from the door and gain an entrance to the channel- way by an opening in the side of the roofing arch. The sill of the door is about 12 feet above the bottom of the channel-way. Those not intended for an entrance stand directly over the top of the Aqueduct and are groined into the roofing arch. Besides these Ventilators, there are openings 2 feet square in the top of the roofing arch, every quarter of a mile : they are covered with a flag stone and the place is marked by a small stone monument projecting above the surface of the ground. These may be useful to obtain entrance to the > 55 U=i <! =1 a ^ @3 S= tel 89 Aqueduct, or to afford increased ventilation should it ever become necessary. CULVERTS. Where streams intersect the line of Aqueduct, culverts are built to allow them to pass under it. They are simply a stone channel-way built under the Aqueduct of such form and dimensions as will allow the stream to pursue its natural direction without causing injury to the work. The founda- tion of these culverts is formed by laying down concrete, upon which an inverted arch of cut stone is laid forming the bottom of the water-way : side walls of stone are built and surmounted by an arch of stone. The span, or width of water way, of the culverts built, varies from 1^ foot to 25 feet. Those of li foot span have a square form for the water-way, and are constructed by making a foundation of concrete, upon which a flooring of well dressed stone is laid forming the bottom of the water-way, and from this, side walls are built and covered by a course of thick stone flag- ging well dressed and closely fitted. At each end of the culvert a deep wall is built underneath so as to prevent the water from doing injury by undermining it. Buttresses and wing walls are built at each end of the culvert to guide the water to and from the channel-way, and a parapet wall is built over the top of the channel-way at each end to sustain the embankment of earth over the culvert. These wing walls and parapets have various forms ; sometimes the pa- rapet is built across the top of the culvert, and the wing walls built at right angles to it, and sloping down to the but- 23 90 tresses, and sometimes the wing walls and parapet form one continuous wall of a semi-circular form, the top sloping up from the buttresses in a plane parallel with the slope of the embankment covering the Aqueduct above. These culverts are permanently constructed, and in preparing the plans for them much skill has been displayed in adapting the form and size which the circumstances required, and much taste displayed in the design for their construction. Plate VII. is an isometrical drawing of one of the culverts with rectangular wings and parapets ; the body of the cul- vert is cut in two in the drawing, showing that it may be of any length, according to the width of the embankment through which it is constructed. The length is generally arranged so that the slope of the embankment may intersect the rear of the top of the parapet and pursue a direction down, parallel with the slope of the top of the wing walls. Gate Chamber at the Head of the Aqueduct and Grade of the Water-way of the Aqueduct. Plate VIII. is a longitudinal section through the tunnel and gate chamber at the head of the Aqueduct showing its connection with the Fountain Reservoir. This gate cham- ber is not in any way connected with the dam itself but stands some distance from it, and the water reaches it by means of the tunnel which leaves the Reservoir above the dam and passes through the solid rock of the hill against which the masonry of the dam is built, a distance of over 200 feet. This tunnel descends into the Reservoir, so that the centre of it at the mouth is about 12 feet below the sur- 91 face of the water ; any floating substance cannot enter it, and during the winter season when the water is frozen over no obstruction can take place to the flow into the Aqueduct, and during the summer season the water will be drawn from a level where it is cooler than at the surface. The gate chamber has two ranges, or sets of gates ; one called regulating gates, and the other guard gates : the regulating gates are made of gun metal, and work in frames of the same material which are fitted to stone jambs and lintels : the guard gates