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of the trusses should be secured by iron strapsand bolts to the king-posts, or rest upon across-piece of timber strapped to them.

It is astonishing to what extent this maybe carried; abeam, trussed in the middle, maybe safely loaded to ten times the weight thatit can bear alone. A bridge on oak trusses, of40 feet span, in which none of the timber ex-ceeds 6 inches square, may thus be made tosupport artillery.

When the width of a river,—the span of abroken arch,—or the distance between rows ofpiles is such that the timber at command willnot reach across with one scarfing, fig. 13, plate10, a truss, fig. 14, may be used: this will ad-mit of the tie-beam being scarfed at O and P.

It is not necessary to enter on the construc-tion of the joints, mortises, tenons, joggles, andother methods of making truss-frames—an armycan always furnish plenty of carpenters, wellversed in this business, if even there should beno corps of military artificers present.

In laying wooden bridges across brokenarches,* it will frequently be more convenient

* The destruction of a stone arch is sometimes a work ofv erv considerable difficulty. In our first campaign in thePeninsula, this was experienced chiefly from want of means,a nd from not having a corps of sappers and miners; but in some

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