Chap. XXII.

CARPENTRY.

1307

For joining two pieces of tim-ber together notching is the mostcommon and simple method;for, when four angles arc to beformed, the surfaces of onepiece are both parallel and per-pendicular to those of the other ;a notch may be cut out of onepiece (fig. 2071.), the breadth of

Fig. 2075.

Fig. 2076.

the other, which may be let down on the first, or the two pieces may be both notchedto each other, and then secured by an oak pin: this is the best practice when each of thetimbers is equally exposed to a strain in any direction. "When one piece has to supportthe other transversely, the upper may have a notch cut across it, to the breadth of two-thirds the thickness of the one below, which must also have a similar notch cut out oil eachupper edge, leaving two-thirds of the breadth of the middle entire, by which means thestrength of the supporting or lower piece is less diminished than if a notch of less depthwere cut the whole breadth : such joints are particularly adapted for purlins, when letdown upon the principal rafters.

Lapping is performed in a variety of ways, either by simply halving the end of each tim-ber, or by halving and dovetailing; in the latter case the timbers act as a tie, and cannotbe readily pulled asunder.

In these joints the greatest attention is required to make the several parts abut com-pletely on each other, as the least play or liability to motion at opce destroys theirefficacy ; the butting joints, being slightly tapered to one side of the beam, require verymoderate blows with a hammer to force them into their place ; if driven too hard, the partswill be liable to strain, and the abutments to split off: it is better sometimes to leave theabutments open, and afterwards drive in a smallwedge, which, if made of hard wood and not likely tocompress, is an excellent substitute ; iron has beensaid to injure the fibres of the timber, from its toogreat hardness, otherwise it is well adapted for thejoggles and wedges.

Two pieces of timber may be united in sucha manner that they preserve the same breadth anddepth throughout, which is of great importancein the construction of beams for bridges or roofs ofconsiderable span: the length to be given to thescarf must depend upon the force that will causethe fibres of the timber to slide upon each other,and that for oak, ash, or elm should be 6 timesthe depth of the timber, in fir 12 times; but wherebolts are used so much is not required in eithercase : the simplest method for uniting the ends oftwo timbers is by cutting away an equal portion ofeach, and letting one down upon the other.

Timbers united together bya number of such cuttings,afterwards united and boltedthrough or hooped round withiron, are capable of sustaininggreat resistance: a stirrup-ironat each end holds the timbers intheir places, and one or morebolts is sufficient to preventtheir being drawn asunder.

Fig. 2077.

Fig. 2078.

The carpenter frequently ex-ercises great ingenuity in join-ing timbers of considerablescantling, and, by the introduc-tion of iron or small cubes ofharder wood into the joints, canprevent their being thrust ordrawn out of their positioneither longitudinally or late-rally.

The Scarfing of Girders andBeams , which the French en-gineers term Traits de Jupiter,

Fig. 2080.