438

HISTORY OF ENGINEERING.

Book I.

Fig. -140. ELEVATION OF ARCH, WATERLOO BRIDGE.

exceedingly small as compared with all previous examples, and it is no more than a fiftiethof the diameter of its circle of curvature. In the theory of the arch the thickness given tothe voussoirs forms the most important consideration; and here we have them admirablyproportioned, so as to diminish the pressure on the arch, and increase in strength towardsthe springing, that is to say, in proportion to the accumulated tangential pressure. Atsixty degrees from the top the voussoirs are lengthened, and continue to receive someaddition down to the springing.

The bridge at Neuilly , built by Perronet, is at the crown 4 feet 8 inches in thickness,and its span is 128 feet, with a versed sine of 52 feet: the crown of the arch being theportion of a circle whose radius is 150 feet, its horizontal thrust is very considerable, andthe key-stone is made 4 feet 8 inches in height; nevertheless, the arch sunk when thecentre was struck nearly 2 feet, whilst those of Waterloo descended only a few inches.

The form given to the starlings at Waterloo Bridge is well contrived to split the streamin its current, and carry the water quietly under the arches.

Fig. 441.

The centres made use of were admirably contrived, and put together in such a mannerthat they could be readily dropped, if required, by means of the wedges introduced uponthe heads of the inclined piles. The piers and abutments being raised to their necessaryheight, to receive the arches, the timber centres or ribs were set up, framed together, asrepresented in the elevation of the arch : these centres were remarkable for their strength,and calculated to support the weight of the voussoirs placed upon them, throughout theentire progress of the construction of the arch until finally keyed by the introduction ofthe last stone. The form of the arches has undergone no change, which is the bestindication that can be adduced of the excellent manner in which the centres were framedand supported. They were laid upon whole timbers, which capped the piles; and undereach set of ribs the wedges were introduced, which were made to extend across the entire