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+Project Gutenberg's The Mechanical Properties of Wood, by Samuel J. Record
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: The Mechanical Properties of Wood
+       Including a Discussion of the Factors Affecting the Mechanical
+       Properties, and Methods of Timber Testing
+       
+
+Author: Samuel J. Record
+
+Release Date: May 8, 2004 [EBook #12299]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE MECHANICAL PROPERTIES OF WOOD ***
+
+
+
+
+Produced by Curtis Weyant, GF Untermeyer and PG Distributed
+Proofreaders.  Scans provided by Case Western Reserve University's
+Preservation Department http://www.cwru.edu/UL/preserve/general.htm
+
+
+
+
+
+
+
+
+
+THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+
+[Illustration: Frontispiece. _Photo by the author_.
+
+Photomicrograph of a small block of western hemlock. At the top
+is the cross section showing to the right the late wood of one
+season's growth, to the left the early wood of the next season.
+The other two sections are longitudinal and show the fibrous
+character of the wood. To the left is the radial section with
+three rays crossing it. To the right is the tangential section
+upon which the rays appear as vertical rows of beads. X 35.]
+
+
+
+
+THE MECHANICAL PROPERTIES OF WOOD
+
+_Including a Discussion of the Factors Affecting the Mechanical
+Properties, and Methods of Timber Testing_
+
+
+BY SAMUEL J. RECORD, M.A., M.F. ASSISTANT PROFESSOR OF FOREST
+PRODUCTS, YALE UNIVERSITY
+
+
+FIRST EDITION FIRST THOUSAND
+
+1914
+
+
+BY THE SAME AUTHOR
+
+Identification of the Economic Woods of the United States.
+8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net.
+
+
+TO THE STAFF OF THE FOREST PRODUCTS LABORATORY, AT MADISON,
+WISCONSIN IN APPRECIATION OF THE MANY OPPORTUNITIES AFFORDED AND
+COURTESIES EXTENDED THE AUTHOR
+
+
+
+
+PREFACE
+
+
+
+This book was written primarily for students of forestry to whom
+a knowledge of the technical properties of wood is essential.
+The mechanics involved is reduced to the simplest terms and
+without reference to higher mathematics, with which the students
+rarely are familiar. The intention throughout has been to avoid
+all unnecessarily technical language and descriptions, thereby
+making the subject-matter readily available to every one
+interested in wood.
+
+Part I is devoted to a discussion of the mechanical properties
+of wood--the relation of wood material to stresses and strains.
+Much of the subject-matter is merely elementary mechanics of
+materials in general, though written with reference to wood in
+particular. Numerous tables are included, showing the various
+strength values of many of the more important American woods.
+
+Part II deals with the factors affecting the mechanical
+properties of wood. This is a subject of interest to all who are
+concerned in the rational use of wood, and to the forester it
+also, by retrospection, suggests ways and means of regulating
+his forest product through control of the conditions of
+production. Attempt has been made, in the light of all data at
+hand, to answer many moot questions, such as the effect on the
+quality of wood of rate of growth, season of cutting, heartwood
+and sapwood, locality of growth, weight, water content,
+steaming, and defects.
+
+Part III describes methods of timber testing. They are for the
+most part those followed by the U.S. Forest Service. In schools
+equipped with the necessary machinery the instructions will
+serve to direct the tests; in others a study of the text with
+reference to the illustrations should give an adequate
+conception of the methods employed in this most important line
+of research.
+
+The appendix contains a copy of the working plan followed by the
+U.S. Forest Service in the extensive investigations covering the
+mechanical properties of the woods grown in the United States.
+It contains many valuable suggestions for the independent
+investigator. In addition four tables of strength values for
+structural timbers, both green and air-seasoned, are included.
+The relation of the stresses developed in different structural
+forms to those developed in the small clear specimens is given.
+
+In the bibliography attempt was made to list all of the
+important publications and articles on the mechanical properties
+of wood, and timber testing. While admittedly incomplete, it
+should prove of assistance to the student who desires a fuller
+knowledge of the subject than is presented here.
+
+The writer is indebted to the U.S. Forest Service for nearly all
+of his tables and photographs as well as many of the data upon
+which the book is based, since only the Government is able to
+conduct the extensive investigations essential to a thorough
+understanding of the subject. More than eighty thousand tests
+have been made at the Madison laboratory alone, and the work is
+far from completion.
+
+The writer also acknowledges his indebtedness to Mr. Emanuel
+Fritz, M.E., M.F., for many helpful suggestions in the
+preparation of Part I; and especially to Mr. Harry Donald
+Tiemann, M.E., M.F., engineer in charge of Timber Physics at the
+Government Forest Products Laboratory, Madison, Wisconsin, for
+careful revision of the entire manuscript.
+
+SAMUEL J. RECORD.
+YALE FOREST SCHOOL, _July 1, 1914_.
+
+
+
+
+CONTENTS
+
+
+
+      PREFACE
+
+       PART I THE MECHANICAL PROPERTIES OF WOOD
+
+              Introduction
+              Fundamental considerations and definitions
+              Tensile strength
+              Compressive or crushing strength
+              Shearing strength
+              Transverse or bending strength: Beams
+              Toughness: Torsion
+              Hardness
+              Cleavability
+
+      PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF
+                WOOD
+
+              Introduction
+              Rate of growth
+              Heartwood and sapwood
+              Weight, density, and specific gravity
+              Color
+              Cross grain
+              Knots
+              Frost splits
+              Shakes, galls, pitch pockets
+              Insect injuries
+              Marine wood-borer injuries
+              Fungous injuries
+              Parasitic plant injuries
+              Locality of growth
+              Season of cutting
+              Water content
+              Temperature
+              Preservatives
+
+     PART III TIMBER TESTING
+
+              Working plan
+              Forms of material tested
+              Size of test specimens
+              Moisture determination
+              Machine for static tests
+              Speed of testing machine
+              Bending large beams
+              Bending small beams
+              Endwise compression
+              Compression across the grain
+              Shear along the grain
+              Impact test
+              Hardness test: Abrasion and indentation
+              Cleavage test
+              Tension test parallel to the grain
+              Tension test at right angles to the grain
+              Torsion test
+              Special tests
+              Spike pulling test
+              Packing boxes
+              Vehicle and implement woods
+              Cross-arms
+              Other tests
+
+     APPENDIX
+
+              Sample working plan of United States Forest
+                Service
+              Strength values for structural timbers
+
+ BIBLIOGRAPHY
+
+      Part I: Some general works on mechanics, materials of
+                construction, and testing of materials
+     Part II: Publications and articles on the mechanical
+                properties of wood, and timber testing
+    Part III: Publications of the United States Government on
+                the mechanical properties of wood, and timber
+                testing
+
+ILLUSTRATIONS
+
+ Frontispiece Photomicrograph of a small block of western
+                hemlock
+           1. Stress-strain diagrams of two longleaf pine beams
+           2. Compression across the grain
+           3. Side view of failures in compression across the
+                grain
+           4. End view of failures in compression across the
+                grain
+           5. Testing a buggy-spoke in endwise compression
+           6. Unequal distribution of stress in a long column due
+                to lateral bending
+           7. Endwise compression of a short column
+           8. Failures of a short column of green spruce
+           9. Failures of short columns of dry chestnut
+          10. Example of shear along the grain
+          11. Failures of test specimens in shear along the
+                grain
+          12. Horizontal shear in a beam
+          13. Oblique shear in a short column
+          14. Failure of a short column by oblique shear
+          15. Diagram of a simple beam
+          16. Three common forms of beams--(1) simple,
+                (2) cantilever, (3) continuous
+          17. Characteristic failures of simple beams
+          18. Failure of a large beam by horizontal shear
+          19. Torsion of a shaft
+          20. Effect of torsion on different grades of hickory
+          21. Cleavage of highly elastic wood
+          22. Cross-sections of white ash, red gum, and eastern
+                hemlock
+          23. Cross-section of longleaf pine
+          24. Relation of the moisture content to the various
+                strength values of spruce
+          25. Cross-section of the wood of western larch showing
+                fissures in the thick-walled cells of the late
+                wood
+          26. Progress of drying throughout the length of a
+                chestnut beam
+          27. Excessive season checking
+          28. Control of season checking by the use of S-irons
+          29. Static bending test on a large beam
+          30. Two methods of loading a beam
+          31. Static bending test on a small beam
+          32. Sample log sheet, giving full details of a
+                transverse bending test on a small pine beam
+          33. Endwise compression test
+          34. Sample log sheet of an endwise compression test on
+                a short pine column
+          35. Compression across the grain
+          36. Vertical section of shearing tool
+          37. Front view of shearing tool
+          38. Two forms of shear test specimens
+          39. Making a shearing test
+          40. Impact testing machine
+          41. Drum record of impact bending test
+          42. Abrasion machine for testing the wearing qualities
+                of woods
+          43. Design of tool for testing the hardness of woods
+                by indentation
+          44. Design of tool for cleavage test
+          45. Design of cleavage test specimen
+          46. Designs of tension test specimens used in United
+                States
+          47. Design of tension test specimen used in New South
+                Wales
+          48. Design of tool and specimen for testing tension at
+                right angles to the grain
+          49. Making a torsion test on hickory
+          50. Method of cutting and marking test specimens
+          51. Diagram of specific gravity apparatus
+
+       TABLES
+
+           I. Comparative strength of iron, steel, and wood
+          II. Ratio of strength of wood in tension and in
+                compression
+         III. Right-angled tensile strength of small clear
+                pieces of 25 woods in green condition
+          IV. Results of compression tests across the grain on
+                51 woods in green condition, and comparison with
+                white oak
+           V. Relation of fibre stress at elastic limit in
+                bending to the crushing strength of blocks cut
+                therefrom in pounds per square inch
+          VI. Results of endwise compression tests on small
+                clear pieces of 40 woods in green condition
+         VII. Shearing strength along the grain of small clear
+                pieces of 41 woods in green condition
+        VIII. Shearing strength across the grain of various
+                American woods
+          IX. Results of static bending tests on small clear
+                beams of 49 woods in green condition
+           X. Results of impact bending tests on small clear
+                beams of 34 woods in green condition
+          XI. Manner of first failure of large beams
+         XII. Hardness of 32 woods in green condition, as
+                indicated by the load required to imbed a
+                0.444-inch steel ball to one-half its diameter
+        XIII. Cleavage strength of small clear pieces of 32
+                woods in green condition
+         XIV. Specific gravity, and shrinkage of 51 American
+                woods
+          XV. Effect of drying on the mechanical properties of
+                wood, shown in ratio of increase due to reducing
+                moisture content from the green condition to
+                kiln-dry
+         XVI. Effect of steaming on the strength of green
+                loblolly pine
+        XVII. Speed-strength moduli, and relative increase in
+                strength at rates of fibre strain increasing in
+                geometric ratio
+       XVIII. Results of bending tests on green structural
+                timbers
+         XIX. Results of compression and shear tests on green
+                structural timbers
+          XX. Results of bending tests on air-seasoned
+                structural timbers
+         XXI. Results of compression and shear tests on
+                air-seasoned structural timbers
+        XXII. Working unit stresses for structural timber
+                expressed in pounds per square inch
+
+
+
+
+PART I THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+INTRODUCTION
+
+
+The mechanical properties of wood are its fitness and ability to
+resist applied or external forces. By external force is meant
+any force outside of a given piece of material which tends to
+deform it in any manner. It is largely such properties that
+determine the use of wood for structural and building purposes
+and innumerable other uses of which furniture, vehicles,
+implements, and tool handles are a few common examples.
+
+Knowledge of these properties is obtained through
+experimentation either in the employment of the wood in practice
+or by means of special testing apparatus in the laboratory.
+Owing to the wide range of variation in wood it is necessary
+that a great number of tests be made and that so far as possible
+all disturbing factors be eliminated. For comparison of
+different kinds or sizes a standard method of testing is
+necessary and the values must be expressed in some defined
+units. For these reasons laboratory experiments if properly
+conducted have many advantages over any other method.
+
+One object of such investigation is to find unit values for
+strength and stiffness, etc. These, because of the complex
+structure of wood, cannot have a constant value which will be
+exactly repeated in each test, even though no error be made. The
+most that can be accomplished is to find average values, the
+amount of variation above and below, and the laws which govern
+the variation. On account of the great variability in strength
+of different specimens of wood even from the same stick and
+appearing to be alike, it is important to eliminate as far as
+possible all extraneous factors liable to influence the results
+of the tests.
+
+The mechanical properties of wood considered in this book are:
+(1) stiffness and elasticity, (2) tensile strength, (3)
+compressive or crushing strength, (4) shearing strength, (5)
+transverse or bending strength, (6) toughness, (7) hardness, (8)
+cleavability, (9) resilience. In connection with these,
+associated properties of importance are briefly treated.
+
+In making use of figures indicating the strength or other
+mechanical properties of wood for the purpose of comparing the
+relative merits of different species, the fact should be borne
+in mind that there is a considerable range in variability of
+each individual material and that small differences, such as a
+few hundred pounds in values of 10,000 pounds, cannot be
+considered as a criterion of the quality of the timber. In
+testing material of the same kind and grade, differences of 25
+per cent between individual specimens may be expected in
+conifers and 50 per cent or even more in hardwoods. The figures
+given in the tables should be taken as indications rather than
+fixed values, and as applicable to a large number collectively
+and not to individual pieces.
+
+
+
+FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS
+
+
+Study of the mechanical properties of a material is concerned
+mostly with its behavior in relation to stresses and strains,
+and the factors affecting this behavior. A ~stress~ is a
+distributed force and may be defined as the mutual action (1) of
+one body upon another, or (2) of one part of a body upon another
+part. In the first case the stress is _external_; in the other
+_internal_. The same stress may be internal from one point of
+view and external from another. An external force is always
+balanced by the internal stresses when the body is in
+equilibrium.
+
+If no external forces act upon a body its particles assume
+certain relative positions, and it has what is called its
+_natural shape and size_. If sufficient external force is
+applied the natural shape and size will be changed. This
+distortion or deformation of the material is known as the
+~strain~. Every stress produces a corresponding strain, and
+within a certain limit (see _elastic limit_, in FUNDAMENTAL
+CONSIDERATIONS AND DEFINITIONS, above) the strain is directly
+proportional to the stress producing it.[1] The same intensity
+of stress, however, does not produce the same strain in
+different materials or in different qualities of the same
+material. No strain would be produced in a perfectly rigid body,
+but such is not known to exist.
+
+[Footnote 1: This is in accordance with the discovery made in
+1678 by Robert Hooke, and is known as _Hooke's law_.]
+
+Stress is measured in pounds (or other unit of weight or force).
+A ~unit stress~ is the stress on a unit of the sectional
+      {                P  }
+area. { Unit stress = --- } For instance, if a load (P) of one
+      {                A  }
+hundred pounds is uniformly supported by a vertical post with a
+cross-sectional area (A) of ten square inches, the unit
+compressive stress is ten pounds per square inch.
+
+Strain is measured in inches (or other linear unit). A ~unit
+strain~ is the strain per unit of length. Thus if a post 10
+inches long before compression is 9.9 inches long under the
+compressive stress, the total strain is 0.1 inch, and the unit
+           l     0.1
+strain is --- = ----- = 0.01 inch per inch of length.
+           L     10
+
+As the stress increases there is a corresponding increase in the
+strain. This ratio may be graphically shown by means of a
+diagram or curve plotted with the increments of load or stress
+as ordinates and the increments of strain as absciss�. This is
+known as the ~stress-strain diagram~. Within the limit mentioned
+above the diagram is a straight line. (See Fig. 1.) If the
+results of similar experiments on different specimens are
+plotted to the same scales, the diagrams furnish a ready means
+for comparison. The greater the resistance a material offers to
+deformation the steeper or nearer the vertical axis will be the
+line.
+
+[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf
+pine beams. E.L. = elastic limit. The areas of the triangles
+0(EL)A and 0(EL)B represent the elastic resilience of the dry
+and green beams, respectively.]
+
+There are three kinds of internal stresses, namely, (1)
+~tensile~, (2) ~compressive~, and (3) ~shearing~. When external
+forces act upon a bar in a direction away from its ends or a
+direct pull, the stress is a tensile stress; when toward the
+ends or a direct push, compressive stress. In the first instance
+the strain is an _elongation_; in the second a _shortening_.
+Whenever the forces tend to cause one portion of the material to
+slide upon another adjacent to it the action is called a
+_shear_. The action is that of an ordinary pair of shears. When
+riveted plates slide on each other the rivets are sheared off.
+
+These three simple stresses may act together, producing compound
+stresses, as in flexure. When a bow is bent there is a
+compression of the fibres on the inner or concave side and an
+elongation of the fibres on the outer or convex side. There is
+also a tendency of the various fibres to slide past one another
+in a longitudinal direction. If the bow were made of two or more
+separate pieces of equal length it would be noted on bending
+that slipping occurred along the surfaces of contact, and that
+the ends would no longer be even. If these pieces were securely
+glued together they would no longer slip, but the tendency to do
+so would exist just the same. Moreover, it would be found in the
+latter case that the bow would be much harder to bend than where
+the pieces were not glued together--in other words, the
+_stiffness_ of the bow would be materially increased.
+
+~Stiffness~ is the property by means of which a body acted upon
+by external forces tends to retain its natural size and shape,
+or resists deformation. Thus a material that is difficult to
+bend or otherwise deform is stiff; one that is easily bent or
+otherwise deformed is _flexible_. Flexibility is not the exact
+counterpart of stiffness, as it also involves toughness and
+pliability.
+
+If successively larger loads are applied to a body and then
+removed it will be found that at first the body completely
+regains its original form upon release from the stress--in other
+words, the body is ~elastic~. No substance known is perfectly
+elastic, though many are practically so under small loads.
+Eventually a point will be reached where the recovery of the
+specimen is incomplete. This point is known as the ~elastic
+limit~, which may be defined as the limit beyond which it is
+impossible to carry the distortion of a body without producing a
+permanent alteration in shape. After this limit has been
+exceeded, the size and shape of the specimen after removal of
+the load will not be the same as before, and the difference or
+amount of change is known as the ~permanent set~.
+
+Elastic limit as measured in tests and used in design may be
+defined as that unit stress at which the deformation begins to
+increase in a faster ratio than the applied load. In practice
+the elastic limit of a material under test is determined from
+the stress-strain diagram. It is that point in the line where
+the diagram begins perceptibly to curve.[2] (See Fig. 1.)
+
+[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.)]
+
+~Resilience~ is the amount of work done upon a body in deforming
+it. Within the elastic limit it is also a measure of the
+potential energy stored in the material and represents the
+amount of work the material would do upon being released from a
+state of stress. This may be graphically represented by a
+diagram in which the absciss� represent the amount of deflection
+and the ordinates the force acting. The area included between
+the stress-strain curve and the initial line (which is zero)
+represents the work done. (See Fig. 1.) If the unit of space is
+in inches and the unit of force is in pounds the result is
+inch-pounds. If the elastic limit is taken as the apex of the
+triangle the area of the triangle will represent the ~elastic
+resilience~ of the specimen. This amount of work can be applied
+repeatedly and is perhaps the best measure of the toughness of
+the wood as a working quality, though it is not synonymous with
+toughness.
+
+Permanent set is due to the ~plasticity~ of the material. A
+perfectly plastic substance would have no elasticity and the
+smallest forces would cause a set. Lead and moist clay are
+nearly plastic and wood possesses this property to a greater or
+less extent. The plasticity of wood is increased by wetting,
+heating, and especially by steaming and boiling. Were it not for
+this property it would be impossible to dry wood without
+destroying completely its cohesion, due to the irregularity of
+shrinkage.
+
+A substance that can undergo little change in shape without
+breaking or rupturing is ~brittle~. Chalk and glass are common
+examples of brittle materials. Sometimes the word _brash_ is
+used to describe this condition in wood. A brittle wood breaks
+suddenly with a clean instead of a splintery fracture and
+without warning. Such woods are unfitted to resist shock or
+sudden application of load.
+
+The measure of the stiffness of wood is termed the ~modulus of
+elasticity~ (or _coefficient of elasticity_). It is the ratio of
+stress per unit of area to the deformation per unit of
+        {      unit stress  }
+length. { E = ------------- } It is a number indicative of
+        {      unit strain  }
+stiffness, not of strength, and only applies to conditions
+within the elastic limit. It is nearly the same whether derived
+from compression tests or from tension tests.
+
+A large modulus indicates a stiff material. Thus in green wood
+tested in static bending it varies from 643,000 pounds per
+square inch for arborvit� to 1,662,000 pounds for longleaf pine,
+and 1,769,000 pounds for pignut hickory. (See Table IX.) The
+values derived from tests of small beams of dry material are
+much greater, approaching 3,000,000 for some of our woods. These
+values are small when compared with steel which has a modulus of
+elasticity of about 30,000,000 pounds per square inch. (See
+Table I.)
+
+|------------------------------------------------------------------------------|
+|                                   TABLE I                                    |
+|------------------------------------------------------------------------------|
+|                COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD                 |
+|------------------------------------------------------------------------------|
+|                         | Sp.  | Modulus of | Tensile  | Crushing | Modulus  |
+|        MATERIAL         | gr., | elasticity | strength | strength |   of     |
+|                         | dry  | in bending |          |          | rupture  |
+|-------------------------+----- +------------+----------+----------+----------|
+|                         |      | Lbs. per   | Lbs. per | Lbs. per | Lbs. per |
+|                         |      |  sq. in.   | sq. in.  | sq. in.  | sq. in.  |
+|                         |      |            |          |          |          |
+| Cast iron, cold blast   |      |            |          |          |          |
+|   (Hodgkinson)          | 7.1  | 17,270,000 |  16,700  |  106,000 |  38,500  |
+| Bessenger steel,        |      |            |          |          |          |
+|   high grade (Fairbain) | 7.8  | 29,215,000 |  88,400  |  225,600 |          |
+| Longleaf pine,          |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .63 |  2,800,000 |          |   13,000 |  21,000  |
+| Redspruce,              |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .41 |  1,800,000 |          |    8,800 |  14,500  |
+| Pignut hickory,         |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .86 |  2,370,000 |          |   11,130 |  24,000  |
+|------------------------------------------------------------------------------|
+| NOTE.--Great variation may be found in different samples of metals as well   |
+| as of wood. The examples given represent reasonable values.                  |
+|------------------------------------------------------------------------------|
+
+
+
+TENSILE STRENGTH
+
+
+~Tension~ results when a pulling force is applied to opposite
+ends of a body. This external pull is communicated to the
+interior, so that any portion of the material exerts a pull or
+tensile force upon the remainder, the ability to do so depending
+upon the property of cohesion. The result is an elongation or
+stretching of the material in the direction of the applied
+force. The action is the opposite of compression.
+
+Wood exhibits its greatest strength in tension parallel to the
+grain, and it is very uncommon in practice for a specimen to be
+pulled in two lengthwise. This is due to the difficulty of
+making the end fastenings secure enough for the full tensile
+strength to be brought into play before the fastenings shear off
+longitudinally. This is not the case with metals, and as a
+result they are used in almost all places where tensile strength
+is particularly needed, even though the remainder of the
+structure, such as sills, beams, joists, posts, and flooring,
+may be of wood. Thus in a wooden truss bridge the tension
+members are steel rods.
+
+The tensile strength of wood parallel to the grain depends upon
+the strength of the fibres and is affected not only by the
+nature and dimensions of the wood elements but also by their
+arrangement. It is greatest in straight-grained specimens with
+thick-walled fibres. Cross grain of any kind materially reduces
+the tensile strength of wood, since the tensile strength at
+right angles to the grain is only a small fraction of that
+parallel to the grain.
+
+|--------------------------------------------------------------|
+|                           TABLE II                           |
+|--------------------------------------------------------------|
+|   RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION    |
+|           (Bul. 10, U. S. Div. of Forestry, p. 44)           |
+|--------------------------------------------------------------|
+|               |       Ratio:    |  A stick 1 square inch in  |
+|               |                 |      cross section.        |
+|               |      Tensile    |                            |
+| KIND OF WOOD  |      strength   |   Weight required to--     |
+|               | R = ----------- +----------------------------|
+|               |     compressive | Pull apart | Crush endwise |
+|               |      strength   |            |               |
+|---------------+-----------------+------------+---------------|
+| Hickory       |        3.7      |     32,000 |     8,500     |
+| Elm           |        3.8      |     29,000 |     7,500     |
+| Larch         |        2.3      |     19,400 |     8,600     |
+| Longleaf Pine |        2.2      |     17,300 |     7,400     |
+|--------------------------------------------------------------|
+| NOTE.--Moisture condition not given.                         |
+|--------------------------------------------------------------|
+
+Failure of wood in tension parallel to the grain occurs
+sometimes in flexure, especially with dry material. The tension
+portion of the fracture is nearly the same as though the piece
+were pulled in two lengthwise. The fibre walls are torn across
+obliquely and usually in a spiral direction. There is
+practically no pulling apart of the fibres, that is, no
+separation of the fibres along their walls, regardless of their
+thickness. The nature of tension failure is apparently not
+affected by the moisture condition of the specimen, at least not
+so much so as the other strength values.[3]
+
+[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.]
+
+Tension at right angles to the grain is closely related to
+cleavability. When wood fails in this manner the thin fibre
+walls are torn in two lengthwise while the thick-walled fibres
+are usually pulled apart along the primary wall.
+
+|--------------------------------------------|
+|                 TABLE III                  |
+|--------------------------------------------|
+|  TENSILE STRENGTH AT RIGHT ANGLES TO THE   |
+| GRAIN OF SMALL CLEAR PIECES OF 25 WOODS IN |
+|              GREEN CONDITION               |
+|         (Forest Service Cir. 213)          |
+|--------------------------------------------|
+|                  |    When    |    When    |
+|   COMMON NAME    | surface of | surface of |
+|   OF SPECIES     | failure is | failure is |
+|                  |   radial   | tangential |
+|------------------+------------+------------|
+|                  |  Lbs. per  |  Lbs. per  |
+|                  |  sq. inch  |  sq. inch  |
+|                  |            |            |
+|    Hardwoods     |            |            |
+|                  |            |            |
+| Ash, white       |      645   |      671   |
+| Basswood         |      226   |      303   |
+| Beech            |      633   |      969   |
+| Birch, yellow    |      446   |      526   |
+| Elm, slippery    |      765   |      832   |
+| Hackberry        |      661   |      786   |
+| Locust, honey    |    1,133   |    1,445   |
+| Maple, sugar     |      610   |      864   |
+| Oak, post        |      714   |      924   |
+|   red            |      639   |      874   |
+|   swamp white    |      757   |      909   |
+|   white          |      622   |      749   |
+|   yellow         |      728   |      929   |
+| Sycamore         |      540   |      781   |
+| Tupelo           |      472   |      796   |
+|                  |            |            |
+|    Conifers      |            |            |
+|                  |            |            |
+| Arborvit�        |      241   |      235   |
+| Cypress, bald    |      242   |      251   |
+| Fir, white       |      213   |      304   |
+| Hemlock          |      271   |      323   |
+| Pine, longleaf   |      240   |      298   |
+|   red            |      179   |      205   |
+|   sugar          |      239   |      304   |
+|   western yellow |      230   |      252   |
+|   white          |      225   |      285   |
+| Tamarack         |      236   |      274   |
+|--------------------------------------------|
+
+
+
+COMPRESSIVE OR CRUSHING STRENGTH
+
+
+~Compression across the grain~ is very closely related to
+hardness and transverse shear. There are two ways in which wood
+is subjected to stress of this kind, namely, (1) with the load
+acting over the entire area of the specimen, and (2) with a load
+concentrated over a portion of the area. (See Fig. 2.) The
+latter is the condition more commonly met with in practice, as,
+for example, where a post rests on a horizontal sill, or a rail
+rests on a cross-tie. The former condition, however, gives the
+true resistance of the grain to simple crushing.
+
+[Illustration: FIG. 2.--Compression across the grain.]
+
+The first effect of compression across the grain is to compact
+the fibres, the load gradually but irregularly increasing as the
+density of the material is increased. If the specimen lies on a
+flat surface and the load is applied to only a portion of the
+upper area, the bearing plate indents the wood, crushing the
+upper fibres without affecting the lower part. (See Fig. 3.) As
+the load increases the projecting ends sometimes split
+horizontally. (See Fig. 4.) The irregularities in the load are
+due to the fact that the fibres collapse a few at a time,
+beginning with those with the thinnest walls. The projection of
+the ends increases the strength of the material directly beneath
+the compressing weight by introducing a beam action which helps
+support the load. This influence is exerted for a short distance
+only.
+
+[Illustration: FIG. 3.--Side view of failures in compression
+across the grain, showing crushing of blocks under bearing
+plate. Specimen at right shows splitting at ends.]
+
+[Illustration: FIG. 4.--End view of failures in compression
+across the grain, showing splitting of the ends of the test
+specimens.]
+
+When wood is used for columns, props, posts, and spokes, the
+weight of the load tends to shorten the material endwise. This
+is ~endwise compression~, or compression parallel to the grain.
+In the case of long columns, that is, pieces in which the length
+is very great compared with their diameter, the failure is by
+sidewise bending or flexure, instead of by crushing or
+splitting. (See Fig. 5.) A familiar instance of this action is
+afforded by a flexible walking-stick. If downward pressure is
+exerted with the hand on the upper end of the stick placed
+vertically on the floor, it will be noted that a definite amount
+of force must be applied in each instance before decided flexure
+takes place. After this point is reached a very slight increase
+of pressure very largely increases the deflection, thus
+obtaining so great a leverage about the middle section as to
+cause rupture.
+
+[Illustration: FIG. 5.--Testing a buggy spoke in endwise
+compression, illustrating the failure by sidewise bending of a
+long column fixed only at the lower end. _Photo by U. S. Forest
+Service_]
+
+The lateral bending of a column produces a combination of
+bending with compressive stress over the section, the
+compressive stress being maximum at the section of greatest
+deflection on the concave side. The convex surface is under
+tension, as in an ordinary beam test. (See Fig. 6.) If the same
+stick is braced in such a way that flexure is prevented, its
+supporting strength is increased enormously, since the
+compressive stress acts uniformly over the section, and failure
+is by crushing or splitting, as in small blocks. In all columns
+free to bend in any direction the deflection will be seen in the
+direction in which the column is least stiff. This sidewise
+bending can be overcome by making pillars and columns thicker in
+the middle than at the ends, and by bracing studding, props, and
+compression members of trusses. The strength of a column also
+depends to a considerable extent upon whether the ends are free
+to turn or are fixed.
+
+[Illustration: FIG. 6.--Unequal distribution of stress in a long
+column due to lateral bending.]
