mum here, or certain depth, to who we should ( nt, se) as to make the strongest compound beam or two rivet balks. Duhamel has made a number of experiment ; with a t kw to determine this point ; and it appears from them, that the depth should he about one-eighth of thr depth of the two balks, or one seventh of ompound beam.
Many artists u .c an oblique scarfing, instead of square tabling, as Fig. 8. This practice does not appear advi sable ; the sharp corner is apt to be crippled. Dote them, as in Fig. 9. seems also injodh ions; thy pieces are then apt to tear each (Aber up. T,ie expe riments of Dohamel appear clearly to fa% our the method of square tabling, and it has the advantage of acting both ways. The dovetailing appears intended to take a hold of the two pieces, and prevent them from parting. There is a very ingenious method of fixing dowel and tree nails in that way, which may be applied to other joints in carpentry. A little split is made in the end of the tree nail, and a small wedge struck in it ; this wedge is driven home N% hen the bolt reaches the bottom, ex pands the end of it, and squeezes it firmly agz.inst the side of the hole.
Two beams arc often connected side by side, w ith the intention of resisting strains in the direction of the plane of the joining, or rather with the view of resisting a strain in every' direction, as is the case tt ith the parts of a mast, or an axle. The ft ather foul groove, used in flouring., is of the first kind, and it is sometimes employed in join ing beams. The feather loosens by the unequal shrink ing of the beams, and then it may slide in the groove. Besides, this form gives a hold only in one way, and is therefore unfit for masts or axles. Tabling the pieces together, or keeping them through in the plane of the joint, would he improper here, so much of the exterior fibres as arc cut across by the tabling are unfit for dis tension, and therefore the compound beam will be weak ened against a strain in the direction of the joining. The one piece may also slide on the other in that line, at least so far as to admit of serious bendings, and the form would have no resistance against ittisting.
The lateral slide is prevented in mast-making. by divid ing the joint along the middle of its breadth, and placing the projecting tablings alternately on tither side (Plate CXIII. Fig. 10.) Thus a, b, c, are projecting parts,
and d, e, f, are depressions, the other piece being form ed in the same way, the projections of the one lit into the depressions of the other, viz. a into d, b into e, and c into dr, which prevents the beams from sliding on each other, either longitudinally or transversely, and resists twisting completely.
Even this form is not without its defects; for. inde pendent of the jagged appearance of the scant. which is unsightly, and lodges water, that part ol the exterior side of the beam which is occupied by the tabling, is not capable of resisting distension ; and this, in the case of a round mast or yard. becomes of much consequence, as it occupies precisely the most favourable situation for that purpose. The mast-maker, therefore, perhaps with no other view than to make a fair joint, does not carry his tabling to the exterior side, but leaves two or three inches, of what he calls good wood, keeping. the coatis at that distance from the edge of the beam. Fig. 11. This is certainly a good practice ; and. of its,df, sufficient to prevent the lateral sliding. The ce k is thereby placed in that part of the beam which is tit at t'le neutral line. It is not subject to strains. for v hich it i-, not calculated, yet is sufficient, by its abutment, both laterally and longitudinally, to make a good connection of the pieces.
Another method, called a running coak, is in frequent use in the dock-yards. The coak is continued the whole length along the middle, but the butts project on each side of the middle line alternately, so as to be one-third their breadth within and without each other. This method has some advantages, as the coatis have a hold both by the bottom and one side, Fig. 12.
There is still another method of forming the alternate tabling, which not only gives a smooth seam, but is also better fitted to resist twisting than the running coak. The tables have their full depth in the axis of the join ing, but are sloped off towards the outside like so many wedges, and end there in a straight edge. This form has the full strength of its fibres at the exterior side against every strain, and the connection formed at the axis is sufficiently firm to make the parts act together, Fig. 13.