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

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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 foundation 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