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[Footnote 1: This is in accordance with the discovery made in 1678 by Robert Hooke, and is known asHooke's law.]
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[Footnote 2: If the straight portion does not pass through the origin, a parallel line should be drawn through the origin, and the load at elastic limit taken from this line. (See Fig. 32.)]
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[Footnote 3: See Brush, Warren D.: A microscopic study of the mechanical failure of wood. Vol. II, Rev. F.S. Investigations, Washington, D.C., 1912, p. 35.]
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[Footnote 4: See Circular No. 18, U.S. Division of Forestry: Progress in timber physics, pp. 13-18; also Bulletin 70, U.S. Forest Service: Effect of moisture on the strength and stiffness of wood, pp. 42, 89-90.]
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[Footnote 5: See Bulletin 70,op. cit., p. 129.]
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[Footnote 6: Jaccard, P.: Étude anatomique des bois comprimés. Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band, 1. Heft. Zurich, 1910, p. 66.]
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[Footnote 7: This does not correspond exactly with the conclusions of A. Thil, who says ("Constitution anatomique du bois," pp. 140-141): "The sides of the medullary rays sometimes produce planes of least resistance varying in size with the height of the rays. The medullary rays assume a direction more or less parallel to the lumen of the cells on which they border; the latter curve to the right or left to make room for the ray and then close again beyond it. If the force acts parallel to the axis of growth, the tracheids are more likely to be displaced if the marginal cells of the medullary rays are provided with weak walls that are readily compressed. This explains why on the radial surface of the test blocks the plane of rupture passes in a direction nearly following a medullary ray, whereas on the tangential surface the direction of the plane of rupture is oblique—but with an obliquity varying with the species and determined by the pitch of the spirals along which the medullary rays are distributed in the stem." See Jaccard,op. cit., pp. 57et seq.]
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[Footnote 8: Shear should not be confused with ordinary cutting or incision.]
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[Footnote 9: While in reality this relationship does not exactly hold, the formulæ for beams are based on its assumption.]
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[Footnote 10: Only this form of beam is considered since it is the simplest. For cantilever and continuous beams, and beams rigidly fixed at one or both ends, as well as for different methods of loading, different forms of cross section, etc., other formulæ are required. See any book on mechanics.]
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[Footnote 11: See Tiemann, Harry D.: Some results of dead load bending tests of timber by means of a recording deflectometer. Proc. Am. Soc. for Testing Materials. Phila. Vol. IX, 1909, pp. 534-548.]
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[Footnote 12: A fourth might be added, namely,vibratory, orharmonic repetition, which is frequently serious in the case of bridges.]
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[Footnote 13: Johnson, J.B.: The materials of construction, pp. 81-82.]
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[Footnote 14: See Tiemann, Harry D.: The theory of impact and its application to testing materials. Jour. Franklin Inst., Oct., Nov., 1909, pp. 235-259, 336-364.]
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[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912, XXIII2, pp. 12-13.]
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[Footnote 16: See articles by Gabriel Janka listed in bibliography,pages 151-152.]
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[Footnote 17: For details regarding the structure of wood see Record, Samuel J.: Identification of the economic woods of the United States. New York, John Wiley & Sons, 1912.]
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[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p. 29.]
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[Footnote 19: Bul. 108, U. S. Forest Service: Tests of structural timbers, p. 37.]
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[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]
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[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp. 20-21.]
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[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
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[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
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[Footnote 24:Loc. cit.]
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[Footnote 25: Although the factor of heart or sapwood does not influence the mechanical properties of the wood and there is usually no difference in structure observable under the microscope, nevertheless sapwood is generally decidedly different from heartwood in its physical properties. It dries better and more easily than heartwood, usually with less shrinkage and little checking or honeycombing. This is especially the case with the more refractory woods, such as white oaks andEucalyptus globulusandviminalis. It is usually much more permeable to air, even in green wood, notably so in loblolly pine and even in white oak. As already stated, it is much more subject to decay. The sapwood of white oak may be impregnated with creosote with comparative ease, while the heartwood is practically impenetrable. These facts indicate a difference in its chemical nature.—H.D. Tiemann.]
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[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
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[Footnote 27: The oaks for some unknown reason fall below the normal strength for weight, whereas the hickories rise above. Certain other woods also are somewhat exceptional to the normal relation of strength and density.]
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[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126, appendix.]
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[Footnote 29: See Burke, H.E.: Black check in western hemlock. Cir. No. 61, U.S. Bu. Entomology, 1905.]
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[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]
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[Footnote 31: See Von Schrenck, Hermann: The "bluing" and the "red rot" of the western yellow pine, with special reference to the Black Hills forest reserve. Bul. No. 36, U.S. Bu. Plant Industry, Washington, 1903, pp. 13-14.
