20

CIRCULATION OF THE SAP.

strength and durability; in the one, Scotch larch, there were three annual rings in five-eighths of an inch, whereas in Italian larch there were twenty-four layers in the same space. In some of the tropical woods the appearance of the rings can scarcely be defined, and in a specimen of the lower or butt end of teak, now before me, three annual rings cover the great space of one inch and three-eighths.

The horizontal section of a tree, occasionally looks as if it were the result of two, three, or more separate shoots or stems consolidated into one; in some of the foreign woods in particular, this irregularity often gives rise to deep indentations, and most strange shapes, which become eventually surrounded by one single covering of sap; so that a stem of considerable girth may yield only an insignificant piece of wood, scarcely available for the smallest purposes of turnery, much less for cabinet work*.

The circulation of the sap is considered to be limited to a few of the external layers, or those of the sap-wood, or alburnum, which are in a less matured state than the perfect wood, or duramen, beneath. The last act of the circulation, as regards the heart-wood, is supposed to be the deposition of the colouring matter, resin or gum, through the agency of the medullary rays, that proceed from the bark towards the center, and leave their contents in the layer outside the true wood perfected the year previous. We may fairly suppose by analogy, that as one ring is added each year, so one is perfected annually, and thrown out of the circulatory system.

That the circulation has ceased in the heart-wood, and that the connexion between it and the bark has become broken, is further proved by the fact, that numbers of trees may be found in tolerably vigorous growth within the bark, whereas at the heart they are decayed and rotten. In fact some of the hardest foreign woods, as king-wood, tulip-wood, and others, are rarely sound in the center, and thus indicate very clearly that

* This is not peculiar to the tropical woods; for example, some of the yew-trees in Hampton Court gardens, appear to have grown in this manner from three or four separate stems, that have joined into one a short distance above the ground. As an instance of the singular manner in which the separate branches of trees thus combine, I may mention that stones and pieces of metal, &c., are occasionally met with in the central parts of timber, from having been accidentally deposited in a cleft, or the fork of a branch, and entirely enclosed or overgrown by the subsequent increase of the plant.

THE TREE WHEN FELLED.

21

their decay commenced whilst they were in their parent soil; and as in these, the appearance of annual rings is scarcely to be made out, this also appears to indicate a great term of age, enough to account for this relatively premature decay.

The quantity of sap-wood is various in different plants, and the line of division is usually most distinctly marked; in some, as boxwood, the sap-wood is very inconsiderable, and together with the bark is on the average only about the thickness of a stout card, whereas in others, as the snake-wood, it constitutes fully two-thirds of the diameter, so that a large tree yields but an inconsiderable stick of wood, of one third or fourth the external diameter.

It may be presumed that in the same variety of wood, about an average number of the layers exist as sap-wood, as in cutting up a number of pieces of the same kind, such as the black Botany-Bay wood, and others, it is found that in those measuring about two inches diameter, the piece of heart-wood is only about as large as the finger, but in pieces one, two, or three inches larger, the heart-wood is also respectively one, two or three inches larger, or nearly to the full extent of the increase of the diameter.

The sap-wood may be therefore, in general, considered as of about an average thickness in each kind of wood: it is mostly softer, lighter, more even in colour, and more disposed to decay than the heart-wood, which prove it to be in a less matured or useful state, whether for mechanical or chemical purposes.

At the time the tree is separated from its root, its organic life ceases, and then commences the gradual evaporation of the sap, and the drying and contracting of the tubes, or tissues, previously distended by its presence.

The woods are in general felled during the cold months, when the vegetative powers of the plant are nearly dormant, and when they are the most free from sap; but none of the woods are fit for use in the state in which they are cut down, for although no distinct circulation is going on within the heart-wood, still the capillary vessels keep the trees continually moist throughout their substance, in which state they should not be employed.

If the green or wet woods are placed in confined situations,

22

GREEN WOOD UNFIT FOR USE.

the tree or plank, first becomes stained or doated, and this speedily leads to its decomposition or decay, effects that are averted by careful drying with free access of air*.

Other mischiefs almost as fatal as decay also occur; blocks cut out of the entire circular stem of green wood, or the same pieces divided into quarterings, split in the direction of the medullary rays, or radially, also though less frequently upon the annual rings. The entire sections or round blocks contract pretty equally, and retain their circular form, but the quarterings become oval from their unequal shrinking.

As a general observation it may be said the woods do not alter in any material degree in respect to length. Boards and flat pieces contract however in width, they warp and twist, and

On this account the timbers for ships are usually cut out to their shape and dimensions for about a year before they are framed together, and they are commonly left a twelvemonth longer in the skeleton state, to complete the seasoning, as in that condition they are more favourably situated as regards exposure to the air than when they are closely covered in with the planking.

Mr. Fincham considers that the destruction of timber by the decay commonly known as dry-rot, cannot occur unless air, moisture, and heat, are each present, and that the entire exclusion of any of the three stays the mischief. By way of experiment, he bored a hole in one of the timbers of an old ship built of oak, whose wood was at the time perfectly sound; the admission of air, the third element, to the central part of the wood, (the two others being to a certain degree present,) caused the hole to be filled up in the course of twenty-four hours with mouldiness, a well-known vegetation, which very speedily became so compact a fungus as to admit of being withdrawn like a stick. He considers the shakes in timber to predispose it to decay, in damp and confined situations, from admitting the air in the same manner.

