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.

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 of the ground.