gates. Aqueduct bridges Where major water courses crossed the line of the Aqueduct it was necessary to build something larger than a culvert, and Jervis was forced to consider aqueduct bridges along the length of the Aqueduct between the Croton Dam and the north bank of the Harlem River. The most impressive of these aqueduct bridges was over Sing Sing Kill, it had a single span arch with a rise of 33 feet and a clear span of 88 feet (figure 27). The sophisticated design in which he used a five centered arch instead of a simple semi-circu- lar span was a solid piece of work founded on rock, with the stone masonry laid out in hydraulic cement. Jervis insisted on the use of hydraulic cement in all of the masonry work on the Aqueduct and its associated structures. Hydraulic cement differs from lime in that it will set under water and is water-proof, properties which Jervis felt were essential for the longevity of the Aqueduct. Jervis' concern for leakage and resulting damage in aqueduct bridge structures can be seen in the Sing Sing Bridge. Rather than having solid spandrel walls filled with earth, individual walls were carried out from the springing of the arch to a series of brick arches just below the line of the Aqueduct. Thus, any water leaking from the Aqueduct would pass into the void area and exit from the masonry work before it could do any dam- age. As further safeguard against leakage, Jervis, learning of the cast iron lining of the Chirk Aqueduct Bridge built by Telford in England, lined the Sing Sing Kill Bridge with cast iron plates. I S G I i I I I C The greatest natural obstruction to carrying the Aqueduct into Manhattan was crossing the Harlem River. There was much discus- sion of the most suitable way of effecting a crossing. An under- ground pipe passing under the river, designated by hydraulic engi- neers as an inverted siphon, was considered, together with a low level bridge carrying the water from the higher elevation of the Aqueduct in iron pipes across the river. There were concerns about the navigation of the river and doubts about a deep siphon because of Jervis' concern for leakage at the bottom, resulting from exces- sively high water pressure. After state legislation was passed requiring either a tunnel or a high-level bridge, Jervis convinced the water commission to build the High Bridge. A viaduct consisting of seven arches, each with an 50-foot span, and eight arches, each with an 80-foot span, was designed by Jervis to cross this river valley (figure 28). Since the level of the bridge was slightly below the grade line of the Aqueduct, Jervis designed twin wrought iron 30-inch diameter pipes to act as an inverted siphon, carrying the water for nearly 1,500 feet across the river at a height of 110 feet. In order to ensure an adequate flow of water, the grade line was dropped 2 feet from one end of the aqueduct bridge to the other. 31 Considering the time of its construction, it was a work on a stupen- dous scale hitherto unknown in America. In contemplating High Bridge, Jervis was aware of and corresponded with the engineers responsible for, the construction of the aqueduct bridge across the Potomac on the Alexandria Canal. He was also aware of the aque- duct bridges built on the suspension system by John A. Roebling, the first at Pittsburgh and four more on the Delaware and Hudson Canal, on which Jervis was employed earlier as an engineer. High Bridge was a far more ambitious engineering achievement, howev- er, comparing favorably with contemporary viaduct railway bridges constructed in Britain in both brick and stone and with classi- cal Roman aqueducts. In fact, it is in a class with the Roquefavour Bridge Aqueduct, which was completed in 1846 in France. The grade line of the aqueduct supplying water to Marseilles was 270 feet above the valley floor and of comparable length to High Bridge. The Croton High Bridge is without peer in antebellum America and is the centerpiece of the Aqueduct. figure 27. above left: Possibly by John B. Jervis, Aqueduct Bridge at Sing Sing (plan and elevation), c. 1837-39, watercolor and ink on paper Courtesy Jervis Public Library, Rome, NY, drawing #317. Photo: G. R. Farley figure 28. below left: Elevation of a Higli Bridge for Crossing Hariaem [sic] River, c.1839-40. watercolor and ink on paper Courtesy Jervis Public Library, Rome, NY, drawing #249. Photo: G. R. Farley figure 29. above: Pian of Effluent Pipes in East Side of Receiving Reservoir at Yorl< Hill, c.1841, watercolor and ink on paper Courtesy Jervis Public Library, Rome. NY. drawing #307. Photo: G. R. Farley Kemp Inverted siphons As indicated, it is possible