498 



SCIENCE. 



[Vol. V., No. 124. 



very large capacity, placed low enough in the 

 Croton valley to increase to 361.82 square 

 miles the available area of the watershed of 

 Croton River. This reservoir is to have a 

 capacity of 3,200,000,000 gallons, — a body of 

 water which would cover 9,400 acres ten feet 

 deep. 



The dam which is to form this reservoir (the 

 Quaker-bridge dam), 178 feet high above the 

 bed of the river, is to be built of solid ma- 

 sonry, and the water behind it is to be 171 

 feet deep. As the foundations of the dam 

 must be extended to the bed-rock, a distance 

 of nearly 100 feet below the bed of the stream, 

 the total height of the masonry structure will 

 consequently be not far from 300 feet for a 

 length of 400 feet in the deeper portion of 

 the valley. On both sides of this deeper por- 

 tion the rock-bottom rises gradually, and the 

 total length of the dam is to be about 1,300 

 feet. 



The height mentioned for a masonry dam 

 is unprecedented ; and the strains which will 

 be transmitted to the base of the structure 

 by the combined action of its own weight and 

 of the water-pressure are such as to require in 

 the design a departure from the methods used 

 and recommended by the engineering author- 

 ities who have studied the question of high 

 masonry dams of lesser magnitude. The 

 width of the dam at its base, although not 

 fully decided upon, is to be more than 200 

 feet. 



The question of providing an overflow to 

 liberate the surplus water which must be 

 wasted over the clam is happily and economi- 

 cally solved by nature, which has provided 

 in the immediate vicinity a depression in the 

 rock-formation, of the required elevation and 

 form for the safe disposal of the freshets. 



The new aqueduct starts from a point near 

 the present Croton dam, and follows a general 

 southerly direction towards the city, to 135th 

 Street, with a length of nearly thirty-one miles. 

 For the remaining distance, from 135th Street 

 to the reservoir in Central Park (two and one- 

 third miles), the water is to be conveyed in 

 pipes. Harlem River is crossed by means of 

 an inverted siphon 150 feet below the water 

 surface. 



With the exception of three points where it 

 comes to the surface of the ground for short 

 distances, the aqueduct is to be wholly in tun- 

 nel ; and from the indications furnished b} T the 

 topographical character of the country, and 

 by numerous borings made with the diamond 

 drill, it is probable that the excavation is to 

 be, almost for the whole length of the aqueduct, 



in solid rock. It is expected that a large pro- 

 portion of the tunnel excavation is to be lined 

 with masonry ; but, wherever the character of 

 the rock is such that it can remain exposed 

 without danger of falls, the masonry is to be 

 dispensed with. If the line of work had been 

 so located as to allow of the construction of 

 the aqueduct in open trenches of moderate 

 depth, it would have been much longer, owing 

 to the necessity of following the contours of 

 the land ; and it would have passed along the 

 east shore of Hudson River, through thickly 

 settled communities, where the land-damages 

 would have been much higher. The tunnel 

 presents also the important advantage of being 

 almost wholly safe from the attacks of a mob 

 or of a military foe. 



From Croton dam to a point south of and 

 near the boundary of the cities of Yonkers 

 and New York, the aqueduct has a maximum 

 flowing capacity of 320,000,000 gallons per 

 day: it is 13.6 feet high and 13.6 feet wide; 

 and its section is that of a semicircular arch, 

 supported on slightly concave sides, the bottom 

 being formed by a flat inverted arch. 



At the point just mentioned, where it is ex- 

 pected that a large distributing reservoir is to 

 be built to supply the annexed district, and 

 where consequently a portion of the supply 

 is to be diverted, the flowing capacity of the 

 aqueduct is reduced to 250,000,000 gallons per 

 day, and its form is circular, with a diameter 

 of twelve feet three inches. 



This part of the aqueduct, over six and a 

 half miles in length, including the inverted 

 siphon under Harlem River, is to be heavily 

 lined with masonr}^ ; and, owing to the insuf- 

 ficient elevation of the land, it is depressed to 

 a considerable depth, presenting the peculiar, 

 and to a certain extent experimental, feature 

 of a masonry channel built in solid rock, and 

 subject to a considerable internal water-press- 

 ure. Its prototype, the tunnel under Dor- 

 chester Bay, which conve} T s the sewage of 

 Boston to Moon Island, has been in successful 

 operation for more than a year, but under 

 somewhat different conditions of location, size, 

 and pressures. 



For the purposes of construction and of 

 future maintenance of the aqueduct, thirty- 

 two shafts are provided, of various. depths, the 

 greatest being 350 feet. Twenty-four shafts 

 are under construction, twelve of which are 

 already completed, or nearly so. 



Six extensive gate-chambers are to be con- 

 structed, in connection with the aqueduct, for 

 the purpose of emptying it when necessary, 

 and of regulating 1 the flow of water from the 



