416

HISTORY OF ENGINEERING.

Book I.

masons of the middle ages; many were built, as has been observed, in the same manner asthe vaults of the chapter houses and cathedral churches ; after the piers were carried above

Fig. 414. OUSE BRIDGE, AND ST. WILLIAM’S CHAPEL, YORK.

the level of the stream, ribs of stone spanned the opening from one pier to the other, andsupported a rubble construction laid above them, an arrangement combining both economy

Fig. 415 OUSE BRIDGE, AND ST. WILLIAM’S CHAPEL, YORK.

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and convenience. In subsequent instances we see one or more rings of voussoirs spanninga river, upon which slabs of stone are laid, and the bridge completed; but it must be bornein mind that such ribs simply serve the purpose of centres, and cannot have the strengthof our modern bridges, where a wedge-like form is given to every portion of the stone.

Between the towers of Lincoln cathedral , and above the vaulting, is a stone girder, com-posed of numerous voussoirs, arranged within the segment of an arch, whose radius is91 feet; the abutments are extremely solid, and the girder, which exhibits a horizontalsurface above and concave below, is about 20 inches deep at its abutments, and 11 inches in