great care being taken to obtain a deep and firm footing for this wall in order to render the work secure. In such a position the Aqueduct is perhaps less secure than in those before described. Where the soil is wet from springs, and the formation clay, there is danger of slides ; and in rainy seasons 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 constructed 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 Reservoir through a portion of the Aqueduct and discharging

;

from these waste-weirs while the remainder of the channelway, 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.