Carpentry is the art of cutting out, framing, and joining large pieces of wood to be used in building. The only difference between Carpentry and Joinery is, that whilst the former includes the larger and rougher description of work, which is essential to the construction and stability of an edifice, the -atter term comprehends the exterior finishing and ornamental wood-work. To enter into a detailed account of the numerous tools used in Carpentry, and the processes of forming, by their application, the almost illimitable variety of matters that are comprehended in constructive carpentry, would of course require a volume to itself; and as the subject is foreign to the leading objects of this work, we shall under this head confine ourselves to some general observations of a practical nature, alike useful to the engineer as well as the carpenter. With regard to the tenacity or strength of wood, it has been found by Muschenbroek and other eminent experimentalists, first, that the wood which surrounds immediately the pith or heart of the tree is the weakest, and that this weakness is greater as the tree is older. It is of importance that this fact be known, as a common notion exists of a contrary nature.

Secondly, the fibres next to the bark, commonly called the white or blea, are also weaker than the rest; and the wood gradually increases in strength as it recedes from the centre to the blea. Thirdly, the wood is stronger in the middle of the trunk than at the springing of the branches, or at the root; and the wood forming a branch is weaker than that of the trunk. Fourthly, the wood on the northern sides of all trees that grow in Europe is the weakest, while that on the south-eastern side is the strongest; this difference is most remarkable in hedge-row trees, and such as grow singly. The heart of a tree never lies in its centre, but always towards its northern side, and the annual coats of wood are thinner on that side. In conformity with this, it is a general opinion of carpenters that timber is stronger in proportion to the thickness of its annual plates. The trachea, or air vessels, being the same in diameter and number of rows in trees of the same species, occasion the visible separation between the annual plates; for which reason, when these are thicker, they contain a greater portion of the simple ligneous fibres. Fifthly, all woods are most tenacious whilst green, but after the trees are felled, that tenacity is considerably diminished by their drying.

By the experiments of Muschenbroek, it appears that the absolute strengths of a square inch of the following different kinds of wood are as stated

Locust

20,100

Jujeb

18,500

Beech and Oak

17,300

Orange

15,500

Alder .....

13,900

Elm.....

13,200

Mulberry and Willow

12,500

Ash

12,000

Plum

11,800

Elder.....

10,000

Pomegranate . . .

9,750

Lemon

9,250

Tamarind

8,750

Fir

8,330

Walnut

8,130

Pitch pine

7,650

Quince

6,750

Cypress

6,000

Poplar

5,500

Cedar

4,800

The woods mentioned were all formed into slips of uniform dimensions, and as much cut away from each, as to form a parallelopiped of one-fifth of an inch, that is, one-twenty-fifth of a square inch in section; and the weights which were required to tear these asunder, formed the data of the above calculations. Muschenbroek gives a very minute detail of his experiments on the ash and walnut, in which he states the weights required to tear asunder slips taken from the four sides of these trees, and on each side in a regular progression from the centre to the circumference. The numbers in the foregoing, corresponding with these two woods, may be considered, therefore, as the average of more than fifty trials of each. He mentions, also, that all the other numbers were calculated with the same care. For these reasons some confidence may be placed in the results, though they carry the degree of tenacity considerably higher than those enumerated by some other writers. Pitot and Parent state, that a weight of 60 lbs. will just tear asunder a square line of sound oak, but that it will bear 50 lbs. with safety.

This gives 8640 for the greatest strength of a square inch, or rather less than one-half of Muschenbroek's estimate; but the latter is, we think, the most entitled to confidence, as the experiments were made upon greater masses of the material. It should, however, be observed, that two-thirds of the actual weight which may be sustained by a body, will greatly impair the strength after a considerable time, and that one-half the absolute tenacity indicated by experiment, should be the utmost that an engineer should calculate upon in his constructions. Woods of the same denomination often differ greatly in their tenacity; those with a very straight fibre suffer less injury from a load that is insufficient to break them immediately. Mr. Emerson mentions the following as the loads which may be safely suspended to an inch square of the several bodies hereafter enumerated; but the term safely can hardly apply to the four first-named bodies.

lbs.

Iron

76,400

Brass

35,600

Hempen rope

19,600

Ivory

15,700

Oak, box, and yew . .

7,850

Elm, ash, and beech . .

6,070

Walnut and plum . . .

5,360

lb>.

Red fir, holly, elder, plane, and crab.............

5,000

Cherry and hazel . . .

4,760

Alder, asp, birch, and willow...............

4,290

Lead.............

450

Freestone...................

914

The same ingenious author has laid down as a practical rule, that a cylinder of one inch diameter will carry, when loaded to one-fourth of its absolute strength, as follows: - iron, 135 cwt.; good rope, 22 cwt.; oak, 14 cwt; and fir, 9 cwt. Parent has shown that the force required to crush a body is nearly equal to that which will tear it asunder. This may be an approximation to the truth as respects woods, but it will not apply to any other bodies. Glass, for instance, will carry a hundred times more on it than oak in this way, but will not bear suspended above four or five times as much. Oak will suspend a great deal more than fir, but fir will carry twice as much as a pillar. Some woods which are very soft, and consequently yield to pressure, possess very strong fibres, and will resist a longitudinal strain. The softness of texture is chiefly owing to the crooked nature of their fibres, and to the existence of considerable vacuities between each fibre, so that they are more easily bent in a lateral direction and crushed.

In all cases where the fibres lie oblique to the strain, the strength is considerably diminished, which may be ascribed to the circumstance, that the parts in such case slide on each other, and the connecting force of the cementing matter is for that reason easier overcome. The strain which most commonly acts on materials of any nature, is that which tends to break them in a transverse direction. For the results derived from experiment in strains of this nature, we must refer the reader to the article Strength of Materials; but the complete investigation of the resistance of materials, according to the direction and situation of the forces applied, would require a volume. The reader who wishes for full information on this subject, cannot do better than consult Mr. Barlow's valuable Essay on the Strength and Stress of Timber. Some excellent practical rules on this important subject will, however, be found in The New Practical Builder, by Nicholson, and in the works of Banks, Emerson, and Roberson.