+
+|-------------------------------------------------------|
+|                       TABLE IV                        |
+|-------------------------------------------------------|
+|   RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON    |
+|   51 WOODS IN GREEN CONDITION, AND COMPARISON WITH    |
+|                       WHITE OAK                       |
+|                 (U. S. Forest Service)                |
+|-------------------------------------------------------|
+|                       | Fibre stress  | Fiber stress  |
+|      COMMON NAME      |  at elastic   | in per cent   |
+|      OF SPECIES       |     limit     | of white oak, |
+|                       | perpendicular | or 853 pounds |
+|                       |   to grain    | per sq. in.   |
+|-----------------------+---------------+---------------|
+|                       |   Lbs. per    |               |
+|                       |   sq. inch    |    Per cent   |
+|                       |               |               |
+| Osage orange          |     2,260     |      265.0    |
+| Honey locust          |     1,684     |      197.5    |
+| Black locust          |     1,426     |      167.2    |
+| Post oak              |     1,148     |      134.6    |
+| Pignut hickory        |     1,142     |      133.9    |
+| Water hickory         |     1,088     |      127.5    |
+| Shagbark hickory      |     1,070     |      125.5    |
+| Mockernut hickory     |     1,012     |      118.6    |
+| Big shellbark hickory |       997     |      116.9    |
+| Bitternut hickory     |       986     |      115.7    |
+| Nutmeg hickory        |       938     |      110.0    |
+| Yellow oak            |       857     |      100.5    |
+| White oak             |       853     |      100.0    |
+| Bur oak               |       836     |       98.0    |
+| White ash             |       828     |       97.1    |
+| Red oak               |       778     |       91.2    |
+| Sugar maple           |       742     |       87.0    |
+| Rock elm              |       696     |       81.6    |
+| Beech                 |       607     |       71.2    |
+| Slippery elm          |       599     |       70.2    |
+| Redwood               |       578     |       67.8    |
+| Bald cypress          |       548     |       64.3    |
+| Red maple             |       531     |       62.3    |
+| Hackberry             |       525     |       61.6    |
+| Incense cedar         |       518     |       60.8    |
+| Hemlock               |       497     |       58.3    |
+| Longleaf pine         |       491     |       57.6    |
+| Tamarack              |       480     |       56.3    |
+| Silver maple          |       456     |       53.5    |
+| Yellow birch          |       454     |       53.2    |
+| Tupelo                |       451     |       52.9    |
+| Black cherry          |       444     |       52.1    |
+| Sycamore              |       433     |       50.8    |
+| Douglas fir           |       427     |       50.1    |
+| Cucumber tree         |       408     |       47.8    |
+| Shortleaf pine        |       400     |       46.9    |
+| Red pine              |       358     |       42.0    |
+| Sugar pine            |       353     |       41.1    |
+| White elm             |       351     |       41.2    |
+| Western yellow pine   |       348     |       40.8    |
+| Lodgepole pine        |       348     |       40.8    |
+| Red spruce            |       345     |       40.5    |
+| White pine            |       314     |       36.8    |
+| Engelman spruce       |       290     |       34.0    |
+| Arborvit�             |       288     |       33.8    |
+| Largetooth aspen      |       269     |       31.5    |
+| White spruce          |       262     |       30.7    |
+| Butternut             |       258     |       30.3    |
+| Buckeye (yellow)      |       210     |       24.6    |
+| Basswood              |       209     |       24.5    |
+| Black willow          |       193     |       22.6    |
+|-------------------------------------------------------|
+
+The complexity of the computations depends upon the way in which
+the stress is applied and the manner in which the stick bends.
+Ordinarily where the length of the test specimen is not greater
+than four diameters and the ends are squarely faced (see Fig.
+7), the force acts uniformly over each square inch of area and
+the crushing strength is equal to the maximum load (P) divided
+                                      {      P  }
+by the area of the cross-section (A). { C = --- }
+                                      {      A  }
+
+[Illustration: FIG. 7.--Endwise compression of a short column.]
+
+It has been demonstrated[4] that the ultimate strength in
+compression parallel to the grain is very nearly the same as the
+extreme fibre stress at the elastic limit in bending. (See Table
+V.) In other words, the transverse strength of beams at elastic
+limit is practically equal to the compressive strength of the
+same material in short columns. It is accordingly possible to
+calculate the approximate breaking strength of beams from the
+compressive strength of short columns except when the wood is
+brittle. Since tests on endwise compression are simpler, easier
+to make, and less expensive than transverse bending tests, the
+importance of this relation is obvious, though it does not do
+away with the necessity of making beam tests.
+
+[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.]
+
+|-------------------------------------------------------------------------------|
+|                                    TABLE V                                    |
+|-------------------------------------------------------------------------------|
+|   RELATION OF FIBRE STRESS AT ELASTIC LIMIT (r) IN BENDING TO THE CRUSHING    |
+|        STRENGTH (C) OF BLOCKS CUT THEREFROM,  IN POUNDS PER SQUARE INCH       |
+|                        (Forest Service Bul. 70, p. 90)                        |
+|-------------------------------------------------------------------------------|
+|                                 LONGLEAF PINE                                 |
+|-------------------------------------------------------------------------------|
+|                          | Soaked | Green  | 14    | 11.5  | 9.5   | Kiln-dry |
+|    MOISTURE CONDITION    | 50 per | 23 per | per   | per   | per   | 6.2 per  |
+|                          | cent   | cent   | cent  | cent  | cent  | cent     |
+| -------------------------+--------+--------+-------+-------+-------+----------|
+| Number of tests averaged |      5 |      5 |     5 |     5 |     4 |        5 |
+| _r_ in bending           |  4,920 |  5,944 | 6,924 | 7,852 | 9,280 |   11,550 |
+| _C_ in compression       |  4,668 |  5,100 | 6,466 | 7,466 | 8,985 |   10,910 |
+| Per cent _r_ is in       |        |        |       |       |       |          |
+|   excess of _C_          |    5.5 |   16.5 |   7.1 |   5.2 |   3.3 |      5.9 |
+|-------------------------------------------------------------------------------|
+|                                    SPRUCE                                     |
+|-------------------------------------------------------------------------------|
+|                                  | Soaked | Green  | 10    | 8.1   | Kiln-dry |
+|        MOISTURE CONDITION        | 30 per | 30 per | per   | per   | 3.9 per  |
+|                                  | cent   | cent   | cent  | cent  | cent     |
+|----------------------------------+--------+--------+-------+-------+----------|
+| Number of tests averaged         |      5 |      4 |     5 |     3 |        4 |
+| _r_ in bending                   |  3,002 |  3,362 | 6,458 | 8,400 |   10,170 |
+| _C_ in compression               |  2,680 |  3,025 | 6,120 | 7,610 |    9,335 |
+| Per cent _r_                     |        |        |       |       |          |
+|   is in excess of _C_            |   12.0 |   11.1 |   5.5 |  10.4 |      9.0 |
+|-------------------------------------------------------------------------------|
+
+When a short column is compressed until it breaks, the manner of
+failure depends partly upon the anatomical structure and partly
+upon the degree of humidity of the wood. The fibres (tracheids
+in conifers) act as hollow tubes bound closely together, and in
+giving way they either (1) buckle, or (2) bend.[5]
+
+[Footnote 5: See Bulletin 70, _op. cit._, p. 129.]
+
+The first is typical of any dry thin-walled cells, as is usually
+the case in seasoned white pine and spruce, and in the early
+wood of hard pines, hemlock, and other species with decided
+contrast between the two portions of the growth ring. As a rule
+buckling of a tracheid begins at the bordered pits which form
+places of least resistance in the walls. In hardwoods such as
+oak, chestnut, ash, etc., buckling occurs only in the
+thinnest-walled elements, such as the vessels, and not in the
+true fibres.
+
+According to Jaccard[6] the folding of the cells is accompanied
+by characteristic alterations of their walls which seem to split
+them into extremely thin layers. When greatly magnified, these
+layers appear in longitudinal sections as delicate threads
+without any definite arrangements, while on cross section they
+appear as numerous concentric strata. This may be explained on
+the ground that the growth of a fibre is by successive layers
+which, under the influence of compression, are sheared apart.
+This is particularly the case with thick-walled cells such as
+are found in late wood.
+
+[Footnote 6: Jaccard, P.: �tude anatomique des bois comprim�s.
+Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band,
+1. Heft. Zurich, 1910, p. 66.]
+
+|-------------------------------------------------------|
+|                       TABLE VI                        |
+|-------------------------------------------------------|
+|  RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR  |
+|         PIECES OF 40 WOODS IN GREEN CONDITION         |
+|               (Forest Service Cir. 213)               |
+|-------------------------------------------------------|
+|                   |   Fibre   |          |  Modulus   |
+|    COMMON NAME    | stress at | Crushing |     of     |
+|    OF SPECIES     |  elastic  | strength | elasticity |
+|                   |   limit   |          |            |
+|-------------------+-----------+----------+------------|
+|                   | Lbs. per  | Lbs. per |  Lbs. per  |
+|                   | sq. inch  | sq. inch |  sq. inch  |
+|                   |           |          |            |
+|    Hardwoods      |           |          |            |
+|                   |           |          |            |
+| Ash, white        |   3,510   |   4,220  |  1,531,000 |
+| Basswood          |     780   |   1,820  |  1,016,000 |
+| Beech             |   2,770   |   3,480  |  1,412,000 |
+| Birch, yellow     |   2,570   |   3,400  |  1,915,000 |
+| Elm, slippery     |   3,410   |   3,990  |  1,453,000 |
+| Hackberry         |   2,730   |   3,310  |  1,068,000 |
+| Hickory,          |           |          |            |
+|   big shellbark   |   3,570   |   4,520  |  1,658,000 |
+|   bitternut       |   4,330   |   4,570  |  1,616,000 |
+|   mockernut       |   3,990   |   4,320  |  1,359,000 |
+|   nutmeg          |   3,620   |   3,980  |  1,411,000 |
+|   pignut          |   3,520   |   4,820  |  1,980,000 |
+|   shagbark        |   3,730   |   4,600  |  1,943,000 |
+|   water           |   3,240   |   4,660  |  1,926,000 |
+| Locust, honey     |   4,300   |   4,970  |  1,536,000 |
+| Maple, sugar      |   3,040   |   3,670  |  1,463,000 |
+| Oak, post         |   2,780   |   3,330  |  1,062,000 |
+|   red             |   2,290   |   3,210  |  1,295,000 |
+|   swamp white     |   3,470   |   4,360  |  1,489,000 |
+|   white           |   2,400   |   3,520  |    946,000 |
+|   yellow          |   2,870   |   3,700  |  1,465,000 |
+| Osage orange      |   3,980   |   5,810  |  1,331,000 |
+| Sycamore          |   2,320   |   2,790  |  1,073,000 |
+| Tupelo            |   2,280   |   3,550  |  1,280,000 |
+|                   |           |          |            |
+|    Conifers       |           |          |            |
+|                   |           |          |            |
+| Arborvit�         |   1,420   |   1,990  |    754,000 |
+| Cedar, incense    |   2,710   |   3,030  |    868,000 |
+| Cypress, bald     |   3,560   |   3,960  |  1,738,000 |
+| Fir, alpine       |   1,660   |   2,060  |    882,000 |
+|   amabilis        |   2,763   |   3,040  |  1,579,000 |
+|   Douglas         |   2,390   |   2,920  |  1,440,000 |
+|   white           |   2,610   |   2,800  |  1,332,000 |
+| Hemlock           |   2,110   |   2,750  |  1,054,000 |
+| Pine, lodgepole   |   2,290   |   2,530  |  1,219,000 |
+|   longleaf        |   3,420   |   4,280  |  1,890,000 |
+|   red             |   2,470   |   3,080  |  1,646,000 |
+|   sugar           |   2,340   |   2,600  |  1,029,000 |
+|   western yellow  |   2,100   |   2,420  |  1,271,000 |
+|   white           |   2,370   |   2,720  |  1,318,000 |
+| Redwood           |   3,420   |   3,820  |  1,175,000 |
+| Spruce, Engelmann |   1,880   |   2,170  |  1,021,000 |
+| Tamarack          |   3,010   |   3,480  |  1,596,000 |
+|-------------------------------------------------------|
+
+The second case, where the fibres bend with more or less regular
+curves instead of buckling, is characteristic of any green or
+wet wood, and in dry woods where the fibres are thick-walled. In
+woods in which the fibre walls show all gradations of
+thickness--in other words, where the transition from the
+thin-walled cells of the early wood to the thick-walled cells of
+the late wood is gradual--the two kinds of failure, namely,
+buckling and bending, grade into each other. In woods with very
+decided contrast between early and late wood the two forms are
+usually distinct. Except in the case of complete failure the
+cavity of the deformed cells remains open, and in hardwoods this
+is true not only of the wood fibres but also of the tube-like
+vessels. In many cases longitudinal splits occur which isolate
+bundles of elements by greater or less intervals. The splitting
+occurs by a tearing of the fibres or rays and not by the
+separation of the rays from the adjacent elements.
+
+[Illustration: FIG. 8.--Failures of short columns of green
+spruce.]
+
+[Illustration: FIG. 9.--Failures of short columns of dry
+chestnut.]
+
+Moisture in wood decreases the stiffness of the fibre walls and
+enlarges the region of failure. The curve which the fibre walls
+make in the region of failure is more gradual and also more
+irregular than in dry wood, and the fibres are more likely to be
+separated.
+
+In examining the lines of rupture in compression parallel to the
+grain it appears that there does not exist any specific type,
+that is, one that is characteristic of all woods. Test blocks
+taken from different parts of the same log may show very decided
+differences in the manner of failure, while blocks that are much
+alike in the size, number, and distribution of the elements of
+unequal resistance may behave very similarly. The direction of
+rupture is, according to Jaccard, not influenced by the
+distribution of the medullary rays.[7] These are curved with the
+bundles of fibres to which they are attached. In any case the
+failure starts at the weakest points and follows the lines of
+least resistance. The plane of failure, as visible on radial
+surfaces, is horizontal, and on the tangential surface it is
+diagonal.
+
+[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. 57 _et seq._]
+
+
+
+SHEARING STRENGTH
+
+
+Whenever forces act upon a body in such a way that one portion
+tends to slide upon another adjacent to it the action is called
+a ~shear~.[8] In wood this shearing action may be (1) ~along the
+grain~, or (2) ~across the grain~. A tenon breaking out its
+mortise is a familiar example of shear along the grain, while
+the shoving off of the tenon itself would be shear across the
+grain. The use of wood for pins or tree-nails involves
+resistance to shear across the grain. Another common instance of
+the latter is where the steel edge of the eye of an axe or
+hammer tends to cut off the handle. In Fig. 10 the action of the
+wooden strut tends to shear off along the grain the portion _AB_
+of the wooden tie rod, and it is essential that the length of
+this portion be great enough to guard against it. Fig. 11 shows
+characteristic failures in shear along the grain.
+
+[Footnote 8: Shear should not be confused with ordinary cutting
+or incision.]
+
+[Illustration: FIG. 10.--Example of shear along the grain.]
+
+[Illustration: FIG. 11.--Failures of test specimens in shear
+along the grain. In the block at the left the surface of failure
+is radial; in the one at the right, tangential]
+
+|---------------------------------------------|
+|                  TABLE VII                  |
+|---------------------------------------------|
+| SHEARING STRENGTH ALONG THE GRAIN OF SMALL  |
+| CLEAR PIECES OF 41 WOODS IN GREEN CONDITION |
+|          (Forest Service Cir. 213)          |
+|---------------------------------------------|
+|                   |    When    |    When    |
+|   COMMON NAME     | surface of | surface of |
+|   OF SPECIES      | failure is | failure is |
+|                   |   radial   | tangential |
+|-------------------+------------+------------|
+|                   |  Lbs. per  |  Lbs. per  |
+|                   |  sq. inch  |  sq. inch  |
+|                   |            |            |
+|    Hardwoods      |            |            |
+|                   |            |            |
+| Ash, black        |      876   |      832   |
+|   white           |    1,360   |    1,312   |
+| Basswood          |      560   |      617   |
+| Beech             |    1,154   |    1,375   |
+| Birch, yellow     |    1,103   |    1,188   |
+| Elm, slippery     |    1,197   |    1,174   |
+|   white           |      778   |      872   |
+| Hackberry         |    1,095   |    1,161   |
+| Hickory,          |            |            |
+|   big shellbark   |    1,134   |    1,191   |
+|   bitternut       |    1,134   |    1,348   |
+|   mockernut       |    1,251   |    1,313   |
+|   nutmeg          |    1,010   |    1,053   |
+|   pignut          |    1,334   |    1,457   |
+|   shagbark        |    1,230   |    1,297   |
+|   water           |    1,390   |    1,490   |
+| Locust, honey     |    1,885   |    2,096   |
+| Maple, red        |    1,130   |    1,330   |
+|   sugar           |    1,193   |    1,455   |
+| Oak, post         |    1,196   |    1,402   |
+|   red             |    1,132   |    1,195   |
+|   swamp white     |    1,198   |    1,394   |
+|   white           |    1,096   |    1,292   |
+|   yellow          |    1,162   |    1,196   |
+| Sycamore          |      900   |    1,102   |
+| Tupelo            |      978   |    1,084   |
+|                   |            |            |
+|    Conifers       |            |            |
+|                   |            |            |
+| Arborvit�         |      617   |      614   |
+| Cedar, incense    |      613   |      662   |
+| Cypress, bald     |      836   |      800   |
+| Fir, alpine       |      573   |      654   |
+|   amabilis        |      517   |      639   |
+|   Douglas         |      853   |      858   |
+|   white           |      742   |      723   |
+| Hemlock           |      790   |      813   |
+| Pine, lodgepole   |      672   |      747   |
+|   longleaf        |    1,060   |      953   |
+|   red             |      812   |      741   |
+|   sugar           |      702   |      714   |
+|   western yellow  |      686   |      706   |
+|   white           |      649   |      639   |
+| Spruce, Engelmann |      607   |      624   |
+| Tamarack          |      883   |      843   |
+|---------------------------------------------|
+
+Both shearing stresses may act at the same time. Thus the weight
+carried by a beam tends to shear it off at right angles to the
+axis; this stress is equal to the resultant force acting
+perpendicularly at any point, and in a beam uniformly loaded and
+supported at either end is maximum at the points of support and
+zero at the centre. In addition there is a shearing force
+tending to move the fibres of the beam past each other in a
+longitudinal direction. (See Fig. 12.) This longitudinal shear
+is maximum at the neutral plane and decreases toward the upper
+and lower surfaces.
+
+[Illustration: FIG. 12.--Horizontal shear in a beam.]
+
+Shearing across the grain is so closely related to compression
+at right angles to the grain and to hardness that there is
+little to be gained by making separate tests upon it. Knowledge
+of shear parallel to the grain is important, since wood
+frequently fails in that way. The value of shearing stress
+parallel to the grain is found by dividing the maximum load in
+pounds (P) by the area of the cross section in inches (A).
+
+     {          P  }
+     { Shear = --- }
+     {          A  }
+
+Oblique shearing stresses are developed in a bar when it is
+subjected to direct tension or compression. The maximum shearing
+stress occurs along a plane when it makes an angle of 45 degrees
+                                                       P
+with the axis of the specimen. In this case, shear = -----. When
+                                                      2 A
+the value of the angle [Greek: theta] is less than 45 degrees,
+                             P
+the shear along the plane = --- sin [Greek: theta] cos [Greek:
+                             A
+theta]. (See Fig. 13.) The effect of oblique shear is often
+visible in the failures of short columns. (See Fig. 14.)
+
+[Illustration: FIG. 13.--Oblique shear in a short column.]
+
+[Illustration: FIG. 14.--Failure of short column by oblique
+shear.]
+
+|---------------------------------------------------------------------------|
+|                                TABLE VIII                                 |
+|---------------------------------------------------------------------------|
+|       SHEARING STRENGTH ACROSS THE GRAIN OF VARIOUS AMERICAN WOODS        |
+|  (J.C. Trautwine. Jour. Franklin Institute. Vol. 109, 1880, pp. 105-106)  |
+|---------------------------------------------------------------------------|
+|     KIND OF WOOD      | Lbs. per |        KIND OF WOOD         | Lbs. per |
+|                       | sq. inch |                             | sq. inch |
+|-----------------------+----------+-----------------------------+----------|
+| Ash                   |   6,280  | Hickory                     |   7,285  |
+| Beech                 |   5,223  | Locust                      |   7,176  |
+| Birch                 |   5,595  | Maple                       |   6,355  |
+| Cedar (white)         |   1,372  | Oak                         |   4,425  |
+| Cedar (white)         |   1,519  | Oak (live)                  |   8,480  |
+| Cedar (Central Amer.) |   3,410  | Pine (white)                |   2,480  |
+| Cherry                |   2,945  | Pine (northern yellow)      |   4,340  |
+| Chestnut              |   1,536  | Pine (southernyellow)       |   5,735  |
+| Dogwood               |   6,510  | Pine (very resinous yellow) |   5,053  |
+| Ebony                 |   7,750  | Poplar                      |   4,418  |
+| Gum                   |   5,890  | Spruce                      |   3,255  |
+| Hemlock               |   2,750  | Walnut (black)              |   4,728  |
+| Hickory               |   6,045  | Walnut (common)             |   2,830  |
+|---------------------------------------------------------------------------|
+| NOTE.--Two specimens of each were tested. All were fairly seasoned and    |
+| without defects. The piece sheared off was 5/8 in. The single circular    |
+| area of each pin was 0.322 sq. in.                                        |
+|---------------------------------------------------------------------------|
+
+
+
+TRANSVERSE OR BENDING STRENGTH: BEAMS
+
+
+When external forces acting in the same plane are applied at
+right angles to the axis of a bar so as to cause it to bend,
+they occasion a shortening of the longitudinal fibres on the
+concave side and an elongation of those on the convex side.
+Within the elastic limit the relative stretching and contraction
+of the fibres is directly[9] proportional to their distances
+from a plane intermediate between them--the ~neutral plane~.
+(N_{1} P in Fig. 15.) Thus the fibres half-way between the
+neutral plane and the outer surface experience only half as much
+shortening or elongation as the outermost or extreme fibres.
+Similarly for other distances. The elements along the neutral
+plane experience no tension or compression in an axial
+direction. The line of intersection of this plane and the plane
+of section is known as the ~neutral axis~ (N A in Fig. 15) of
+the section.
+
+[Footnote 9: While in reality this relationship does not exactly
+hold, the formul� for beams are based on its assumption.]
+
+[Illustration: FIG. 15.--Diagram of a simple beam. N_{1} P =
+neutral plane, N A = neutral axis of section R S.]
+
+If the bar is symmetrical and homogeneous the neutral plane is
+located half-way between the upper and lower surfaces, so long
+as the deflection does not exceed the elastic limit of the
+material. Owing to the fact that the tensile strength of wood is
+from two to nearly four times the compressive strength, it
+follows that at rupture the neutral plane is much nearer the
+convex than the concave side of the bar or beam, since the sum
+of all the compressive stresses on the concave portion must
+always equal the sum of the tensile stresses on the convex
+portion. The neutral plane begins to change from its central
+position as soon as the elastic limit has been passed. Its
+location at any time is very uncertain.
+
+The external forces acting to bend the bar also tend to rupture
+it at right angles to the neutral plane by causing one
+transverse section to slip past another. This stress at any
+point is equal to the resultant perpendicular to the axis of the
+forces acting at this point, and is termed the ~transverse
+shear~ (or in the case of beams, ~vertical shear~).
+
+In addition to this there is a shearing stress, tending to move
+the fibres past one another in an axial direction, which is
+called ~longitudinal shear~ (or in the case of beams,
+~horizontal shear~). This stress must be taken into
+consideration in the design of timber structures. It is maximum
+at the neutral plane and decreases to zero at the outer elements
+of the section. The shorter the span of a beam in proportion to
+its height, the greater is the liability of failure in
+horizontal shear before the ultimate strength of the beam is
+reached.
+
+
+_Beams_
+
+There are three common forms of beams, as follows:
+
+(1) ~Simple beam~--a bar resting upon two supports, one near
+each end. (See Fig. 16, No. 1.)
+
+(2) ~Cantilever beam~--a bar resting upon one support or
+fulcrum, or that portion of any beam projecting out of a wall or
+beyond a support. (See Fig. 16, No. 2.)
+
+(3) ~Continuous beam~--a bar resting upon more than two
+supports. (See Fig. 16, No. 3.)
+
+[Illustration: FIG. 16.--Three common forms of beams. 1. Simple.
+2. Cantilever. 3. Continuous.]
+
+
+_Stiffness of Beams_
+
+The two main requirements of a beam are stiffness and strength.
+The formul� for the _modulus of elasticity (E)_ or measure of
+stiffness of a rectangular prismatic simple beam loaded at the
+centre and resting freely on supports at either end is:[10]
+
+[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 formul� are required. See any book on mechanics.]
+
+           P' l^{3}
+     E = -------------
+          4 D b h^{3}
+
+   b = breadth or width of beam, inches.
+   h = height or depth of beam, inches.
+   l = span (length between points of supports) of beam, inches.
+   D = deflection produced by load P', inches.
+  P' = load at or below elastic limit, pounds.
+
+From this formul� it is evident that for rectangular beams of
+the same material, mode of support, and loading, the deflection
+is affected as follows:
+
+(1) It is inversely proportional to the width for beams of the
+same length and depth. If the width is tripled the deflection is
+one-third as great.
+
+(2) It is inversely proportional to the cube of the depth for
+beams of the same length and breadth. If the depth is tripled
+the deflection is one twenty-seventh as great.
+
+(3) It is directly proportional to the cube of the span for
+beams of the same breadth and depth. Tripling the span gives
+twenty-seven times the deflection.
+
+The number of pounds which concentrated at the centre will
+deflect a rectangular prismatic simple beam one inch may be
+found from the preceding formul� by substituting D = 1" and
+solving for P'. The formul� then becomes:
+
+                              4 E b h^{3}
+     Necessary weight (P') = -------------
+                                 l^{3}
+
+In this case the values for E are read from tables prepared from
+data obtained by experimentation on the given material.
+
+
+_Strength of Beams_
+
+The measure of the breaking strength of a beam is expressed in
+terms of unit stress by a _modulus of rupture_, which is a
+purely hypothetical expression for points beyond the elastic
+limit. The formul� used in computing this modulus is as follows:
+
+          1.5 P l
+     R = ---------
+          b h{^2}
+
+  b, h, l = breadth, height, and span, respectively, as in
+    preceding formul�.
+  R = modulus of rupture, pounds per square inch.
+  P = maximum load, pounds.
+
+In calculating the fibre stress at the elastic limit the same
+formul� is used except that the load at elastic limit (P_{1}) is
+substituted for the maximum load (P).
+
+From this formul� it is evident that for rectangular prismatic
+beams of the same material, mode of support, and loading, the
+load which a given beam can support varies as follows:
+
+(1) It is directly proportional to the breadth for beams of the
+same length and depth, as is the case with stiffness.
+
+(2) It is directly proportional to the square of the height for
+beams of the same length and breadth, instead of as the cube of
+this dimension as in stiffness.
+
+(3) It is inversely proportional to the span for beams of the
+same breadth and depth and not to the cube of this dimension as
+in stiffness.
+
+The fact that the strength varies as the _square_ of the height
+and the stiffness as the _cube_ explains the relationship of
+bending to thickness. Were the law the same for strength and
+stiffness a thin piece of material such as a sheet of paper
+could not be bent any further without breaking than a thick
+piece, say an inch board.
+|-------------------------------------------------------------------------------------|
+|                                      TABLE IX                                       |
+|-------------------------------------------------------------------------------------|
+| RESULTS OF STATIC BENDING TESTS ON SMALL CLEAR BEAMS OF 49 WOODS IN GREEN CONDITION |
+|                             (Forest Service Cir. 213)                               |
+|-------------------------------------------------------------------------------------|
+|                 |   Fibre   |          |            |        Work in Bending        |
+|   COMMON NAME   | stress at | Modulus  |  Modulus   |-------------------------------|
+|   OF SPECIES    |  elastic  |    of    |     of     |    To    |    To    |         |
+|                 |   limit   | rupture  | elasticity | elastic  | maximum  |  Total  |
+|                 |           |          |            |  limit   |   load   |         |
+|-----------------+-----------+----------+------------+----------+----------+---------|
+|                 |           |          |            | In.-lbs. | In.-lbs. | In.-lb. |
+|                 | Lbs. per  | Lbs. per |  Lbs. per  | per cu.  | per cu.  | per     |
+|                 | sq. in.   | sq. in.  |  sq. in.   | inch     | inch     | inch    |
+|                 |           |          |            |          |          |         |
+|   Hardwoods     |           |          |            |          |          |         |
+|                 |           |          |            |          |          |         |
+| Ash, black      |    2,580  |   6,000  |    960,000 |    0.41  |    13.1  |   38.9  |
+|   white         |    5,180  |   9,920  |  1,416,000 |    1.10  |    20.0  |   43.7  |
+| Basswood        |    2,480  |   4,450  |   842,000  |     .45  |     5.8  |    8.9  |
+| Beech           |    4,490  |   8,610  |  1,353,000 |     .96  |    14.1  |   31.4  |
+| Birch, yellow   |    4,190  |   8,390  |  1,597,000 |     .62  |    14.2  |   31.5  |
+| Elm, rock       |    4,290  |   9,430  |  1,222,000 |     .90  |    19.4  |   47.4  |
+|   slippery      |    5,560  |   9,510  |  1,314,000 |    1.32  |    11.7  |   44.2  |
+|   white         |    2,850  |   6,940  |  1,052,000 |     .44  |    11.8  |   27.4  |
+| Gum, red        |    3,460  |   6,450  |  1,138,000 |          |          |         |
+| Hackberry       |    3,320  |   7,800  |  1,170,000 |     .56  |    19.6  |   52.9  |
+| Hickory,        |           |          |            |          |          |         |
+|   big shellbark |    6,370  |  11,110  |  1,562,000 |    1.47  |    24.3  |   78.0  |
+|   bitternut     |    5,470  |  10,280  |  1,399,000 |    1.22  |    20.0  |   75.5  |
+|   mockernut     |    6,550  |  11,110  |  1,508,000 |    1.50  |    31.7  |   84.4  |
+|   nutmeg        |    4,860  |   9,060  |  1,289,000 |    1.06  |    22.8  |   58.2  |
+|   pignut        |    5,860  |  11,810  |  1,769,000 |    1.12  |    30.6  |   86.7  |
+|   shagbark      |    6,120  |  11,000  |  1,752,000 |    1.22  |    18.3  |   72.3  |
+|   water         |    5,980  |  10,740  |  1,563,000 |    1.29  |    18.8  |   52.9  |
+| Locust, honey   |    6,020  |  12,360  |  1,732,000 |    1.28  |    17.3  |   64.4  |
+| Maple, red      |    4,450  |   8,310  |  1,445,000 |     .78  |     9.8  |   17.1  |
+|   sugar         |    4,630  |   8,860  |  1,462,000 |     .88  |    12.7  |   32.0  |
+| Oak, post       |    4,720  |   7,380  |    913,000 |    1.39  |     9.1  |   17.4  |
+|   red           |    3,490  |   7,780  |  1,268,000 |     .60  |    11.4  |   26.0  |
+|   swamp white   |    5,380  |   9,860  |  1,593,000 |    1.05  |    14.5  |   37.6  |
+|   tanbark       |    6,580  |  10,710  |  1,678,000 |    1.49  |          |         |
+|   white         |    4,320  |   8,090  |  1,137,000 |     .95  |    12.1  |   36.7  |
+|   yellow        |    5,060  |   8,570  |  1,219,000 |    1.20  |    11.7  |   30.7  |
+| Osage orange    |    7,760  |  13,660  |  1,329,000 |    2.53  |    37.9  |  101.7  |
+| Sycamore        |    2,820  |   6,300  |    961,000 |     .51  |     7.1  |   13.6  |
+| Tupelo          |    4,300  |   7,380  |  1,045,000 |    1.00  |     7.8  |   20.9  |
+|                 |           |          |            |          |          |         |
+|   Conifers      |           |          |            |          |          |         |
+|                 |           |          |            |          |          |         |
+| Arborvit�       |    2,600  |   4,250  |    643,000 |     .60  |     5.7  |    9.5  |
+| Cedar, incense  |    3,950  |   6,040  |    754,000 |          |          |         |
+| Cypress, bald   |    4,430  |   7,110  |  1,378,000 |     .96  |     5.1  |   15.4  |
+| Fir, alpine     |    2,366  |   4,450  |    861,000 |     .66  |     4.4  |    7.4  |
+|   amabilis      |    4,060  |   6,570  |  1,323,000 |          |          |         |
+|   Douglas       |    3,570  |   6,340  |  1,242,000 |     .59  |     6.6  |   13.6  |
+|   white         |    3,880  |   5,970  |  1,131,000 |     .77  |     5.2  |   14.9  |
+| Hemlock         |    3,410  |   5,770  |    917,000 |     .73  |     6.6  |   12.9  |
+| Pine, lodgepole |    3,080  |   5,130  |  1,015,000 |     .54  |     5.1  |    7.4  |
+|   longleaf      |    5,090  |   8,630  |  1,662,000 |     .88  |     8.1  |   34.8  |
+|   red           |    3,740  |   6,430  |  1,384,000 |     .59  |     5.8  |   28.0  |
+|   shortleaf     |    4,360  |   7,710  |  1,395,000 |          |          |         |
+|   sugar         |    3,330  |   5,270  |    966,000 |     .66  |     5.0  |   11.6  |
+|   west, yellow  |    3,180  |   5,180  |  1,111,000 |     .52  |     4.3  |   15.6  |
+| White           |    3,410  |   5,310  |  1,073,000 |     .62  |     5.9  |   13.3  |
+| Redwood         |    4,530  |   6,560  |  1,024,000 |          |          |         |
+| Spruce,         |           |          |            |          |          |         |
+|   Engelmann     |    2,740  |   4,550  |    866,000 |     .50  |     4.8  |    6.1  |
+|   red           |    3,440  |   5,820  |  1,143,000 |     .62  |     6.0  |         |
+|   white         |    3,160  |   5,200  |    968,000 |     .58  |     6.6  |         |
+| Tamarack        |    4,200  |   7,170  |  1,236,000 |     .84  |     7.2  |   30.0  |
+|-------------------------------------------------------------------------------------|
+
+
+_Kinds of Loads_
+
+There are various ways in which beams are loaded, of which the
+following are the most important:
+
+(1) ~Uniform load~ occurs where the load is spread evenly over
+the beam.
