472

HISTORY OF ENGINEERING.

Book I.

delivered upon the abutments, and are generally contained in vertical planes, lying parallelto the sides of the roadway: such arches, intersected by numerous planes, transmit theirpressure from one to the other, in a direction perpendicular to those pressures.

Bridge over the Ouse near York , constructed for the Great North of England Railway,consists of three arches, each 66 feet span; the piers are 10 feet in thickness, and the archmeasured on the soffite 28 feet 7 inches. The thickness at the keystone is 3 feet 6 inches,and the voussoirs gradually increase towards the springing.

Tliis is another excellent example of a bridge of three arches, the curvature of which inthe outer is continued to the footings of the abutments; by which means the voussoirs atthe springing are extended considerably in length. The centering made use of was well

Fig. 474. BRIDGE OVER THE Ot-SE ; PLAN AND ELEVATION.

put together, and rested upon piles inclined towards the foundations on which they wereplaced; and the piers are capped with a single stone, from which the voussoirs commence,their thickness gradually decreasing towards the key, which lessens the pressure andincreases the strength of the arch. The starlings or cutwaters are formed of two arcs of

Fig. 475. BRIDGE OVER THE OCSE ; ARCH AND CENTRE.

60 degrees, described from the two angles of the piers, which perhaps are the bestadapted for a rapid stream. It is generally supposed by most practical men that thestrongest angle is that formed by an isosceles right-angled triangle, having its right anglefacing the stream.