Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/546

Rh 486 BUILDING [JOINERY. 38. Materials. There is no art in which it is required that the structure and properties of wood should be so thoroughly under stood as in joinery. The practical joiner, who has made the nature of timber his study, has always a most decided advantage over those who have neglected this most im portant part of the art. Woods It is well known that wood contracts less in proportion, shrink in diameter, than it does in circumference ; hence a whole irregularly. tree a i wavs splits in drying. Mr Knight has shown that, in consequence of this irregular contraction, a board may be cut from a tree that can scarcely be made by any means to retain the same form and position when subjected to various degrees of heat and moisture. From the ash and the beech he cut some thin boards in different directions relatively to their tranverse septa, so that the septa crossed the middle of some of the boards at right angles, and lay nearly parallel with the surfaces of others. Both kinds were placed in a warm room, under perfectly similar cir cumstances. Those which had been formed by cutting across the tranverse septa, as at A in fig. 38, soon changed their form very considerably, the one side becoming hollow, and the other round ; and in drying, they contracted nearly 14 per cent, in width. The other kind, in which the septa were nearly parallel to the surfaces of the boards, as at B, retained, with very little variation, their primary form, and did not contract in drying more than 3^- per cent, in width. (Philosophical Transactions, part ii. for 1817 ; Philosophical Magazine, vol. 1. p. 437.) As Mr Knight had not tried resinous woods, two specimens were cut from a piece of Meniel timber ; and to render the result of the observations more clear, conceive fig. 38 to represent the section of a tree, the annual rings being shown by circles. BD represents the manner in which one of our pieces was cut, and AC the other. The board AC contracted 3 75 per cent, in width, and became hollow on the side marked b. The board BD retained its original straightness and contracted only O7 per cent. The difference in the quantity of contraction is still greater than in hard woods. From these experiments, the advan tages to be obtained merely by a proper attention in cutting out boards for panels, &c., will be obvious ; and it will also &quot;be found that panels cut so that the septa are nearly parallel to their faces, will appear of a finer and more even grain, and require less labour to make their surfaces even and smooth. But as this system would necessitate the rejection of all but the heart of the tree for superior work, a method has been adopted which it is said was first used by the billiard-table makers. Let AC, fig. 39, represent the piece above referred to by the same letters. It will become hollow on the side marked b, no doubt because the rings of the wood when cut across are relieved from tension, and endeavour to expand themselves. To counteract this it is customary, in all good work, to rip the plank down the centre, and then to &quot; turn the stuff inside out &quot; as it is popularly called. This is done by reversing the wood, end for end, so as to bring the heart against heart, arid the outside against outside, as is shown in fig. 40 (without which the glue joints are sometimes liable to fly), and also so as to reverse the circular parts of the grain. In wood that has the larger transverse septa, as the oak, for example, boards cut as BD will be figured, while those cut as AC will be plain. There is another kind of contraction in wood whilst drying, which causes it to become curved in the direction of its length. In the long styles of framing we have often observed it ; indeed, on this account, it is difficult to pre vent the style of a door, hung with centres, from curving, so as to rub against the jamb. A very satisfactory reason for this kind of curving has been given by Mr Knight, which also points out the manner of cutting out wood, so as to be less subject to this defect, which it is most desir able to avoid. The interior layers of wood, being older, are more compact and solid than the exterior layers of the same tree ; consequently, in drying, the latter contract more in length than the former. This irregularity of con traction causes the wood to curve in the direction of its length, and it may be avoided by cutting the wood so that the parts of each piece shall be as nearly of the same age as possible. But as this would also necessitate the rejec tion of a great deal of stuff, a simpler method is found, which is always to turn the heart of the wood outwards. Thus, in framing a door, the heart should always go against the jambs, and the sap side to the panels. Besides the contraction which takes place in drying, wood undergoes a considerable change in bulk with the variations of the atmosphere. In straight-grained woods the change in length is nearly insensible, and hence they are sometimes employed for pendulum rods; but the lateral dimensions vary so much that a wide piece of wood will serve as a rude hygrometer. The extent of variation decreases in a few seasons, but it is of some importance to the joiner to be aware that even in very old wood, when the surface is removed, the extent of variation is nearly the same as in new wood. It appears from Rondelet s experiments (L Art de bdtir, article &quot; Menuiserie,&quot; torn. iv. p. 425, 1814), that in wood of a mean degree of dryness, the extent of contraction and expansion, produced by the usual changes in the state of the atmosphere, was, in fir wood, from -Q^ to -^ of its width, and in oak, from T J-g- to ^-. Consequently, the mean extent of variation in fir is yi^, and in oak, -j-j-^ ; and, at this mean rate, in a fir board about 1 2^- inches wide, the difference in width would be y^th of an inch. This will show the importance of attending to the maxims of construction we have already laid before the reader ; for, if a board of that width should be fixed at both edges, it must unavoidably split from one end to the other. The kinds of wood commonly employed in joinery are Kinds the oak, the different species of pine, mahogany, and wood - sometimes lime-tree and poplar. Of the oak there are two species common in Britain ; that which Linnaeus has named Qucrcus Robur is the most valuable for joiners work ; it is of a finer grain, less tough, and not so sub ject to twist as the other kind. Oak is also imported from the Baltic ports, from Germany (that known as wainscot), and from America. These foreign kinds being free from knots, of a straighter grain, and less difficult to work, they are used in preference to British species. The greater part of joiners work is executed in fir imported from the north of Europe. Yellow fir is used for outside work, as doors and sashes, and for floors where there is likely to be much wear. Very good red pine deals have been imported from Canada. Inside work is almost always framed of white fir. Some very good panels when not too wide, and excellent mouldings, are made of American pine. White fir is often used for internal work, and yellow pine is much used for mouldings. The forest of Braemov, in Aberdeenshirc, furnishes yellow fir of an excellent quality, little inferior to the best foreign kinds. For the general purposes of joinery, the wood of the larch seems to be the best; this useful tree thrives well on the Scottish hills. Some fine specimens of it have been obtained from Blair-