1342

Book II.

THEORY AND PRACTICE OF ENGINEERING.

thoroughly seasoned, by payingattention to the work some timeafter its execution, the ligatureswhich strap or hold the severalpieces together may be tightened,or wedges introduced, so thateach plank may be brought to itsbearing.

Where brick or stone can be readilyobtained, it is often far less expensive toconstruct a series of arches in one orother of these materials, and to lay thepurlins on them to support the commonrafters : many custom-houses in France ,and buildings destined to receive mer-chandise, are so formed; semicircularbrick or stone arches, 100 feet in diameteror more, span the entire building at every15 or 20 feet of its length, and these alonesupport and carry the roof. There isin such an arrangement far less labourrequired, and as great durability, andthe centres used for their construc-tion are of a very simple kind, oneserving for each successive arch; run-ning on wheels upon strut timbers ortwo iron rails they can be readilymoved; and as the first course is con-tinued through or over its entire surface,the rib is wedged up, and the weighttaken off the rollers beneath it. Toform a covering with a flat arch uponthis principle, three curved ribs havebeen employed, which strutted andbolted together have the strength of asolid timber. Bent timbers are madeuse of for a variety of purposes, andcare should always be taken, when heatis applied, to give them the necessarycurvature, that it is general and equalthroughout.

Fig. 2160.

LlBOmNE.