The side walls of stone and brick are bonded together by headers of brick entering the stone walls as shown in the drawing, and the walls of stone are built closely against the sides of the rock and forming a junction with it. On the exterior of the roofing arch a heavy spandrel of stone masonry (of the same character as the stone walls beneath it) is built, filling the space between the arch and the rock. After the masonry is finished, the rock cut above it is filled

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Scale of Feet

Scale of Feet

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Scale of Feet Scale of Feet

Cun&tr. FlB Tower

with earth to the same height above the roofing arch as mentioned for earth excavation. Plate III. is a section of the Aqueduct in tunnel cutting

in rock.

The width of the tunnel excavation in rock is the same as that of open excavation in rock ; and the manner of building the masonry to form the channel-way is the same, with the exception that the rock roof of the tunnel serves as the roof of the channel-way, where it is sound, but in cases where the rock is soft and liable to fall, a brick arch is built over the channel-way, and the space between its extrados, or outer surface, and the rock roof is filled with earth closely rammed in. In some instances where the tunnel perforated rock which was at first quite hard, the roofing has by exposure to the air, become soft and insecure, so as to render it