through which Chicago and other cities on the Great Lakes draw their supply. These intakes, however, hardly come in the same category as the aqueducts described here.
The Old Croton Aqueduct, supplying New York City, has a total length of 38.1 miles and a total fall of 43.7 feet, the ordinary grade being 1.1088 feet per mile. It is of brick-lined masonry, the bottom being an inverted arch of 6.75 feet chord, 0.75 feet versed sine; sides, 4 feet high, battered to 7.42 feet apart at top; covered with semi-circular arch, giving total interior height of 8.64 feet and cross-sectional area of 53.34 square feet. The Harlem River is crossed on a granite masonry arched bridge, 100 feet high in the clear, and about 1400 feet long, the water being conveyed in two 36-inch cast-iron and one 90½-inch wrought-iron pipe, the latter added in 1860. The Manhattan Valley is crossed by inverted cast-iron pipe siphons, two miles long, the original two 36-inch pipes being supplemented by a 48-inch in 1853 and a 60-inch in 1861, the latter being reported as the largest iron pipe cast up to that time. The aqueduct was designed to carry 72,000,000 gallons a day. In 1865, the portion of aqueduct below Ninety-second Street was replaced by two 72-inch cast-iron pipes, for which three 48-inch pipes were substituted later on. In 1870 another length of aqueduct within the city was replaced by six parallel lines of 48-inch cast-iron pipe, ¾ mile long. This aqueduct was carried as near the surface as the grades would permit. The New Croton Aqueduct, like the old one, begins at Croton Lake, formed by a dam on the Croton River, and extends to 135th Street, New York City. Its total length is 30.87 miles, or 33.25 miles if the pipe line extension to the Central Park reservoir is included. Of the masonry aqueduct, 29.63 miles is in tunnel, requiring shafts from 18 to 402 feet deep for its construction. In general the aqueduct is shaped like a horseshoe, 13.53 feet high and 13.6 feet wide, has a fall of 0.7 foot per mile and an original rated carrying capacity (see below) of about 318,000,000 gallons a day. At the Jerome Park storage reservoir, in the north part of the city, and some 23 miles from the upper end, it is reduced to a rated capacity of 250,000,000 gallons a day and changed to a circular section, 12½ feet in diameter, for over 6 miles. It crosses beneath the Harlem River, still as a masonry aqueduct, under 55 pounds pressure, when full, the aqueduct here being 10½ feet in diameter, lined with cast iron. The cost of the aqueduct varied from $89.98 to $123.25 per lineal foot in different sections and under varying conditions. When the new aqueduct was designed it was estimated that it would carry 318,000,000 gallons a day, when flowing to a depth of 12.842 feet in the horseshoe sections. Gaugings after its completion fixed the carrying capacity at about 302,500,000 gallons. Careful studies made by Mr. John R. Freeman in 1899 (Report Upon New York's Water Supply, New York, 1900) led him to conclude that the aqueduct was then carrying 16 per cent. less for stated depths than shown by the earlier gaugings, part of the difference being due to deterioration of the inner surface.
The Wachusett Aqueduct for Boston and vicinity has a rated daily capacity of 300,000,000 gallons. It is 12 miles long, if the 3 miles of canal at its lower end are included, and leads from the site of a proposed masonry dam on the Nashua River, at Clinton, Mass., to the Sudbury reservoir, a part of the old Boston water-works now controlled by the Metropolitan Water Board. From this reservoir the water flows through the old Sudbury aqueduct, completed in Boston in 1878. The first two miles of the Wachusett aqueduct is in tunnel, through rock so compact that about one-half of it required no lining. Where lining was needed brick was used. The floor of the tunnel is of brick, with a slope of 1 foot in 5000 feet. After the tunnel comes 7 miles of aqueduct, with a grade of I foot in 2500 feet, built in embankment or in excavation. Both tunnel and covered aqueduct were built in the general shape of a horseshoe, from 11½ to 13½ feet wide and from 10½ to 11 feet 10 inches high, and were of concrete, with the lower portion lined with one course of brick. Below the section just described there are 3 miles of open channel, or canal. The aqueduct is carried over the Assabet River on a handsome granite masonry bridge of seven 29½-feet spans.
The Cabin John Arch, which carries the first Washington aqueduct across a creek of the same name, was for many years the largest single-span masonry bridge in the world, having a length of 220 feet, and rising to a height of 101 feet in the clear. The rise of the arch, from the spring line, is 57½ feet. The bridge is 20 feet wide and its total length is 420 feet. It was built of large granite blocks, with sandstone parapets and coping. It cost $237,000.
Consult: Frontinus, De Aquæductis (edited by Herschel, New York, 1900); Friedländer, Darstellungen aus der Sittengeschichte Roms (Leipzig, 1888-90); and Leger, Les travaux publics des Romains (Paris, 1875).
A'QUEOUS HU'MOR. The fluid which occupies the space in the eye between the back of the cornea and the front of the lens, which in fœtal life is divided into an anterior and a posterior chamber by the membrana pupillaris (q.v.), and in adult life by the iris. It consists of water, with, according to Berzelius, about a fiftieth of its weight made up of chloride of sodium and extractive matters held in solution. This watery secretion is produced by epithelial cells covering
