This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
A piece or plate of lead used formerly to be sometimes inserted between the abutting surfaces of timber joints to equalise if possible, and more fairly distribute, transmitted pressure, but there does not appear to be much ground for recommending the practice on that score, though with respect to the ravages occasioned by damp, the presence of lead in the joint has been known to exercise highly favourable functions. For example, in the Bridge of Bamberg, with a span of 208 ft. over the Regnitz, the timbers built into the abutments were thoroughly soaked in hot oil and then covered with sheet lead. Alfred Bartholomew, whose talent as a constructor is indisputable, and who well understood what he wrote about, though perhaps some of his ideas may appear to be slightly antiquated when viewed in the light of comparatively recent improvements and discoveries in building materials, appliances, and devices, observes with respect to bedding timber: - "If the plates lie upon stonework, there should always be sheets of lead placed between the stonework and all the timber, as stonework in general imbibes enough moisture to rot all wood in contact with it." The utility of the lead is no doubt connected in some way or other with the retention of the carbon in the timber, which is generally pretty safe from decay when dry and properly seasoned. Perfectly seasoned wood, however, of proper scantling for girders and the ribs of timber bridges is not easily procurable at the moment when wanted, and bedding timber upon lead prevents a fresh accession of moisture reaching the timber from the interior of the stone, whilst at the same time its non-porosity is equally efficacious in preserving it from the deleterious effects of sweat or a deposition of moisture attributable in certain conditions of the atmosphere to the conductivity and weak absorptive power of the stone. The plate of lead being cut to the exact size of the bearing end of a beam supported, 'for instance, by a stone corbel, would just raise the timber sufficiently above the top of the corbel to place it beyond the reach of any condensed vapour that might accumulate thereon, whilst all passage of wet from the wall to the beam would be peremptorily checked.
Longitudinal Joint is one in which the fibres of both timbers run parallel to the direction of the joint or seam, and is common in fishing and scarfing as performed to resist either a tensile, compressive, or bending stress. A lateral joint, as elsewhere observed, is likewise a longitudinal joint.
Mortise Joint is the junction of a tenon and mortise at any given angle. It is often called a mortise and tenon joint, under which term it is explained.