In mortise and tenon joining, four sides of the mortise are always supposed to lie at right angles to each other and to the sorfare whence it is recessed, and two of these sides to lie parallel with each of the sides, which form a right allele with the side from which the mortise is made : the fifth plane, \Villa is the bottom of the mortise. is parallel with the other. With respect to the tenon, four of its sides are parallel with the four sides of the piece.
In the application of timbers to buildings, it is here sup posed that all pieces eut fbr use have a rectangular section, and when laid horizontally have their si les perlendicnlar to, and parallel with, the horizon.
If t pieces of timber arc to he joined at four cut a notch fn one piece equal to the breadth of the other, so as to leave the remaining tart of the thickness stiffieiently strong, a very small excavation being sufficient.; then insert the other piece in the notch : or it' the work he required to be very firm, notelt each piece reciprocally to each other's breadth, and fasten them together by pins, spikes, or bolts, as the case may require : this form is applicable where the pieces are equally exposed to a strain.
NV11011 one piece has to sustain another over it, transversely, and if only the upper be required to support a Weight, cut a notch frfon its lower side, equal in breadth to about three fourths of that of the lower piece, and as deep as the vertical distance that it is to be h-t down ; then the lower piece must have a notch cut ill its vertical side, leaving the middle of the tipper face entire to three-quarters of its breadth. and the lower parts of the vertical side entire, so that the vertical depth of each notch may he the same as that of the upper notch : by this means the strength of the supporting or lower pieces is diminished in a much less degree than if' the notch were cut out the whole breadth. This method is applicable to roofing, and naked The framing of timber by dovetail notching is chiefly applicable, to horizontal training, where the lower timber is sufficiently supported ; but Where the lower timber is unsup ported, it is common to use mortise and tenon, which does not weaken the timber in any considerable degree; Where the tinth,T is notched from the upper side, the operation reduces its thickness. and consequently impairs its strength; though it may he said, if the solid of one piece fill the exca vation of the other, and both be tightly driven or tbrool together (if we can place implicit confidence in the experi ments of I>11 I I amel) and it' the pieces be not cut more than one-third through, there will lie rather an accession than a loss I strength. It may lie observed, however. that in large works, %, here heavy timbers are employed, it is diffi•ult, almost impossible. to lit them with due accuser ; mid even
the joints eloselv fitted at first, the shrinking, would oc•a-ion cavities on the sides, that would render the tenons of no avail, because the axis of fracture would lie nearer to the breaking, or under side of the supporting piece.
What has been observed with regard to horizontal pieces of ti•dier. apples to framing in every position. where the tir•e is to fill on the plane of the sides ; and if a number of pieces thus liable to latend pressure on either side, are to lie framed into two other stiff pieces, the mortise and tenon will prove best fbr the When joists are framed into triimners, the usual method is to make. the mortise on the tenon with a plain shoulder, ill the middle of the sides its respective timber : this mode is particularly used in letting down bridging joists upon binding joists. and small rafters upon puffins.
It' it be required to join two pieces of timber, to two right :nudes, so as to be immovable when the transverse is held or fixed fast, and the standing piece pulled in a direc tion of its length ; cut a dovetail notch across the breadth of the transverse piece. and notch out the vertical sides of the standing piece at the end. so as to form a et frresponding similar and equal solid. In some pieces of work. besides the dovetail, an additional notch is call, to receive the shoulder of the lower piece. If the position of these pieces lie hori zontal, and the app: r of sufficient weight. or pressed down by any considerable tbrec. when the pkees are put together ill their place, the dovetail will be sufficiently strong without the assistan•e of pins, spikes, or hobs. This construction requires the timbers to be well seasoned, for otherwise the shrinking will permit the standing piece to be drawn out of the transverse, an 1 thus defeat the which the con struction was intended to answer. The Iiillowing method of remedying this defect will lie found effectual :—Cut the transverse piece in two exravations from the upper side, so that if the breadth be supposed to be di% ided into five equal parts. and a notch equal in breadth to three parts be cut next to the outer vertical side, and the other notch be made equal to tiv breadth of one part, and each notch depressed from the upper face about one-third the thickness of the piece, so as to leave the second part on the upper surface next to the inner vertical si le. and the two-thirds of the depth of each vertical side next to the lower si le entire : then the corers. ponding single notch being made on the standing piec• to the solid left on the surf tee of the transverse piece, the two pieces will reviprovally receiVe each other.