+
+(2) ~Concentrated load~ occurs where the load is applied at
+single point or points.
+
+(3) ~Live~ or ~immediate load~ is one of momentary or short
+duration at any one point, such as occurs in crossing a bridge.
+
+(4) ~Dead~ or ~permanent load~ is one of constant and
+indeterminate duration, as books on a shelf. In the case of a
+bridge the weight of the structure itself is the dead load. All
+large beams support a uniform dead load consisting of their own
+weight.
+
+The effect of dead load on a wooden beam may be two or more
+times that produced by an immediate load of the same weight.
+Loads greater than the elastic limit are unsafe and will
+generally result in rupture if continued long enough. A beam may
+be considered safe under permanent load when the deflections
+diminish during equal successive periods of time. A continual
+increase in deflection indicates an unsafe load which is almost
+certain to rupture the beam eventually.
+
+Variations in the humidity of the surrounding air influence the
+deflection of dry wood under dead load, and increased
+deflections during damp weather are cumulative and not recovered
+by subsequent drying. In the case of longleaf pine, dry beams
+may with safety be loaded permanently to within three-fourths of
+their elastic limit as determined from ordinary static tests.
+Increased moisture content, due to greater humidity of the air,
+lowers the elastic limit of wood so that what was a safe load
+for the dry material may become unsafe.
+
+When a dead load not great enough to rupture a beam has been
+removed, the beam tends gradually to recover its former shape,
+but the recovery is not always complete. If specimens from such
+a beam are tested in the ordinary testing machine it will be
+found that the application of the dead load did not affect the
+stiffness, ultimate strength, or elastic limit of the material.
+In other words, the deflections and recoveries produced by live
+loads are the same as would have been produced had not the beam
+previously been subjected to a dead load.[11]
+
+[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.]
+
+~Maximum load~ is the greatest load a material will support and
+is usually greater than the load at rupture.
+
+~Safe load~ is the load considered safe for a material to
+support in actual practice. It is always less than the load at
+elastic limit and is usually taken as a certain proportion of
+the ultimate or breaking load.
+
+The ratio of the breaking to the safe load is called the factor
+of safety. (Factor of safety = ultimate strength / safe load) In
+order to make due allowance for the natural variations and
+imperfections in wood and in the aggregate structure, as well as
+for variations in the load, the factor of safety is usually as
+high as 6 or 10, especially if the safety of human life depends
+upon the structure. This means that only from one-sixth to
+one-tenth of the computed strength values is considered safe to
+use. If the depth of timbers exceeds four times their thickness
+there is a great tendency for the material to twist when loaded.
+It is to overcome this tendency that floor joists are braced at
+frequent intervals. Short deep pieces shear out or split before
+their strength in bending can fully come into play.
+
+
+_Application of Loads_
+
+There are three[12] general methods in which loads may be
+applied to beams, namely:
+
+[Footnote 12: A fourth might be added, namely, ~vibratory~, or
+~harmonic repetition~, which is frequently serious in the case
+of bridges.]
+
+(1) ~Static loading~ or the gradual imposition of load so that
+the moving parts acquire no appreciable momentum. Loads are so
+applied in the ordinary testing machine.
+
+(2) ~Sudden imposition of load without initial velocity.~ "Thus
+in the case of placing a load on a beam, if the load be brought
+into contact with the beam, but its weight sustained by external
+means, as by a cord, and then this external support be
+_suddenly_ (instantaneously) removed, as by quickly cutting the
+cord, then, although the load is already touching the beam (and
+hence there is no real impact), yet the beam is at first
+offering no resistance, as it has yet suffered no deformation.
+Furthermore, as the beam deflects the resistance increases, but
+does not come to be equal to the load until it has attained its
+normal deflection. In the meantime there has been an unbalanced
+force of gravity acting, of a constantly diminishing amount,
+equal at first to the entire load, at the normal deflection. But
+at this instant the load and the beam are in motion, the
+hitherto unbalanced force having produced an accelerated
+velocity, and this velocity of the weight and beam gives to them
+an energy, or _vis viva_, which must now spend itself in
+overcoming an _excess_ of resistance over and above the imposed
+load, and the whole mass will not stop until the deflection (as
+well as the resistance) has come to be equal to _twice_ that
+corresponding to the static load imposed. Hence we say the
+effect of a suddenly imposed load is to produce twice the
+deflection and stress of the same load statically applied. It
+must be evident, however, that this case has nothing in common
+with either the ordinary 'static' tests of structural materials
+in testing-machines, or with impact tests."[13]
+
+[Footnote 13: Johnson, J.B.: The materials of construction, pp.
+81-82.]
+
+(3) ~Impact, shock,~ or ~blow.~[14] There are various common
+uses of wood where the material is subjected to sudden shocks
+and jars or impact. Such is the action on the felloes and spokes
+of a wagon wheel passing over a rough road; on a hammer handle
+when a blow is struck; on a maul when it strikes a wedge.
+
+[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.]
+
+Resistance to impact is resistance to energy which is measured
+by the product of the force into the space through which it
+moves, or by the product of one-half the moving mass which
+causes the shock into the square of its velocity. The work done
+upon the piece at the instant the velocity is entirely removed
+from the striking body is equal to the total energy of that
+body. It is impossible, however, to get all of the energy of the
+striking body stored in the specimen, though the greater the
+mass and the shorter the space through which it moves, or, in
+other words, the greater the proportion of weight and the
+smaller the proportion of velocity making up the energy of the
+striking body, the more energy the specimen will absorb. The
+rest is lost in friction, vibrations, heat, and motion of the
+anvil.
+
+In impact the stresses produced become very complex and
+difficult to measure, especially if the velocity is high, or the
+mass of the beam itself is large compared to that of the weight.
+
+The difficulties attending the measurement of the stresses
+beyond the elastic limit are so great that commonly they are not
+reckoned. Within the elastic limit the formul� for calculating
+the stresses are based on the assumption that the deflection is
+proportional to the stress in this case as in static tests.
+
+A common method of making tests upon the resistance of wood to
+shock is to support a small beam at the ends and drop a heavy
+weight upon it in the middle. (See Fig. 40.) The height of the
+weight is increased after each drop and records of the
+deflection taken until failure. The total work done upon the
+specimen is equal to the area of the stress-strain diagram plus
+the effect of local inertia of the molecules at point of
+contact.
+
+The stresses involved in impact are complicated by the fact that
+there are various ways in which the energy of the striking body
+may be spent:
+
+(_a_) It produces a local deformation of both bodies at the
+surface of contact, within or beyond the elastic limit. In
+testing wood the compression of the substance of the steel
+striking-weight may be neglected, since the steel is very hard
+in comparison with the wood. In addition to the compression of
+the fibres at the surface of contact resistance is also offered
+by the inertia of the particles there, the combined effect of
+which is a stress at the surface of contact often entirely out
+of proportion to the compression which would result from the
+action of a static force of the same magnitude. It frequently
+exceeds the crushing strength at the extreme surface of contact,
+as in the case of the swaging action of a hammer on the head of
+an iron spike, or of a locomotive wheel on the steel rail. This
+is also the case when a bullet is shot through a board or a pane
+of glass without breaking it as a whole.
+
+(_b_) It may move the struck body as a whole with an accelerated
+velocity, the resistance consisting of the inertia of the body.
+This effect is seen when a croquet ball is struck with a mallet.
+
+(_c_) It may deform a fixed body against its external supports
+and resistances. In making impact tests in the laboratory the
+test specimen is in reality in the nature of a cushion between
+two impacting bodies, namely, the striking weight and the base
+of the machine. It is important that the mass of this base be
+sufficiently great that its relative velocity to that of the
+common centre of gravity of itself and the striking weight may
+be disregarded.
+
+(_d_) It may deform the struck body as a whole against the
+resisting stresses developed by its own inertia, as, for
+example, when a baseball bat is broken by striking the ball.
+
+|-------------------------------------------------------|
+|                        TABLE X                        |
+|-------------------------------------------------------|
+| RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS  |
+|            OF 34 WOODS IN GREEN CONDITION             |
+|               (Forest Service Cir. 213)               |
+|-------------------------------------------------------|
+|                   |   Fibre   |            | Work in  |
+|    COMMON NAME    | stress at | Modulus of | bending  |
+|    OF SPECIES     |  elastic  | elasticity |    to    |
+|                   |   limit   |            | elastic  |
+|                   |           |            |  limit   |
+|-------------------+-----------+------------+----------|
+|                   |           |            | In.-lbs. |
+|                   | Lbs. per  |  Lbs. per  | per cu.  |
+|                   | sq. in.   |  sq. in.   | inch     |
+|                   |           |            |          |
+|    Hardwoods      |           |            |          |
+|                   |           |            |          |
+| Ash, black        |    7,840  |    955,000 |    3.69  |
+|   white           |   11,710  |  1,564,000 |    4.93  |
+| Basswood          |    5,480  |    917,000 |    1.84  |
+| Beech             |   11,760  |  1,501,000 |    5.10  |
+| Birch, yellow     |   11,080  |  1,812,000 |    3.79  |
+| Elm, rock         |   12,090  |  1,367,000 |    6.52  |
+|   slippery        |   11,700  |  1,569,000 |    4.86  |
+|   white           |    9,910  |  1,138,000 |    4.82  |
+| Hackberry         |   10,420  |  1,398,000 |    4.48  |
+| Locust, honey     |   13,460  |  2,114,000 |    4.76  |
+| Maple, red        |   11,670  |  1,411,000 |    5.45  |
+|   sugar           |   11,680  |  1,680,000 |    4.55  |
+| Oak, post         |   11,260  |  1,596,000 |    4.41  |
+|   red             |   10,580  |  1,506,000 |    4.16  |
+|   swamp white     |   13,280  |  2,048,000 |    4.79  |
+|   white           |    9,860  |  1,414,000 |    3.84  |
+|   yellow          |   10,840  |  1,479,000 |    4.44  |
+| Osage orange      |   15,520  |  1,498,000 |    8.92  |
+| Sycamore          |    8,180  |  1,165,000 |    3.22  |
+| Tupelo            |    7,650  |  1,310,000 |    2.49  |
+|                   |           |            |          |
+|    Conifers       |           |            |          |
+|                   |           |            |          |
+| Arborvit�         |    5,290  |    778,000 |    2.04  |
+| Cypress, bald     |    8,290  |  1,431,000 |    2.71  |
+| Fir, alpine       |    5,280  |    980,000 |    1.59  |
+|   Douglas         |    8,870  |  1,579,000 |    2.79  |
+|   white           |    7,230  |  1,326,000 |    2.21  |
+| Hemlock           |    6,330  |  1,025,000 |    2.19  |
+| Pine, lodgepole   |    6,870  |  1,142,000 |    2.31  |
+|   longleaf        |    9,680  |  1,739,000 |    3.02  |
+|   red             |    7,480  |  1,438,000 |    2.18  |
+|   sugar           |    6,740  |  1,083,000 |    2.34  |
+|   western yellow  |    7,070  |  1,115,000 |    2.51  |
+|   white           |    6,490  |  1,156,000 |    2.06  |
+| Spruce, Engelmann |    6,300  |  1,076,000 |    2.09  |
+| Tamarack          |    7,750  |  1,263,000 |    2.67  |
+|-------------------------------------------------------|
+
+Impact testing is difficult to conduct satisfactorily and the
+data obtained are of chief value in a relative sense, that is,
+for comparing the shock-resisting ability of woods of which like
+specimens have been subjected to exactly identical treatment.
+Yet this test is one of the most important made on wood, as it
+brings out properties not evident from other tests. Defects and
+brittleness are revealed by impact better than by any other kind
+of test. In common practice nearly all external stresses are of
+the nature of impact. In fact, no two moving bodies can come
+together without impact stress. Impact is therefore the
+commonest form of applied stress, although the most difficult to
+measure.
+
+
+_Failures in Timber Beams_
+
+If a beam is loaded too heavily it will break or fail in some
+characteristic manner. These failures may be classified
+according to the way in which they develop, as tension,
+compression, and horizontal shear; and according to the
+appearance of the broken surface, as brash, and fibrous. A
+number of forms may develop if the beam is completely ruptured.
+
+Since the tensile strength of wood is on the average about three
+times as great as the compressive strength, a beam should,
+therefore, be expected to fail by the formation in the first
+place of a fold on the compression side due to the crushing
+action, followed by failure on the tension side. This is usually
+the case in green or moist wood. In dry material the first
+visible failure is not infrequently on the lower or tension
+side, and various attempts have been made to explain why such is
+the case.[15]
+
+[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912,
+XXIII_{2}, pp. 12-13.]
+
+Within the elastic limit the elongations and shortenings are
+equal, and the neutral plane lies in the middle of the beam.
+(See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) Later the
+top layer of fibres on the upper or compression side fail, and
+on the load increasing, the next layer of fibres fail, and so
+on, even though this failure may not be visible. As a result the
+shortenings on the upper side of the beam become considerably
+greater than the elongations on the lower side. The neutral
+plane must be presumed to sink gradually toward the tension
+side, and when the stresses on the outer fibres at the bottom
+have become sufficiently great, the fibres are pulled in two,
+the tension area being much smaller than the compression area.
+The rupture is often irregular, as in direct tension tests.
+Failure may occur partially in single bundles of fibres some
+time before the final failure takes place. One reason why the
+failure of a dry beam is different from one that is moist, is
+that drying increases the stiffness of the fibres so that they
+offer more resistance to crushing, while it has much less effect
+upon the tensile strength.
+
+There is considerable variation in tension failures depending
+upon the toughness or the brittleness of the wood, the
+arrangement of the grain, defects, etc., making further
+classification desirable. The four most common forms are:
+
+(1)~Simple tension,~ in which there is a direct pulling in two
+of the wood on the under side of the beam due to a tensile
+stress parallel to the grain, (See Fig. 17, No. 1.) This is
+common in straight-grained beams, particularly when the wood is
+seasoned.
+
+[Illustration: FIG. 17.--Characteristic failures of simple
+beams.]
+
+(2)~Cross-grained tension,~ in which the fracture is caused by a
+tensile force acting oblique to the grain. (See Fig. 17, No. 2.)
+This is a common form of failure where the beam has diagonal,
+spiral or other form of cross grain on its lower side. Since the
+tensile strength of wood across the grain is only a small
+fraction of that with the grain it is easy to see why a
+cross-grained timber would fail in this manner.
+
+(3)~Splintering tension,~ in which the failure consists of a
+considerable number of slight tension failures, producing a
+ragged or splintery break on the under surface of the beam. (See
+Fig. 17, No. 3.) This is common in tough woods. In this case the
+surface of fracture is fibrous.
+
+(4)~Brittle tension,~ in which the beam fails by a clean break
+extending entirely through it. (See Fig. 17, No. 4.) It is
+characteristic of a brittle wood which gives way suddenly
+without warning, like a piece of chalk. In this case the surface
+of fracture is described as brash.
+
+~Compression failure~ (see Fig. 17, No. 5) has few variations
+except that it appears at various distances from the neutral
+plane of the beam. It is very common in green timbers. The
+compressive stress parallel to the fibres causes them to buckle
+or bend as in an endwise compressive test. This action usually
+begins on the top side shortly after the elastic limit is
+reached and extends downward, sometimes almost reaching the
+neutral plane before complete failure occurs. Frequently two or
+more failures develop at about the same time.
+
+~Horizontal shear failure,~ in which the upper and lower
+portions of the beam slide along each other for a portion of
+their length either at one or at both ends (see Fig. 17, No. 6),
+is fairly common in air-dry material and in green material when
+the ratio of the height of the beam to the span is relatively
+large. It is not common in small clear specimens. It is often
+due to shake or season checks, common in large timbers, which
+reduce the actual area resisting the shearing action
+considerably below the calculated area used in the formul� for
+horizontal shear. (See page 98 for this formul�.) For this
+reason it is unsafe, in designing large timber beams, to use
+shearing stresses higher than those calculated for beams that
+failed in horizontal shear. The effect of a failure in
+horizontal shear is to divide the beam into two or more beams
+the combined strength of which is much less than that of the
+original beam. Fig. 18 shows a large beam in which two failures
+in horizontal shear occurred at the same end. That the parts
+behave independently is shown by the compression failure below
+the original location of the neutral plane.
+
+[Illustration: FIG. 18.--Failure of a large beam by horizontal
+shear. _Photo by U. S, Forest Service._]
+
+Table XI gives an analysis of the causes of first failure in 840
+large timber beams of nine different species of conifers. Of the
+total number tested 165 were air-seasoned, the remainder green.
+The failure occurring first signifies the point of greatest
+weakness in the specimen under the particular conditions of
+loading employed (in this case, third-point static loading).
+
+|-----------------------------------------------------------|
+|                         TABLE XI                          |
+|-----------------------------------------------------------|
+|          MANNER OF FIRST FAILURE OF LARGE BEAMS           |
+|             (Forest Service Bul. 108, p. 56)              |
+|-----------------------------------------------------------|
+|                  | Total  | Per cent of total failing by  |
+|   COMMON NAME    | number |---------+-------------+-------|
+|   OF SPECIES     | of     | Tension | Compression | Shear |
+|                  | tests  |         |             |       |
+|------------------+--------+---------+-------------+-------|
+| Longleaf pine:   |        |         |             |       |
+|   green          |    17  |    18   |      24     |   58  |
+|   dry            |     9  |    22   |      22     |   56  |
+| Douglas fir:     |        |         |             |       |
+|   green          |   191  |    27   |      72     |    1  |
+|   dry            |    91  |    19   |      76     |    5  |
+| Shortleaf pine:  |        |         |             |       |
+|   green          |    48  |    27   |      56     |   17  |
+|   dry            |    13  |    54   |             |   46  |
+| Western larch:   |        |         |             |       |
+|   green          |    62  |    23   |      71     |    6  |
+|   dry            |    52  |    54   |      19     |   27  |
+| Loblolly pine:   |        |         |             |       |
+|   green          |   111  |    40   |      53     |    7  |
+|   dry            |    25  |    60   |      12     |   28  |
+| Tamarack:        |        |         |             |       |
+|   green          |    30  |    37   |      53     |   10  |
+|   dry            |     9  |    45   |      22     |   33  |
+| Western hemlock: |        |         |             |       |
+|   green          |    39  |    21   |      74     |    5  |
+|   dry            |    44  |    11   |      66     |   23  |
+| Redwood:         |        |         |             |       |
+|   green          |    28  |    43   |      50     |    7  |
+|   dry            |    12  |    83   |      17     |       |
+| Norway pine:     |        |         |             |       |
+|   green          |    49  |    18   |      76     |    6  |
+|   dry            |    10  |    30   |      60     |   10  |
+|-----------------------------------------------------------|
+| NOTE.--These tests were made on timbers ranging in cross  |
+| section from 4" x 10" to 8" x 16", and with a span of 15  |
+| feet.                                                     |
+|-----------------------------------------------------------|
+
+
+
+TOUGHNESS: TORSION
+
+
+Toughness is a term applied to more than one property of wood.
+Thus wood that is difficult to split is said to be tough. Again,
+a tough wood is one that will not rupture until it has deformed
+considerably under loads at or near its maximum strength, or one
+which still hangs together after it has been ruptured and may be
+bent back and forth without breaking apart. Toughness includes
+flexibility and is the reverse of brittleness, in that tough
+woods break gradually and give warning of failure. Tough woods
+offer great resistance to impact and will permit rougher
+treatment in manipulations attending manufacture and use.
+Toughness is dependent upon the strength, cohesion, quality,
+length, and arrangement of fibre, and the pliability of the
+wood. Coniferous woods as a rule are not as tough as hardwoods,
+of which hickory and elm are the best examples.
+
+The torsion or twisting test is useful in determining the
+toughness of wood. If the ends of a shaft are turned in opposite
+directions, or one end is turned and the other is fixed, all of
+the fibres except those at the axis tend to assume the form of
+helices. (See Fig. 19.) The strain produced by torsion or
+twisting is essentially shear transverse and parallel to the
+fibres, combined with longitudinal tension and transverse
+compression. Within the elastic limit the strains increase
+directly as the distance from the axis of the specimen. The
+outer elements are subjected to tensile stresses, and as they
+become twisted tend to compress those near the axis. The
+elongated elements also contract laterally. Cross sections which
+were originally plane become warped. With increasing strain the
+lateral adhesion of the outer fibres is destroyed, allowing them
+to slide past each other, and reducing greatly their power of
+resistance. In this way the strains on the fibres nearer the
+axis are progressively increased until finally all of the
+elements are sheared apart. It is only in the toughest materials
+that the full effect of this action can be observed. (See Fig.
+20.) Brittle woods snap off suddenly with only a small amount of
+torsion, and their fracture is irregular and oblique to the axis
+of the piece instead of frayed out and more nearly perpendicular
+to the axis as is the case with tough woods.
+
+[Illustration: FIG. 19.--Torsion of a shaft.]
+
+[Illustration: FIG. 20.--Effect of torsion on different grades
+of hickory. _Photo by U. S. Forest Service._]
+
+
+
+HARDNESS
+
+
+The term _hardness_ is used in two senses, namely: (1)
+resistance to indentation, and (2) resistance to abrasion or
+scratching. In the latter sense hardness combined with toughness
+is a measure of the wearing ability of wood and is an important
+consideration in the use of wood for floors, paving blocks,
+bearings, and rollers. While resistance to indentation is
+dependent mostly upon the density of the wood, the wearing
+qualities may be governed by other factors such as toughness,
+and the size, cohesion, and arrangement of the fibres. In use
+for floors, some woods tend to compact and wear smooth, while
+others become splintery and rough. This feature is affected to
+some extent by the manner in which the wood is sawed; thus
+edge-grain pine flooring is much better than flat-sawn for
+uniformity of wear.
+
+|-------------------------------------------------------------------|
+|                             TABLE XII                             |
+|-------------------------------------------------------------------|
+|             HARDNESS OF 32 WOODS IN GREEN CONDITION,              |
+|            AS INDICATED BY THE LOAD REQUIRED TO IMBED             |
+|         A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER          |
+|                     (Forest Service Cir. 213)                     |
+|-------------------------------------------------------------------|
+| COMMON NAME OF SPECIES | Average |   End   | Radial  | Tangential |
+|                        |         | surface | surface |  surface   |
+|------------------------+---------+---------+---------+------------|
+|                        |  Pounds |  Pounds |  Pounds |   Pounds   |
+|                        |         |         |         |            |
+|    Hardwoods           |         |         |         |            |
+|                        |         |         |         |            |
+|  1 Osage orange        |   1,971 |   1,838 |   2,312 |    1,762   |
+|  2 Honey locust        |   1,851 |   1,862 |   1,860 |    1,832   |
+|  3 Swamp white oak     |   1,174 |   1,205 |   1,217 |    1,099   |
+|  4 White oak           |   1,164 |   1,183 |   1,163 |    1,147   |
+|  5 Post oak            |   1,099 |   1,139 |   1,068 |    1,081   |
+|  6 Black oak           |   1,069 |   1,093 |   1,083 |    1,031   |
+|  7 Red oak             |   1,043 |   1,107 |   1,020 |    1,002   |
+|  8 White ash           |   1,046 |   1,121 |   1,000 |    1,017   |
+|  9 Beech               |     942 |   1,012 |     897 |      918   |
+| 10 Sugar maple         |     937 |     992 |     918 |      901   |
+| 11 Rock elm            |     910 |     954 |     883 |      893   |
+| 12 Hackberry           |     799 |     829 |     795 |      773   |
+| 13 Slippery elm        |     788 |     919 |     757 |      687   |
+| 14 Yellow birch        |     778 |     827 |     768 |      739   |
+| 15 Tupelo              |     738 |     814 |     666 |      733   |
+| 16 Red maple           |     671 |     766 |     621 |      626   |
+| 17 Sycamore            |     608 |     664 |     560 |      599   |
+| 18 Black ash           |     551 |     565 |     542 |      546   |
+| 19 White elm           |     496 |     536 |     456 |      497   |
+| 20 Basswood            |     239 |     273 |     226 |      217   |
+|                        |         |         |         |            |
+|    Conifers            |         |         |         |            |
+|                        |         |         |         |            |
+|  1 Longleaf pine       |    532  |     574 |     502 |      521   |
+|  2 Douglas fir         |    410  |     415 |     399 |      416   |
+|  3 Bald cypress        |    390  |     460 |     355 |      354   |
+|  4 Hemlock             |    384  |     463 |     354 |      334   |
+|  5 Tamarack            |    384  |     401 |     380 |      370   |
+|  6 Red pine            |    347  |     355 |     345 |      340   |
+|  7 White fir           |    346  |     381 |     322 |      334   |
+|  8 Western yellow pine |    328  |     334 |     307 |      342   |
+|  9 Lodgepole pine      |    318  |     316 |     318 |      319   |
+| 10 White pine          |    299  |     304 |     294 |      299   |
+| 11 Engelmann pine      |    266  |     272 |     253 |      274   |
+| 12 Alpine fir          |    241  |     284 |     203 |      235   |
+|-------------------------------------------------------------------|
+| NOTE.--Black locust and hickory are not included in this table,   |
+| but their position would be near the head of the list.            |
+|-------------------------------------------------------------------|
+
+Tests for either form of hardness are of comparative value only.
+Tests for indentation are commonly made by penetrations of the
+material with a steel punch or ball.[16] Tests for abrasion are
+made by wearing down wood with sandpaper or by means of a sand
+blast.
+
+[Footnote 16: See articles by Gabriel Janka listed in
+bibliography, pages 151-152.]
+
+
+
+CLEAVABILITY
+
+
+_Cleavability_ is the term used to denote the facility with
+which wood is split. A splitting stress is one in which the
+forces act normally like a wedge. (See Fig. 21.) The plane of
+cleavage is parallel to the grain, either radially or
+tangentially.
+
+[Illustration: FIG. 21.--Cleavage of highly elastic wood. The
+cleft runs far ahead of the wedge.]
+
+This property of wood is very important in certain uses such as
+firewood, fence rails, billets, and squares. Resistance to
+splitting or low cleavability is desirable where wood must hold
+nails or screws, as in box-making. Wood usually splits more
+readily along the radius than parallel to the growth rings
+though exceptions occur, as in the case of cross grain.
+
+Splitting involves transverse tension, but only a portion of the
+fibres are under stress at a time. A wood of little stiffness
+and strong cohesion across the grain is difficult to split,
+while one with great stiffness, such as longleaf pine, is easily
+split. The form of the grain and the presence of knots greatly
+affect this quality.