The woods differ amazingly in their resistance to decay; some perish in one or two years, whereas others are very durable, and even preserve their fragrance when they are opened after many years, or almost centuries.

Mr. G. Loddiges says, the oak boxes, for the plants in his green-houses, decay in two or three years, whereas he has found those of teak to last fully six or seven times as long: the situation is one of severe trial for the wood.

The process of Kyanizing is intended to prevent the re-vegetation of timber, by infusing into its pores an antiseptic salt: the corrosive sublimate is generally employed, other metallic salts are also considered to be applicable, but the general utility of the process, especially in thick timbers, or those exposed to much wet, is still unsettled amongst practical men.

The Kyanizing is sometimes done in open tanks, at others, (by Timperley's process, Hull and Selby Railway,) in close vessels from which the air is first exhausted to the utmost, and the fluid is then admitted under a pressure of about 100 pounds on the inch.-See Minutes of Proceedings Inst. Civ. Eng., p. 83, 1841.

There are two quarto works on dry-rot; the one by Mr. M'William, 1818; the other by Mr. John Knowles, Surveyor of Her Majesty's Navy, 1821.

SEASONING THE WOODS.

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when they are fitted as panels into loose grooves, they shrink away from that edge which happens to be the most slightly held; but when restrained by nails, mortices, or other unyielding attachments, which do not allow them the power of contraction, they split with irresistible force, and the materials and labour thus improperly employed will render no useful service.

In general, the softest woods shrink the most in width, but no correct observations on this subject have been published. Mr. Fincham considers the rock-elm to shrink as much as any wood, namely, about half an inch in the foot, whereas the teak scarcely shrinks at all; in the "Tortoise" store-ship, when fifty years old, no openings were found to exist between the boards.

In the woods that have been partially dried, some of these effects are lessened when they are defended by paint or varnish, but they do not then cease, and with dry wood, every time a new surface is exposed to the air, even should the work have been made for many years, these perplexing alterations will in a degree recommence, even independently of the changes of the atmosphere, the fluctuations of which the woods are at all times too freely disposed to obey.

The disposition to shrink and warp from atmospheric influence, appears indeed to be never entirely subdued; some bogoak, supposed to have been buried in the island of Sheppy, not less than a thousand years, was dried for many months, and ultimately made into chairs and furniture; it was still found to shrink and cast, when divided into the small pieces required for the work.

SECT. II.SEASONING AND PREPARING THE WOODS.

HAVING briefly alluded to the mischiefs consequent upon the use of woods in an improper condition, I shall proceed to describe the general modes pursued for avoiding such mischiefs by a proper course of preparation.

The woods immediately after being felled, are sometimes immersed in running water for a few days, weeks, or months, at other times they are boiled or steamed; this appears to be done under the expectation of diluting and washing out the sap, after which it is said the drying is more rapidly and better accomplished, and also that the colours of the white woods are im

proved, (see article HOLLY in Catalogue, also EBONY,) but the ordinary course is simply to expose the logs to the air, the effect of which is assisted by the preparation of the wood into smaller pieces, approaching to the sizes and forms in which they will be ultimately used, such as square logs and beams, planks or boards of various thicknesses, short lengths or quarterings, &c.

The stems and branches of the woods of our own country, such as alder, birch, beech, &c., that are used by the turner, frequently require no reduction in diameter; but when they are beyond the size of the work, they are split into quarterings and stacked in heaps to dry, which latter proceeding should never be forgotten under any circumstances.

We know but little of the early treatment of the foreign woods used for cabinet-work and turning; some few of them, as mahogany and satin-wood, are imported in square logs; others, as rosewood, ebony, Coromandel, &c., are sometimes shipped in the halves of trees, or in thick planks; but the generality of those used for turning are small, and do not require this reduction; these only reach us in billets, sometimes with the rind or bark upon them, and sometimes cleaned or trimmed.

The smaller hard woods are very much more wasteful than the timber woods; in many of the former, independently of their thick bark, the section is very far from circular, as they are often exceedingly irregular, indented, and ill-defined; others are almost constantly unsound in their growth, and either present central hollows and cavities, or cracks and radial divisions, which separate the stem into three or four irregular pieces.

Probably none of the hard woods are so defective as the black Botany-Bay wood, in which the available produce, when it is trimmed ready for the lathe, may be considered to be about one third or fourth of the original weight, sometimes still less; but unfortunately many others approach too nearly to this condition, as a very large proportion of them partake of the imperfections referred to, more especially the cracks; the larger hard woods are by comparison much less wasteful.

All the harder woods require increased care in the seasoning, which is often badly begun by exposure to the sun or hot winds in their native climates: their greater impenetrability to the air the more disposes them to crack, and their comparative scarcity and expense, are also powerful arguments on the score of precau