Weiss, Howard, and Barnum, Charles T.: The prevention of sapstain in lumber. Cir. 192, U.S. Forest Service, Washington, 1911, pp. 16-17.]
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[Footnote 32: See Standard classification of structural timber. Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303. Contains three plates showing standard defects.]
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[Footnote 33: Bul. 108, pp. 52et seq.]
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[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical properties of western hemlock, p. 20.]
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[Footnote 35: Hartig, R.: The diseases of trees (trans. by Somerville and Ward), London and New York, 1894, pp. 282-294.]
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[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss. Centralb., XXXII, 2, 1910, pp. 74-81.]
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[Footnote 37: For detailed information regarding insect injuries, the reader is referred to the various publications of the U.S. Bureau of Entomology, Washington, D.C.]
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[Footnote 38: See Smith, C. Stowell: Preservation of piling against marine wood borers. Cir. 128, U.S. Forest Service, 1908, pp. 15.]
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[Footnote 39: See Von Schrenck, H.: The decay of timber and methods of preventing it. Bul. 14, U.S. Bu. Plant Industry, Washington, D.C., 1902. Also Buls. 32, 114, 214, 266.
Meineoke, E.P.: Forest tree diseases common in California and Nevada, U.S. Forest Service, Washington, D.C., 1914.
Hartig, R.: The diseases of trees. London and New York, 1894.]
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[Footnote 40: Dry rot in factory timbers, by Inspection Dept. Associated Factory Mutual Fire Insurance Cos., 31 Milk Street, Boston, 1913.]
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[Footnote 41: Falck, Richard: Die Meruliusfaüle des Bauholzes, Hausschwammforschungen, 6. Heft., Jena, 1912.]
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[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
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[Footnote 43: A culture of fungus placed in a glass jar and the air pumped out ceases to grow, but will start again as soon as oxygen is admitted.]
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[Footnote 44: Experiments in kiln-dryingEucalyptusin Berkeley, U.S. Forest Service.]
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[Footnote 45: See Anderson, Paul J.: The morphology and life history of the chestnut blight fungus. Bul. No. 7, Penna. Chestnut Tree Blight Com., Harrisburg, 1914, p. 17.]
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[Footnote 46: See York, Harlan H.: The anatomy and some of the biological aspects of the "American mistletoe." Bul. 120, Sci. Ser. No. 13, Univ. of Texas, Austin, 1909.
Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166, U.S. Bu. Plant Ind., Washington, 1910.
Meinecke, E.P.: Forest tree diseases common in California and Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.]
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[Footnote 47: See Record, S.J.: Sap in relation to the properties of wood. Proc. Am. Wood Preservers' Assn., Baltimore, Md., 1913, pp. 160-166.
Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am. Ry. Eng. Assn., 1913, p. 214.]
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[Footnote 48: See Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U.S. Forest Service, Washington, D.C., 1906; also Cir. 108, 1907.]
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[Footnote 49: The wood ofEucalyptus globulus(blue gum) appears to be an exception to this rule. Tiemann says: "The wood of blue gum begins to shrink immediately from the green condition, even at 70 to 90 per cent moisture content, instead of from 30 or 25 per cent as in other species of hardwoods." Proc. Soc. Am. For., Washington, Vol. VIII, No. 3, Oct., 1913, p. 313.]
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[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev. ed.), p. 75.]
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[Footnote 51: Cir. 39. Experiments on the strength of treated timber, p. 18.]
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[Footnote 52:Ibid., p. 21. See also Cir. 108, p. 19, table 5.]
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[Footnote 53: Hatt, W. K.: Experiments on the strength of treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]
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[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p. 7.]
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[Footnote 55: Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U. S. Forest Service, 1907, pp. 122-123, tables 43-44.]
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[Footnote 56: The methods of timber testing described here are for the most part those employed by the U. S. Forest Service. See Cir. 38 (rev. ed.), 1909.]
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[Footnote 57: Bul. 108, U. S. Forest Service: Tests of structural timbers, pp. 53-54.]
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[Footnote 58: See Tiemann, Harry Donald: The effect of the speed of testing upon the strength and the standardization of tests for speed. Proc. Am. Soc. for Testing Materials, Vol. VIII, Philadelphia, 1908.]
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[Footnote 59: For description of U.S. Forest Service automatic and autographic impact testing machine, see Proc. Am. Soc. for Testing Materials, Vol. VIII, 1908, pp. 538-540.]
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[Footnote 60: See Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers. Dept. For., N.S.W., Sydney, 1911, pp. 88-95.]
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[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]
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[Footnote 62: Cir. 38: Instructions to engineers of timber tests, 1906, p. 24.]
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[Footnote 63: Warren, W.H.: The strength, elasticity, and other properties of New South Wales hardwood timbers, 1911, pp. 58-62.]
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[Footnote 64: Wood is so seldom subjected to a pure stress of this kind that the torsion test is usually omitted.]