j$72

THEORY AND PRACTICE OF ENGINEERING.

Book II.

cular position, the struts will lose their abut-ments, and the balance between the timberswhich constitutes the entire strength of thebridge will be destroyed; an iron tie wouldinsure greater stability, and might be intro-duced across the stream, under each side of theplatform, without changing the disposition ofany of the timbers.

In some of the timber bridges on the con-tinent, where the oak platform that sustains theroad has been in a state of decay, and re-quiring constant repair, stone pavement hasbeen substituted, laid from bearer to bearer,and in some instances iron plates perforated with holes, to allow the water to pass through ;but the additional weight produced by either of these is so considerable, that it is aquestion whether any benefit has been obtained.

Bridge at Tete in Picardy , erected by M. Coffinet; its span is nearly 126 feet, and itsversed sine scarcely an eighth of the span.

Fig 2245.

BRIDGE AT TETB IN PICARDY.

Fig. 2243.

— b<T*I -

Fig. 2247.

Fig. 2246.

Fig. 2248

BRIDGE AT TETE IN PICARDY.

Perronefs Timber Bridges were upon the principle of the polygonal arch ; the spans ofthe arches were equal; the framing of the timbers that carried the roadway extended

Fig. 2249. perronet’s timber bridges.

through the parapet or railing on each side of the bridge, by which means the form of a

mma

SP^i

terronet’s timber bridges.

Fig.2250.