the aqueduct from the Croton river to the north bank of the Harlem River 33 miles, and determined the grade of the aqueduct at about 13' /4 inches to the mile. It was, in the main, well located. In regard to plans of work, he proposed a cross section of the masonry of the conduit with which some modification was adopted. So far as I have known, he prepared no specifications for the work that were approved by the commissioners.'^ Jervis noted that Douglass did not proceed with the design of any of the structures for the Aqueduct. Jervis described the cross section used and also provided an illustration (figure 14): The form and dimensions of the interior of the aqueduct are as fol- lows: The bottom is an inverted arch; the chord or span line is 6 feet and 9 inches, and the versed sine 9 inches. The masonry of the side walls rises 4 feet above the springing line of the inverted arch, with a 27 bevel of 1 inch to a foot rise, or 4 inches on each side, which brings the width at the top of the side walls 7 feet and 5 inches; forming the abutments of the roofing arch, which is a semicircle, having a radius of 3 feet S'h inches, or a chord line of 7 feet 5 inches. It will therefore he perceived, the greatest interior width is 7 feet 5 inches, and greatest height 8 feet 5'/i inches. The area of the interior is 53.34 square feet. In rock tunnels the roofing arch is generally dispensed with, but the bottom and sides are formed with masonry similar to that above described. There is an exception to this form in the first 4.949 miles of the upper end of the aqueduct, where the side walls have an extra height, on account of the bottom being depressed, to draw the water at a lower level from the Croton Reservoir.^ In addition to this description and illustration, Jervis also provid- ed a table delineating the masonry and piping which carried the water over the length of the Aqueduct: The length of aqueduct from the Croton dam to the distributing reser- voir is 40.562 miles — to wit: ^ Masonry conduit in Westchester county 32.880 [Ditto ditto] on New-York Island 4.187 Total Length 37.067 Receiving reservoir from end of aqueduct to south-eastern effluent gate house 0.172 Distributing reservoir 0.080 Iron pipes on bridge over Harlem valley 0.275 [Ditto ditto) across Manhattan valley 0.792 [Ditto ditto] between reservoirs 2.176 40.562 The various sections mentioned in the table are shown in figure 16. The route of the Aqueduct was over very rugged ground necessitating an irregular path in order to take advantage of the contour of the land. In order to maintain the constant slope of the Aqueduct from the Dam on the Croton River to the Harlem River, Jervis constructed a number of tunnels, culverts, embankments, and aqueduct bridges. In addition, ventilators were required to ensure that the water would flow under atmospheric pressure only and also to permit access so that the Aqueduct could be inspected all along its length. Croton Aqueduct structures Work began on the Aqueduct in 1837. One of the first tasks was to raise a dam on the Croton River to ensure a head of 40 feet above the river and to provide an intake for the Aqueduct. Work was well underway when a freshet struck the Croton River watershed. The dam was breached when the water rose 15 feet above the spillway of the dam. The masonry part was on solid rock and was not dis- turbed. Having inspected the damage, Jervis wrote to the Board to state the facts of the disaster. In many large public works this kind of disaster had resulted in the discharge of the chief engineer. That this did not happen to Jervis is probably due to his outstanding rep- utation, although he readily admitted that he did not anticipate such a flood in the 3 years that he had observed the hydrology of the area. It is fair to say in his defense that rainfall data was simply not available at the time. In 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