+
+|---------------------------------------------|
+|                 TABLE XIII                  |
+|---------------------------------------------|
+| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF  |
+|         32 WOODS IN GREEN CONDITION         |
+|          (Forest Service Cir. 213)          |
+|---------------------------------------------|
+|                   |    When    |    When    |
+|   COMMON NAME     | surface of | surface of |
+|   OF SPECIES      | failure is | failure is |
+|                   |   radial   | tangential |
+|-------------------+------------+------------|
+|                   |  Lbs. per  |  Lbs. per  |
+|                   |  sq. inch  |  sq. inch  |
+|                   |            |            |
+|    Hardwoods      |            |            |
+|                   |            |            |
+| Ash, black        |     275    |     260    |
+|   white           |     333    |     346    |
+| Bashwood          |     130    |     168    |
+| Beech             |     339    |     527    |
+| Birch, yellow     |     294    |     287    |
+| Elm, slippery     |     401    |     424    |
+|   white           |     210    |     270    |
+| Hackberry         |     422    |     436    |
+| Locust, honey     |     552    |     610    |
+| Maple, red        |     297    |     330    |
+|   sugar           |     376    |     513    |
+| Oak, post         |     354    |     487    |
+|   red             |     380    |     470    |
+|   swamp white     |     428    |     536    |
+|   white           |     382    |     457    |
+|   yellow          |     379    |     470    |
+| Sycamore          |     265    |     425    |
+| Tupelo            |     277    |     380    |
+|                   |            |            |
+|    Conifers       |            |            |
+|                   |            |            |
+| Arborvit�         |     148    |     139    |
+| Cypress, bald     |     167    |     154    |
+| Fir, alpine       |     130    |     133    |
+|   Douglas         |     139    |     127    |
+|   white           |     145    |     187    |
+| Hemlock           |     168    |     151    |
+| Pine, lodgepole   |     142    |     140    |
+|   longleaf        |     187    |     180    |
+|   red             |     161    |     154    |
+|   sugar           |     168    |     189    |
+|   western yellow  |     162    |     187    |
+|   white           |     144    |     160    |
+| Spruce, Engelmann |     110    |     135    |
+| Tamarack          |     167    |     159    |
+|---------------------------------------------|
+
+
+
+
+PART II FACTORS  AFFECTING THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+INTRODUCTION
+
+
+Wood is an organic product--a structure of infinite variation of
+detail and design.[17] It is on this account that no two woods
+are alike--in reality no two specimens from the same log are
+identical. There are certain properties that characterize each
+species, but they are subject to considerable variation. Oak,
+for example, is considered hard, heavy, and strong, but some
+pieces, even of the same species of oak, are much harder,
+heavier, and stronger than others. With hickory are associated
+the properties of great strength, toughness, and resilience, but
+some pieces are comparatively weak and brash and ill-suited for
+the exacting demands for which good hickory is peculiarly
+adapted.
+
+[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.]
+
+It follows that no definite value can be assigned to the
+properties of any wood and that tables giving average results of
+tests may not be directly applicable to any individual stick.
+With sufficient knowledge of the intrinsic factors affecting the
+results it becomes possible to infer from the appearance of
+material its probable variation from the average. As yet too
+little is known of the relation of structure and chemical
+composition to the mechanical and physical properties to permit
+more than general conclusions.
+
+
+
+RATE OF GROWTH
+
+
+To understand the effect of variations in the rate of growth it
+is first necessary to know how wood is formed. A tree increases
+in diameter by the formation, between the old wood and the inner
+bark, of new woody layers which envelop the entire stem, living
+branches, and roots. Under ordinary conditions one layer is
+formed each year and in cross section as on the end of a log
+they appear as rings--often spoken of as _annual rings_. These
+growth layers are made up of wood cells of various kinds, but
+for the most part fibrous. In timbers like pine, spruce,
+hemlock, and other coniferous or softwood species the wood cells
+are mostly of one kind, and as a result the material is much
+more uniform in structure than that of most hardwoods. (See
+Frontispiece.) There are no vessels or pores in coniferous wood
+such as one sees so prominently in oak and ash, for example.
+(See Fig. 22.)
+
+[Illustration: FIG. 22.--Cross sections of a ring-porous
+hardwood (white ash), a diffuse-porous hardwood (red gum), and a
+non-porous or coniferous wood (eastern hemlock). X 30.
+_Photomicrographs by the author._]
+
+The structure of the hardwoods is more complex. They are more or
+less filled with vessels, in some cases (oak, chestnut, ash)
+quite large and distinct, in others (buckeye, poplar, gum) too
+small to be seen plainly without a small hand lens. In
+discussing such woods it is customary to divide them into two
+large classes--_ring-porous_ and _diffuse-porous_. (See Fig.
+22.) In ring-porous species, such as oak, chestnut, ash, black
+locust, catalpa, mulberry, hickory, and elm, the larger vessels
+or pores (as cross sections of vessels are called) become
+localized in one part of the growth ring, thus forming a region
+of more or less open and porous tissue. The rest of the ring is
+made up of smaller vessels and a much greater proportion of wood
+fibres. These fibres are the elements which give strength and
+toughness to wood, while the vessels are a source of weakness.
+
+In diffuse-porous woods the pores are scattered throughout the
+growth ring instead of being collected in a band or row.
+Examples of this kind of wood are gum, yellow poplar, birch,
+maple, cottonwood, basswood, buckeye, and willow. Some species,
+such as walnut and cherry, are on the border between the two
+classes, forming a sort of intermediate group.
+
+If one examines the smoothly cut end of a stick of almost any
+kind of wood, he will note that each growth ring is made up of
+two more or less well-defined parts. That originally nearest the
+centre of the tree is more open textured and almost invariably
+lighter in color than that near the outer portion of the ring.
+The inner portion was formed early in the season, when growth
+was comparatively rapid and is known as _early wood_ (also
+spring wood); the outer portion is the _late wood_, being
+produced in the summer or early fall. In soft pines there is not
+much contrast in the different parts of the ring, and as a
+result the wood is very uniform in texture and is easy to work.
+In hard pine, on the other hand, the late wood is very dense and
+is deep-colored, presenting a very decided contrast to the soft,
+straw-colored early wood. (See Fig. 23.) In ring-porous woods
+each season's growth is always well defined, because the large
+pores of the spring abut on the denser tissue of the fall
+before. In the diffuse-porous, the demarcation between rings is
+not always so clear and in not a few cases is almost, if not
+entirely, invisible to the unaided eye. (See Fig. 22.)
+
+[Illustration: FIG. 23.--Cross section of longleaf pine showing
+several growth rings with variations in the width of the
+dark-colored late wood. Seven resin ducts are visible. X 33.
+_Photomicrograph by U.S. Forest Service_]
+
+If one compares a heavy piece of pine with a light specimen it
+will be seen at once that the heavier one contains a larger
+proportion of late wood than the other, and is therefore
+considerably darker. The late wood of all species is denser than
+that formed early in the season, hence the greater the
+proportion of late wood the greater the density and strength.
+When examined under a microscope the cells of the late wood are
+seen to be very thick-walled and with very small cavities, while
+those formed first in the season have thin walls and large
+cavities. The strength is in the walls, not the cavities. In
+choosing a piece of pine where strength or stiffness is the
+important consideration, the principal thing to observe is the
+comparative amounts of early and late wood. The width of ring,
+that is, the number per inch, is not nearly so important as the
+proportion of the late wood in the ring.
+
+It is not only the proportion of late wood, but also its
+quality, that counts. In specimens that show a very large
+proportion of late wood it may be noticeably more porous and
+weigh considerably less than the late wood in pieces that
+contain but little. One can judge comparative density, and
+therefore to some extent weight and strength, by visual
+inspection.
+
+The conclusions of the U.S. Forest Service regarding the effect
+of rate of growth on the properties of Douglas fir are
+summarized as follows:
+
+"1. In general, rapidly grown wood (less than eight rings per
+inch) is relatively weak. A study of the individual tests upon
+which the average points are based shows, however, that when it
+is not associated with light weight and a small proportion of
+summer wood, rapid growth is not indicative of weak wood.
+
+"2. An average rate of growth, indicated by from 12 to 16 rings
+per inch, seems to produce the best material.
+
+"3. In rates of growths lower than 16 rings per inch, the
+average strength of the material decreases, apparently
+approaching a uniform condition above 24 rings per inch. In such
+slow rates of growth the texture of the wood is very uniform,
+and naturally there is little variation in weight or strength.
+
+"An analysis of tests on large beams was made to ascertain if
+average rate of growth has any relation to the mechanical
+properties of the beams. The analysis indicated conclusively
+that there was no such relation. Average rate of growth [without
+consideration also of density], therefore, has little
+significance in grading structural timber."[18] This is because
+of the wide variation in the percentage of late wood in
+different parts of the cross section.
+
+[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.
+29.]
+
+Experiments seem to indicate that for most species there is a
+rate of growth which, in general, is associated with the
+greatest strength, especially in small specimens. For eight
+conifers it is as follows:[19]
+
+[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, p. 37.]
+
+                  Rings per inch
+Douglas fir             24
+Shortleaf pine          12
+Loblolly pine            6
+Western larch           18
+Western hemlock         14
+Tamarack                20
+Norway pine             18
+Redwood                 30
+
+No satisfactory explanation can as yet be given for the real
+causes underlying the formation of early and late wood. Several
+factors may be involved. In conifers, at least, rate of growth
+alone does not determine the proportion of the two portions of
+the ring, for in some cases the wood of slow growth is very hard
+and heavy, while in others the opposite is true. The quality of
+the site where the tree grows undoubtedly affects the character
+of the wood formed, though it is not possible to formulate a
+rule governing it. In general, however, it may be said that
+where strength or ease of working is essential, woods of
+moderate to slow growth should be chosen. But in choosing a
+particular specimen it is not the width of ring, but the
+proportion and character of the late wood which should govern.
+
+In the case of the ring-porous hardwoods there seems to exist a
+pretty definite relation between the rate of growth of timber
+and its properties. This may be briefly summed up in the general
+statement that the more rapid the growth or the wider the rings
+of growth, the heavier, harder, stronger, and stiffer the wood.
+This, it must be remembered, applies only to ring-porous woods
+such as oak, ash, hickory, and others of the same group, and is,
+of course, subject to some exceptions and limitations.
+
+In ring-porous woods of good growth it is usually the middle
+portion of the ring in which the thick-walled, strength-giving
+fibres are most abundant. As the breadth of ring diminishes,
+this middle portion is reduced so that very slow growth produces
+comparatively light, porous wood composed of thin-walled vessels
+and wood parenchyma. In good oak these large vessels of the
+early wood occupy from 6 to 10 per cent of the volume of the
+log, while in inferior material they may make up 25 per cent or
+more. The late wood of good oak, except for radial grayish
+patches of small pores, is dark colored and firm, and consists
+of thick-walled fibres which form one-half or more of the wood.
+In inferior oak, such fibre areas are much reduced both in
+quantity and quality. Such variation is very largely the result
+of rate of growth.
+
+Wide-ringed wood is often called "second-growth," because the
+growth of the young timber in open stands after the old trees
+have been removed is more rapid than in trees in the forest, and
+in the manufacture of articles where strength is an important
+consideration such "second-growth" hardwood material is
+preferred. This is particularly the case in the choice of
+hickory for handles and spokes. Here not only strength, but
+toughness and resilience are important. The results of a series
+of tests on hickory by the U.S. Forest Service show that "the
+work or shock-resisting ability is greatest in wide-ringed wood
+that has from 5 to 14 rings per inch, is fairly constant from 14
+to 38 rings, and decreases rapidly from 38 to 47 rings. The
+strength at maximum load is not so great with the most
+rapid-growing wood; it is maximum with from 14 to 20 rings per
+inch, and again becomes less as the wood becomes more closely
+ringed. The natural deduction is that wood of first-class
+mechanical value shows from 5 to 20 rings per inch and that
+slower growth yields poorer stock. Thus the inspector or buyer
+of hickory should discriminate against timber that has more than
+20 rings per inch. Exceptions exist, however, in the case of
+normal growth upon dry situations, in which the slow-growing
+material may be strong and tough."[20]
+
+[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]
+
+The effect of rate of growth on the qualities of chestnut wood
+is summarized by the same authority as follows: "When the rings
+are wide, the transition from spring wood to summer wood is
+gradual, while in the narrow rings the spring wood passes into
+summer wood abruptly. The width of the spring wood changes but
+little with the width of the annual ring, so that the narrowing
+or broadening of the annual ring is always at the expense of the
+summer wood. The narrow vessels of the summer wood make it
+richer in wood substance than the spring wood composed of wide
+vessels. Therefore, rapid-growing specimens with wide rings have
+more wood substance than slow-growing trees with narrow rings.
+Since the more the wood substance the greater the weight, and
+the greater the weight the stronger the wood, chestnuts with
+wide rings must have stronger wood than chestnuts with narrow
+rings. This agrees with the accepted view that sprouts (which
+always have wide rings) yield better and stronger wood than
+seedling chestnuts, which grow more slowly in diameter."[21]
+
+[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.
+20-21.]
+
+In diffuse-porous woods, as has been stated, the vessels or
+pores are scattered throughout the ring instead of collected in
+the early wood. The effect of rate of growth is, therefore, not
+the same as in the ring-porous woods, approaching more nearly
+the conditions in the conifers. In general it may be stated that
+such woods of medium growth afford stronger material than when
+very rapidly or very slowly grown. In many uses of wood,
+strength is not the main consideration. If ease of working is
+prized, wood should be chosen with regard to its uniformity of
+texture and straightness of grain, which will in most cases
+occur when there is little contrast between the late wood of one
+season's growth and the early wood of the next.
+
+
+
+HEARTWOOD AND SAPWOOD
+
+
+Examination of the end of a log of many species reveals a
+darker-colored inner portion--the _heartwood_, surrounded by a
+lighter-colored zone--the _sapwood_. In some instances this
+distinction in color is very marked; in others, the contrast is
+slight, so that it is not always easy to tell where one leaves
+off and the other begins. The color of fresh sapwood is always
+light, sometimes pure white, but more often with a decided tinge
+of green or brown.
+
+Sapwood is comparatively new wood. There is a time in the early
+history of every tree when its wood is all sapwood. Its
+principal functions are to conduct water from the roots to the
+leaves and to store up and give back according to the season the
+food prepared in the leaves. The more leaves a tree bears and
+the more thrifty its growth, the larger the volume of sapwood
+required, hence trees making rapid growth in the open have
+thicker sapwood for their size than trees of the same species
+growing in dense forests. Sometimes trees grown in the open may
+become of considerable size, a foot or more in diameter, before
+any heartwood begins to form, for example, in second-growth
+hickory, or field-grown white and loblolly pines.
+
+As a tree increases in age and diameter an inner portion of the
+sapwood becomes inactive and finally ceases to function. This
+inert or dead portion is called heartwood, deriving its name
+solely from its position and not from any vital importance to
+the tree, as is shown by the fact that a tree can thrive with
+its heart completely decayed. Some, species begin to form
+heartwood very early in life, while in others the change comes
+slowly. Thin sapwood is characteristic of such trees as
+chestnut, black locust, mulberry, Osage orange, and sassafras,
+while in maple, ash, gum, hickory, hackberry, beech, and
+loblolly pine, thick sapwood is the rule.
+
+There is no definite relation between the annual rings of growth
+and the amount of sapwood. Within the same species the
+cross-sectional area of the sapwood is roughly proportional to
+the size of the crown of the tree. If the rings are narrow, more
+of them are required than where they are wide. As the tree gets
+larger, the sapwood must necessarily become thinner or increase
+materially in volume. Sapwood is thicker in the upper portion of
+the trunk of a tree than near the base, because the age and the
+diameter of the upper sections are less.
+
+When a tree is very young it is covered with limbs almost, if
+not entirely, to the ground, but as it grows older some or all
+of them will eventually die and be broken off. Subsequent growth
+of wood may completely conceal the stubs which, however, will
+remain as knots. No matter how smooth and clear a log is on the
+outside, it is more or less knotty near the middle. Consequently
+the sapwood of an old tree, and particularly of a forest-grown
+tree, will be freer from knots than the heartwood. Since in most
+uses of wood, knots are defects that weaken the timber and
+interfere with its ease of working and other properties, it
+follows that sapwood, because of its position in the tree, may
+have certain advantages over heartwood.
+
+It is really remarkable that the inner heartwood of old trees
+remains as sound as it usually does, since in many cases it is
+hundreds of years, and in a few instances thousands of years,
+old. Every broken limb or root, or deep wound from fire,
+insects, or falling timber, may afford an entrance for decay,
+which, once started, may penetrate to all parts of the trunk.
+The larv� of many insects bore into the trees and their tunnels
+remain indefinitely as sources of weakness. Whatever advantages,
+however, that sapwood may have in this connection are due solely
+to its relative age and position.
+
+If a tree grows all its life in the open and the conditions of
+soil and site remain unchanged, it will make its most rapid
+growth in youth, and gradually decline. The annual rings of
+growth are for many years quite wide, but later they become
+narrower and narrower. Since each succeeding ring is laid down
+on the outside of the wood previously formed, it follows that
+unless a tree materially increases its production of wood from
+year to year, the rings must necessarily become thinner. As a
+tree reaches maturity its crown becomes more open and the annual
+wood production is lessened, thereby reducing still more the
+width of the growth rings. In the case of forest-grown trees so
+much depends upon the competition of the trees in their struggle
+for light and nourishment that periods of rapid and slow growth
+may alternate. Some trees, such as southern oaks, maintain the
+same width of ring for hundreds of years. Upon the whole,
+however, as a tree gets larger in diameter the width of the
+growth rings decreases.
+
+It is evident that there may be decided differences in the grain
+of heartwood and sapwood cut from a large tree, particularly one
+that is overmature. The relationship between width of growth
+rings and the mechanical properties of wood is discussed under
+Rate of Growth. In this connection, however, it may be stated
+that as a general rule the wood laid on late in the life of a
+tree is softer, lighter, weaker, and more even-textured than
+that produced earlier. It follows that in a large log the
+sapwood, because of the time in the life of the tree when it was
+grown, may be inferior in hardness, strength, and toughness to
+equally sound heartwood from the same log.
+
+After exhaustive tests on a number of different woods the U.S.
+Forest Service concludes as follows: "Sapwood, except that from
+old, overmature trees, is as strong as heartwood, other things
+being equal, and so far as the mechanical properties go should
+not be regarded as a defect."[22] Careful inspection of the
+individual tests made in the investigation fails to reveal any
+relation between the proportion of sapwood and the breaking
+strength of timber.
+
+[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
+
+In the study of the hickories the conclusion was: "There is an
+unfounded prejudice against the heartwood. Specifications place
+white hickory, or sapwood, in a higher grade than red hickory,
+or heartwood, though there is no inherent difference in
+strength. In fact, in the case of large and old hickory trees,
+the sapwood nearest the bark is comparatively weak, and the best
+wood is in the heart, though in young trees of thrifty growth
+the best wood is in the sap."[23] The results of tests from
+selected pieces lying side by side in the same tree, and also
+the average values for heartwood and sapwood in shipments of the
+commercial hickories without selection, show conclusively that
+"the transformation of sapwood into heartwood does not affect
+either the strength or toughness of the wood.... It is true,
+however, that sapwood is usually more free from latent defects
+than heartwood."[24]
+
+[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
+
+[Footnote 24: _Loc. cit._]
+
+Specifications for paving blocks often require that longleaf
+pine be 90 per cent heart. This is on the belief that sapwood is
+not only more subject to decay, but is also weaker than
+heartwood. In reality there is no sound basis for discrimination
+against sapwood on account of strength, provided other
+conditions are equal. It is true that sapwood will not resist
+decay as long as heartwood, if both are untreated with
+preservatives. It is especially so of woods with deep-colored
+heartwood, and is due to infiltrations of tannins, oils, and
+resins, which make the wood more or less obnoxious to
+decay-producing fungi. If, however, the timbers are to be
+treated, sapwood is not a defect; in fact, because of the
+relative ease with which it can be impregnated with
+preservatives it may be made more desirable than heartwood.[25]
+
+[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 and _Eucalyptus globulus_ and _viminalis_. 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.]
+
+In specifications for structural timbers reference is sometimes
+made to "boxheart," meaning the inclusion of the pith or centre
+of the tree within a cross section of the timber. From numerous
+experiments it appears that the position of the pith does not
+bear any relation to the strength of the material. Since most
+season checks, however, are radial, the position of the pith may
+influence the resistance of a seasoned beam to horizontal shear,
+being greatest when the pith is located in the middle half of
+the section.[26]
+
+[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
+
+
+
+WEIGHT, DENSITY, AND SPECIFIC GRAVITY
+
+
+From data obtained from a large number of tests on the strength
+of different woods it appears that, other things being equal,
+the crushing strength parallel to the grain, fibre stress at
+elastic limit in bending, and shearing strength along the grain
+of wood vary in direct proportion to the weight of dry wood per
+unit of volume when green. Other strength values follow
+different laws. The hardness varies in a slightly greater ratio
+than the square of the density. The work to the breaking point
+increases even more rapidly than the cube of density. The
+modulus of rupture in bending lies between the first power and
+the square of the density. This, of course, is true only in case
+the greater weight is due to increase in the amount of wood
+substance. A wood heavy with resin or other infiltrated
+substance is not necessarily stronger than a similar specimen
+free from such materials. If differences in weight are due to
+degree of seasoning, in other words, to the relative amounts of
+water contained, the rules given above will of course not hold,
+since strength increases with dryness. But of given specimens of
+pine or of oak, for example, in the green condition, the
+comparative strength may be inferred from the weight. It is not
+permissible, however, to compare such widely different woods as
+oak and pine on a basis of their weights.[27]
+
+[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.]
+
+The weight of wood substance, that is, the material which
+composes the walls of the fibres and other cells, is practically
+the same in all species, whether pine, hickory, or cottonwood,
+being a little greater than half again as heavy as water. It
+varies slightly from beech sapwood, 1.50, to Douglas fir
+heartwood, 1.57, averaging about 1.55 at 30� to 35� C., in terms
+of water at its greatest density 4� C. The reason any wood
+floats is that the air imprisoned in its cavities buoys it up.
+When this is displaced by water the wood becomes water-logged
+and sinks. Leaving out of consideration infiltrated substances,
+the reason a cubic foot of one kind of dry wood is heavier than
+that of another is because it contains a greater amount of wood
+substance.  ~Density~ is merely the weight of a unit of volume,
+as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.
+~Specific gravity~ or relative density is the ratio of the
+density of any material to the density of distilled water at 4�
+C. (39.2� F.). A cubic foot of distilled water at 4� C. weighs
+62.43 pounds. Hence the specific gravity of a piece of wood with
+a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight
+per cubic foot when the specific gravity is given, simply
+multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric
+system, since the weight of a cubic centimetre of pure water is
+one gram, the density in grams per cubic centimetre has the same
+numerical value as the specific gravity.
+
+Since the amount of water in wood is extremely variable it
+usually is not satisfactory to refer to the density of green
+wood. For scientific purposes the density of "oven-dry" wood is
+used; that is, the wood is dried in an oven at a temperature of
+100�C. (212�F.) until a constant weight is attained. For
+commercial purposes the weight or density of air-dry or
+"shipping-dry" wood is used. This is usually expressed in pounds
+per thousand board feet, a board foot being considered as
+one-twelfth of a cubic foot.
+
+Wood shrinks greatly in drying from the green to the oven-dry
+condition. (See Table XIV.) Consequently a block of wood
+measuring a cubic foot when green will measure considerably less
+when oven-dry. It follows that the density of oven-dry wood does
+not represent the weight of the dry wood substance in a cubic
+foot of green wood. In other words, it is not the weight of a
+cubic foot of green wood minus the weight of the water which it
+contains. Since the latter is often a more convenient figure to
+use and much easier to obtain than the weight of oven-dry wood,
+it is commonly expressed in tables of "specific gravity or
+density of dry wood."
+
+|----------------------------------------------------------------------------|
+|                                 TABLE XIV                                  |
+|----------------------------------------------------------------------------|
+|            SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS            |
+|                         (Forest Service Cir. 213)                          |
+|----------------------------------------------------------------------------|
+|                 |         |  Specific gravity  | Shrinkage from green to   |
+|                 | Mois-   | oven-dry, based on | oven-dry condition        |
+|   COMMON NAME   |  ture   |--------------------+---------------------------|
+|   OF SPECIES    | content | Volume  |  Volume  | In     |        | Tangen- |
+|                 |         |   when  |   when   | volume | Radial | tial    |
+|                 |         |  green  | oven-dry |        |        |         |
+|-----------------+---------+---------+----------+--------+--------+---------|
+|                 |   Per   |         |          |  Per   |  Per   |   Per   |
+|                 |   cent  |         |          |  cent  |  cent  |   cent  |
+|                 |         |         |          |        |        |         |
+|    Hardwoods    |         |         |          |        |        |         |
+|                 |         |         |          |        |        |         |
+| Ash, black      |     77  |  0.466  |          |        |        |         |
+|   white         |     38  |   .550  |   0.640  |  12.6  |   4.3  |    6.4  |
+|     "           |     47  |   .516  |    .590  |  11.7  |        |         |
+| Basswood        |    110  |   .315  |    .374  |  14.5  |   6.2  |    8.4  |
+| Beech           |     61  |   .556  |    .669  |  16.5  |   4.6  |   10.5  |
+| Birch, yellow   |     72  |   .545  |    .661  |  17.0  |   7.9  |    9.0  |
+| Elm, rock       |     46  |   .578  |          |        |        |         |
+|   slippery      |     57  |   .541  |    .639  |  15.5  |   5.1  |    9.9  |
+|   white         |     66  |   .430  |          |        |        |         |
+| Gum, red        |     71  |   .434  |          |        |        |         |
+| Hackberry       |     50  |   .504  |    .576  |  14.0  |   4.2  |    8.9  |
+| Hickory,        |         |         |          |        |        |         |
+|   big shellbark |     64  |   .601  |          |  17.6  |   7.4  |   11.2  |
+|    "      "     |     55  |   .666  |          |  20.9  |   7.9  |   14.2  |
+|   bitternut     |     65  |   .624  |          |        |        |         |
+|   mockernut     |     64  |   .606  |          |  16.5  |   6.9  |   10.4  |
+|       "         |     57  |   .662  |          |  18.9  |   8.4  |   11.4  |
+|       "         |     48  |   .666  |          |        |        |         |
+|   nutmeg        |     76  |   .558  |          |        |        |         |
+|   pignut        |     59  |   .627  |          |  15.0  |   5.6  |    9.8  |
+|     "           |     54  |   .667  |          |  15.3  |   6.3  |    9.5  |
+|     "           |     55  |   .667  |          |  16.9  |   6.8  |   10.9  |
+|     "           |     52  |   .667  |          |  21.2  |   8.5  |   13.8  |
+|   shagbark      |     65  |   .608  |          |  16.0  |   6.5  |   10.2  |
+|      "          |     58  |   .646  |          |  18.4  |   7.9  |   11.4  |
+|      "          |     64  |   .617  |          |        |        |         |
+|      "          |     60  |   .653  |          |  15.5  |   6.5  |    9.7  |
+|   water         |     74  |   .630  |          |        |        |         |
+| Locust, honey   |     53  |   .695  |    .759  |   8.6  |        |         |
+| Maple, red      |     69  |   .512  |          |        |        |         |
+|   sugar         |     57  |   .546  |    .643  |  14.3  |   4.9  |    9.1  |
+|     "           |     56  |   .577  |          |        |        |         |
+| Oak, post       |     64  |   .590  |    .732  |  16.0  |   5.7  |   10.6  |
+|   red           |     80  |   .568  |    .660  |  13.1  |   3.7  |    8.3  |
+|   swamp white   |     74  |   .637  |    .792  |  17.7  |   5.5  |   10.6  |
+|   tanbark       |     88  |   .585  |          |        |        |         |
+|   white         |     58  |   .594  |    .704  |  15.8  |   6.2  |    8.3  |
+|     "           |     62  |   .603  |    .696  |  14.3  |   4.9  |    9.0  |
+|     "           |     78  |   .600  |    .708  |  16.0  |   4.8  |    9.2  |
+|   yellow        |     77  |   .573  |    .669  |  14.2  |   4.5  |    9.7  |
+|      "          |     80  |   .550  |          |        |        |         |
+| Osage orange    |     31  |   .761  |    .838  |   8.9  |        |         |
+| Sycamore        |     81  |   .454  |    .526  |  13.5  |   5.0  |    7.3  |
+| Tupelo          |    121  |   .475  |    .545  |  12.4  |   4.4  |    7.9  |
+|----------------------------------------------------------------------------|
+
+|----------------------------------------------------------------------------|
+|                             TABLE XIV (CONT.)                              |
+|----------------------------------------------------------------------------|
+|            SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS            |
+|                         (Forest Service Cir. 213)                          |
+|----------------------------------------------------------------------------|
+|                 |         |  Specific gravity  | Shrinkage from green to   |
+|                 |         | oven-dry, based on |    oven-dry condition     |
+|   COMMON NAME   |         |--------------------+---------------------------|
+|   OF SPECIES    |  Mois-  | Volume  |  Volume  |   In   |        | Tangen- |
+|                 |   ture  |   when  |   when   | volume | Radial |  tial   |
+|                 | content |  green  | oven-dry |        |        |         |
+|-----------------+---------+---------+----------+--------+--------+---------|
+|                 |   Per   |         |          |  Per   |  Per   |   Per   |
+|                 |   cent  |         |          |  cent  |  cent  |   cent  |
+|                 |         |         |          |        |        |         |
+|    Conifers     |         |         |          |        |        |         |
+|                 |         |         |          |        |        |         |
+| Arborvit�       |     55  |   .293  |    .315  |   7.0  |   2.1  |    4.9  |
+| Cedar, incense  |     80  |   .363  |          |        |        |         |
+| Cypress, bald   |     79  |   .452  |    .513  |  11.5  |   3.8  |    6.0  |
+| Fir, alpine     |     47  |   .306  |    .321  |   9.0  |   2.5  |    7.1  |
+|   amabilis      |    117  |   .383  |          |        |        |         |
+|   Douglas       |     32  |   .418  |    .458  |  10.9  |   3.7  |    6.6  |
+|   white         |    156  |   .350  |    .437  |  10.2  |   3.4  |    7.0  |
+| Hemlock (east.) |    129  |   .340  |    .394  |   9.2  |   2.3  |    5.0  |
+| Pine, lodgepole |     44  |   .370  |    .415  |  11.3  |   4.2  |    7.1  |
+|           "     |     58  |   .371  |    .407  |  10.1  |   3.6  |    5.9  |
+|   longleaf      |     63  |   .528  |    .599  |  12.8  |   6.0  |    7.6  |
+|   red or Nor    |     54  |   .440  |    .507  |  11.5  |   4.5  |    7.2  |
+|   shortleaf     |     52  |   .447  |          |        |        |         |
+|   sugar         |    123  |   .360  |    .386  |   8.4  |   2.9  |    5.6  |
+|   west yellow   |     98  |   .353  |    .395  |   9.2  |   4.1  |    6.4  |
+|     "    "      |    125  |   .377  |    .433  |  11.5  |   4.3  |    7.3  |
+|     "    "      |     93  |   .391  |    .435  |   9.9  |   3.8  |    5.8  |
+|   white         |     74  |   .363  |    .391  |   7.8  |   2.2  |    5.9  |
+| Redwood         |     81  |   .334  |          |        |        |         |
+|    "            |     69  |   .366  |          |        |        |         |
+| Spruce,         |         |         |          |        |        |         |
+|   Engelmann     |     45  |   .325  |    .359  |  10.5  |   3.7  |    6.9  |
+|       "         |    156  |   .299  |    .335  |  10.3  |   3.0  |    6.2  |
+|   red           |     31  |   .396  |          |        |        |         |
+|   white         |     41  |   .318  |          |        |        |         |
+| Tamarack        |     52  |   .491  |    .558  |  13.6  |   3.7  |    7.4  |
+|----------------------------------------------------------------------------|
+
+This weight divided by 62.43 gives the specific gravity per
+green volume. It is purely a fictitious quantity. To convert
+this figure into actual density or specific gravity of the dry
+wood, it is necessary to know the amount of shrinkage in volume.
+If S is the percentage of shrinkage from the green to the
+oven-dry condition, based on the green volume; D, the density of
+the dry wood per cubic foot while green; and d the actual
+                                   D
+density of oven-dry wood, then ---------- = d.
