506. On examining the transverse section of a tree a number of layers or rings will be seen, as stated in Art. 439, regularly disposed around the pith which is generally near the centre of the section; and radiating from the pith towards the bark will also be seen a number of fine divisions called medullary rays, with pores or cells between them, often empty but sometimes filled: in the class of wood to which pine belongs these pores appear to be nearly all filled with resinous matter, and a part of each layer or annual ring consists of a hard compact and dark-coloured substance, the other part being lighter-coloured and softer.

Besides the fine divisions or rays referred to, there are in some descriptions of wood other divisions very large and distinct, which radiate in the same way, but are generally of a light silvery colour. These form what is called the "silver grain" of the wood, and produce that fine flowered appearance seen in oak when cut through this grain.

In some kinds of wood, while a part of each annual layer is nearly compact, the remainder of it presents the appearance of a circle of empty pores, of which we have an example in the ash, and which shows remarkably distinct in the section of the Arbutus given by Hill.*

In other kinds of wood the annual layers seem to be nearly uniform in texture, and the line of separation between them is not very distinct. Mahogany is an example of this structure, and the Robinia Caragna of Hill is another of the same kind.

507. The classification of timber given in the following Table is based on the foregoing distinctions.†

* ' Construction of Timber,' p. 136.

† The Table is a modification, suggested by Professor Rankine, of that given in a former edition of this work.

Class I. - Pine Wood (Natural Order Coniferae)

Annual rings very distinct, pores filled with resinous matter, one part of the ring hard, and dark-coloured; the other soft and light-coloured.

Pine, Fir,

Larch, Cedar,

Cowrie, etc, etc.

Class II. - Leaf Wood (Non-Coniferous)

The only other properties of wood that seem to require explanation are the cohesive force, modulus of elasticity, stiffness, hardness, and toughness.

508. The cohesive force of a bar or beam is equal to the power or weight that would pull it asunder in the direction of its length. The weight that would pull asunder a bar of an inch square of different kinds of wood has been ascertained by experiments, which have been made by Muschcn-broek,* Emerson,† Rondelet,‡ Anderson, Barlow,§ Hodgkin-son,|| and others. Of these experiments the highest and lowest result have been taken for each kind of wood.

509. The modulus of elasticity or weight in lbs. required to extend a bar of 1 inch square to double its original length (Art. 79) is usually taken as the standard by which the elastic force of one substance is compared with another. Dr. Thomas Young has by means of it given some very elegant demonstrations of the laws of resistance,* and its use must be evident when it is considered that it is only the elastic force of timber that is employed in resisting the usual strains in carpentry. The constant numbers in the rules for the stiffness of timber have the modulus of elasticity for one of their elements.

* 'Intro, ad Phil. Nat.' † ' Mechanics,' 4to edit., sect. viii.

‡'LArt de Bath.' § 'Essay on the Strength of Timber,' etc.

|| ' Phil. Trans' 1840.

By means of the modulus of elasticity the comparative stiffness of bodies can be ascertained. For instance, its weight for cast iron is 17,000,000 lbs., and its weight for oak is 1,714,500 lbs. Hence it appears that the modulus for Cast iron is nearly ten times that of oak, and therefore a piece of cast iron is ten times as stiff as a piece of oak of the same dimensions and bearings.

510. A hard body is that which yields but little to any stroke or impressive force; and it may be shown, by the principles of mechanics, that in uniform bodies the degree of yielding is always proportional to the weight of the modulus of elasticity; therefore a table containing the weights of the modulus of elasticity of such bodies shows also their relative hardness and stiffness.

The relative hardness may be determined by means of the modulus of elasticity; but the methods used for ascertaining the hardness of mineral bodies is very defective; and the method proposed by Dr. O. Gregory,† from the theory of percussion, is not susceptible of any tolerable degree of accuracy, from the difficulty of making correct experiments.

As the hardness follows the same laws as the stiffness, cast iron is ten times as hard as oak. But it is necessary to inform the reader, that when the substance is not uniform, the hardness thus found is that of the hardest part. Thus, in fir it is the darker part of the annual ring that is the hardest, and which determines the extent to which a beam will bend without fracture. Dry wood is harder than green, consequently it is more difficult to work. The labour of sawing dry oak is to that of sawing green as 4 is to 3,* nearly.

* 'Lectures on Natural Philosophy.'

† 'Treatise on Mechanics,' vol. i., art. 348.

511. In respect to the toughness of woods, that wood is the toughest which combines the greatest degree of strength and flexibility; hence that wood which bears the greatest load and bends the most at the time of fracture is the toughest. The comparative toughness has been ascertained from the data obtained in the course of the author's experiments, except in a few instances, where he had not specimens sufficiently long for the purpose. In such cases they have been calculated from Barlow's experiments.

512. The opposite to hardness is softness, the opposite to toughness is brittleness, and the opposite to stiffness is flexibility; therefore, when the hardness, toughness, or stiffness of wood is expressed by a low number, it may be considered to have the opposite quality.

513. Oak has been made the standard of comparison both for strength, toughness, and stiffness, and is taken = 100; the mean strength of oak being taken at 11,880 lbs. per square inch, and its modulus of elasticity at 1,714,500 lbs. for a square inch.

Having thus laid before the reader the means adopted for arriving at the properties of timber, it is scarcely necessary to say, that it is those properties which determine its fitness. for the different purposes of carpentry. In some cases stiff woods are required, as in the joists and rafters of a building; in other cases tough woods should be employed, as for the shafts of carriages; and in other cases strength is necessary, as in ties, and other timbers strained in the direction of their length.

* Belidor, * Architecture Hydraulique,' tom, i., p. 342.

Tough woods, which are also hard, are the most difficult to work, especially if cross-grained; on the contrary, brittle woods work easily; and hard woods preserve the best surface.

In general, where straightness is desirable, stiff woods should be preferred; where sudden shocks are to be sustained, tough woods are the best; where little strength is required, but much labour is to be put upon it, a brittle wood is the most economical; and where a fine surface is to be preserved, a hard wood should be chosen.