which it is applied, it seldom arrives at maturity. The proper time to cut down ash is in winter, when the sap is at rest.

The quality is nearly the same through the whole substance of the tree, but the outside is rather the toughest. It soon rots when exposed to the weather, in a state of rest, but will last very long in constant use, if properly taken care of. It is of a porous structure; and that of which the fibres are long and straight is always considered the best,

190.-ELM is another tough and strong species of wood; it is, also, very useful for the husbandman, many implements being made of it; and, indeed, it is often preferred, for particular purposes, to ash itself. This is a very common wood, and is mostly found in hedge-rows, or around the skirts of plantations. On the demesne lands of Springwood Park, in the neighbourhood of Kelso, trees of this kind are very large, high, and branching, and contain a very great quantity of valuable timber. This circumstance authorises us to conclude, that a rich and loamy soil is the best for its production; as, in this particular place, the land is of such a quality, and also extremely fertile, the elms reared on it may be compared with any in the kingdom.

191. FIR.-The next species to which our attention shall be directed is FIR, than which there is no kind of timber more useful, or applied to more useful purposes. This, however, arises, not from its superior strength or durability, but from its being cheap, and yielding easily to the tools of the workman. It is common in almost all northern countries, and is brought, in great quantities, from Norway, Russia, Prussia, Sweden, and North-America.

The fir which is mostly used in carpentry is distinguished by the name of Memel, Dantzic, and Riga Fir. Norway Fir is also much used for smaller timbers, and answers extremely well when exposed to the air, or when kept under ground. The fir from North-America is softer than any just mentioned; it is likewise more free from knots, and, of course, suitable for the finer parts of joinery, such as panels and mouldings, and is called pine wood. What is termed in England white deal, is a species of spruce fir, and is very durable when kept dry, and for that reason is much used by cabinet-makers; but, as it does not stand the weather, it is used only for internal work in joinery.

In former times the Highlands of Scotland abounded in forests of fir-trees, as appears from the great number of stumps and roots still existing in the bogs and morasses. Above LochielHouse, in Invernesshire, along the whole extent of Loch-Aghrigh, or Arkeig, is a forest of fir, in which many of the trees are yet in a high state of health, and of a great size: this wood is very strong, and so full of rosin, that many of the inhabitants use it in lieu of candles, it giving such a brilliant light as to render the use of tallow unnecessary.

192.-BIRCH is also a very common wood, and in the North of Scotland the dwarf kind grows spontaneously, in great abundance. The quality of birch is nearly the same quite across the tree; it is very tough, but will stand the weather, and worms are very hurtful to it. Birch is often used in works which lie under water. Some beautiful species of birch are imported from North-America in large logs; and they are much used for cabinet-work. The one of these species is called brown birch, and is frequently figured with dapples. When this kind is properly stained, it has much the appearance of mahogany.

193.-POPLAR is a tree that thrives well on wet ground, and is very often found on wet spots about gentlemen's seats. In beams it is liable to the same objections as beech, but it is well adapted for floors and stairs; but it rots when exposed to the weather.

The poplar and the aspen resemble each other; the latter is tough and soft, lasts better when exposed to the weather, and is equally good throughout the body of the tree.

194. SYCAMORE and LIME.-For the large timbers of roofing and flooring the SYCAMORE and LIME are subject to the same objections as the beech and poplar. The lime is, however, suitable for furniture; and makes good floors, being smooth, when wrought. When Sycamore is obtained of a considerable degree of whiteness and figured, it is much esteemed for cabinet-work.

195. WALNUT and CHESNUT.-We have also the WALNUT-TREE and the CHESNUT; the former of which has become too valuable in Britain, in consequence of the great consumption for gun-stocks; and mahogany has now nearly superseded its use in furniture.

The Spanish or Sweet Chesnut is frequently found in old buildings in England; it is very like oak, and is often confounded with it; but, notwithstanding, it differs from it in this, that, when a nail or bolt has been driven into oak before it was dry, a black stain appears round the iron, which in chesnut is not the case.

196. MAHOGANY is chiefly used in furniture, and sometimes also in doors and windowsashes, it is sawn out and seasoned by being kept under cover yet exposed freely to the air: it is extremely valuable, and grows in Jamaica. There is another kind, from Yucatan, called Honduras mahogany, but that of Jamaica is much the most beautiful and durable. The pores of the Honduras appear quite black; those of the Jamaica kind appear as if filled with chalk.

General Cautions and Remarks respecting Timber.

