THE most elastic woods are those in which the annual or longitudinal fibres are the straightest, and the least interwoven with the medullary rays, and which are the least interrupted by the presence of knots; such woods arc also the most easily rent, and the plainest in figure, as the lanccwood, hickory and ash; whereas other woods, in which the fibres are more crossed and interlaced, are considerably tougher and more rigid; they arc also less disposed to split in a straight or economical manner, as oak, beech and mahogany, which, although moderately elastic, do not bend with the facility of those before named.
Fishing-rods, unless made of bamboo, have generally ash for the lower joint, hickory for the two middle pieces, and a strip cut of a bamboo of three or four inches diameter as the top joint. Archery bows are another example of elastic works; the "single-piece bow" is made of one rod of hickory, lanccwood, or yew tree, which last, if perfectly free from knots, is considered the most suitable wood: the "back or union bow" is made of two or sometimes three pieces glued together. The back-piece, or that furthest from the string, is of rectangular section, and always of lanccwood or hickory; the belly, which is nearly of semicircular section, is made of any hardwood that can be obtained straight and clean, as ruby-wood, rose-wood, green-heart, king-wood, snake-wood, and several others: it is in a great measure a matter of taste, as the elasticity is principally due to the back-piece; the palmyra is also used for bows.*
The elasticity or rather the flexibility of the woods, is greatly increased for the time, when they are heated by steaming or boiling: the process is continually employed for bending the oak two other sets of experiments on Indian timber woods, by Captain H. C. Baker, late of the Bengal Artillery, superintendent of the half-wrought timber-yard, Calcutta, at pp. 123 and 230 of the Gleanings of Science, published at Calcutta, 1829. * The union bow is considered to be "softer," that is more agreeably elastic than the single-piece bow, even when the two require the same weight to draw them to the length of the arrow. In the act of bending the bow, the back is put into tension, and the inner piece into a state of compression, and each wood is then employed in its most suitable manner. Sometimes the union bow is imitate by one solid piece of straight cocoa-wood, (of the West Indies, not that of the cocoa-nut palm,) in which case the tough fibrous sap is used for the back, and in its nature sufficiently resembles the lance-wood more generally used.
The woods are steamed in suitable vessels, and are screwed or wedged, at short intervals throughout their length, in contact with rigid patterns or moulds, and whilst under this restraint they are allowed to become perfectly cold; the pieces are then released. These bent works suffer very little departure from the forms thus given, and they possess the great advantage of the grain being parallel with the curve, which adds materially to their strength, saves much cost of material and time in the preparation, and gives in fact a new character to the timber.
The inner and outer plankings of ships are steamed or boiled before they are applied; they are brought into contact with the ribs by temporary screw-bolts which are ultimately replaced by the copper bolts inserted through the three thicknesses and riveted: or they are secured by oak or locust tree-nails, which are caulked at each end.*
Boiling and steaming are likewise employed for softening the woods, to facilitate the cutting as well as bending of them.†
When the two sets of fibres meet in confused angular directions, they produce the tough cross-grained woods, such as lignum-vitae, elm, etc, and, like the diagonal braces in carpentry and shipping, they deprive the mass of elasticity, and dispose it rather to break than to bend, especially when the pieces are thin, and the fibres crop out on both sides of the same; the confusion of the fibres is, at the same time, a fertile source of beauty in appearance to most woods.
* See the description of Mr. William Hookey's apparatus for bending ships' timbers, rewarded by the Society of Arts, and described in their Trans., vol. 32, p. 91.
Preference is now given to the "Steam Kiln " over the "Water Kiln," and the time allowed is one hour for every inch of the thickness of the timber; it loses much extractive matter in the process, which is never attempted a second time, as the wood then becomes brittle.
Colonel G. A. Lloyd devised an ingenious and economical mode of bending the timbers to constitute the ribs of a teak-bridge which he built in the Mauritius. Every rib was about 180 ft. long, and of 8 ft. rise, and consisted of five thicknesses of wood of various lengths and widths. The wood had been cut down about a month; it was well steamed and brought into contact with a strong mould, by means of an iron chain attached to a hook at the one extremity of the mould and passed under a roller fixed at the other; the chain was drawn tight by a powerful capstan. Whilst under restraint the neighbouring pieces were pinned together by tree-nails, after which a further portion of the rib was proceeded with: the seasoning of the timber was also effected by the process.
† Thus in Taylor's Patent Machinery for making casks, the blocks intended for the staves are cut out of white Canada oak to the size of thirty inches by five, and smaller. They are well steamed, and then sliced into pieces one-half or five-eighths inch thick, at the rate of 200 in each minute, by a process far more rapid and economical than sawing; the instrument being a revolving iron plate of 12 or 14 feet diameter, with two radial knives, arranged somewhat like the irons of an ordinary plane or spokeshave.
Elm is perhaps the toughest of the European woods; it is considered to bear the driving of bolts and nails better than any other, and it is on this account, and also for its great durability under water, constantly employed for the keels of ships, for boat-building, and a variety of works requiring great strength and exposure to wet.
A similar rigidity is also found to exist in the crooked and knotted limbs of trees from the confusion amongst the fibres, and such gnarled pieces of timber, especially those of oak, were in former days particularly valued for the knees of ships: of later years they have been in a great measure superseded by iron knees, which can be more accurately and effectively moulded at the forge to suit their respective places, and they cause a very great saving in the available room of the vessel.
The lipium-vitae is a most peculiar wood, as its fibres seem arranged in moderately thick layers,crossing each other obliquely, often at as great an angle as thirty degrees with the axis of the tree; when the wood is split, it almost appears as if the one layer of annual fibres grew after the manner of an ordinary screw, and the succeeding layer wound the other way so as to cross them like a left hand screw. The interlacement of the fibres in lignnm-vitae is so rigid and decided, although irregular, that it exceeds all other woods in resistance to splitting, which cannot be effected with economy; the wood is consequently always prepared with the saw. It is used for works that have to sustain great pressure and rough usage, several examples of which are given under the head Lignum-vitae:, in the Catalogue already referred to.