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natural impediments too wide to be spannedby any application of pontoon balks.

Timber may be very readily united by scarf-ing, or by fishing, or by bolts and screws; butthe method of uniting timber by wedges, asapplied in the famous wooden arch of 250 feetspan across Portsmouth river, in North Ame­ rica , is a very useful expedient for building-beams, with very portable pieces. I broughtfrom America an accurate drawing of that in-genious work, as represented in fig. 9, 10, 11, 12,plate 11. The arch is composed of three sets ofconcentric arcs, A B C, D E F, G H I, of whichthat in the centre, D E F, and the correspondingarches in the other two sets, support the floor •

of the bridge. The circular beams ABC,

DEF, GHI, are connected with each otherby pieces of hard wood a c, a c, and a wedge b,fig. 10, at the parts 1, 2, 3, 4, &c. fig. 9, wherecorresponding mortises are prepared. Thewedge secures these triple tenons in their mor-tises, and connects the centre beam A B, fig. 10,

(D E F, fig. 9,) with the other two beams, bythe dovetail tenons a c, ac.

If this structure be braced by diagonal beams,as dotted in fig. 9, it would be more capableof bearing unequal strains; but as it is, thearch is exceedingly flexible.