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Wooden bridges across rivers of great width are supportedon piles driven into the bottom of the river, of which kind thelargest in Scotland is that erected about eighteen years since atMontTOse, by the late Mr Stevens, over an inlet of the sea, 600feet across and 32 feet deep at high water ; a structure of greatbenefit and consequence to all the surrounding country; but, fromthe perishable nature of the materials, requiring constant repair.It may, however, be worthy of remark, respecting this bridge,that the failure is principally in the fir piles in the middle, whichare about 60 feet long and 2 feet square, being rapidly destroy-ed by an insect, in the space between low and high water mark.The piles of oak have not been destroyed in the same manner,and all the knots in the fir remain Untouched.
The introduction of cast-iron for forming bridges constitutesa new era in that science, and is wholly a British invention.The first of this material was erected over the Severn near Cole-brookdale, county of Salop, in 1777, under the direction of A-braham Derby. The span is 100 feet 6 inches, the rise 45 feet.Since which several others of larger dimensions have been exe-cuted, particularly that at Sunderland, w r hose span is 236 feet,the versed line or rise of the arch 34 feet, and the height abovelow water 100 feet. The first bridge is still standing: great im-provements have since been made in the construction, and thereis no dou' many more will still be added.
One great cause of weakness in these bridges is the difficultyof securing the abutments. The great span, and the elasticityof the materials of such bridges, subject them to great vibra-tion, which constantly acting upon the abutments is progressive-ly working their destruction. Every means must therefore beused to make the abutments firm. The most effectual wouldbe to make the arch broader at the spring than in the middle,perhaps double the breadth, which would very much tend tolessen the vibration, and render the whole structure firm andsecure.
Bridges of Stone.
Of all the materials with which bridges may be formed, noneCan be compared with stone, for strength and durability.
The most essential parts in the construction of stone bridgesare the foundation of the piers and abutments, and the form of thearch. Bridges of more than one arch should not, if it possiblyCan be avoided, be built where the current is rapid. All foun-dations not upon rock; and exposed to the current of the water,should be well protected by embankments formed with stones,and kept together with piles and cramps. The piers of all bridgeswith flat arches should be of a height equal to the surface of the
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