One of the greatest and most important modern masonry aqueducts is that known as the Nadria Bridge which carries the great irri gation canals conveying the water of the lower Ganges canal over the Kala Naddi. Its length is very nearly one-quarter of a mile and the width between vertical ends of the arches is 150 feet. It is constructed with 15 arches of 60 feet span. The foundations were the most troublesome feature of its construction, as they consisted of 268 circular, concrete-filled, brick cylinders carried to a depth of about 50' feet below the bed of the river. The aqueduct was opened in 1889 and is said to have a capacity of 4,100 cubic feet per second.
The evidence of early aqueducts on the western continent is meager, and the remains of them so far discovered are few. It seems to be well established that the Indians of the western part of the United States constructed canals of considerable magnitude to supply water for irrigation. Recent exploration in Peru, in the heart of the Andes, has disclosed canals and aqueducts of great extent, built by the Incas or their predecessors, some of which are wonderful constructions considering their antiquity and the difficulties to be overcome. Consult Cook, 0. F., in National Geographic Magazine (May 1916).
The Spanish during their occupation of Mexico built aqueducts of considerable magni tude to supply the City of Mexico with water. The remains of at least one of these still exist in and near the city, consisting of an arched masonry structure quite like its Roman prede cessors. It terminated in the city with a masonry fountain structure of no little artistic merit.
There are three aqueducts now supplying water to New York city. The first, or old Cro ton Aqueduct, was begun in 1837 and com pleted in 1843. It is mainly a masonry con duit, following the surface topography wher ever practicable, but there are tvho stretches of cast-iron pipe. It takes water from the Croton Reservoir, 41 miles north of the city. The Harlem River is crossed at a point about 11 miles north of the Battery by a masonry via duct, known as High Bridge, 1,450 feet long, the greatest height above the foundations being about 150 feet. There are eight semi-circular arches of 80 feet span and seven of 50 feet span. Both in design and construction it is a beautiful and substantial structure, a fitting monument to its builders. The aqueduct has a
capacity of 80,000,000 gallons per day and cost between 11 and 12 millions of dollars.
The new Croton Aqueduct was begun in 1887 and water was turned into it 15 July 1890. The water is taken from the same source the Croton Reservoir (or lake)— as the Old Croton Aqueduct, the two inlets being near each other. The new aqueduct follows the same general course, but is much more direct than the old and crosses the Harlem River near the masonry structure of the earlier Croton aqueduct. While the aqueduct proper ends at 135th street, the water is carried from there to the Central Park Reservoir through eight lines of 48-inch cast-iron pipe. The length is 30.87 miles, of which all but slightly over one mile is in tunnel. The capacity of the aqueduct is about 300,000,000 gallons per 24 hours.
The tunnels are lined with masonry and concrete, the section for 23.7 miles being of horseshoe form, 13 feet- 7 inches wide, and about the same in height. The remainder is of circular section, varying in diameter from 14 feet 3 inches to 10 feet 6 inches under the Har lem River. The difference in elevation between the crest of the new Croton dam and the sur face of the water in the Central Park Reser voir is 33.65 feet, and as the total distance is miles, the average hydraulic gradient is about 3.3 inches per thousand feet.
The most interesting construction feature of this aqueduct is the crossing of the Harlem River. At the eastern approach to this cross ing, where the tunnel is about 150 feet below the surface, it drops on a 15 per cent descend ing grade for about 850 feet, followed by a 2 per cent descending grade for 200 feet, where it enters a vertical shaft 12% feet in diameter and 168 feet deep. From the bottom of this shaft the tunnel crosses under the river upon a 1 per cent descending grade with a diameter of feet for 1,270 feet, and rises in a ver tical shaft 321 feet deep, from the top of which it .descends on a slight falling grade to 135th street. The bottom of the tunnel at the river crossing is 300 feet below mean tide. This aqueduct was, at the time, by far the greatest engineering feat of its kind in America, if not in the world. Its cost was very close to $30, 000,000.