+                                1 - .0 S
+
+This relation becomes clearer from the following analysis:
+Taking V and W as the volume and weight, respectively, when
+green, and v and w as the corresponding volume and weight when
+                     w         W         V - v
+oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and
+                     v         V           V
+     V - v
+s = ------- X 100, in which S is the percentage of shrinkage
+       v
+from the green to the oven-dry condition, based on the green
+volume, and s the same based on the oven-dry volume.
+
+In tables of specific gravity or density of wood it should
+always be stated whether the dry weight per unit of volume when
+green or the dry weight per unit of volume when dry is intended,
+since the shrinkage in volume may vary from 6 to 50 per cent,
+though in conifers it is usually about 10 per cent, and in
+hardwoods nearer 15 per cent. (See Table XIV.)
+
+
+
+COLOR
+
+
+In species which show a distinct difference between heartwood
+and sapwood the natural color of heartwood is invariably darker
+than that of the sapwood, and very frequently the contrast is
+conspicuous. This is produced by deposits in the heartwood of
+various materials resulting from the process of growth,
+increased possibly by oxidation and other chemical changes,
+which usually have little or no appreciable effect on the
+mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD,
+above.) Some experiments[28] on very resinous longleaf pine
+specimens, however, indicate an increase in strength. This is
+due to the resin which increases the strength when dry. Spruce
+impregnated with crude resin and dried is greatly increased in
+strength thereby.
+
+[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126,
+appendix.]
+
+Since the late wood of a growth ring is usually darker in color
+than the early wood, this fact may be used in judging the
+density, and therefore the hardness and strength of the
+material. This is particularly the case with coniferous woods.
+In ring-porous woods the vessels of the early wood not
+infrequently appear on a finished surface as darker than the
+denser late wood, though on cross sections of heartwood the
+reverse is commonly true. Except in the manner just stated the
+color of wood is no indication of strength.
+
+Abnormal discoloration of wood often denotes a diseased
+condition, indicating unsoundness. The black check in western
+hemlock is the result of insect attacks.[29] The reddish-brown
+streaks so common in hickory and certain other woods are mostly
+the result of injury by birds.[30] The discoloration is merely
+an indication of an injury, and in all probability does not of
+itself affect the properties of the wood. Certain rot-producing
+fungi impart to wood characteristic colors which thus become
+criterions of weakness. Ordinary sap-staining is due to fungous
+growth, but does not necessarily produce a weakening effect.[31]
+
+[Footnote 29: See Burke, H.E.: Black check in western hemlock.
+Cir. No. 61, U.S. Bu. Entomology, 1905.]
+
+[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees
+and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]
+
+[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.]
+
+
+
+CROSS GRAIN
+
+
+_Cross grain_ is a very common defect in timber. One form of it
+is produced in lumber by the method of sawing and has no
+reference to the natural arrangement of the wood elements. Thus
+if the plane of the saw is not approximately parallel to the
+axis of the log the grain of the lumber cut is not parallel to
+the edges and is termed diagonal. This is likely to occur where
+the logs have considerable taper, and in this case may be
+produced if sawed parallel to the axis of growth instead of
+parallel to the growth rings.
+
+Lumber and timber with diagonal grain is always weaker than
+straight-grained material, the extent of the defect varying with
+the degree of the angle the fibres make with the axis of the
+stick. In the vicinity of large knots the grain is likely to be
+cross. The defect is most serious where wood is subjected to
+flexure, as in beams.
+
+_Spiral grain_ is a very common defect in a tree, and when
+excessive renders the timber valueless for use except in the
+round. It is produced by the arrangement of the wood fibres in a
+spiral direction about the axis instead of exactly vertical.
+Timber with spiral grain is also known as "torse wood." Spiral
+grain usually cannot be detected by casual inspection of a
+stick, since it does not show in the so-called visible grain of
+the wood, by which is commonly meant a sectional view of the
+annual rings of growth cut longitudinally. It is accordingly
+very easy to allow spiral-grained material to pass inspection,
+thereby introducing an element of weakness in a structure.
+
+There are methods for readily detecting spiral grain. The
+simplest is that of splitting a small piece radially. It is
+necessary, of course, that the split be radial, that is, in a
+plane passing through the axis of the log, and not tangentially.
+In the latter case it is quite probable that the wood would
+split straight, the line of cleavage being between the growth
+rings.
+
+In inspection, the elements to examine are the rays. In the case
+of oak and certain other hardwoods these rays are so large that
+they are readily seen not only on a radial surface, but on the
+tangential as well. On the former they appear as flakes, on the
+latter as short lines. Since these rays are between the fibres
+it naturally follows that they will be vertical or inclined
+according as the tree is straight-grained or spiral-grained.
+While they are not conspicuous in the softwoods, they can be
+seen upon close scrutiny, and particularly so if a small hand
+magnifier is used.
+
+When wood has begun to dry and check it is very easy to see
+whether or not it is straight- or spiral-grained, since the
+checks will for the most part follow along the rays. If one
+examines a row of telephone poles, for example, he will probably
+find that most of them have checks running spirally around them.
+If boards were sawed from such a pole after it was badly checked
+they would fall to pieces of their own weight. The only way to
+get straight material would be to split it out.
+
+It is for this reason that split billets and squares are
+stronger than most sawed material. The presence of the spiral
+grain has little, if any, effect on the timber when it is used
+in the round, but in sawed material the greater the pitch of the
+spiral the greater is the defect.
+
+
+
+KNOTS
+
+
+_Knots_ are portions of branches included in the wood of the
+stem or larger branch. Branches originate as a rule from the
+central axis of a stem, and while living increase in size by the
+addition of annual woody layers which are a continuation of
+those of the stem. The included portion is irregularly conical
+in shape with the tip at the pith. The direction of the fibre is
+at right angles or oblique to the grain of the stem, thus
+producing local cross grain.
+
+During the development of a tree most of the limbs, especially
+the lower ones, die, but persist for a time--often for years.
+Subsequent layers of growth of the stem are no longer intimately
+joined with the dead limb, but are laid around it. Hence dead
+branches produce knots which are nothing more than pegs in a
+hole, and likely to drop out after the tree has been sawed into
+lumber. In grading lumber and structural timber, knots are
+classified according to their form, size, soundness, and the
+firmness with which they are held in place.[32]
+
+[Footnote 32: See Standard classification of structural timber.
+Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
+Contains three plates showing standard defects.]
+
+Knots materially affect checking and warping, ease in working,
+and cleavability of timber. They are defects which weaken timber
+and depreciate its value for structural purposes where strength
+is an important consideration. The weakening effect is much more
+serious where timber is subjected to bending and tension than
+where under compression. The extent to which knots affect the
+strength of a beam depends upon their position, size, number,
+direction of fibre, and condition. A knot on the upper side is
+compressed, while one on the lower side is subjected to tension.
+The knot, especially (as is often the case) if there is a season
+check in it, offers little resistance to this tensile stress.
+Small, knots, however, may be so located in a beam along the
+neutral plane as actually to increase the strength by tending to
+prevent longitudinal shearing. Knots in a board or plank are
+least injurious when they extend through it at right angles to
+its broadest surface. Knots which occur near the ends of a beam
+do not weaken it. Sound knots which occur in the central portion
+one-fourth the height of the beam from either edge are not
+serious defects.
+
+Extensive experiments by the U.S. Forest Service[33] indicate
+the following effects of knots on structural timbers:
+
+[Footnote 33: Bul. 108, pp. 52 _et seq._]
+
+(1) Knots do not materially influence the stiffness of
+structural timber.
+
+(2) Only defects of the most serious character affect the
+elastic limit of beams. Stiffness and elastic strength are more
+dependent upon the quality of the wood fibre than upon defects
+in the beam.
+
+(3) The effect of knots is to reduce the difference between the
+fibre stress at elastic limit and the modulus of rupture of
+beams. The breaking strength is very susceptible to defects.
+
+(4) Sound knots do not weaken wood when subject to compression
+parallel to the grain.[34]
+
+[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
+properties of western hemlock, p. 20.]
+
+
+
+FROST SPLITS
+
+
+A common defect in standing timber results from radial splits
+which extend inward from the periphery of the tree, and almost,
+if not always, near the base. It is most common in trees which
+split readily, and those with large rays and thin bark. The
+primary cause of the splitting is frost, and various theories
+have been advanced to explain the action.
+
+R. Hartig[35] believes that freezing forces out a part of the
+imbibition water of the cell walls, thereby causing the wood to
+shrink, and if the interior layers have not yet been cooled,
+tangential strains arise which finally produce radial clefts.
+
+[Footnote 35: Hartig, R.: The diseases of trees (trans. by
+Somerville and Ward), London and New York, 1894, pp. 282-294.]
+
+Another theory holds that the water is not driven out of the
+cell walls, but that difference in temperature conditions of
+inner and outer layers is itself sufficient to set up the
+strains, resulting in splitting. An air temperature of 14�F. or
+less is considered necessary to produce frost splits.
+
+A still more recent theory is that of Busse[36] who considers
+the mechanical action of the wind a very important factor. He
+observed: (_a_) Frost splits sometimes occur at higher
+temperatures than 14�F. (_b_) Most splits take place shortly
+before sunrise, _i.e._, at the time of lowest air and soil
+temperature; they are never heard to take place at noon,
+afternoon, or evening. (_c_) They always occur between two roots
+or between the collars of two roots, (_d_) They are most
+frequent in old, stout-rooted, broad-crowned trees; in younger
+stands it is always the stoutest members that are found with
+frost splits, while in quite young stands they are altogether
+absent, (_e_) Trees on wet sites are most liable to splits, due
+to difference in wood structure, just as difference in wood
+structure makes different species vary in this regard. (_f_)
+Frost splits are most numerous less than three feet above the
+ground.
+
+[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
+Centralb., XXXII, 2, 1910, pp. 74-81.]
+
+When a tree is swayed by the wind the roots are counteracting
+forces, and the wood fibres are tested in tension and
+compression by the opposing forces; where the roots exercise
+tension stresses most effectively the effect of compression
+stresses is at a minimum; only where the pressure is in excess
+of the tension, _i.e._, between the roots, can a separation of
+the fibre result. Hence, when by frost a tension on the entire
+periphery is established, and the wind localizes additional
+strains, failure occurs. The stronger the compression and
+tension, the severer the strains and the oftener failures occur.
+The occurrence of reports of frost splits on wind-still days is
+believed by Busse to be due to the opening of old frost splits
+where the tension produced by the frost alone is sufficient.
+
+Frost splits may heal over temporarily, but usually open up
+again during the following winter. The presence of old splits is
+often indicated by a ridge of callous, the result of the
+cambium's effort to occlude the wound. Frost splits not only
+affect the value of lumber, but also afford an entrance into the
+living tree for disease and decay.
+
+
+
+SHAKES, GALLS, PITCH POCKETS
+
+
+_Heart shake_ occurs in nearly all overmature timber, being more
+frequent in hardwoods (especially oak) than in conifers. In
+typical heart shake the centre of the hole shows indications of
+becoming hollow and radial clefts of varying size extend outward
+from the pith, being widest inward. It frequently affects only
+the butt log, but may extend to the entire hole and even the
+larger branches. It usually results from a shrinkage of the
+heartwood due probably to chemical changes in the wood.
+
+When it consists of a single cleft extending across the pith it
+is termed _simple heart shake_. Shake of this character in
+straight-grained trees affects only one or two central boards
+when cut into lumber, but in spiral-grained timber the damage is
+much greater. When shake consists of several radial clefts it is
+termed _star shake_. In some instances one or more of these
+clefts may extend nearly to the bark. In felled or converted
+timber clefts due to heart shake may be distinguished from
+seasoning cracks by the darker color of the exposed surfaces.
+Such clefts, however, tend to open up more and more as the
+timber seasons.
+
+_Cup_ or _ring shake_ results from the pulling apart of two or
+more growth rings. It is one of the most serious defects to
+which sound timber is subject, as it seriously reduces the
+technical properties of wood. It is very common in sycamore and
+in western larch, particularly in the butt portion. Its
+occurrence is most frequent at the junction of two growth layers
+of very unequal thickness. Consequently it is likely to occur in
+trees which have grown slowly for a time, then abruptly
+increased, due to improved conditions of light and food, as in
+thinning. Old timber is more subject to it than young trees. The
+damage is largely confined to the butt log. Cup shake is often
+associated with other forms of shake, and not infrequently shows
+traces of decay.
+
+The causes of cup shake are uncertain. The swaying action of the
+wind may result in shearing apart the growth layers, especially
+in trees growing in exposed places. Frost may in some instances
+be responsible for cup shake or at least a contributing factor,
+although trees growing in regions free from frost often have
+ring shake. Shrinkage of the heartwood may be concentric as well
+as radial in its action, thus producing cup shake instead of, or
+in connection with, heart shake.
+
+A local defect somewhat similar in effect to cup shake is known
+as _rind gall_. If the cambium layer is exposed by the removal
+of the entire bark or rind it will die. Subsequent growth over
+the damaged portion does not cohere with the wood previously
+formed by the old cambium. The defect resulting is termed rind
+gall. The most common causes of it are fire, gnawing, blazing,
+chipping, sun scald, lightning, and abrasions.
+
+_Heart break_ is a term applied to areas of compression failure
+along the grain found in occasional logs. Sometimes these breaks
+are invisible until the wood is manufactured into the finished
+article. The occurrence of this defect is mostly limited to the
+dense hardwoods, such as hickory and to heavy tropical species.
+It is the source of considerable loss in the fancy veneer
+industry, as the veneer from valuable logs so affected drops to
+pieces.
+
+The cause of heart break is not positively known. It is highly
+probable, however, that when the tree is felled the trunk
+strikes across a rock or another log, and the impact causes
+actual failure in the log as in a beam.
+
+_Resin_ or _pitch pockets_ are of common occurrence in the wood
+of larch, spruce, fir, and especially of longleaf and other hard
+pines. They are due to accumulations of resin in openings
+between adjacent layers of growth. They are more frequent in
+trees growing alone than in those of dense stands. The pockets
+are usually a few inches in greatest dimension and affect only
+one or two growth layers. They are hidden until exposed by the
+saw, rendering it impossible to cut lumber with reference to
+their position. Often several boards are damaged by a single
+pocket. In grading lumber, pitch pockets are classified as
+small, standard, and large, depending upon their width and
+length.
+
+
+
+INSECT INJURIES[37]
+
+
+[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.]
+
+The larv� of many insects are destructive to wood. Some attack
+the wood of living trees, others only that of felled or
+converted material. Every hole breaks the continuity of the
+fibres and impairs the strength, and if there are very many of
+them the material may be ruined for all purposes where strength
+is required.
+
+Some of the most common insects attacking the wood of living
+trees are the oak timber worm, the chestnut timber worm,
+carpenter worms, ambrosia beetles, the locust borer, turpentine
+beetles and turpentine borers, and the white pine weevil.
+
+The insect injuries to forest products may be classed according
+to the stage of manufacture of the material. Thus round timber
+with the bark on, such as poles, posts, mine props, and sawlogs,
+is subject to serious damage by the same class of insects as
+those mentioned above, particularly by the round-headed borers,
+timber worms, and ambrosia beetles. Manufactured unseasoned
+products are subject to damage from ambrosia beetles and other
+wood borers. Seasoned hardwood lumber of all kinds, rough
+handles, wagon stock, etc., made partially or entirely of
+sapwood, are often reduced in value from 10 to 90 per cent by a
+class of insects known as powder-post beetles. Finished hardwood
+products such as handles, wagon, carriage and machinery stock,
+especially if ash or hickory, are often destroyed by the
+powder-post beetles. Construction timbers in buildings, bridges
+and trestles, cross-ties, poles, mine props, fence posts, etc.,
+are sometimes seriously injured by wood-boring larv�, termites,
+black ants, carpenter bees, and powder-post beetles, and
+sometimes reduced in value from 10 to 100 per cent. In tropical
+countries termites are a very serious pest in this respect.
+
+
+
+MARINE WOOD-BORER INJURIES
+
+
+Vast amounts of timber used for piles in wharves and other
+marine structures are constantly being destroyed or seriously
+injured by marine borers. Almost invariably they are confined to
+salt water, and all the woods commonly used for piling are
+subject to their attacks. There are two genera of mollusks,
+_Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria,
+Chelura_, and _Sphoeroma_, that do serious damage in many places
+along both the Atlantic and Pacific coasts.
+
+These mollusks, which are popularly known as "shipworms," are
+much alike in structure and mode of life. They attack the
+exposed surface of the wood and immediately begin to bore. The
+tunnels, often as large as a lead pencil, extend usually in a
+longitudinal direction and follow a very irregular, tangled
+course. Hard woods are apparently penetrated as readily as soft
+woods, though in the same timber the softer parts are preferred.
+The food consists of infusoria and is not obtained from the wood
+substance. The sole object of boring into the wood is to obtain
+shelter.
+
+Although shipworms can live in cold water they thrive best and
+are most destructive in warm water. The length of time required
+to destroy an average barked, unprotected pine pile on the
+Atlantic coast south from Chesapeake Bay and along the entire
+Pacific coast varies from but one to three years.
+
+Of the crustacean borers, _Limnoria_, or the "wood louse," is
+the only one of great importance, although _Sphoeroma_ is
+reported destructive in places. _Limnoria_ is about the size of
+a grain of rice and tunnels into the wood for both food and
+shelter. The galleries extend inward radially, side by side, in
+countless numbers, to the depth of about one-half inch. The thin
+wood partitions remaining are destroyed by wave action, so that
+a fresh surface is exposed to attack. Both hard and soft woods
+are damaged, but the rate is faster in the soft woods or softer
+portions of a wood.
+
+Timbers seriously attacked by marine borers are badly weakened
+or completely destroyed. If the original strength of the
+material is to be preserved it is necessary to protect the wood
+from the borers. This is sometimes accomplished by proper
+injection of creosote oil, and more or less successfully by the
+use of various kinds of external coatings.[38] No treatment,
+however, has proved entirely satisfactory.
+
+[Footnote 38: See Smith, C. Stowell: Preservation of piling
+against marine wood borers. Cir. 128, U.S. Forest Service, 1908,
+pp. 15.]
+
+
+
+FUNGOUS INJURIES[39]
+
+
+[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.]
+
+Fungi are responsible for almost all decay of wood. So far as
+known, all decay is produced by living organisms, either fungi
+or bacteria. Some species attack living trees, sometimes killing
+them, or making them hollow, or in the case of pecky cypress and
+incense cedar filling the wood with galleries like those of
+boring insects. A much larger variety work only in felled or
+dead wood, even after it is placed in buildings or manufactured
+articles. In any case the process of destruction is the same.
+The mycelial threads penetrate the walls of the cells in search
+of food, which they find either in the cell contents (starches,
+sugars, etc.), or in the cell wall itself. The breaking down of
+the cell walls through the chemical action of so-called
+"enzymes" secreted by the fungi follows, and the eventual
+product is a rotten, moist substance crumbling readily under the
+slightest pressure. Some species remove the ligneous matter and
+leave almost pure cellulose, which is white, like cotton; others
+dissolve the cellulose, leaving a brittle, dark brown mass of
+ligno-cellulose. Fungi (such as the bluing fungus) which merely
+stain wood usually do not affect its mechanical properties
+unless the attacks are excessive.
+
+It is evident, then, that the action of rot-causing fungi is to
+decrease the strength of wood, rendering it unsound, brittle,
+and dangerous to use. The most dangerous kinds are the so-called
+"dry-rot" fungi which work in many kinds of lumber after it is
+placed in the buildings. They are particularly to be dreaded
+because unseen, working as they do within the walls or inside of
+casings. Several serious wrecks of large buildings have been
+attributed to this cause. It is stated[40] that in the three
+years (1911-1913) more than $100,000 was required to repair
+damage due to dry rot.
+
+[Footnote 40: Dry rot in factory timbers, by Inspection Dept.
+Associated Factory Mutual Fire Insurance Cos., 31 Milk Street,
+Boston, 1913.]
+
+Dry rot develops best at 75�F. and is said to be killed by a
+temperature of 110�F.[41] Fully 70 per cent humidity is
+necessary in the air in which a timber is surrounded for the
+growth of this fungus, and probably the wood must be quite near
+its fibre saturation condition. Nevertheless _Merulius
+lacrymans_ (one of the most important species) has been found to
+live four years and eight months in a dry condition.[42]
+Thorough kiln-drying will kill this fungus, but will not prevent
+its redevelopment. Antiseptic treatment, such as creosoting, is
+the best prevention.
+
+[Footnote 41: Falck, Richard: Die Meruliusfa�le des Bauholzes,
+Hausschwammforschungen, 6. Heft., Jena, 1912.]
+
+[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
+
+All fungi require moisture and air[43] for their growth.
+Deprived of either of these the fungus dies or ceases to
+develop. Just what degree of moisture in wood is necessary for
+the "dry-rot" fungus has not been determined, but it is
+evidently considerably above that of thoroughly air-dry timber,
+probably more than 15 per cent moisture. Hence the importance of
+free circulation of air about all timbers in a building.
+
+[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.]
+
+Warmth is also conducive to the growth of fungi, the most
+favorable temperature being about 90�F. They cannot grow in
+extreme cold, although no degree of cold such as occurs
+naturally will kill them. On the other hand, high temperature
+will kill them, but the spores may survive even the boiling
+temperature. Mould fungus has been observed to develop rapidly
+at 130�F. in a dry kiln in moist air, a condition under which an
+animal cannot live more than a few minutes. This fungus was
+killed, however, at about 140� or 145�F.[44]
+
+[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in
+Berkeley, U.S. Forest Service.]
+
+The fungus (_Endothia parasitica_ And.) which causes the
+chestnut blight kills the trees by girdling them and has no
+direct effect upon the wood save possibly the four or five
+growth rings of the sapwood.[45]
+
+[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.]
+
+
+
+PARASITIC PLANT INJURIES.[46]
+
+
+[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.]
+
+The most common of the higher parasitic plants damaging timber
+trees are mistletoes. Many species of deciduous trees are
+attacked by the common mistletoe (_Phoradendron flavescens_). It
+is very prevalent in the South and Southwest and when present in
+sufficient quantity does considerable damage. There is also a
+considerable number of smaller mistletoes belonging to the genus
+_Razoumofskya (Arceuthobium)_ which are widely distributed
+throughout the country, and several of them are common on
+coniferous trees in the Rocky Mountains and along the Pacific
+coast.
+
+One effect of the common mistletoe is the formation of large
+swellings or tumors. Often the entire tree may become stunted or
+distorted. The western mistletoe is most common on the branches,
+where it produces "witches' broom." It frequently attacks the
+trunk as well, and boards cut from such trees are filled with
+long, radial holes which seriously damage or destroy the value
+of the timber affected.
+
+
+
+LOCALITY OF GROWTH
+
+
+The data available regarding the effect of the locality of
+growth upon the properties of wood are not sufficient to warrant
+definite conclusions. The subject has, however, been kept in
+mind in many of the U.S. Forest Service timber tests and the
+following quotations are assembled from various reports:
+
+"In both the Cuban and longleaf pine the locality where grown
+appears to have but little influence on weight or strength, and
+there is no reason to believe that the longleaf pine from one
+State is better than that from any other, since such variations
+as are claimed can be found on any 40-acre lot of timber in any
+State. But with loblolly and still more with shortleaf this
+seems not to be the case. Being widely distributed over many
+localities different in soil and climate, the growth of the
+shortleaf pine seems materially influenced by location. The wood
+from the southern coast and gulf region and even Arkansas is
+generally heavier than the wood from localities farther north.
+Very light and fine-grained wood is seldom met near the southern
+limit of the range, while it is almost the rule in Missouri,
+where forms resembling the Norway pine are by no means rare. The
+loblolly, occupying both wet and dry soils, varies accordingly."
+Cir. No. 12, p. 6.
+
+" ... It is clear that as all localities have their heavy and
+their light timber, so they all share in strong and weak, hard
+and soft material, and the difference in quality of material is
+evidently far more a matter of individual variation than of soil
+or climate." _Ibid._, p.22
+
+"A representative committee of the Carriage Builders'
+Association had publicly declared that this important industry
+could not depend upon the supplies of southern timber, as the
+oak grown in the South lacked the necessary qualities demanded
+in carriage construction. Without experiment this statement
+could be little better than a guess, and was doubly unwarranted,
+since it condemned an enormous amount of material, and one
+produced under a great variety of conditions and by at least a
+dozen species of trees, involving, therefore, a complexity of
+problems difficult enough for the careful investigator, and
+entirely beyond the few unsystematic observations of the members
+of a committee on a flying trip through one of the greatest
+timber regions of the world.
+
+"A number of samples were at once collected (part of them
+supplied by the carriage builders' committee), and the fallacy
+of the broad statement mentioned was fully demonstrated by a
+short series of tests and a more extensive study into structure
+and weight of these materials. From these tests it appears that
+pieces of white oak from Arkansas excelled well-selected pieces
+from Connecticut, both in stiffness and endwise compression (the
+two most important forms of resistance)." Report upon the
+forestry investigations of the U.S.D.A. 1877-1898, p. 331. See
+also Rep. of Div. of For., 1890, p. 209.
+
+"In some regions there are many small, stunted hickories, which
+most users will not touch. They have narrow sap, are likely to
+be birdpecked, and show very slow growth. Yet five of these
+trees from a steep, dry south slope in West Virginia had an
+average strength fully equal to that of the pignut from the
+better situation, and were superior in toughness, the work to
+maximum load being 36.8 as against 31.2 for pignut. The trees
+had about twice as many rings per inch as others from better
+situations.
+
+"This, however, is not very significant, as trees of the same
+species, age, and size, growing side by side under the same
+conditions of soil and situation, show great variation in their
+technical value. It is hard to account for this difference, but
+it seems that trees growing in wet or moist situations are
+rather inferior to those growing on fresher soil; also, it is
+claimed by many hickory users that the wood from limestone soils
+is superior to that from sandy soils.
+
+"One of the moot questions among hickory men is the relative
+value of northern and southern hickory. The impression prevails
+that southern hickory is more porous and brash than hickory from
+the north. The tests ... indicate that southern hickory is as
+tough and strong as northern hickory of the same age. But the
+southern hickories have a greater tendency to be shaky, and this
+results in much waste. In trees from southern river bottoms the
+loss through shakes and grub-holes in many cases amounts to as
+much as 50 per cent.
+
+"It is clear, therefore, that the difference in northern and
+southern hickory is not due to geographic location, but rather
+to the character of timber that is being cut. Nearly all of that
+from southern river bottoms and from the Cumberland Mountains is
+from large, old-growth trees; that from the north is from
+younger trees which are grown under more favorable conditions,
+and it is due simply to the greater age of the southern trees
+that hickory from that region is lighter and more brash than
+that from the north." Bul. 80, pp. 52-55.
+
+
+
+SEASON OF CUTTING
+
+
+It is generally believed that winter-felled timber has decided
+advantages over that cut at other seasons of the year, and to
+that cause alone are frequently ascribed much greater
+durability, less liability to check and split, better color, and
+even increased strength and toughness. The conclusion from the
+various experiments made on the subject is that while the time
+of felling may, and often does, affect the properties of wood,
+such result is due to the weather conditions rather than to the
+condition of the wood.
+
+There are two phases of this question. One is concerned with the
+physiological changes which might take place during the year in
+the wood of a living tree. The other deals with the purely
+physical results due to the weather, as differences in
+temperature, humidity, moisture, and other features to be
+mentioned later.
+
+Those who adhere to the first view maintain that wood cut in
+summer is quite different in composition from that cut in
+winter. One opinion is that in summer the "sap is up," while in
+winter it is "down," consequently winter-felled timber is drier.
+A variation of this belief is that in summer the sap contains
+certain chemicals which affect the properties of wood and does
+not contain them in winter. Again it is sometimes asserted that
+wood is actually denser in winter than in summer, as part of the
+wood substance is dissolved out in the spring and used for plant
+food, being restored in the fall.
+
+It is obvious that such views could apply only to sapwood, since
+it alone is in living condition at the time of cutting.
+Heartwood is dead wood and has almost no function in the
+existence of the tree other than the purely mechanical one of
+support. Heartwood does undergo changes, but they are gradual
+and almost entirely independent of the seasons.
+
+Sapwood might reasonably be expected to respond to seasonal
+changes, and to some extent it does. Just beneath the bark there
+is a thin layer of cells which during the growing season have
+not attained their greatest density. With the exception of this
+one annual ring, or portion of one, the density of the wood
+substance of the sapwood is nearly the same the year round.
+Slight variations may occur due to impregnation with sugar and
+starch in the winter and its dissolution in the growing season.
+The time of cutting can have no material effect on the inherent
+strength and other mechanical properties of wood except in the
+outermost annual ring of growth.
+
+The popular belief that sap is up in the spring and summer and
+is down in the winter has not been substantiated by experiment.
+There are seasonal differences in the composition of sap, but so
+far as the amount of sap in a tree is concerned there is fully
+as much, if not more, during the winter than in summer.
+Winter-cut wood is not drier, to begin with, than
+summer-felled--in reality, it is likely to be wetter.[47]
+
+[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.]
+
+The important consideration in regard to this question is the
+series of circumstances attending the handling of the timber
+after it is felled. Wood dries more rapidly in summer than in
+winter, not because there is less moisture at one time than
+another, but because of the higher temperature in summer. This
+greater heat is often accompanied by low humidity, and
+conditions are favorable for the rapid removal of moisture from
+the exposed portions of wood. Wood dries by evaporation, and
+other things being equal, this will proceed much faster in hot
+weather than in cold.
+
+It is a matter of common observation that when wood dries it
+shrinks, and if shrinkage is not uniform in all directions the
+material pulls apart, causing season checks. (See Fig. 27.) If
+evaporation proceeds more rapidly on the outside than inside,
+the greater shrinkage of the outer portions is bound to result
+in many checks, the number and size increasing with the degree
+of inequality of drying.
+
+In cold weather, drying proceeds slowly but uniformly, thus
+allowing the wood elements to adjust themselves with the least
+amount of rupturing. In summer, drying proceeds rapidly and
+irregularly, so that material seasoned at that time is more
+likely to split and check.
+
+There is less danger of sap rot when trees are felled in winter
+because the fungus does not grow in the very cold weather, and
+the lumber has a chance to season to below the danger point
+before the fungus gets a chance to attack it. If the logs in
+each case could be cut into lumber immediately after felling and
+given exactly the same treatment, for example, kiln-dried, no
+difference due to the season of cutting would be noted.
+
+
+
+WATER CONTENT[48]
+
+
+[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.]
+
+Water occurs in living wood in three conditions, namely: (1) in
+the cell walls, (2) in the protoplasmic contents of the cells,
+and (3) as free water in the cell cavities and spaces. In
+heartwood it occurs only in the first and last forms. Wood that
+is thoroughly air-dried retains from 8 to 16 per cent of water
+in the cell walls, and none, or practically none, in the other
+forms. Even oven-dried wood retains a small percentage of
+moisture, but for all except chemical purposes, may be
+considered absolutely dry.
