Chap. XXII.

CARPENTRY.

1309

can just pass; when it is in its place, a wooden key or wedge is driven tightly on thestraight side, which forces the tenon against one side of the mortise-hole, and preventsit effectually from being drawn out: oak or iron may be added, or an iron strap may beapplied.

Tenons may be wedged at the end, but to dothis they must be made long enough to pass en-tirely through the mortise: two saw cuts are thenmade across it, and the wedges arc driven home :the tenon sometimes splits, but not sufficiently toinjure its strength. When in machinery it is notpracticable to cut the mortise through, the fox-tailwedging is adopted; the tenon is made to fit themortise exactly; the wedges are loosely put intothe saw-cuts, as before, and the whole is drivento its place: when the wedges touch the bottomof the mortise, they cause it to spread, and thushold the tenon firmly in its place.

Dovetailing in some degree resembles mortisingand tenoning, and is more adapted to uniting toge-ther the angles of framework : the feet of the raftersrequire the mortise and tenon to be carefully made,and the thrust is destroyed to a certain extent toobtain greater strength : a portion of the rafter istenoned into the tie-beam, and another small part islet into the upper part of it; both rafter and tenonare cut at right angles with the inclination of theroof: in one example the rafter has two bearingshoulders in its depth, one behind the other, inaddition to the tenon which unites them. Strutsand braces which are loaded require but littlemortising to keep them from sliding out of theirplaces; the more flat their ends can be cut, themore efficient will they be : the shrinking of tim-bers sometimes occasions them to become loose,particularly where there is not much stress uponthem.

King-posts, queens, and principal rafters, whichare subject to great strains, should have iron straps ortiesr when they unite with the tie-beam, and an ironstrap should embrace the head of the kings and queens,and unite with the principal rafters, the feet of whichin large buildings sometimeshave their abutment in a cast-iron shoe, which prevents thesplitting off the end of thetie-beam.

The ends of king or queenposts may have a screw-boltpassed into them, whichallows the nut to be turnedat pleasure, and thus theframing may be tightenedagain, when shrinking of thetimbers renders it necessary:this in many instances ispreferable to the iron strap,and keys or screws put inthe ordinary way.

Whatever form we adoptfor the butting joint, we mustbe careful that all parts bearalike, for in the general com-pression the greater surfaceswill be less affected and the smaller undergo the greatest change ; when all have cometo their bearing they should exhibit an equally close joint, and as large timbers are movedwith some difficulty, the joint cannot be often put to the test of trying whether it fitsuieely ; it must therefore be set out with great precision, and worked with regard to its

Fig 2C8G.

PRINCIPAL RAFTEnS

Fig. 20S7.

Fig. 2088.

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L-

KING-POSTS.

Fig.2090.

QUEEN* POSTS.