Specific Gravities Of Various Timbers. 1. Oaks - English, 829; Memel, 727; Dantzic, 720; Italian, 796; American white, 779; American red, 952; African live, 1160; 2. Firs - American white pine, 432; American red do., 576 ; American yellow do., 508; pitch do., 740; Archangel do., 551; Dantzic, 649; Memel, 601; Prussian, 596; Riga, 654; spruce, American, 772, 503; Mar Forest,. 698; Norway spar, 577; Deal, Christiania, 689. 3. Ashes - English, 760; American, 626; American swamp, 925; do., black, 533. 4. Beeches - English, 696; American, white, 711; do., red, 775. 5. Birches - English, 711; American, black, 670; American, yellow, 756. 6. Elms - English, 579; Canada rock, 725. 7. Cedars - Lebanon, 330; Bermuda, 748; American, white, 354; Guadaloupe, 756. 8. Miscellaneous - Larch, 556; lignum vitae, 1082; teak, 729 ; iron-wood, 879 ; green heart, 985; Honduras mahogany, 608; soft maple, 675; acacia, 710; hickory (American), 831; hemlock, 911; Canada balsam, 548. Carpentry work, as interior, or of limited area, is measured by the square foot or the square yard; where the area or extent is large, as in roofs, it is measured by what is called the " square " of a hundred square feet.

69. The following is the bearing value of various timbers, the numbers representing the "constant" used in calculating the breaking weight of beams : - pitch pine, 629 ; red pine, 467; Baltic pine, 444; yellow pine, 358.5; English oak, 1079.5; American do., 653.5; English elm, 595.25; American do., 631.5; ash, 517.75.

70. If it is required to find the dimensions of a rectangular post, pillar, or column of timber, square the length, and multiply the weight in pounds which it has to support by this, and the product of the two by .0015, if the post is of oak; by .00152, if of Riga fir; by .00133, if of Memel; and by .00142, if of spruce timber. Divide the product obtained by the number of inches in the breadth of the post, and the thickness will be equal to the cube root of the quotient. If the pillar or post is to be square, the product obtained by the multiplying of the weight (in pounds) by the square of the length in feet, is to be multiplied by four times the "constants" above named (.0015 being that for oak, etc.); the fourth root of the product will give the length in inches of the diagonal of the square. We have stated that practically the tensile strength of timber beams does not often enter into calculations, as the force required to tear asunder the fibres is very great. The experiments made in order to find the tenacity of various timbers have shown great discrepancies, so that not much reliance can be placed upon them. The following is a rule to find the sectional area of a beam subjected to a certain tension or tensile strain of so many pounds weight, taking for oak 10,000, and for fir 12,000; divide the weight of either of these numbers, as the case may be, four times the quotient will be the area required; or if the dimensions are given, multiply the area of section by the numbers, as above, either for oak or for fir, and the result will be the resistance in pounds which the beam will oppose to the tensile strain. As before, the factor of safety-should be not less than one-third of the results obtained by calculation.

Fig. 473.

Fig. 474.

When a rectangular beam of timber, as a b, fig. 473, is loaded in the centre c, while supported at both ends e f, the number of pounds which it will bear without breaking maybe found by the following rule : - Find the sectional area of the beam in inches (by multiplying the depth a b, fig. 473, by the breadth d a), which call (a), and reduce the length of the beam or span in feet to inches, which call (i), and the depth of the beam also in inches (d), (s) the " constant;" for oak 1181, red pine 1341, pitch pine 1631, fir (Riga) 1108, Memel 1731; then multiply (a) by four times (d) and by (s), and divide the result by (i); the quotient will give the highest number of pounds which the beam will bear in the centre, or twice this equally distributed. One-third of this - some authorities give a broader factor of safety, and say one-fourth of the weights thus found - will be the safe load for the beam. The best proportion of breadth, e f, fig. 474, to depth g e in a rectangular beam is 6 (d c) to 10 (a b), when the beam is fixed at one end and loaded at the other or outer extremity. Taking the letters to represent certain values, as above, then multiply (a) by (s) and by (d) and divide by l; the quotient will be the breaking weight of the beam. The following will be found useful in calculating the dimensions of the various members of a roof, etc.

To find the breadth of a pine girder, where the bearing or distance between the walls and the depth is given - Take the square of the bearing in feet, and divide by the cube of the depth, and multiply the result by 74.

To find the depth of a pine girder, the bearing and breadth and thickness being given - Divide by the breadth the square of the bearing, then multiply by 4.2, the cube root of the quotient.

To find the breadth of a binding joist, the bearing in feet, or the length and depth in inches being given - Cube the depth, and divide by it the square of the bearing, and multiply the result by 40.

To find the depth in inches of a binding joist, the bearing in feet and the breadth in inches being given - Divide by the breadth the square of the bearing, and multiply the cube root of the result by 3.42.

To find the depth in inches of a tie beam of pine, in which the bearing in feet and the breadth in inches are given - Take the cube root of the breadth and divide the bearing by it, and multiply the result by 1.47.