+
+The general effect of the water content upon the wood substance
+is to render it softer and more pliable. A similar effect of
+common observation is in the softening action of water on
+rawhide, paper, or cloth. Within certain limits the greater the
+water content the greater its softening effect.
+
+Drying produces a decided increase in the strength of wood,
+particularly in small specimens. An extreme example is the case
+of a completely dry spruce block two inches in section, which
+will sustain a permanent load four times as great as that which
+a green block of the same size will support.
+
+The greatest increase due to drying is in the ultimate crushing
+strength, and strength at elastic limit in endwise compression;
+these are followed by the modulus of rupture, and stress at
+elastic limit in cross-bending, while the modulus of elasticity
+is least affected. These ratios are shown in Table XV, but it is
+to be noted that they apply only to wood in a much drier
+condition than is used in practice. For air-dry wood the ratios
+are considerably lower, particularly in the case of the ultimate
+strength and the elastic limit. Stiffness (within the elastic
+limit), while following a similar law, is less affected. In the
+case of shear parallel to the grain, the general effect of
+drying is to increase the strength, but this is often offset by
+small splits and checks caused by shrinkage.
+
+|----------------------------------------------------------------------|
+|                               TABLE XV                               |
+|----------------------------------------------------------------------|
+|   EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN    |
+|       RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM        |
+|            THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT)            |
+|                   (Forest Service Bul. 70, p. 89)                    |
+|----------------------------------------------------------------------|
+|      KIND OF STRENGTH      |  Longleaf   |   Spruce    |  Chestnut   |
+|                            |    pine     |             |             |
+|----------------------------+-------------+-------------+-------------|
+|                            |  (1)    (2) |  (1)    (2) |  (1)   (2)  |
+|                            |             |             |             |
+| Crushing strength parallel |             |             |             |
+|   to grain                 | 2.89   2.60 | 3.71   3.41 | 2.83   2.55 |
+| Elastic limit in           |             |             |             |
+|   compression              |             |             |             |
+|   parallel to grain        | 2.60   2.34 | 3.80   3.49 | 2.40   2.26 |
+| Modulus of rupture in      |             |             |             |
+|   bending                  | 2.50   2.20 | 2.81   2.50 | 2.09   1.82 |
+| Stress at elastic limit in |             |             |             |
+|   bending                  | 2.90   2.55 | 2.90   2.58 | 2.30   2.00 |
+| Crushing strength at right |             |             |             |
+|   angles to grain          |             | 2.58   2.48 |             |
+| Shearing strength parallel |             |             |             |
+|   to grain                 | 2.01   1.91 | 2.03   1.95 | 1.55   1.47 |
+| Modulus of elasticity in   |             |             |             |
+|   compression parallel to  |             |             |             |
+|   grain                    | 1.63   1.47 | 2.26   2.08 | 1.43   1.29 |
+| Modulus of elasticity in   |             |             |             |
+| bending                    | 1.59   1.35 | 1.43   1.23 | 1.44   1.21 |
+|----------------------------------------------------------------------|
+| NOTE.--The figures in the first column show the relative increase in |
+| strength between a green specimen and a kiln-dry specimen of equal   |
+| size. The figures in the second column show the relative increase of |
+| strength of the same block after being dried from a green condition  |
+| to 3.5 per cent moisture, correction having been made for shrinkage. |
+| That is, in the first column the strength values per actual unit of  |
+| area are used; in the second the values per unit of area of green    |
+| wood which shrinks to smaller size when dried.                       |
+|                                                                      |
+| See also Cir. 108, Fig. 1, p. 8.                                     |
+|----------------------------------------------------------------------|
+
+The moisture content has a decided bearing also upon the manner
+in which wood fails. In compression tests on very dry specimens
+the entire piece splits suddenly into pieces before any buckling
+takes place (see Fig. 9.), while with wet material the block
+gives way gradually, due to the buckling or bending of the walls
+of the fibres along one or more shearing planes. (See Fig. 14.)
+In bending tests on wet beams, first failure occurs by
+compression on top of the beam, gradually extending downward
+toward the neutral axis. Finally the beam ruptures at the
+bottom. In the case of very dry beams the failure is usually by
+splitting or tension on the under side (see Fig. 17.), without
+compression on the upper, and is often sudden and without
+warning, and even while the load is still increasing. The effect
+varies somewhat with different species, chestnut, for example,
+becoming more brittle upon drying than do ash, hemlock, and
+longleaf pine. The tensile strength of wood is least affected by
+drying, as a rule.
+
+In drying wood no increase in strength results until the free
+water is evaporated and the cell walls begin to dry[49]. This
+critical point has been called the _fibre-saturation point_.
+(See Fig. 24.) Conversely, after the cell walls are saturated
+with water, any increase in the amount of water absorbed merely
+fills the cavities and intercellular spaces, and has no effect
+on the mechanical properties. Hence, soaking green wood does not
+lessen its strength unless the water is heated, whereupon a
+decided weakening results.
+
+[Footnote 49: The wood of _Eucalyptus 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.]
+
+[Illustration: FIG. 24.--Relation of the moisture content to the
+various strength values of spruce. FSP = fibre-saturation
+point.]
+
+The strengthening effects of drying, while very marked in the
+case of small pieces, may be fully offset in structural timbers
+by inherent weakening effects due to the splitting apart of the
+wood elements as a result of irregular shrinkage, and in some
+cases also to the slitting of the cell walls (see Fig. 25).
+Consequently with large timbers in commercial use it is unsafe
+to count upon any greater strength, even after seasoning, than
+that of the green or fresh condition.
+
+[Illustration: FIG. 25.--Cross section of the wood of western
+larch showing fissures in the thick-walled cells of the late
+wood. Highly magnified. _Photo by U. S. Forest Service._]
+
+In green wood the cells are all intimately joined together and
+are at their natural or normal size when saturated with water.
+The cell walls may be considered as made up of little particles
+with water between them. When wood is dried the films of water
+between the particles become thinner and thinner until almost
+entirely gone. As a result the cell walls grow thinner with loss
+of moisture,--in other words, the cell shrinks.
+
+It is at once evident that if drying does not take place
+uniformly throughout an entire piece of timber, the shrinkage as
+a whole cannot be uniform. The process of drying is from the
+outside inward, and if the loss of moisture at the surface is
+met by a steady capillary current of water from the inside, the
+shrinkage, so far as the degree of moisture affected it, would
+be uniform. In the best type of dry kilns this condition is
+approximated by first heating the wood thoroughly in a moist
+atmosphere before allowing drying to begin.
+
+In air-seasoning and in ordinary dry kilns this condition too
+often is not attained, and the result is that a dry shell is
+formed which encloses a moist interior. (See Fig. 26.)
+Subsequent drying out of the inner portion is rendered more
+difficult by this "case-hardened" condition. As the outer part
+dries it is prevented from shrinking by the wet interior, which
+is still at its greatest volume. This outer portion must either
+check open or the fibres become strained in tension. If this
+outer shell dries while the fibres are thus strained they become
+"set" in this condition, and are no longer in tension. Later
+when the inner part dries, it tends to shrink away from the
+hardened outer shell, so that the inner fibres are now strained
+in tension and the outer fibres are in compression. If the
+stress exceeds the cohesion, numerous cracks open up, producing
+a "honey-combed" condition, or "hollow-horning," as it is
+called. If such a case-hardened stick of wood be resawed, the
+two halves will cup from the internal tension and external
+compression, with the concave surface inward.
+
+[Illustration: FIG. 26.--Progress of drying throughout the
+length of a chestnut beam, the black spots indicating the
+presence of free water in the wood. The first section at the
+left was cut one-fourth inch from the end, the next one-half
+inch, the next one inch, and all the others one inch apart. The
+illustration shows case-hardening very clearly. _Photo by U. S.
+Forest Service._]
+
+For a given surface area the loss of water from wood is always
+greater from the ends than from the sides, due to the fact that
+the vessels and other water-carriers are cut across, allowing
+ready entrance of drying air and outlet for the water vapor.
+Water does not flow out of boards and timbers of its own accord,
+but must be evaporated, though it may be forced out of very
+sappy specimens by heat. In drying a log or pole with the bark
+on, most of the water must be evaporated through the ends, but
+in the case of peeled timbers and sawn boards the loss is
+greatest from the surface because the area exposed is so much
+greater.
+
+The more rapid drying of the ends causes local shrinkage, and
+were the material sufficiently plastic the ends would become
+bluntly tapering. The rigidity of the wood substance prevents
+this and the fibres are split apart. Later, as the remainder of
+the stick dries many of the checks will come together, though
+some of the largest will remain and even increase in size as the
+drying proceeds. (See Fig. 27.)
+
+[Illustration: FIG. 27.--Excessive season checking. _Photo by U.
+S. Forest Service._]
+
+A wood cell shrinks very little lengthwise. A dry wood cell is,
+therefore, practically of the same length as it was in a green
+or saturated condition, but is smaller in cross section, has
+thinner walls, and a larger cavity. It is at once evident that
+this fact makes shrinkage more irregular, for wherever cells
+cross each other at a decided angle they will tend to pull apart
+upon drying. This occurs wherever pith rays and wood fibres
+meet. A considerable portion of every wood is made up of these
+rays, which for the most part have their cells lying in a radial
+direction instead of longitudinally. (See Frontispiece.) In
+pine, over 15,000 of these occur on a square inch of a
+tangential section, and even in oak the very large rays which
+are readily visible to the eye as flakes on quarter-sawed
+material represent scarcely one per cent of the number which the
+microscope reveals.
+
+A pith ray shrinks in height and width, that is, vertically and
+tangentially as applied to the position in a standing tree, but
+very little in length or radially. The other elements of the
+wood shrink radially and tangentially, but almost none
+lengthwise or vertically as applied to the tree. Here, then, we
+find the shrinkage of the rays tending to shorten a stick of
+wood, while the other cells resist it, and the tendency of a
+stick to get smaller in circumference is resisted by the endwise
+reaction or thrust of the rays. Only in a tangential direction,
+or around the stick in direction of the annual rings of growth,
+do the two forces coincide. Another factor to the same end is
+that the denser bands of late wood are continuous in a
+tangential direction, while radially they are separated by
+alternate zones of less dense early wood. Consequently the
+shrinkage along the rings (tangential) is fully twice as much as
+toward the centre (radial). (See Table XIV.) This explains why
+some cracks open more and more as drying advances. (See Fig.
+27.)
+
+Although actual shrinkage in length is small, nevertheless the
+tendency of the rays to shorten a stick produces strains which
+are responsible for some of the splitting open of ties, posts,
+and sawed timbers with box heart. At the very centre of a tree
+the wood is light and weak, while farther out it becomes denser
+and stronger. Longitudinal shrinkage is accordingly least at the
+centre and greater toward the outside, tending to become
+greatest in the sapwood. When a round or a box-heart timber
+dries fast it splits radially, and as drying continues the cleft
+widens partly on account of the greater tangential shrinkage and
+also because the greater contraction of the outer fibres warps
+the sections apart. If a small hardwood stem is split while
+green for a short distance at the end and placed where it can
+dry out rapidly, the sections will become bow-shaped with the
+concave sides out. These various facts, taken together, explain
+why, for example, an oak tie, pole, or log may split open its
+entire length if drying proceeds rapidly and far enough. Initial
+stresses in the living trees produce a similar effect when the
+log is sawn into boards. This is especially so in _Eucalyptus
+globulus_ and to a less extent with any rapidly grown wood.
+
+The use of S-shaped thin steel clamps to prevent large checks
+and splits is now a common practice in this country with
+crossties and poles as it has been for a long time in European
+countries. These devices are driven into the butts of the
+timbers so as to cross incipient checks and prevent their
+widening. In place of the regular S-hook another of crimped iron
+has been devised. (See Fig. 28.) Thin straps of iron with one
+tapered edge are run between intermeshing cogs and crimped,
+after which they may be cut off any length desired. The time for
+driving S-irons of either form is when the cracks first appear.
+
+[Illustration: FIG. 28.--Control of season checking by the use
+of S-irons. _Photo by U. S. Forest Service._]
+
+The tendency of logs to split emphasizes the importance of
+converting them into planks or timbers while in a green
+condition. Otherwise the presence of large checks may render
+much lumber worthless which might have been cut out in good
+condition. The loss would not be so great if logs were perfectly
+straight-grained, but this is seldom the case, most trees
+growing more or less spirally or irregularly. Large pieces crack
+more than smaller ones, quartered lumber less than that sawed
+through and through, thin pieces, especially veneers, less than
+thicker boards.
+
+In order to prevent cracks at the ends of boards, small straps
+of wood may be nailed on them or they may be painted. This
+method is usually considered too expensive, except in the case
+of valuable material. Squares used for shuttles, furniture,
+gun-stocks, and tool handles should always be protected at the
+ends. One of the best means is to dip them into melted
+paraffine, which seals the ends and prevents loss of moisture
+there. Another method is to glue paper on the ends. In some
+cases abroad paper is glued on to all the surfaces of valuable
+exotic balks. Other substances sometimes employed for the
+purpose of sealing the wood are grease, carbolineum, wax, clay,
+petroleum, linseed oil, tar, and soluble glass. In place of
+solid beams, built-up material is often preferable, as the
+disastrous results of season checks are thereby largely overcome
+or minimized.
+
+
+
+TEMPERATURE
+
+
+The effect of temperature on wood depends very largely upon the
+moisture content of the wood and the surrounding medium. If
+absolutely dry wood is heated in absolutely dry air the wood
+expands. The extent of this expansion is denoted by a
+coefficient corresponding to the increase in length or other
+dimensions for each degree rise in temperature divided by the
+original length or other dimension of the specimen. The
+coefficient of linear expansion of oak has been found to be
+.00000492; radial expansion, .0000544, or about eleven times the
+longitudinal. Spruce expands less than oak, the ratio of radial
+to longitudinal expansion being about six to one. Metals and
+glass expand equally in all directions, since they are
+homogeneous substances, while wood is a complicated structure.
+The coefficient of expansion of iron is .0000285, or nearly six
+times the coefficient of linear expansion of oak and seven times
+that of spruce[50].
+
+[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev.
+ed.), p. 75.]
+
+Under ordinary conditions wood contains more or less moisture,
+so that the application of heat has a drying effect which is
+accompanied by shrinkage. This shrinkage completely obscures the
+expansion due to the heating.
+
+Experiments made at the Yale Forest School revealed the effect
+of temperature on the crushing strength of wet wood. In the case
+of wet chestnut wood the strength decreases 0.42 per cent for
+each degree the water is heated above 60� F.; in the case of
+spruce the decrease is 0.32 per cent.
+
+The effects of high temperature on wet wood are very marked.
+Boiling produces a condition of great pliability, especially in
+the case of hardwoods. If wood in this condition is bent and
+allowed to dry, it rigidly retains the shape of the bend, though
+its strength may be somewhat reduced. Except in the case of very
+dry wood the effect of cold is to increase the strength and
+stiffness of wood. The freezing of any free water in the pores
+of the wood will augment these conditions.
+
+The effect of steaming upon the strength of cross-ties was
+investigated by the U.S. Forest Service in 1904. The conclusions
+were summarized as follows:
+
+"(1) The steam at pressure up to 40 pounds applied for 4 hours,
+or at a pressure of 20 pounds up to 20 hours, increases the
+weight of ties. At 40 pounds' pressure applied for 4 hours and
+at 20 pounds for 5 hours the wood began to be scorched.
+
+"(2) The steamed and saturated wood, when tested immediately
+after treatment, exhibited weaknesses in proportion to the
+pressure and duration of steaming. (See Table XVI.) If allowed
+to air-dry subsequently the specimens regained the greater part
+of their strength, provided the pressure and duration had not
+exceeded those cited under (1). Subsequent immersion in water of
+the steamed wood and dried specimens showed that they were
+weaker than natural wood similarly dried and resoaked."[51]
+
+[Footnote 51: Cir. 39. Experiments on the strength of treated
+timber, p. 18.]
+
+|------------------------------------------------------------------------------------------|
+|                                        TABLE XVI                                         |
+|------------------------------------------------------------------------------------------|
+|                EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE                 |
+|                                (Forest Service, Cir. 39)                                 |
+|------------------------------------------------------------------------------------------|
+|           |       Cylinder conditions       |                  Strength                  |
+|           |---------------------------------+--------------------------------------------|
+|           |            Steaming             |       Static        |  Impact  |           |
+|           |---------------------------------+---------------------+----------|  Average  |
+| Treatment |        |          |             | Bending  | Compres- |  Height  |  of the   |
+|           |        |          |             | modulus  |  sion    | of drop  |   three   |
+|           | Period | Pressure | Temperature |   of     | parallel | causing  | strengths |
+|           |        |          |             | rupture  | to grain | complete |           |
+|           |        |          |             |          |          | failure  |           |
+|-----------+--------+----------+-------------+----------+----------+----------+-----------|
+|           |        | Lbs. per |             | Per cent | Per cent | Per cent | Per cent  |
+|           |  Hrs.  | sq. inch |     �F.     |          |          |          |           |
+|           |        |          |             |           Untreated wood = 100%            |
+|           |        |          |             |          |          |          |           |
+| Steam,    |    4   |          |     230[a]  |   91.3   |   79.1   |   96.4   |    88.9   |
+| at        |    4   |     10   |     238     |   78.2   |   93.7   |   93.3   |    88.4   |
+| various   |    4   |     20   |     253     |   83.3   |   84.2   |   91.4   |    80.8   |
+| pressures |    4   |     30   |     269     |   80.4   |   78.4   |   89.8   |    82.9   |
+|           |    4   |     40   |     283     |   78.1   |   74.4   |   74.0   |    75.5   |
+|           |    4   |     50   |     292     |   75.8   |   71.5   |   63.9   |    70.4   |
+|           |    4   |    100   |     337     |   41.4   |   65.0   |   55.2   |    53.9   |
+|-----------+--------+----------+-------------+----------+----------+----------+-----------|
+| Steam,    |    1   |     20   |     257     |  100.6   |   98.6   |   86.7   |    95.3   |
+| for       |    2   |     20   |     267     |   88.4   |   93.0   |  107.0   |    96.1   |
+| various   |    3   |     20   |     260     |   90.0   |   93.6   |   84.1   |    89.2   |
+| periods   |    4   |     20   |     253     |   83.3   |   84.2   |   91.4   |    86.3   |
+|           |    5   |     20   |     253     |   85.0   |   78.1   |   84.2   |    82.4   |
+|           |    6   |     20   |     242     |   95.2   |   89.8   |   76.0   |    87.0   |
+|           |   10   |     20   |     255     |   73.7   |   82.0   |   76.0   |    77.2   |
+|           |   20   |     20   |     258     |   67.5   |   65.0   |   99.0   |    77.2   |
+|------------------------------------------------------------------------------------------|
+| [Footnote a: It will be noted that the temperature was 230�. This is the maximum         |
+| temperature by the maximum-temperature recording thermometer, and is due to the handling |
+| of the exhaust valve. The average temperature was that of exhaust steam.]                |
+|------------------------------------------------------------------------------------------|
+
+"(3) A high degree of steaming is injurious to wood in strength
+and spike-holding power. The degree of steaming at which
+pronounced harm results will depend upon the quality of the wood
+and its degree of seasoning, and upon the pressure (temperature)
+of steam and the duration of its application. For loblolly pine
+the limit of safety is certainly 30 pounds for 4 hours, or 20
+pounds for 6 hours."[52]
+
+[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table
+5.]
+
+Experiments made at the Yale Forest School showed that steaming
+above 30 pounds' gauge pressure reduces the strength of wood
+permanently while wet from 25 to 75 per cent.
+
+
+
+PRESERVATIVES
+
+
+The exact effects of chemical impregnation upon the mechanical
+properties of wood have not been fully determined, though they
+have been the subject of considerable investigation.[53] More
+depends upon the method of treatment than upon the preservatives
+used. Thus preliminary steaming at too high pressure or for too
+long a period will materially weaken the wood, (See TEMPERATURE,
+above.)
+
+[Footnote 53: Hatt, W. K.: Experiments on the strength of
+treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]
+
+The presence of zinc chloride does not weaken wood under static
+loading, although the indications are that the wood becomes
+brittle under impact. If the solution is too strong it will
+decompose the wood.
+
+Soaking in creosote oil causes wood to swell, and accordingly
+decreases the strength to some extent, but not nearly so much so
+as soaking in water.[54]
+
+[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by
+the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p.
+7.]
+
+Soaking in kerosene seems to have no significant weakening
+effect.[55]
+
+[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.]
+
+
+
+
+PART III TIMBER TESTING[56]
+
+
+
+[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.]
+
+
+
+WORKING PLAN
+
+
+Preliminary to making a series of timber tests it is very
+important that a working plan be prepared as a guide to the
+investigation. This should embrace: (1) the purpose of the
+tests; (2) kind, size, condition, and amount of material needed;
+(3) full description of the system of marking the pieces; (4)
+details of any special apparatus and methods employed; (5)
+proposed method of analyzing the data obtained and the nature of
+the final report. Great care should be taken in the preparation
+of this plan in order that all problems arising may be
+anticipated so far as possible and delays and unnecessary work
+avoided. A comprehensive study of previous investigations along
+the same or related lines should prove very helpful in outlining
+the work and preparing the report. (For sample working plan see
+Appendix.)
+
+
+
+FORMS OF MATERIAL TESTED
+
+
+In general, four forms of material are tested, namely: (1) large
+timbers, such as bridge stringers, car sills, large beams, and
+other pieces five feet or more in length, of actual sizes and
+grades in common use; (2) built-up structural forms and
+fastenings, such as built-up beams, trusses, and various kind of
+joints; (3) small clear pieces, such as are used in compression,
+shear, cleavage, and small cross-breaking tests; (4)
+manufactured articles, such as axles, spokes, shafts,
+wagon-tongues, cross-arms, insulator pins, barrels, and packing
+boxes.
+
+As the moisture content is of fundamental importance (see WATER
+CONTENT, above.), all standard tests are usually made in the
+green condition. Another series is also usually run in an
+air-dry condition of about 12 per cent moisture. In all cases
+the moisture is very carefully determined and stated with the
+results in the tables.
+
+
+
+SIZE OF TEST SPECIMENS
+
+
+The size of the test specimen must be governed largely by the
+purpose for which the test is made. If the effect of a single
+factor, such as moisture, is the object of experiment, it is
+necessary to use small pieces of wood in order to eliminate so
+far as possible all disturbing factors. If the specimens are too
+large, it is impossible to secure enough perfect pieces from one
+tree to form a series for various tests. Moreover, the drying
+process with large timbers is very difficult and irregular, and
+requires a long period of time, besides causing checks and
+internal stresses which may obscure the results obtained.
+
+On the other hand, the smaller the dimensions of the test
+specimen the greater becomes the relative effect of the inherent
+factors affecting the mechanical properties. For example, the
+effect of a knot of given size is more serious in a small stick
+than in a large one. Moreover, the smaller the specimen the
+fewer growth rings it contains, hence there is greater
+opportunity for variation due to irregularities of grain.
+
+Tests on large timbers are considered necessary to furnish
+designers data on the probable strength of the different sizes
+and grades of timber on the market; their coefficients of
+elasticity under bending (since the stiffness rather than the
+strength often determines the size of a beam); and the manner of
+failure, whether in bending fibre stress or horizontal shear. It
+is believed that this information can only be obtained by direct
+tests on the different grades of car sills, stringers, and other
+material in common use.
+
+When small pieces are selected for test they very often are
+clear and straight-grained, and thus of so much better grade
+than the large sticks that tests upon them may not yield unit
+values applicable to the larger sizes. Extensive experiments
+show, however, (1) that the modulus of elasticity is
+approximately the same for large timbers as for small clear
+specimens cut from them, and (2) that the fibre stress at
+elastic limit for large beams is, except in the weakest timbers,
+practically equal to the crushing strength of small clear pieces
+of the same material.[57]
+
+[Footnote 57: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, pp. 53-54.]
+
+
+
+MOISTURE  DETERMINATION
+
+
+In order for tests to be comparable, it is necessary to know the
+moisture content of the specimens at the zone of failure. This
+is determined from disks an inch thick cut from the timber
+immediately after testing.
+
+In cases, as in large beams, where it is desirable to know not
+only the average moisture content but also its distribution
+through the timber, the disks are cut up so as to obtain an
+outside, a middle, and an inner portion, of approximately equal
+areas. Thus in a section 10" x 12" the outer strip would be one
+inch wide, and the second one a little more than an inch and a
+quarter. Moisture determinations are made for each of the three
+portions separately.
+
+The procedure is as follows:
+
+(1) Immediately after sawing, loose splinters are removed and
+each section is weighed.
+
+(2) The material is put into a drying oven at 100� C. (212� F.)
+and dried until the variation in weight for a period of
+twenty-four hours is less than 0.5 per cent.
+
+(3) The disk is again carefully weighed.
+
+(4) The loss in weight expressed in per cent of the dry weight
+indicates the moisture content of the specimen from which the
+specimen was cut.
+
+
+
+MACHINE FOR STATIC TESTS
+
+
+The standard screw machines used for metal tests are also used
+for wood, but in the case of wood tests the readings must be
+taken "on the fly," and the machine operated at a uniform speed
+without interruption from beginning to end of the test. This is
+on account of the time factor in the strength of wood. (See
+SPEED OF TESTING MACHINE, below.)
+
+The standard machines for static tests can be used for
+transverse bending, compression, tension, shear, and cleavage. A
+common form consists of three main parts, namely: (1) the
+straining mechanism, (2) the weighing apparatus, and (3) the
+machinery for communicating motion to the screws.
+
+The straining mechanism consists of two parts, one of which is a
+movable crosshead operated by four (sometimes two or three)
+upright steel straining screws which pass through openings in
+the platform and bear upward on the bed of the machine upon
+which the weighing platform rests as a fulcrum. At the lower
+ends of these screws are geared nuts all rotated simultaneously
+by a system of gears which cause the movable crosshead to rise
+and fall as desired.
+
+The stationary part of the straining mechanism, which is used
+only for tension and cleavage tests, consists of a steel cage
+above the movable crosshead and rests directly upon the weighing
+platform. The top of the cage contains a square hole into which
+one end of the test specimen may be clamped, the crosshead
+containing a similar clamp for the other end, in making tension
+tests.
+
+For testing long beams a special form of machine with an
+extended platform is used. (See Fig. 29.)
+
+The weighing platform rests upon knife edges carried by primary
+levers of the weighing apparatus, the fulcrum being on the bed
+of the machine, and any pressure upon it is directly transmitted
+through a series of levers to the weighing beam. This beam is
+adjusted by means of a poise running on a screw. In operation
+the beam is kept floating by means of another poise moved back
+and forth by a screw which is operated by a hand wheel or
+automatically. The larger units of stress are read from the
+graduations along the side of the beam, while the intermediate
+smaller weights are observed on the dial on the rear end of the
+beam.
+
+The machine is driven by power from a shaft or a motor and is so
+geared that various speeds are obtainable. One man can operate
+it.
+
+In making tests the operation of the straining screws is always
+downward so as to bring pressure to bear upon the weighing
+platform. For tests in tension and cleavage the specimen is
+placed between the top of the stationary cage and the movable
+head and subjected to a pull. For tests in transverse bending,
+compression, and cleavage the specimen is placed between the
+movable head and the platform, and a direct compression force
+applied.
+
+Testing machines are usually calibrated to a portion of their
+capacity before leaving the factory. The delicacy of the
+weighing levers is verified by determining the number of pounds
+necessary to move the beam between the stops while a load of
+1,000 pounds rests on the platform. The usual requirement is
+that ten pounds should accomplish this movement.
+
+The size of machine suitable for compression tests on 2" X 2"
+sticks or for 2" X 2" beams with 26 to 36-inch span has a
+capacity of 30,000 pounds.
+
+
+
+SPEED OF TESTING MACHINE
+
+
+In instructions for making static tests the rate of application
+of the stress, _i.e._, the speed of the machine, is given
+because the strength of wood varies with the speed at which the
+fibres are strained. The speed of the crosshead of the testing
+machine is practically never constant, due to mechanical defects
+of the apparatus and variations in the speed of the motor, but
+so long as it does not exceed 25 per cent the results will not
+be appreciably affected. In fact, a change in speed of 50 per
+cent will not cause the strength of the wood to vary more than 2
+per cent.[58]
+
+[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.]
+
+Following are the formul� used in determining the speed of the
+movable head of the machine in inches per minute (n):
+
+(1) For endwise compression          n = Z l
+
+                                          Z l^{2}
+(2) For beams (centre loading)       n = ---------
+                                            6h
+
+                                          Z l^{2}
+(3) For beams (third-pointloading)   n = ---------
+                                           5.4h
+
+  Z = rate of fibre strain per inch of fibre length.
+  l = span of beam or length of compression specimen.
+  h = height of beam.
+
+The values commonly used for Z are as follows:
+
+  Bending large beams                      Z = 0.0007
+  Bending small beams                      Z = 0.0015
+  Endwise compression-large specimens      Z = 0.0015
+  Endwise compression-small       "        Z = 0.003
+  Right-angled compression-large  "        Z = 0.007
+  Right-angled compression-small  "        Z = 0.015
+  Shearing parallel to the grain           Z = 0.015
+
+Example: At what speed should the crosshead move to give the
+required rate of fibre strain in testing a small beam 2" X 2" X
+30". (Span = 28".) Substituting these values in equation (2)
+above:
+          (0.0015 X 28^2)
+     n = ----------------- = 0.1 inch per minute.
+              (6 X 2)
+
+In order that tests may be intelligently compared, it is
+important that account be taken of the speed at which the stress
+was applied. In determining the basis for a ratio between time
+and strength the rate of strain, which is controllable, and not
+the ratio of stress, which is circumstantial, should be used. In
+other words, the rate at which the movable head of the testing
+machine descends and not the rate of increase in the load is to
+be regulated. This ratio, to which the name _speed-strength
+modulus_ has been given, may be expressed as a coefficient
+which, if multiplied into any proportional change in speed, will
+give the proportional change in strength. This ratio is derived
+from empirical curves. (See Table XVII.)