197. LAY your timber up, when perfectly dry, in an airy place, that it may not be exposed to the sun and wind, and taking care that it does not stand upright, but let it be laid along, one piece upon another; interposing, here and there, some short blocks, to keep them apart, and prevent that mouldiness which is usually contracted when the planks sweat.

Some persons, in the first stages of seasoning, keep their timber as moist as they can by submerging it in water, with a view to prevent it from cleaving. This is good in fir, and also in some other timbers, Lay your planks in a stream of running water for a fortnight, and then set them up in the sun and wind, so that the air may freely pass between them, and turn them frequently. Boards thus seasoned will floor much better than those which have been kept many years in a dry place.

But, to prevent all possible accidents, when you lay your floors, let the joints be fitted and tacked down only for the first year, nailing them close down the next; and, by this method, they will lie without shrinking in the least.

Amongst wheelwrights the water-seasoning is of special regard, and of such esteem amongst some, that the Venetians lay their oak some years in water before they employ it.

Elm felled ever so green, if kept four or five days in water, obtains a good seasoning, anu is rendered more fit for immediate use. This water-seasoning is not only a remedy against the worm, but it also prevents distortions and warping. Some persons recommend burying timber in the earth, and others will have their timbers covered-in to heat; and we likewise see that scorching and hardening in the fire renders piles durable, especially those which are to stand in earth or water.

Green timber is sometimes used by those who carve and turn: but this for doors, windows, floors, and other close works, is altogether to be rejected; especially if walnut be the material, as they will be sure to shrink. It is, therefore, best to choose such as has had two or three years' seasoning, and which is neither too dry nor moist.

Where huge massy columns are to be used, it is a good plan to bore them right through, from end to end, as it prevents their splitting.

R

Timbers occasionally laid in mortar, or in any part contiguous to lime, as doors, window. cases, ground-sills, and the extremities of beams, &c., have sometimes been capped with melted pitch, as a preserver from the destructive effects of the lime; but it has been found to be rather hurtful than otherwise.

For all uses, that timber is the best which is the most compact and free from knots. As to the place of growth, that is generally esteemed the best which grows most in the sun; but, as we have already hinted, this is not always the case. The climate, however, contributes much to its quality, and perhaps a northern situation is preferable to all others.

198. Foreign Timber is always sufficiently seasoned to cut into scantlings; but home timber requires to be seasoned some time after felling, before it be cut up, otherwise it will warp, split, and often become unfit for use. After timber is felled, it should remain at least six months in the tree, and during that time it should be raised off the ground to admit a free circulation of air round each piece; and it should remain about one year in scantlings, before it be used in buildings. 199. The wood for Joiners' work should be all cut to the proper thicknesses, so as to allow them the most time possible to dry; by such arrangements, on the commencement of a building, every thing will be ready for its respective use, and well fitted for ensuring soundness and durability.

200. Respecting the selection of timber for principal beams we cannot offer much, in addition to the judicious directions of the Italian architect, Alberti, which are here given nearly in his own words.

Beams ought to be perfectly sound and clean; and, especially about the middle of their length, they ought to be free from the least defect. Placing your ear at one end of a beam while the other is struck, if the sound come to you dead and flat, it is a sign of some private infirmity. Beams that have knots in them are absolutely to be rejected, if there be many, or if they be crowded together in clusters. Whichsoever side of a beam has a defect, that runs crossways of it, let that side be laid uppermost; also, if there be a crack lengthways, do not venture it on the side, but lay it either uppermost or undermost. If you happen to have occasion to bore a hole in the beam, or to make any opening, never meddle with the middle of its length, nor its lower superficies. If, as in churches and large houses, the beams are to be laid in pairs, leave a space of some inches between them that they may have room to exhale, and not be spoiled by heating one another, and it will not be amiss to lay the two beams of the same pair different ways, that both their heads may not lie upon the same pillow; but where one has its head, the other may have its foot; for, by this means, the strength of the one's foot will assist the weakness of the other's head, and so, vice versa. Let the plates for the beams be exactly level, and perfectly firm and strong; and in laying them take care that the timber does not touch any lime, and let it have clear and open space all about it, that it may not be tainted by the contact of any other materials, nor decay by being too closely shut up.

Contraction and Expansion of Timber.

