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The most ancient bridges of Scotland were built at a greaterexpense, than at present, considering the then value of money;In 1320, Bishop Cheyne built the bridge of Don, near Aber-deen, from the emoluments of his See, while it was sequestrat-ed by King Robert Bruce , and he was reinstated by that King.Bishop Elphinston left money for building a bridge of 7 archesover the Dee, near Aberdeen, which was accomplished by Bi-shop Gavin Dunbar about 1510. And the oldest toll in Scot-land is for building a bridge over the Earn, which was executedby the Magistrates of Perth about 1200 years ago. The build-ing of bridges is now better understood.
Bridges of Wood.
This material is used either for cheapness* where the price ofstone cannot be obtained; for expedition in the formation, incases where the time of constructing one of more durable ma-terials cannot be allowed, or when other materials would not suitthe purpose, as in cases where the distance of the supports isvery great.
The great inferiority of bridges of wood consists in the wantof durability. Particular attention should therefore be given,that none but the very best red wood be used, entirely free ofsap ; all the ends of the beams to be covered by the abutments,or driven into the soil, should be charred and covered with boil-ed tar ; all the joints should be painted with white lead and lint-seed oil before they are fastened, and the whole structure eitherpainted with the same materials, or at least covered with boiledtar. The covering for forming the road should not be laid flatUpon the beams, but raised about one inch above them by nar-row fillets, so as no lodgment may be formed for moisture to re-main upon any part of the timbers, and all the parts to be soconstructed as to receive at all times a free current of air.
Bridges of this material may, where the stretch is short, beformed by simple straight beams resting upon the abutments ateach end, with the covering laid across. Mr Nasmyth, landscapepainter, Edinburgh * has suggested an ingenious method of con-structing bridges by layers of thin plank laid together in the man-ner of a spring for a carriage, and by these forming an arch ofthe extent required, similar to a bow with its cord ; the planksbeing kept in their bent position by their fixtures at the abut-ments, and by a beam or wood, cord, or bar of iron, fixed tothe opposite ends, and placed in a straight line betwixt them,with ties to the arch formed by the planks at proper distances,to keep them in a regular curve.
For stretches not exceeding 30 or 40 feet, a bridge may beformed in a cheap and expeditious manner by four beams ortrunks of trees with two cross bearers, placed as here represent-ed. A B the opposite banks ; C, C, C, C, four beams or trunks