The following table exhibits the tensile strength of an inch-square rod of each of the kinds of wood in common use; or, in other words, the power each will resist when so applied as to tend to tear it asunder in the direction of its length: -

Problem I. To Determine The Tensile Strength Of A Rectangular Timber

Rule. - Multiply the thickness of the piece in inches by its depth * in inches, and the product by the weight set against the kind of wood in the table. The product so obtained will be the force in pounds the piece will resist.

Example. - What force will be required to pull asunder a tie-beam of spruce, 7 inches thick and 10 inches deep ?

Thickness, 7 10,260 Breaking-power.

Depth, 10 70

- ______

70 Ans. 718,200 lbs.

* The distance across the top of the beam, when it is in a horizontal position, is commonly called its thickness; and that of the side, from the top to the tinder part, its depth.

Problem II. To Determine The Dimensions Of A Piece Of Timber That Will Resist A Given Strain, One Side Only Being Given

Rule. - Multiply the sum set against the kind of wood in the table by the given side in inches, and divide the force to be resisted by this product. The quotient will be the dimension, in inches, of the side required.

Example. - What must be the depth of a beam of white pine, 4 inches thick, to resist a strain of 232,400 pounds ?

8,300 Breaking-weight. 33,200) 232,400 (7 inches.

4 Thickness. 232,400

---------33,200 Ans. 4 by 7 inches.

The following table exhibits the tensile strength of various kinds of wood, as given by the authors named: -

Timber-Strains

The following corollaries, in relation to the strength of timber, have been established by experiment: 1st, A piece of timber should not be subjected to a permanent strain of more than a fourth of the power that would break it.

2d, A piece of perfect timber, while in a level position and properly'supported, is supposed to be of equal tensile strength throughout; and, whether the piece be long or short, it is liable to part in one place nearly as quick as in another.

3c?, A piece of perfect timber, in a vertical position, is in tensile strength proportionate to its length; a short piece being stronger, since a long one must, in addition to the power applied to the lower end, sustain its own weight; and hence, when it breaks, will part near the top.

4th, In calculating the strength of any piece of timber, only so much of the wood should be measured as is continued throughout the entire stick. For instance, a tie-beam measuring eight by ten inches, having an inch-and-a-half rod passing through it, should be considered as measuring but six inches and a half thick; and if the ends of struts, or any thing of the kind, be cut down, into and across the top of the beam, two inches, it would then measure but eight inches deep.

5th, A rectangular beam supported at both ends, with its diagonal placed vertically, will thereby be reduced, in cross-strength, one-tenth.

6th, The tough and hard woods, as oak and chestnut, are about an eighth, and the soft ones, as spruce, pine, and hemlock, from a sixteenth to a twentieth, as strong, when the power is applied at right angles to the fibres, as when applied to their length. This power is that which a pin exerts on the wood of a post through which it has been driven, when the tenon, which is pinned in, tends to drag it out, and thereby split the wood.