+
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XVII                                                                                TABLE XVII                                                                                            |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                            SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, _loc. cit._)                              |
+|                                                                            (Values in parentheses are approximate)                                                                              |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Rate of fibre strain.   |                       |                       |                       |                       |                       |                       |                       |
+|   Ten-thousandths inch  |          2/3          |            2          |            6          |           18          |           54          |          162          |          486          |
+|   per minute per inch   |                       |                       |                       |                       |                       |                       |                       |
+|-------------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| C | Speed of crosshead. |                       |                       |                       |                       |                       |                       |                       |
+| O |   Inches per minute |       0.000383        |         0.00115       |         0.00345       |         0.0103        |         0.0310        |         0.0931        |         .279          |
+| M |                     |                       |                       |                       |                       |                       |                       |                       |
+| P |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| R | Specimens           |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |
+| E |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
+| S | Relative            |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| S |   crushing          |                       | 100.0 | 100.0 | 100.0 | 103.4 | 100.8 | 101.5 | 107.5 | 102.7 | 103.8 | 113.9 | 105.5 | 107.9 | 121.3 | 108.3 | 116.4 | 128.8 | 110.0 |118.9  |
+| I |   strength          |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| O |                     |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| N | Speed-strength      |                       | 0.017 |(0.006)|(0.009)| 0.033 | 0.012 | 0.016 | 0.047 | 0.021 | 0.029 | 0.053 | 0.027 | 0.039 | 0.060 | 0.023 | 0.049 |(0.052)|(0.015)|(0.040)|
+|   |   modulus, _T_      |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|---+---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+|   | Speed of crosshead. |                       |                       |                       |                       |                       |                       |                       |
+|   |   Inches per minute |        0.0072         |         0.0216        |         0.0648        |         0.194         |         0.583         |         1.75          |         5.25          |
+| B |                     |                       |                       |                       |                       |                       |                       |                       |
+| E |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| N | Specimens           |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |
+| D |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
+| I | Relative            |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| N |   crushing          |  97.4 |  99.0 |  98.2 | 100.0 | 100.0 | 100.0 | 105.1 | 102.1 | 103.7 | 111.3 | 105.8 | 108.1 | 117.9 | 108.6 | 112.7 | 123.7 | 109.6 | 116.3 | 126.3 | 110.3 | 118.9 |
+| G |   strength          |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|   |                     |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|   | Speed-strength      |(0.014)|(0.005)| 0.012 | 0.033 | 0.014 | 0.026 | 0.049 | 0.026 | 0.037 | 0.053 | 0.033 | 0.038 | 0.049 | 0.014 | 0.035 | 0.038 | 0.006 | 0.025 |(0.023)|(0.004)|(0.014)|
+|   |   modulus, _T_      |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| NOTE.--The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain                                                                                           |
+| of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively.                                                                                                     |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+
+
+
+BENDING LARGE BEAMS
+
+
+_Apparatus_: A static bending machine (described above), with a
+special crosshead for third-point loading and a long platform
+bearing knife-edge supports, is required. (See Fig. 29.)
+
+[Illustration: FIG. 29.--Static bending test on large beam. Note
+arrangement of wire and scale for measuring deflection; also
+method of applying load at "third-points."]
+
+_Preparing the material_: Standard sizes and grades of beams and
+timbers in common use are employed. The ends are roughly squared
+and the specimen weighed and measured, taking the
+cross-sectional dimensions midway of the length. Weights should
+be to the nearest pound, lengths to the nearest 0.1 inch, and
+cross-sectional dimensions to the nearest 0.01 inch.
+
+_Marking and sketching_: The butt end of the beam is marked _A_
+and the top end _B_. While facing _A_, the top side is marked
+_a_, the right hand _b_, the bottom _c_, the left hand _d_.
+Sketches are made of each side and end, showing (1) size,
+location, and condition of knots, checks, splits, and other
+defects; (2) irregularities of grain; (3) distribution of
+heartwood and sapwood; and on the ends: (4) the location of the
+pith and the arrangement of the growth rings, (5) number of
+rings per inch, and (6) the proportion of late wood.
+
+The number of rings per inch and the proportion of late wood
+should always be determined along a radius or a line normal to
+the rings. The average number of rings per inch is the total
+number of rings divided by the length of the line crossing them.
+The proportion of late wood is equal to the sum of the widths of
+the late wood crossed by the line, divided by the length of the
+line. Rings per inch should be to the nearest 0.1; late wood to
+the nearest 0.1 per cent.
+
+Since in large beams a great variation in rate of growth and
+relative amount of late wood is likely in different parts of the
+section, it is advisable to consider the cross section in three
+volumes, namely, the upper and lower quarters and the middle
+half. The determination should be made upon each volume
+separately, and the average for the entire cross section
+obtained from these results.
+
+At the conclusion of the test the failure, as it appears on each
+surface, is traced on the sketches, with the failures numbered
+in the order of their occurrence. If the beam is subsequently
+cut up and used for other tests an additional sketch may be
+desirable to show the location of each piece.
+
+_Adjusting specimen in machine_: The beam is placed in the
+machine with the side marked _a_ on top, and with the ends
+projecting equally beyond the supports. In order to prevent
+crushing of the fibre at the points where the stress is applied
+it is necessary to use bearing blocks of maple or other hard
+wood with a convex surface in contact with the beam. Roller
+bearings should be placed between the bearing blocks and the
+knife edges of the crosshead to allow for the shortening due to
+flexure. (See Fig. 29.) Third-point loading is used, that is,
+the load is applied at two points one-third the span of the beam
+apart. (See Fig. 30.) This affords a uniform bending moment
+throughout the central third of the beam.
+
+[Illustration: FIG. 30.--Two methods of loading a beam, namely,
+third-point loading (upper), and centre loading (lower).]
+
+_Measuring the deflection_: The method of measuring the
+deflection should be such that any compression at the points of
+support or at the application of the load will not affect the
+reading. This may be accomplished by driving a small nail near
+each end of the beam, the exact location being on the neutral
+plane and vertically above each knife-edge support. Between
+these nails a fine wire is stretched free of the beam and kept
+taut by means of a rubber band or coiled spring on one end.
+Behind the wire at a point on the beam midway between the
+supports a steel scale graduated to hundredths of an inch is
+fastened vertically by means of thumb-tacks or small screws
+passing through holes in it. Attachment should be made on the
+neutral plane.
+
+The first reading is made when the scale beam is balanced at
+zero load, and afterward at regular increments of the load which
+is applied continuously and at a uniform speed. (See SPEED OF
+TESTING MACHINE, above.) If desired, however, the load may be
+read at regular increments of deflection. The deflection
+readings should be to the nearest 0.01 inch. To avoid error due
+to parallax, the readings may be taken by means of a reading
+telescope about ten feet distant and approximately on a level
+with the wire. A mirror fastened to the scale will increase the
+accuracy of the readings if the telescope is not used. As in all
+tests on timber, the strain must be continuous to rupture, not
+intermittent, and readings must be taken "on the fly." The
+weighing beam is kept balanced after the yield point is reached
+and the maximum load, and at least one point beyond it, noted.
+
+_Log of the test_: The proper log sheet for this test consists
+of a piece of cross-section paper with space at the margin for
+notes. (See Fig. 32.) The load in some convenient unit (1,000 to
+10,000 pounds, depending upon the dimensions of the specimen) is
+entered on the ordinates, the deflection in tenths of an inch on
+the absciss�. The increments of load should be chosen so as to
+furnish about ten points on the stress-strain diagram below the
+elastic limit.
+
+As the readings of the wire on the scale are made they are
+entered directly in their proper place on the cross-section
+paper. In many cases a test should be continued until complete
+failure results. The points where the various failures occur are
+indicated on the stress-strain diagram. A brief description of
+the failure is made on the margin of the log sheet, and the form
+traced on the sketches.
+
+_Disposal of the specimen_: Two one-inch sections are cut from
+the region of failure to be used in determining the moisture
+content. (See MOISTURE DETERMINATION, above.) A two-inch section
+may be cut for subsequent reference and identification, and
+possible microscopic study. The remainder of the beam may be cut
+into small beams and compression pieces.
+
+_Calculating the results_: The formul� used in calculating the
+results of tests on large rectangular simple beams loaded at
+third points of the span are as follows:
+
+              0.75 P
+     (1) J = --------
+               b h
+
+              l (P_{1} + 0.75 W)
+     (2) r = --------------------
+                   b h^{2}
+
+              l (P + 0.75 W)
+     (3) R = ----------------
+                 b h^{2}
+
+               P_{1} l^{3}
+     (4) E = ---------------
+              4.7 D b h^{3}
+
+              0.87 P_{1} D
+     (5) S = --------------
+                  2 V
+
+  b, h, l = breadth, height, and span of specimen, inches.
+  D = total deflection at elastic limit, inches.
+  P = maximum load, pounds.
+  P_{1} = load at elastic limit, pounds.
+  E = modulus of elasticity, pounds per square inch.
+  r = fibre stress at elastic limit, pounds per sq. inch.
+  R = modulus of rupture, pounds per square inch.
+  S = elastic resilience or work to elastic limit, inch-pounds
+        per cu. in.
+  J = greatest calculated longitudinal shear, pounds per square
+        inch.
+  V = volume of beam, cubic inches.
+  W = weight of the beam.
+
+In large beams the weight should be taken into account in
+calculating the fibre stress. In (2) and (3) three-fourths of
+the weight of the beam is added to the load for this reason.
+
+
+
+BENDING SMALL BEAMS
+
+
+_Apparatus_: An ordinary static bending machine, a steel I-beam
+bearing two adjustable knife-edge supports to rest on the
+platform, and a special deflectometer, are required. (See Fig.
+31.)
+
+[Illustration: FIG. 31.--Static bending test on small beam. Note
+the use of the deflectometer with indicator and dial for
+measuring the deflection; also roller bearings between beam and
+supports.]
+
+_Preparing the material_: The specimens may be of any convenient
+size, though beams 2" X 2" X 30" tested over a 28-inch span, are
+considered best. The beams are surfaced on all four sides, care
+being taken that they are not damaged by the rollers of the
+surfacing machine. Material for these tests is sometimes cut
+from large beams after failure. The specimens are carefully
+weighed in grams, and all dimensions measured to the nearest
+0.01 inch. If to be tested in a green or fresh condition the
+specimens should be kept in a damp box or covered with moist
+sawdust until needed. No defects should be allowed in these
+specimens.
+
+_Marking and sketching_: Sketches are made of each end of the
+specimen to show the character of the growth, and after testing,
+the manner of failure is shown for all four sides. In obtaining
+data regarding the rate of growth and the proportion of late
+wood the same procedure is followed as with large beams.
+
+_Adjusting specimen in machine_: The beam should be correctly
+centred in the machine and each end should have a plate with
+roller bearings between it and the support. Centre loading is
+used. Between the movable head of the machine and the specimen
+is placed a bearing block of maple or other hard wood, the lower
+surface of which is curved in a direction along the beam, the
+curvature of which should be slightly less than that of the beam
+at rupture, in order to prevent the edges from crushing into the
+fibres of the test piece.
+
+_Measuring the deflection_: The method of measuring deflection
+of large beams can be used for small sizes, but because of the
+shortness of the span and consequent slight deformation in the
+latter, it is hardly accurate enough for good work. The special
+deflectometer shown in Fig. 31 allows closer reading, as it
+magnifies the deflection ten times. It rests on two small nails
+driven in the beam on the neutral plane and vertically above the
+supports. The fine wire on the wheel at the base of the
+indicator is attached to another small nail driven in the beam
+on the neutral plane midway between the end nails. All three
+nails should be in place before the beam is put into the
+machine. The indicator is adjustable by means of a thumb-screw
+at the base and is set at zero before the load is applied.
+Deflections are read to the nearest 0.001 inch. For rate of
+application of load see SPEED OF TESTING MACHINE, above. The
+speed should be uniform from start to finish without stopping.
+Readings must be made "on the fly."
+
+_Log of the test_: The log sheets used for small beams (see Fig.
+32) are the same as for large sizes and the procedure is
+practically identical. The stress-strain diagram is continued to
+or beyond the maximum load, and in a portion of the tests should
+be continued to six-inch deflection or until the specimen fails
+to support a load of 200 pounds. Deflection readings for equal
+increments of load are taken until well beyond the elastic
+limit, after which the scale beam is kept balanced and the load
+read for each 0.1 inch deflection. The load and deflection at
+first failure, the maximum load, and any points of sudden change
+should be shown on the diagram, even though they do not occur at
+one of the regular points. A brief description of the failure
+and the nature of any defects is entered on the log sheet.
+
+[Illustration: FIG. 32.--Sample log sheet, giving full details
+of a transverse bending test on a small pine beam.]
+
+_Calculating the results_: The formul� used in calculating the
+results of tests on small rectangular simple beams are as
+follows:
+
+              0.75 P
+     (1) J = --------
+               b h
+
+              1.5 P_{1} l
+     (2) r = -------------
+                b h^{2}
+
+              1.5 P l
+     (3) R = ---------
+              b h^{2}
+
+              P_{1} l^{3}
+     (4) E = -------------
+              4 D b h^{3}
+
+              P_{1} D
+     (5) S = ---------
+                2 V
+
+The same legend is used as in BENDING LARGE BEAMS. The weight of
+the beam itself is disregarded.
+
+
+
+ENDWISE COMPRESSION
+
+
+_Apparatus_: An ordinary static testing machine and a
+compressometer are required. (See Fig. 33.)
+
+[Illustration: FIG. 33.--Endwise compression test, showing
+method of measuring the deformation by means of a
+compressometer.]
+
+_Preparing the material_: Two classes of specimens are commonly
+used, namely, (1) posts 24 inches in length, and (2) small clear
+blocks approximately 2" X 2" X 8". The specimens are surfaced on
+all four sides and both ends squared smoothly and evenly. They
+are carefully weighed, measured, rate of growth and proportion
+of late wood determined, as in bending tests. After the test a
+moisture section is cut and weighed. Ordinarily these specimens
+should be free from defects.
+
+_Sketching_: Sketches are made of each end of the specimens to
+show the character of the growth. After testing, the manner of
+failure is shown for all four sides, and the various parts of
+the failure are numbered in the order of their occurrence.
+
+_Adjusting specimen in machine_: The compressometer collars are
+adjusted, the distance between them being 20 inches for the
+posts and 6 inches for the blocks. If the two ends of the blocks
+are not exactly parallel a ball-and-socket block can be placed
+between the upper end of the specimen and the movable head of
+the machine to overcome the irregularity. If the blocks are true
+they can simply be stood on end upon the platform and the
+movable head allowed to press directly upon the upper end.
+
+_Measuring the deformation_: The deformation is measured by a
+compressometer. (See Fig. 33.) The latter registers to 0.001
+inch. In the case of posts the compression between the collars
+is communicated to the four points on the arms by means of brass
+rods; with short blocks, as in Fig. 33, the points of the arms
+are in direct contact with the collars. The operator lowers the
+fulcrum of the apparatus by moving the micrometer screws at such
+a rate that the set-screw in the rear end of the upper lever is
+kept barely touching the fixed arm below it, being guided by a
+bell operated by electric contact.
+
+_Log of the test_: The load is applied continuously at a uniform
+rate of speed. (See SPEED OF TESTING MACHINE, above.) Readings
+are taken from the scale of the compressometer at regular
+increments of either load or compression. The stress-strain
+diagram is continued to at least one deformation point beyond
+the maximum load, and in event of sudden failure, the direction
+of the curve beyond the maximum point is indicated. A brief
+description of the failure is entered on the log sheet. (See
+Fig. 34.)
+
+[Illustration: FIG. 34.--Sample log sheet of an endwise
+compression test on a short pine column.]
+
+In short specimens the failure usually occurs in one or several
+planes diagonal to the axis of the specimen. If the ends are
+more moist than the middle a crushing may occur on the extreme
+ends in a horizontal plane. Such a test is not valid and should
+always be culled. If the grain is diagonal or the stress is
+unevenly applied a diagonal shear may occur from top to bottom
+of the test specimen. Such tests are also invalid and should be
+culled. When the plane (or several planes) of failure occurs
+through the body of the specimen the test is valid. It may
+sometimes be advantageous to allow the extreme ends to dry
+slightly before testing in order to bring the planes of failure
+within the body. This is a perfectly legitimate procedure
+provided no drying is allowed from the sides of the specimen,
+and the moisture disk is cut from the region of failure.
+
+_Calculating the results:_ The formul� used in calculating the
+results of tests on endwise compression are as follows:
+
+              P
+     (1) C = -----
+              A
+
+              P_{1}
+     (2) c = -------
+                A
+
+              P_{1} l
+     (3) E = ---------
+                A D
+
+              P D
+     (4) S = -----
+              2 V
+
+  C = crushing strength, pounds per square inch.
+  c = fibre strength at elastic limit, pounds per square inch.
+  A = area of cross section, square inches.
+  l = distance between centres of collars, inches.
+  D = total shortening at elastic limit, inches.
+  V = volume of specimen, cubic inches.
+
+Remainder of legend as in BENDING LARGE BEAMS, above.
+
+
+
+COMPRESSION ACROSS THE GRAIN
+
+
+_Apparatus_: An ordinary static testing machine, a bearing
+plate, and a deflectometer are required. (See Fig. 35.)
+
+[Illustration: FIG. 35.--Compression across the grain. Note
+method of measuring the deformation by means of a
+deflectomoter.]
+
+_Preparing the material_: Two classes of specimens are used,
+namely, (1) sections of commercial sizes of ties, beams, and
+other timbers, and (2) small, clear specimens with the length
+several times the width. Sometimes small cubes are tested, but
+the results are hardly applicable to conditions in practice. In
+(2) the sides are surfaced and the ends squared. The specimens
+are then carefully measured and weighed, defects noted, rate of
+growth and proportion of late wood determined, as in bending
+tests. (See BENDING LARGE BEAMS, above.) After the test a
+moisture section is cut and weighed.
+
+_Sketching_: Sketches are made as in endwise compression tests.
+(See ENDWISE COMPRESSION, above.)
+
+_Adjusting specimen in machine_: The specimen is laid
+horizontally upon the platform of the machine and a steel
+bearing plate placed on its upper surface immediately beneath
+the centre of the movable head. For the larger specimens this
+plate is six inches wide; for the smaller sizes, two inches
+wide. The plate in all cases projects over the edges of the test
+piece, and in no case should the length of the latter be less
+than four times the width of the plate.
+
+_Measuring the deformation_: The compression is measured by
+means of a deflectometer (see Fig. 35), which, after the first
+increment of load is applied, is adjusted (by means of a small
+set screw) to read zero. The actual downward motion of the
+movable head (corresponding to the compression of the specimen)
+is multiplied ten times on the scale from which the readings are
+made.
+
+_Log of the test_: The load is applied continuously and at
+uniform speed (see SPEED OF TESTING MACHINE, above), until well
+beyond the elastic limit. The compression readings are taken at
+regular load increments and entered on the cross-section paper
+in the usual way. Usually there is no real maximum load in this
+case, as the strength continually increases as the fibres are
+crushed more compactly together.
+
+_Calculating the results_: Ordinarily only the fibre stress at
+the elastic limit (c) is computed. It is equal to the load at
+elastic limit (P_{1}) divided by the area under the plate (B).
+{      P_{1}  }
+{ c = ------- }
+{        B    }
+
+
+
+SHEAR ALONG THE GRAIN
+
+
+_Apparatus_: An ordinary static testing machine and a special
+tool designed for producing single shear are required. (See
+Figs. 36 and 37.) This shearing apparatus consists of a solid
+steel frame with set screws for clamping the block within it
+firmly in a vertical position. In the centre of the frame is a
+vertical slot in which a square-edged steel plate slides freely.
+When the testing block is in position, this plate impinges
+squarely along the upper surface of the tenon or lip, which, as
+vertical pressure is applied, shears off.
+
+[Illustration: Fig. 36.--Vertical section of shearing tool.]
+
+[Illustration: FIG. 37.--Front view of shearing tool with test
+specimen and steel plate in position for testing.]
+
+_Preparing the material_: The specimens are usually in the form
+of small, clear, straight-grained blocks with a projecting tenon
+or lip to be sheared off. Two common forms and sizes are shown
+in Figure 38. Part of the blocks are cut so that the shearing
+surface is parallel to the growth rings, or tangential; others
+at right angles to the growth rings, or radial. It is important
+that the upper surface of the tenon or lip be sawed exactly
+parallel to the base of the block. When the form with a tenon is
+used the under cut is extended a short distance horizontally
+into the block to prevent any compression from below.
+
+[Illustration: FIG. 38.--Two forms of shear test specimens.]
+
+In designing a shearing specimen it is necessary to take into
+consideration the proportions of the area of shear, since, if
+the length of the portion to be sheared off is too great in the
+direction of the shearing face, failure would occur by
+compression before the piece would shear. Inasmuch as the
+endwise compressive strength is sometimes not more than five
+times the shearing strength, the shearing surface should be less
+than five times the surface to which the load is applied. This
+condition is fulfilled in the specimens illustrated.
+
+Shearing specimens are frequently cut from beams after testing.
+In this case the specific gravity (dry), proportion of late
+wood, and rate of growth are assumed to be the same as already
+recorded for the beams. In specimens not so taken, these
+quantities are determined in the usual way. The sheared-off
+portion is used for a moisture section.
+
+_Adjusting specimen in machine_: The test specimen is placed in
+the shearing apparatus with the tenon or lip under the sliding
+plate, which is centred under the movable head of the machine.
+(See Fig. 39.) In order to reduce to a minimum the friction due
+to the lateral pressure of the plate against the bearings of the
+slot, the apparatus is sometimes placed upon several parallel
+steel rods to form a roller base. A slight initial load is
+applied to take up the lost motion of the machinery, and the
+beam balanced.
+
+[Illustration: FIG. 39.--Making a shearing test.]
+
+_Log of the test_: The load is applied continuously and at a
+uniform rate until failure, but no deformations are measured.
+The points noted are the maximum load and the length of time
+required to reach it. Sketches are made of the failure. If the
+failure is not pure shear the test is culled.
+
+The shearing strength per square inch is found by dividing the
+                                          {      P  }
+maximum load by the cross-sectional area. { Q = --- }
+                                          {      A  }
+
+
+
+IMPACT TEST
+
+
+_Apparatus_: There are several types of impact testing
+machines.[59] One of the simplest and most efficient for use
+with wood is illustrated in Figure 40. The base of the machine
+is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500
+pounds. Two upright columns, each 8 feet long, act as guides for
+the striking head. At the top of the column is the hoisting
+mechanism for raising or lowering the striking weights. The
+power for operating the machine is furnished by a motor set on
+the top. The hoisting-mechanism is all controlled by a single
+operating lever, shown on the side of the column, whereby the
+striking weight may be raised, lowered, or stopped at the will
+of the operator. There is an automatic safety device for
+stopping the machine when the weight reaches the top.
+
+[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.]
+
+[Illustration: FIG. 40.--Impact testing machine.]
+
+The weight is lifted by a chain, one end of which passes over a
+sprocket wheel in the hoisting mechanism. On the lower end of
+the chain is hung an electro-magnet of sufficient magnetic
+strength to support the heaviest striking weights. When it is
+desired to drop the striking weight the electric current is
+broken and reversed by means of an automatic switch and current
+breaker. The height of drop may be regulated by setting at the
+desired height on one of the columns a tripping pin which throws
+the switch on the magnet and so breaks and reverses the current.
+
+There are four striking weights, weighing respectively 50, 100,
+250, and 500 pounds, any one of which may be used, depending
+upon the desired energy of blow. When used for compression tests
+a flat steel head six inches in diameter is screwed into the
+lower end of the weight. For transverse tests, a well-rounded
+knife edge is screwed into the weight in place of the flat head.
+Knife edges for supporting the ends of the specimen to be
+tested, are securely bolted to the base of the machine.
+
+The record of the behavior of the specimen at time of impact is
+traced upon a revolving drum by a pencil fixed in the striking
+head. (See Fig. 41.) When a drop is made the pencil comes in
+contact with the drum and is held in place by a spring. The drum
+is revolved very slowly, either automatically or by hand. The
+speed of the drum can be recorded by a pencil in the end of a
+tuning fork which gives a known number of vibrations per second.
+
+[Illustration: FIG. 41.--Drum record of impact bending test.]
+
+One size of this machine will handle specimens for transverse
+tests 9 inches wide and 6-foot span; the other, 12 inches wide
+and 8-foot span. For compression tests a free fall of about 6.5
+feet may be obtained. For transverse tests the fall is a little
+less, depending upon the size of the specimen.
+
+The machine is calibrated by dropping the hammer upon a copper
+cylinder. The axial compression of the plug is noted. The energy
+used in static tests to produce this axial compression under
+stress in a like piece of metal is determined. The external
+energy of the blow (_i.e._, the weight of the hammer X the
+height of drop) is compared with the energy used in static tests
+at equal amounts of compression. For instance:
+
+Energy delivered, impact test            35,000 inch-pounds
+Energy computed from static test        .26,400   "    "
+Efficiency of blow of hammer            .75.3 per cent.
+
+_Preparing the material_: The material used in making impact
+tests is of the same size and prepared in the same way as for
+static bending and compression tests. Bending in impact tests is
+more commonly used than compression, and small beams with
+28-inch span are usually employed.
+
+_Method_: In making an impact bending test the hammer is allowed
+to rest upon the specimen and a zero or datum line is drawn. The
+hammer is then dropped from increasing heights and drum records
+taken until first failure. The first drop is one inch and the
+increase is by increments of one inch until a height of ten
+inches is reached, after which increments of two inches are used
+until complete failure occurs or 6-inch deflection is secured.
+
+The 50-pound hammer is used when with drops up to 68 inches it
+is reasonably certain it will produce complete failure or 6-inch
+deflection in the case of all specimens of a species; for all
+other species a 100-pound hammer is used.
+
+_Results_: The tracing on the drum (see Fig. 41) represents the
+actual deflection of the stick and the subsequent rebounds for
+each drop. The distance from the lowest point in each case to
+the datum line is measured and its square in tenths of a square
+inch entered as an abscissa on cross-section paper, with the
+height of drop in inches as the ordinate. The elastic limit is
+that point on the diagram where the square of the deflection
+begins to increase more rapidly than the height of drop. The
+difference between the datum line and the final resting point
+after each drop represents the set the material has received.
+
+The formul� used in calculating the results of impact tests in
+bending when the load is applied at the centre up to the elastic
+limit are as follows:
+
+               3 W H l
+     (1) r = -----------
+              D b h^{2}
+
+              F S l^{2}
+     (2) E = -----------
+                6 D h
+
+               W H
+     (3) S = -------
+              l b h
+
+  H = height of drop of hammer, including deflection, inches.
+  S = modulus of elastic resilience, inch-pounds per cubic inch.
+  W = weight of hammer, pounds.
+
+Remainder of legend as in BENDING LARGE BEAMS, above.
+
+
+
+HARDNESS TEST: ABRASION AND INDENTATION
+
+
+_Abrasion_: The machine used by the U.S. Forest Service is a
+modified form of the Dorry abrasion machine. (See Fig. 42.) Upon
+the revolving horizontal disk is glued a commercial sandpaper,
+known as garnet paper, which is commonly employed in factories
+in finishing wood.
+
+[Illustration: FIG. 42.--Abrasion machine for testing the
+wearing qualities of woods.]
+
+A small block of the wood to be tested is fixed in one clamp and
+a similar block of some wood chosen as a standard, as sugar
+maple, at 10 per cent moisture, in the opposite, and held
+against the same zone of sandpaper by a weight of 26 pounds
+each. The size of the section under abrasion for each specimen
+is 2" X 2". The conditions for wear are the same for both
+specimens. The speed of rotation is 68 revolutions a minute.
+
+The test is continued until the standard specimen is worn a
+specified amount, which varies with the kind of wood under test.
+A comparison of the wear of the two blocks affords a fair idea
+of their relative resistance to abrasion.
+
+Another method makes use of a sand blast to abrade the woods and
+is the one employed in New South Wales.[60] The apparatus
+consists essentially of a nozzle through which sand can be
+propelled at a high velocity against the test specimen by means
+of a steam jet.
+
+[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.]
+
+The wood to be tested is cut into blocks 3" X 3" X 1', and these
+are weighed to the nearest grain just before placing in the
+apparatus. Steam from the boiler at a pressure of about 43
+pounds per square inch is ejected from a nozzle in such a way
+that particles of fine quartz sand are caught up and thrown
+violently against the block which is being rotated. Only
+superheated steam strikes the block, thus leaving the wood dry.
+The test is continued for two minutes, after which the specimen
+is removed and immediately weighed.
+
+By comparison with the original weight the loss from abrasion is
+determined, and by comparison with a certain wood chosen as a
+standard, a coefficient of wear-resistance can be obtained. The
+amount of wear will vary more or less according to the surface
+exposed, and in these tests quarter-sawed material was used with
+the edge grain to the blast.
+
+_Indentation_: The tool used for this test consists of a punch
+with a hemispherical end or steel ball having a diameter of
+0.444 inch, giving a surface area of one-fourth square inch. It
+is fitted with a guard plate, which works loosely until the
+penetration has progressed to a depth of 0.222 inch, whereupon
+it tightens. (See Fig. 43.) The effect is that of sinking a ball
+half its diameter into the specimen. This apparatus is fitted
+into the movable head of the static testing machine.
+
+[Illustration: FIG. 43.--Design of tool for testing the hardness
+of woods by indentation.]
+
+The wood to be tested is cut square with the grain into
+rectangular blocks measuring 2" X 2" X 6". A block is placed on
+the platform and the end of the punch forced into the wood at
+the rate of 0.25 inch per minute. The operator keeps moving the
+small handle of the guard plate back and forth until it
+tightens. At this instant the load is read and recorded.
+
+Two penetrations each are made on the tangential and radial
+surfaces, and one on each end of every specimen tested.
+
+In choosing the places on the block for the indentations, effort
+should be made to get a fair average of heartwood and sapwood,
+fine and coarse grain, early and late wood.
+
+Another method of testing by indentation involves the use of a
+right-angled cone instead of a ball. For details of this test as
+used in New South Wales see _loc. cit._, pp. 86-87.
+
+
+
+CLEAVAGE TEST
+
+
+A static testing machine and a special cleavage testing device
+are required. (See Fig. 44.) The latter consists essentially of
+two hooks, one of which is suspended from the centre of the top
+of the cage, the other extended above the movable head.
+
+[Illustration: FIG. 44.--Design of tool for cleavage test.]
+
+The specimens are 2" X 2" X 3.75". At one end a one-inch hole is
+bored, with its centre equidistant from the two sides and 0.25
+inch from the end. (See Fig. 45.) This makes the cross section
+to be tested 2" X 3". Some of the blocks are cut radially and
+some tangentially, as indicated in the figure.
+
+[Illustration: FIG. 45.--Design of cleavage test specimen.]
+
+The free ends of the hooks are fitted into the notch in the end
+of the specimen. The movable head of the machine is then made to
+descend at the rate of 0.25 inch per minute, pulling apart the
+hooks and splitting the block. The maximum load only is taken
+and the result expressed in pounds per square inch of width. A
+piece one-half inch thick is split off parallel to the failure
+and used for moisture determination.
+
+
+
+TENSION TEST PARALLEL TO THE GRAIN
+
+
+Since the tensile strength of wood parallel to the grain is
+greater than the compressive strength, and exceedingly greater
+than the shearing strength, it is very difficult to make
+satisfactory tension tests, as the head and shoulders of the
+test specimen (which is subjected to both compression and shear)
+must be stronger than the portion subjected to a pure tensile
+stress.
