1348

THEORY AND PRACTICE OF ENGINEERING.

Rook IE

L’OME OP THE CHURCH OP THE SALUTATION AT VENICE.

and half an inch thick, secured by an iron pin to each rib : stout laths are nailed on thebacks of the ninety-six ribs to carry the lead covering ; and where the joints are formed,additional wooden rolls are fixed to dress the lead round.

CHAP. XXIII.

TIMBER BRIDGES. -SWING BRIDGES.-DRAWBRIDGES.-ROPE BRIDGES.—

CENTERING. — SCAFFOLDING.

T imber bridges are general in all countries where timber abounds, and where cheap-ness and expedition are important; it has its objections in being less durable than eitherbrick or stone, but, if covered and protected from the weather, may be made to lasta considerable time. Bridges constructed of timber, so that they may be easily repairedwhen decay takes place, have also their inconveniences, as the traffic across them must bestopped whilst the workmen are employed *, they are undoubtedly of very early invention,and may be considered as strong platforms crossing rivers, and preserving an uninter-rupted communication. Among those built by the Romans, wc have the description left