designing a new dam with a much larg- er spillway capable of taking a larger quantity of flood water, Jervis was forced to build the Dam, in part, on a gravel bottom since the rock ledge did not extend across the river valley. Even though Jervis designed what he called an artificial foundation, he was very con- cerned that the water over the spillway not scour the foot of the dam and ultimately lead to its destruction. He faced this new chal- lenge with a very elegant solution that consisted of two parts. Rather than letting the water simply fall over the spillway, he designed a spillway to match the lower nape of the discharge (fig- ure 24). This design made it possible to ease the water down the face of the Dam and discharge it horizontally onto an apron (figure 25). In addition, much energy could be dissipated in a back water pool. This Jervis provided by constructing a rock-timber cribbed dam of such a height to extend a back water to the foot of the Dam. It is believed that this is the first ogee shaped dam cross section used in America. Jervis reported that it worked well under both high and low flows over the spillway. There was little evidence of erosion of the stonework or of the apron dunng all of the years that the Dam functioned. When additional water was needed for New York, a sec- ond Croton Dam was built, which flooded Jervis' original. Although intact underwater, it no longer functions as designed. Ventilators and tunnels Since the Aqueduct was to be completely enclosed to protect it from the elements and from any possible contamination resulting from intrusion of surface waters, it was necessary to ensure that the inside of the Aqueduct was at atmospheric pressure. To this 29 end, a series of ventilators were erected along the course of the Aqueduct (figure 26). A total of 33 ventilators were constructed, roughly one every mile in the upper portion of the Aqueduct. Of the total, only 11 of them had access doors for inspection purposes. Because the waste weirs provided ventilation, ventilators were not necessary in these locations. When the line of the Aqueduct intersected a ridge it was neces- sary to construct a tunnel. There were 16 tunnels varying in length from 160 to 1,263 feet, giving a total length of 6,841 feet. In those situations in which the tunnel was excavated through sound rock, the roof of the Aqueduct was not lined. The walls and bottom, however, were constructed in the same manner as the rest of the Aqueduct, providing exactly the same flow characteristics through- out the length of the conduit to the north of the Harlem River. Embankments and culverts In order to preclude any frost damage to the aqueduct masonry, Jervis preferred to use large embankments with the conduit buhed figure 24, above left: Ogee Shape of the Weir Overflow figure 25. below left: Lt. Theophilus Schramke. Croton Dam Design — Cross Section of Ogee Curve, engraving In Description of the Sen' York Croton Aqueduct in English, German and French, 1846 Courtesy William Lee Frost figure 26, above: James Renwick, Jr., Details of Ventilators. watercolor and ink in "Letter Books of J. B. Jervis" (vol. Ill) Courtesy Jervis Public Library, (not in exhibition) CoBfriqlirMl minrHl in the apex of the earthwork rather than resort to aqueduct bridges, which he feared might cause problems with frost action and atten- dant leakage. By using large embankments across the valleys, it was necessary to construct 114 culverts to allow water to pass from one side of the embankment to the other. These culverts var- ied from miniscule spans of 1^2 feet to 25 feet. The total length of all the culverts was 7,959 feet. Other notable structures in the form of culverts or small bridges were built at Mill River, Hastings, north of Tarrytown, and at Yonkers (figure 23). In addition to the culverts, six waste weirs were constructed to allow water to pass out of the Aqueduct when it rose above a certain control height. The flow out of the Aqueduct over the waste weirs was controlled by hand oper- ated 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