OF JOINTS.
Having now considered in general the various kinds of strains which occur in the works of the carpenter, and having endeavoured, as far as was in our power, to how the strength of the timber is made to resist them, and how this resistance can be brought under calculation, we proceed to examine upon similar principles, the modes of connection by which these strains are propa gated through the different parts of a framing. Upon this part of our subject we shall be the more brief, as it is handled in more detail in the practical part of this ar ticle, by a gentleman of well known ability. Another opportunity will occur for treating of the more compli cated framings, as trusses, roofs, centres, bridges, ship frames, &c. where we can consider this subject upon principles that are common both to carpentry and iron work. In the latter, we can form frames composed of one single piece, and we can, in like manner, accommo date the thickness and breadth or the parts, or their length, to the strength we require, or the uses they are applied to, without any weaker connecting. parts. lint in the former, we must accommodate our framings to the size of our materials ; and when these are too small, we must endeavour to join or piece them, till the requisite magnitude or strength be attained. The best method of constructing these joinings, is one of the most delicate parts of the art of carpentry.
The different forms are very numerous. Each of them has its advantages in particular cases. The lead ing principle with the carpenter and joiner should be, to make every joint, even those intended only to please the eye, as strong as the case will admit. 11e will attend to the changes that may take place in his work, by swell ing, or shrinking ; for as this is a force altogether insu p•rable, unless the change in the whole assemblage he made to act in the same direction, he cannot prevent the smaller parts from crushing or tearing away.
When the thickness or depth of a beam is to be in creased, other pieces are applied over or under it. If the strength of the beam is to be combined with pliabi lity, these pieces are not fixed to the beam, but bend and yield gradually like the plates of a coach-spring, Plate CX1II. Fig. 3. Where the assemblage is intended to
act like an entire log, the parts must be attached to gether. Iron or wooden bolts may be driven through them, or they may be within, by letting pieces of tough wood into the solid on each side of the joint, Fig. 4, 5. The pieces may be fitted into the face of the joint like tenons, or driven across it like keys or wedges. They may he also attached by cooking, or tabling, Fig. 6, 7, Cc. which is uniting two or more pieces together in the middle by small tabular parts formed from the solid of one, and sunk into corresponding cavities in the other: the butts or ends of these prevent the pieces drawing asunder lengthwise.
When the butting surfaces brought into action are equal, dowelling (Plate CXIII. Fig. 5.) is certainly pre ferable to coaking or tabling, for this obvious reason, that the depths of the two beams are preserved entire ; and we know that the thicker beam will he stronger, in proportion to the greater distance of the compressed or dilated fibres from the neutral line. Observe, however, that the beam is not stronger in proportion to the square of this greater depth, for the quantity of fibres left en tire is supposed, in both cases, to be the same. If the fibres which arc cut across be contained that part of the beam that is compressed, and if close work he made, the assemblage will, indeed, he nearly as power ful as a solid beam or equal depth, which is a still strong er reason for preferring the method of dowelling ; net •r theless, perhaps, because it appears more ingenious, many carpenters appear to prefer the other.
The intention of both these methods of joining, is to prevent the two pieces from slipping on each other. Where there is no great risk of that, the indenting need not be very deep ; at all events, when the butting sur faces are so great as to be able, without crippling, to shove off the coati along the plane, there can be nothing gained by making them deeper. When the one piece is tabled into the other, we hat e an increase of hold and consequent strength, by deepening the indenting so far ; hut we have also a diminution of strength, by lessening the scantling of the beam. There is therefore a maxi .