by using large stones laid in positions to give proper bond, and to allow small broken stone to be closely packed in, filling up all the interstices so as to form a compact and uniform mass. The wall is generally allowed to stand some months after it is completed, before the masonry of the Aqueduct is commenced upon it, lest by this weight being placed upon it before it has found its bearing, it should settle and cause cracks in the masonry. That such settlement should in some instances occur, even after the Aqueduct is completed, is not surprising, for passing over so many different elevations, and encountering such numerous transitions from a hard soil, or from rock, to valleys of alluvial deposit, it would be beyond human powers of foresight and vigilance to pre- vent it. To render the Aqueduct more secure in such positions, the concrete foundation has an increased thickness, and in preparing it a greater proportion of hydraulic lime is used ; the proportion being two and a half parts of sand to one of lime. The dimensions of the stone side walls and the span- drel backing of the roofing arch, are also increased ; and the proportion of hydraulic lime to the sand in the mortar for these is increased. Another precaution has been taken to render the work secure, by plastering the interior of the Aqueduct over these foundation walls. The embankment adjacent to foundation walls has various slopes according to 87 circumstances, and is generally protected with a dry stone wall on the face, and is carried up of sufficient width to insure the requisite covering over the Aqueduct masonry. Along side hills an excavation is made for the Aqueduct into the hill, and a protection wall of stone built on the lower side so as to support a covering of earth over the masonry ; great care being taken to obtain a deep and firm footing for this wall in order to render the work secure. In such a po- sition the Aqueduct is perhaps less secure than in those be- fore described. Where the soil is wet from springs, and the formation clay, there is danger of slides ; and in rainy sea- sons there is danger from the torrents which gather on the hill sides and come down with destructive force : the earth covering is liable to be carried away, and the Aqueduct itself to be undermined. Great care has, however, been used in such cases to form strong paved channels for the passage of the water over the top of the Aqueduct, or by culverts to pass it underneath. WASTE-WEIRS. At suitable places on the line of the Aqueduct, waste-weirs are constructed to discharge surplus water. They are con- structed in one side of the channel-way, in such manner as to allow the water to flow off when it rises above a given level, and arrangements are also made at these places to close the channel-way entirely, by means of stop planks, and to discharge the whole of the water through waste-gates ; so that the water might be running from the Fountain Re- servoir through a portion of the Aqueduct and discharging 88 from these waste-weirs while the remainder of the channel- way, or portions of it, would be drained so as to admit of inspection or repairs. There are six of these waste-weirs constructed for the Aqueduct. VENTILATORS. For the purpose of ventilation hollow cylinders of stone are erected over the top of the Aqueduct and rising about 14 feet above the surface of the ground, or earth covering. These occur every mile, and every third one is constructed with a door to afford an entrance to the Aqueduct. Those allowing an entrance have an interior diameter of 4 feet, and the others have an interior diameter of 2 feet ; each, however, slightly diminishing towards the top. An iron grating covers the top to prevent any thing being thrown in. Plate VI. is a view of an entrance ventilator ; this stands on one side of the Aqueduct, where the masonry of the side wall is enlarged for its base ; we can descend 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