494

HISTORY OF ENGINEERING.

Book I.

In the spandrills are introduced rings of cast-iron, and a railing of the same metal runsalong each side of the roadway.

The ribs are nearly semicircular ; the height from the ordinary low water to the springingplate is about 10 feet, making a total height from low water to the soffite of the arch of 55feet.

The weight of the whole of the iron work is 378J tons.

Fig. 487.

COLEBROOK DALE DRJDGK.

Behind the iron work, at each extremity of the arch, the abutments are carried upperpendicularly, and the square stone facing backed with rubble work.

Iron Bridge at Buildwas> finished in the year 1796, after the designs of Mr. Telford, issituated about three miles above that at Colebrook Dale. In this example the portion thatsustains the road is, as it were, suspended from two large ribs, placed on each side of the

Fig. 488.

BtTILDWAS BRIDGE.

bridge; the span of the arch is 130 feet, and the versed sine of the ribs, which bear thecovering'plates, is 17 feet; the breadth across the sofhtes is 18 feet, and the height from theordinary low water to the soffite is 30 feet.

This novel mode of construction, which allows the roadway to be kept low, on the sus-pending principle, was perhaps the first adopted in this country. The bearing-ribs have acurve of one-eighth of their span, or 17 in 130; they rise about one-fourth of their span,or 34 feet, and are maintained securely, both by horizontal ties and cast-iron braces. Thewhole is covered with 46 plates of iron, 1 inch in thickness, each 18 feet in length ; they