1S84
THEORY AND PRACTICE OF ENGINEERING.
Book II.
Fig. 2285.
TRANSVERSE SECTION, RICHMOND
abutments, it causes a depression at one partand an elevation at the other, which, althoughapparently trifling, disturbs the equilibrium,and racks the framing in such a manneras to be highly detrimental to its stability.Cross-framing, by halving scantlings together,and attaching them strongly to a top andbottom beam, has long been practised, andfound to constitute a very effectual truss:but wherever they are used, they should becovered by a roof, or protected from theeffects of the weather ; for a framing com-posed of a great number of pieces would haveits form greatly changed by any trifling con-traction or shrinking of the several parts.The horizontal timbers are more subject tosuch an effect than those which are placedvertically, or that receive their pressure in thedirection of their length; consequently, inlattice bridges, great care is requisite to havethat portion of the timbers well seasonedupon which the abutments of the severalstruts are secured.
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