201. It is well known that a tree contracts less in proportion, in diameter, than it does in circumference; hence a whole tree always splits in drying; and 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 ash and beech trees he cut some thin boards, in different directions relatively to their structure, so that the rings in the wood crossed the middle of some of

the boards at right-angles, and lay nearly parallel with the surface of others. Both kinds were placed in a warm room, under perfectly similar circumstances; those which had been formed by cutting the boards, so that the rings were nearly parallel, as at A in fig. 1, pl. XLIII, soon changed their form very considerably, the one side becoming hollow, and the other round; and, in drying, they contracted nearly 4th in width. The other kind, in which the rings were nearly at right-angles to the surfaces of the boards, as at B, in the figure, retained with very little variation their primary form, and did not contract in drying more than about th part of their width. As Mr. Knight had not tried resinous woods, the subject was further investigated by Mr. Tredgold, who had two specimens cut from a piece of Memel timber; and, to render the result of the observation more clear, conceive the figure to represent the section of a tree, the annual rings being shown by circles; BD represents the manner in which one of the pieces was cut, and AC the other; the board AC contracted about,th part in width, and became hollow on the side marked b; the board BD retained its original straightness, and contracted only aboutth part of its width; the difference in the quantity of contraction being still greater than in hard woods.

From these experiments the advantages to be obtained, merely by a proper attention in cutting out boards for pannels, &c., will be obvious; and it will also be found, that pannels cut so that the rings are nearly perpendicular to their faces, will appear of a finer and more even grain, and require less labour to make their surfaces even and smooth. The results of these experiments are not less interesting to cabinet-makers than to joiners, particularly in the construction of billiard-tables, card-tables, and indeed every kind of table in use. For such purposes, the planks should be cut so as to cross the rings, as nearly in the direction, BD, as possible. We have no doubt that it is the knowledge of this property of wood, that renders the billiard-tables of some makers so far superior to those of others. In wood that has the large radiating lines, as the oak, for example, boards cut as BD will be figured, while those cut at AC will be plain.

202. 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 prevent 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 desirable 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 contraction causes the wood to curve, in 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.

203. 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; hence they are sometimes employed for pendulum rods; but the lateral dimensions vary so much, by the dampness or dryness of the air, 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 by damp is nearly the same as in new wood. It appears, from Rondelet's experiments, 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 weather, was,

Consequently, the mean extent of variation in fir is 24, and in oak Ti; and at this mean rate. in a fir board, about 12 inches wide, the difference in width would beth of an inch. This will show the importance of attending to the maxims of construction we shall have to place before the reader, in treating of framing in joinery; for, if a board of that width should be fixed at both its edges, it must unavoidably split from one end to the other, by the mere effect of the weather.

The importance of a knowledge of these properties of timber is considerable in all arts where wood is operated upon in considerable pieces, and for good work; and we have to acknowledge the assistance in treating them we have received from the Treatise on Joinery in the Encyclopædia Britannica, written by Mr. Tredgold.

ON THE STRENGTH OF TIMBER.

204. THE strength of materials used in mechanical constructions is exerted in five different ways; that is to say, in resisting a direct pull, in resistance to compression, to a transverse strain, to torsion, or twisting, and to percussion.

The strength which resists extension is the effect of cohesion. If cohesion opposes the extension, both act according to the same law, being as the extension, while the forces exerted are not great. There is a certain limit beyond which cohesion does not act and if it be exceeded, a total separation takes place.

205. Materials also should be considered in relation to the effect a strain produces on them; they bend, they suffer alteration, and they break.

Bending may be occasioned either by a transverse or by a longitudinal force: when the force is transverse, the extent of the bending is nearly proportional to the force; but when it is longitudinal, there is a certain degree of force which must be exceeded, in order to produce, or rather to continue, the bending, if the force be applied exactly at the axis. But it is equally true, that the slightest possible force applied at a distance from the axis, however minute, or with an obliquity however small, or to a beam already a little curved, will produce a certain degree of bending; and this observation will serve to explain some of the difficulties and irregularities which have occurred in making experiments on beams exposed to longitudinal pressure.

206. Alteration, Dr. Young truly remarks, is often an intermediate step between a temporary change of form and a complete fracture. There are many substances, which, after bending to a certain extent, are no longer capable of resuming their original form and in such cases it generally happens that the alteration may be increased without limit, until complete fracture takes place by the continued operation of the same force which has begun it, or by a force a little greater. Those substances which are the most capable of this change, are called ductile, and the most remarkable are gold, and a spider's web. When a substance has undergone an alteration by means of its ductility, its stiffness, in resisting small changes on either side, remains little, or not at all altered. Thus, if the stiffness of a spider's web, in resisting torsion, were sufficient, at the commencement of an experiment, to cause it to recover itself, after being twisted in an angle of ten degrees, it would return ten degrees, and not more, after having been twisted round a thousand times. The ductility of all substances, capable of being annealed, is