+
+Various designs of test specimens have been made. The one first
+employed by the Division of Forestry[61] was prepared as
+follows: Sticks were cut measuring 1.5" X 2.5" X 16". The
+thickness at the centre was then reduced to three-eighths of an
+inch by cutting out circular segments with a band saw. This left
+a breaking section of 2.5" X 0.375". Care was taken to cut the
+specimen as nearly parallel to the grain as possible, so that
+its failure would occur in a condition of pure tension. The
+specimen was then placed between the plane wedge-shaped steel
+grips of the cage and the movable head of the static machine and
+pulled in two. Only the maximum load was recorded. (See Fig. 46,
+No. 1.)
+
+[Illustration: FIG. 46.--Designs of tension test specimens used
+in United States.]
+
+[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]
+
+The difficulty of making such tests compared with the minor
+importance of the results is so great that they are at present
+omitted by the U.S. Forest Service. A form of specimen is
+suggested, however, and is as follows: "A rod of wood about one
+inch in diameter is bored by a hollow drill from the stick to be
+tested. The ends of this rod are inserted and glued in
+corresponding holes in permanent hardwood wedges. The specimen
+is then submitted to the ordinary tension test. The broken ends
+are punched from the wedges."[62] (See Fig. 46, No. 2.)
+
+[Footnote 62: Cir. 38: Instructions to engineers of timber
+tests, 1906, p. 24.]
+
+The form used by the Department of Forestry of New South
+Wales[63] is as shown in Fig. 47. The specimen has a total
+length of 41 inches and is circular in cross section. On each
+end is a head 4 inches in diameter and 7 inches long. Below each
+head is a shoulder 8.5 inches long, which tapers from a diameter
+of 2.75 inches to 1.25 inches. In the middle is a cylindrical
+portion 1.25 inches in diameter and 10 inches long.
+
+[Illustration: FIG. 47.--Design of tension test specimen used in
+New South Wales.]
+
+[Footnote 63: Warren, W.H.: The strength, elasticity, and other
+properties of New South Wales hardwood timbers, 1911, pp.
+58-62.]
+
+In making the test the specimen is fitted in the machine, and an
+extensometer attached to the middle portion and arranged to
+record the extension between the gauge points 8 inches apart.
+The area of the cross section then is 1.226 square inches, and
+the tensile strength is equal to the total breaking load applied
+divided by this area.
+
+
+
+TENSION TEST AT RIGHT ANGLES TO THE GRAIN
+
+
+A static testing machine and a special testing device (see Fig.
+48) are required. The latter consists essentially of two double
+hooks or clamps, one of which is suspended from the centre of
+the top of the cage, the other extended above the movable head.
+The specimens are 2" X 2" X 2.5". At each end a one-inch hole is
+bored with its centre equidistant from the two sides and 0.25
+inch from the ends. This makes the cross section to be tested 1"
+X 2".
+
+[Illustration: FIG. 48.--Design of tool and specimen for testing
+tension at right angles to the grain.]
+
+The free ends of the clamps are fitted into the notches in the
+ends of the specimen. The movable head of the machine is then
+made to descend at the rate of 0.25 inch per minute, pulling the
+specimen in two at right angles to the grain. The maximum load
+only is taken and the result expressed in pounds per inch of
+width. A piece one-half inch thick is split off parallel to the
+failure and used for moisture determination.
+
+
+
+TORSION TEST[64]
+
+
+[Footnote 64: Wood is so seldom subjected to a pure stress of
+this kind that the torsion test is usually omitted.]
+
+_Apparatus_: The torsion test is made in a Riehle-Miller
+torsional testing machine or its equivalent. (See Fig. 49.)
+
+[Illustration: FIG. 49.--Making a torsion test on hickory.]
+
+_Preparation of material_: The test pieces are cylindrical, 1.5
+inches in diameter and 18 inches gauge length, with squared ends
+4 inches long joined to the cylindrical portion with a fillet.
+The dimensions are carefully measured, and the usual data
+obtained in regard to the rate of growth, proportion of late
+wood, location and kind of defects. The weight of the
+cylindrical portion of the specimen is obtained after the test.
+
+_Making the test_: After the specimen is fitted in the machine
+the load is applied continuously at the rate of 22� per minute.
+A troptometer is used in measuring the deformation. Readings are
+made until failure occurs, the points being entered on the
+cross-section paper. The character of the failure is described.
+Moisture determinations are made by the disk method.
+
+_Results_: The conditions of ultimate rupture due to torsion
+appear not to be governed by definite mathematical laws; but
+where the material is not overstrained, laws may be assumed
+which are sufficiently exact for practical cases. The formul�
+commonly used for computations are as follows:
+
+              5.1 M
+     (1) T = -------
+              c^{3}
+
+              114.6 T f
+     (2) G = -----------
+                 a c
+
+  a = angle measured by troptometer at elastic limit, in
+        degrees.
+  c = diameter of specimen, inches.
+  f = gauge length of specimen, inches. _G_ = modulus of
+        elasticity in shear across the grain, pounds per square
+        inch.
+  M = moment of torsion at elastic limit, inch-pounds.
+  T = outer fibre torsional stress at elastic limit, pounds per
+        square inch.
+
+
+
+SPECIAL TESTS
+
+
+_Spike-pulling Test_
+
+Spike-pulling tests apply to problems of railroad maintenance,
+and the results are used to compare the spike-holding powers of
+various woods, both untreated and treated with different
+preservatives, and the efficiency of various forms of spikes.
+Special tests are also made in which the spike is subjected to a
+transverse load applied repetitively by a blow.
+
+For details of tests and results see:
+
+Cir. 38,  U.S.F.S.: Instructions to engineers of timber tests,
+p. 26. Cir. 46,  U.S.F.S.: Holding force of railroad spikes in
+wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of
+cross-ties, pp. 37-40.
+
+
+_Packing Boxes_
+
+Special tests on the strength of packing boxes of various woods
+have been made by the U.S. Forest Service to determine the
+merits of different kinds of woods as box material with the view
+of substituting new kinds for the more expensive ones now in
+use. The methods of tests consisted in applying a load along the
+diagonal of a box, an action similar to that which occurs when a
+box is dropped on one of its corners. The load was measured at
+each one-fourth inch in deflection, and notes were made of the
+primary and subsequent failures.
+
+For details of tests and results, see:
+
+Cir. 47,  U.S.F.S.: Strength of packing boxes of various woods.
+Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.
+
+
+_Vehicle and Implement Woods_
+
+Tests were made by the U.S. Forest Service to obtain a better
+knowledge of the mechanical properties of the woods at present
+used in the manufacture of vehicles and implements and of those
+which might be substituted for them. Tests were made upon the
+following materials: hickory buggy spokes (see Fig. 5); hickory
+and red oak buggy shafts; wagon tongues; Douglas fir and
+southern pine cultivator poles.
+
+Details of the tests and results may be found in:
+
+Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.
+
+
+_Cross-arms_
+
+In tests by the U.S. Forest Service on cross-arms a special
+apparatus was devised in which the load was distributed along
+the arm as in actual practice. The load was applied by rods
+passing through the pinholes in the arms. Nuts on these rods
+pulled down on the wooden bearing-blocks shaped to fit the upper
+side of the arm. The lower ends of these rods were attached to a
+system of equalizing levers, so arranged that the load at each
+pinhole would be the same. In all the tests the load was applied
+vertically by means of the static machine.
+
+See Cir. 204, U.S.F.S.: Strength tests of cross-arms.
+
+
+_Other Tests_
+
+Many other kinds of tests are made as occasion demands. One kind
+consists of barrels and liquid containers, match-boxes, and
+explosive containers. These articles are subjected to shocks
+such as they would receive in transit and in handling, and also
+to hydraulic pressure.
+
+One of the most important tests from a practical standpoint is
+that of built-up structures such as compounded beams composed of
+small pieces bolted together, mortised joints, wooden trusses,
+etc. Tests of this kind can best be worked out according to the
+specific requirements in each case.
+
+
+
+
+APPENDIX
+
+
+
+SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE
+
+MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES
+
+Working Plan No. 124
+
+
+PURPOSE OF WORK
+
+It is the general purpose of the work here outlined to provide:
+
+(_a_) Reliable data for comparing the mechanical properties of
+various species;
+
+(_b_) Data for the establishment of correct strength functions
+or working stresses;
+
+(_c_) Data upon which may be based analyses of the influence on
+the mechanical properties of such factors as:
+
+Locality;
+
+Distance of timber from the pith of the tree;
+
+Height of timber in the tree;
+
+Change from the green to the air-dried condition, etc.
+
+The mechanical properties which will be considered and the
+principal tests used to determine them are as follows:
+
+Strength and stiffness--
+  Static bending;
+  Compression parallel to grain;
+  Compression perpendicular to grain;
+  Shear.
+
+Toughness--
+  Impact bending;
+  Static bending;
+  Work to maximum load and total work.
+
+Cleavability--
+  Cleavage test.
+
+Hardness--
+  Modification of Janka ball test for surface hardness.
+
+
+MATERIAL
+
+
+_Selection and Number of Trees_
+
+The material will be from trees selected in the forest by one
+qualified to determine the species. From each locality, three to
+five dominant trees of merchantable size and approximately
+average age will be so chosen as to be representative of the
+dominant trees of the species. Each species will eventually be
+represented by trees from five to ten localities. These
+localities will be so chosen as to be representative of the
+commercial range of the species. Trees from one to three
+localities will be used to represent each species until most of
+the important species have been tested.
+
+The 16-foot butt log will be taken from each tree selected and
+the entire merchantable hole of one average tree for each
+species.
+
+
+_Field Notes and Shipping Instructions_
+
+Field notes as outlined in Form--_a_ Shipment Description,
+Manual of the Branch of Products, will be fully and carefully
+made by the collector. The age of each tree selected will be
+recorded and any other information likely to be of interest or
+importance will also be made a part of these field notes. Each
+log will have the bark left on. It will be plainly marked in
+accordance with directions given under Detailed Instructions.
+All material will be shipped to the laboratory immediately after
+being cut. No trees will be cut until the collector is notified
+that the laboratory is ready to receive the material.
+
+
+DETAILED INSTRUCTIONS
+
+
+_Part of Tree to be Tested_
+
+(_a_) For determining the value of tree and locality and the
+influence on the mechanical properties of distance from the
+pith, a 4-foot bolt will be cut from the top end of each 16-foot
+butt log.
+
+(_b_) For investigating the variation of properties with the
+height of timber in the tree, all the logs from one average tree
+will be used.
+
+(_c_) For investigating the effect of drying the wood, the bolt
+next below that provided for in (_a_) will be used in the case
+of one tree from each locality.
+
+
+_Marking and Grouping of Material_
+
+The marking will be standard except as noted. Each log will be
+considered a "piece." The piece numbers will be plainly marked
+upon the butt end of each log by the collector. The north side
+of each log will also be marked.
+
+When only one bolt from a tree is used it will be designated by
+the number of the log from which it is cut. Whenever more than
+one bolt is taken from a tree, each 4-foot bolt or length of
+trunk will be given a letter (mark), _a, b, c,_ etc., beginning
+at the stump.
+
+All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the
+sticks marked according to the sketch, Fig. 50. The letters _N,
+E, S,_ and _W_ indicate the cardinal points when known; when
+these are unknown, _H, K, L,_ and _M_ will be used. Thus, _N5,
+K8, S7, M4_ are stick numbers, the letter being a part of the
+stick number.
+
+[Illustration: FIG. 50.--Method of cutting and marking test
+specimens.]
+
+Only straight-grained specimens, free from defects which will
+affect their strength, will be tested.
+
+
+_Care of Material_
+
+No material will be kept in the bolt or log long enough to be
+damaged or disfigured by checks, rot, or stains.
+
+_Green material_: The material to be tested green will be kept
+in a green state by being submerged in water until near the time
+of test. It will then be surfaced, sawed to length, and stored
+in damp sawdust at a temperature of 70�F. (as nearly as
+practicable) until time of test. Care should be taken to avoid
+as much as possible the storage of green material in any form.
+
+_Air-dry material_: The material to be air-dried will be cut
+into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will
+be paraffined to prevent checking. This material will be so
+piled as to leave an air space of at least one-half inch on each
+side of each stick, and in such a place that it will be
+protected from sunshine, rain, snow, and moisture from the
+ground. The sticks will be surfaced and cut to length just
+previous to test.
+
+
+_Order of Tests_
+
+The order of tests in all cases will be such as to eliminate so
+far as possible from the comparisons the effect of changes of
+condition of the specimens due to such factors as storage and
+weather conditions.
+
+The material used for determining the effect of height in tree
+will be tested in such order that the average time elapsing from
+time of cutting to time of test will be approximately the same
+for all bolts from any one tree.
+
+
+_Tests on Green Material_
+
+The tests on all bolts, except those from which a comparison of
+green and dry timber is to be gotten, will be as follows:
+
+_Static bending_: One stick from each pair. A pair consists of
+two adjacent sticks equidistant from the pith, as _N_7 and _N_8,
+or _H_5 and _H_6.
+
+_Impact bending_: Four sticks; one to be taken from near the
+pith; one from near the periphery; and two representative of the
+cross section.
+
+_Compression parallel to grain_: One specimen from each stick.
+These will be marked "1" in addition to the number of the stick
+from which they are taken.
+
+_Compression perpendicular to grain_: One specimen from each of
+50 per cent of the static bending sticks. These will be marked
+"2" in addition to the number of the stick from which they are
+cut.
+
+_Hardness_: One specimen from each of the other 50 per cent of
+the static bending sticks. These specimens will be marked "4."
+
+_Shear_: Six specimens from sticks not tested in bending or from
+the ends cut off in preparing the bending specimens. Two
+specimens will be taken from near the pith; two from near the
+periphery; and two that are representative of the average
+growth. One of each two will be tested in radial shear and the
+other in tangential shear. These specimens will have the mark
+"3."
+
+_Cleavage_: Six specimens chosen and divided just as those for
+shearing. These specimens will have the mark "5." (For sketches
+showing radial and tangential cleavage, see Fig. 45.)
+
+When it is impossible to secure clear specimens for all of the
+above tests, tests will have precedence in the order in which
+they are named.
+
+
+_Tests to Determine the Effect of Air-drying_
+
+These tests will be made on material from the adjacent bolts
+mentioned in "_c_" under Part of Tree to be Tested. Both bolts
+will be cut as outlined above. One-half the sticks from each
+bolt will be tested green, the other half will be air-dried and
+tested. The division of green and air-dry will be according to
+the following scheme:
+
+                     STICK  NUMBERS
+
+Lower bolt,  1,       4, 5,       8, 9,            } Tested
+                                            etc.   } green
+Upper bolt,     2, 3,       6, 7,       10,        }
+
+Lower bolt,     2, 3,       6, 7,       10,        } Air-dried
+                                            etc.   } and
+Upper bolt,  1,       4, 5,       8, 9,            } tested
+
+All green sticks from these two bolts will be tested as if they
+were from the same bolt and according to the plan previously
+outlined for green material from single bolts. The tests on the
+air-dried material will be the same as on the green except for
+the difference of seasoning.
+
+The material will be tested at as near 12 per cent moisture as
+is practicable. The approximate weight of the air-dried
+specimens at 12 per cent moisture will be determined by
+measuring while green 20 per cent of the sticks to be air-dried
+and assuming their dry gravity to be the same as that of the
+specimens tested green. This 20 per cent will be weighed as
+often as is necessary to determine the proper time of test.
+
+
+_Methods of Test_
+
+All tests will be made according to Circular 38 except in case
+of conflict with the instructions given below:
+
+_Static bending_: The tests will be on specimens 2" X 2" X 30"
+on 28-inch span. Load will be applied at the centre.
+
+In all tests the load-deflection curve will be carried to or
+beyond the maximum load. In one-third of the tests the
+load-deflection curve will be continued to 6-inch deflection, or
+till the specimen fails to support a 200-pound load. Deflection
+readings for equal increments of load will be taken until well
+past the elastic limit, after which the scale beam will be kept
+balanced and the load read for each 0.1-inch deflection. The
+load and deflection at first failure, maximum load and points of
+sudden change, will be shown on the curve sheet even if they do
+not occur at one of the regular load or deflection increments.
+
+_Impact bending_: The impact bending tests will be on specimens
+of the same size as those used in static bending. The span will
+be 28 inches.
+
+The tests will be by increment drop. The first drop will be 1
+inch and the increase will be by increments of 1 inch till a
+height of 10 inches is reached, after which increments of 2
+inches will be used until complete failure occurs or 6-inch
+deflection is secured.
+
+A 50-pound hammer will be used when with drops up to 68 inches
+it is practically certain that it will produce complete failure
+or 6-inch deflection in the case of all specimens of a species.
+For all other species, a 100-pound hammer will be used.
+
+In all cases drum records will be made until first failure. Also
+the height of drop causing complete failure or 6-inch deflection
+will be noted.
+
+_Compression parallel to grain_: This test will be on specimens
+2" X 2" X 8" in size. On 20 per cent of these tests
+load-compression curves for a 6-inch centrally located gauge
+length will be taken. Readings will be continued until the
+elastic limit is well passed. The other 80 per cent of the tests
+will be made for the purpose of obtaining the maximum load only.
+
+_Compression perpendicular to grain_: This test will be on
+specimens 2" X 2" X 6" in size. The bearing plates will be 2
+inches wide. The rate of descent of the moving head will be
+0.024 inch per minute. The load-compression curve will be
+plotted to 0.1 inch compression and the test will then be
+discontinued.
+
+_Hardness_: The tool shown in Fig. 43 (an adaptation of the
+apparatus used by the German investigator, Janka) will be used.
+The rate of descent of the moving head will be 0.25 inch per
+minute. When the penetration has progressed to the point at
+which the plate "_a_" becomes tight, due to being pressed
+against the wood, the load will be read and recorded.
+
+Two penetrations will be made on a tangential surface, two on a
+radial, and one on each end of each specimen tested. The choice
+between the two radial and between the two tangential surfaces
+and the distribution of the penetrations over the surfaces will
+be so made as to get a fair average of heart and sap, slow and
+fast growth, and spring and summer wood. Specimens will be 2" X
+2" X 6".
+
+_Shear_: The tests will be made with a tool slightly modified
+from that shown in Circular 38. The speed of descent of head
+will be 0.015 inch per minute. The only measurements to be made
+are those of the shearing area. The offset will be 1/8 inch.
+Specimens will be 2" X 2" X 2-1/2" in size. (For definition of
+offset and form of test specimen, see Fig. 38.)
+
+_Cleavage_: The cleavage tests will be made on specimens of the
+form and size shown in Fig. 45. The apparatus will be as shown
+in Fig. 44. The maximum load only will be taken and the result
+expressed in pounds per inch of width. The speed of the moving
+head will be 0.25 inch per minute.
+
+
+_Moisture Determinations_
+
+Moisture determinations will be made on all specimens tested
+except those to be photographed or kept for exhibit. A 1-inch
+disk will be cut from near the point of failure of bending and
+compression parallel specimens, from the portion under the plate
+in the case of the compression perpendicular specimens, and from
+the centre of the hardness test specimens. The beads from the
+shear specimens will be used as moisture disks. In the case of
+the cleavage specimens a piece 1/2 inch thick will be split off
+parallel to the failure and used as a moisture disk.
+
+
+RECORDS
+
+
+All records will be standard.
+
+
+PHOTOGRAPHS
+
+
+_Cross Sections_
+
+Just before cutting into sticks, the freshly cut end of at least
+one bolt from each tree will be photographed. A scale of inches
+will be shown in this photograph.
+
+
+_Specimens_
+
+Three photographs will be made of a group consisting of four 2"
+X 2" X 30" specimens chosen from the material from each
+locality. Two of these specimens will be representative of
+average growth, one of fast and one of slow growth. These
+photographs will show radial, tangential, and end surfaces for
+each specimen.
+
+
+_Failures_
+
+Typical and abnormal failures of material from each site will be
+photographed.
+
+
+_Disposition of Material_
+
+The specimens photographed to show typical and abnormal failures
+will be saved for purposes of exhibit until deemed by the person
+in charge of the laboratory to be of no further value.
+
+
+
+SHRINKAGE AND SPECIFIC GRAVITY
+
+Appendix to Working Plan 124
+
+
+PURPOSE OF WORK
+
+
+It is the purpose of this work to secure data on the shrinkage
+and specific gravity of woods tested under Project 124. The
+figures to be obtained are for use as average working values
+rather than as the basis for a detailed study of the principles
+involved.
+
+
+MATERIAL
+
+
+The material will be taken from that provided for mechanical
+tests.
+
+
+RADIAL AND TANGENTIAL SHRINKAGE
+
+
+_Specimens_
+
+_Preparation_: Two specimens 1 inch thick, 4 inches wide, and 1
+inch long will be obtained from near the periphery of each "_d_"
+bolt. These will be cut from the sector-shaped sections left
+after securing the material for the mechanical tests or from
+disks cut from near the end of the bolt. They will be taken from
+adjoining pieces chosen so that the results will be comparable
+for use in determining radial and tangential shrinkage. (When a
+disk is used, care must be taken that it is green and has not
+been affected by the shrinkage and checking near the end of the
+bolt.)
+
+One of these specimens will be cut with its width in the radial
+direction and will be used for the determination of radial
+shrinkage. The other will have its width in the tangential
+direction and will be used for tangential shrinkage. These
+specimens will not be surfaced.
+
+_Marking_: The shrinkage specimens will retain the shipment and
+piece numbers and marks of the bolts from which they are taken,
+and will have the additional mark _7_R or _7_T according as
+their widths are in the radial or tangential direction.
+
+
+_Shrinkage measurements_: The shrinkage specimens will be
+carefully weighed and measured soon after cutting. Rings per
+inch, per cent sap, and per cent summer wood will be measured.
+They will then be air-dried in the laboratory to constant
+weight, and afterward oven-dried at 100�C. (212�F.), when they
+will again be weighed and measured.
+
+
+VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY
+
+
+_Specimens_
+
+_Selection and preparation_: Four 2" X 2" X 6" specimens will be
+cut from the mechanical test sticks of each "_d_" bolt; also
+from each of the composite bolts used in getting a comparison of
+green and air-dry. One of these specimens will be taken from
+near the pith and one from near the periphery; the other two
+will be representative of the average growth of the bolt. The
+sides of these specimens will be surfaced and the ends smooth
+sawn.
+
+_Marking_: Each specimen will retain the shipment, piece, and
+stick numbers and mark of the stick from which it is cut, and
+will have the additional mark "_S_."
+
+_Manipulation_: Soon after cutting, each specimen will be
+weighed and its volume will be determined by the method
+described below. The rings per inch and per cent summer wood,
+where possible, will be determined, and a carbon impression of
+the end of the specimen made. It will then be air-dried in the
+laboratory to a constant weight and afterward oven-dried at
+100�C. When dry, the specimen will be taken from the oven,
+weighed, and a carbon impression of its end made. While still
+warm the specimen will be dipped in hot paraffine. The volume
+will then be determined by the following method:
+
+On one pan of a pair of balances is placed a container having in
+it water enough for the complete submersion of the test
+specimen. This container and water is balanced by weights placed
+on the other scale pan. The specimen is then held completely
+submerged and not touching the container while the scales are
+again balanced. The weight required to balance is the weight of
+water displaced by the specimen, and hence if in grams is
+numerically equal to the volume of the specimen in cubic
+centimetres. A diagrammatic sketch of the arrangement of this
+apparatus is shown in Fig. 51.
+
+[Illustration: FIG. 51.--Diagram of specific gravity apparatus,
+showing a balance with container (_c_) filled with water in
+which the test block (_b_) is held submerged by a light rod
+(_a_) which is adjustable vertically and provided with a sharp
+point to be driven into the specimen.]
+
+Air-dry specimens will be dipped in water and then wiped dry
+after the first weighing and just before being immersed for
+weighing their displacement. All displacement determinations
+will be made as quickly as possible in order to minimize the
+absorption of water by the specimen.
+
+
+
+STRENGTH VALUES FOR STRUCTURAL TIMBERS
+
+(From Cir. 189, U.S. Forest Service)
+
+
+The following tables bring together in condensed form the
+average strength values resulting from a large number of tests
+made by the Forest Service on the principal structural timbers
+of the United States. These results are more completely
+discussed in other publications of the Service, a list of which
+is given in BIBLIOGRAPHY, PART III.
+
+The tests were made at the laboratories of the U.S. Forest
+Service, in cooperation with the following institutions: Yale
+Forest School, Purdue University, University of California,
+University of Oregon, University of Washington, University of
+Colorado, and University of Wisconsin.
+
+Tables XVIII and XIX give the average results obtained from
+tests on green material, while Tables XX and XXI give average
+results from tests on air-seasoned material. The small
+specimens, which were invariably 2" X 2" in cross section, were
+free from defects such as knots, checks, and cross grain; all
+other specimens were representative of material secured in the
+open market. The relation of stresses developed in different
+structural forms to those developed in the small clear specimens
+is shown for each factor in the column headed "Ratio to 2" X
+2"." Tests to determine the mechanical properties of different
+species are often confined to small, clear specimens. The ratios
+included in the tables may be applied to such results in order
+to approximate the strength of the species in structural sizes,
+and containing the defects usually encountered, when tests on
+such forms are not available.
+
+A comparison of the results of tests on seasoned material with
+those from tests on green material shows that, without
+exception, the strength of the 2" X 2" specimens is increased by
+lowering the moisture content, but that increase in strength of
+other sizes is much more erratic. Some specimens, in fact, show
+an apparent loss in strength due to seasoning. If structural
+timbers are seasoned slowly, in order to avoid excessive
+checking, there should be an increase in their strength. In the
+light of these facts it is not safe to base working stresses on
+results secured from any but green material. For a discussion of
+factors of safety and safe working stresses for structural
+timbers see the Manual of the American Railway Engineering
+Association, Chicago, 1911. A table from that publication,
+giving working unit stresses for structural timber, is
+reproduced in this book, see Table XXII.
+
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XVIII                                                TABLE XVIII                                                            |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                                                  BENDING TESTS ON GREEN MATERIAL                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                 |      Sizes      |       |       |       |   F.S. at E.L.  |     M. of R.    |     M. of E.    |   Calculated    |
+|                 |-----------------| Num-  | Per   | Rings |                 |                 |                 |      shear      |
+|     Species     |          |      |  ber  | cent  |  per  |-----------------+-----------------+-----------------+-----------------|
+|                 |  Cross   | Span | of    | mois- | inch  | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio |
+|                 | Section  |      | tests |  ture |       | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" |
+|                 |          |      |       |       |       |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |
+|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------|
+|                 |          |      |       |       |       |         |       |         |       |  1,000  |       |         |       |
+|                 |  Inches  | Ins. |       |       |       |   Lbs.  |       |   Lbs.  |       |   lbs.  |       |   Lbs.  |       |
+|                 |          |      |       |       |       |         |       |         |       |         |       |         |       |
+| Longleaf pine   | 12 by 12 |  138 |    4  |  28.6 |   9.7 |  4,029  |  0.83 |  6,710  |  0.74 |  1,523  |  0.99 |    261  |  0.86 |
+|                 | 10 by 16 |  168 |    4  |  26.8 |  16.7 |  6,453  |   .85 |  6,453  |   .71 |  1,626  |  1.05 |    306  |  1.01 |
+|                 |  8 by 16 |  156 |    7  |  28.4 |  14.6 |  3,147  |   .64 |  5,439  |   .60 |  1,368  |   .89 |    390  |  1.29 |
+|                 |  6 by 16 |  132 |    1  |  40.3 |  21.8 |  4,120  |   .83 |  6,460  |   .71 |  1,190  |   .77 |    378  |  1.25 |
+|                 |  6 by 10 |  180 |    1  |  31.0 |   6.2 |  3,580  |   .72 |  6,500  |   .72 |  1,412  |   .92 |    175  |   .58 |
+|                 |  6 by  8 |  180 |    2  |  27.0 |   8.2 |  3,735  |   .75 |  5,745  |   .63 |  1,282  |   .83 |    121  |   .40 |
+|                 |  2 by  2 |   30 |   15  |  33.9 |  14.1 |  4,950  |  1.00 |  9,070  |  1.00 |  1,540  |  1:00 |    303  |  1.00 |
+| Douglas fir     |  8 by 16 |  180 |  191  |  31.5 |  11.0 |  3,968  |   .76 |  5,983  |   .72 |  1,517  |   .95 |    269  |   .81 |
+|                 |  5 by  8 |  180 |   84  |  30.1 |  10.8 |  3,693  |   .71 |  5,178  |   .63 |  1,533  |   .96 |    172  |   .52 |
+|                 |  2 by 12 |  180 |   27  |  35.7 |  20.3 |  3,721  |   .71 |  5,276  |   .64 |  1,642  |  1.03 |    256  |   .77 |
+|                 |  2 by 10 |  180 |   26  |  32.9 |  21.6 |  3,160  |   .60 |  4,699  |   .57 |  1,593  |  1.00 |    189  |   .57 |
+|                 |  2 by  8 |  180 |   29  |  33.6 |  17.6 |  3,593  |   .69 |  5,352  |   .65 |  1,607  |  1.01 |    171  |   .51 |
+|                 |  2 by  2 |   24 |  568  |  30.4 |  11.6 |  5,227  |  1.00 |  9,070  |  1.00 |  1,540  |  1.00 |    303  |  1.00 |
+| Douglas fir     |          |      |       |       |       |         |       |         |       |         |       |         |       |
+|   (fire-killed) |  8 by 16 |  180 |   30  |  36.8 |  10.9 |  3,503  |   .80 |  4,994  |   .64 |  1,531  |   .94 |    330  |  1.19 |
+|                 |  2 by 12 |  180 |   32  |  34.2 |  17.7 |  3,489  |   .80 |  5,085  |   .66 |  1,624  |   .99 |    247  |   .89 |
+|                 |  2 by 10 |  180 |   32  |  38.9 |  18.1 |  3,851  |   .88 |  5,359  |   .69 |  1,716  |  1.05 |    216  |   .78 |
+|                 |  2 by  8 |  180 |   31  |  37.0 |  15.7 |  3,403  |   .78 |  5,305  |   .68 |  1,676  |  1.02 |    169  |   .61 |
+|                 |  2 by  2 |   30 |  290  |  33.2 |  17.2 |  4,360  |  1.00 |  7,752  |  1.00 |  1,636  |  1.00 |    277  |  1.00 |
+| Shortleaf pine  |  8 by 16 |  180 |   12  |  39.5 |  12.1 |  3,185  |   .73 |  5,407  |   .70 |  1,438  |  1.03 |    362  |  1.40 |
+|                 |  8 by 14 |  180 |   12  |  45.8 |  12.7 |  3,234  |   .74 |  5,781  |   .75 |  1,494  |  1.07 |    338  |  1.31 |
+|                 |  8 by 12 |  180 |   24  |  52.2 |  11.8 |  3,265  |   .75 |  5,503  |   .71 |  1,480  |  1.06 |    277  |  1.07 |
+|                 |  5 by  8 |  180 |   24  |  47.8 |  11.5 |  3,519  |   .81 |  5,732  |   .74 |  1,485  |  1.06 |    185  |   .72 |
+|                 |  2 by  2 |   30 |  254  |  51.7 |  13.6 |  4,350  |  1.00 |  7,710  |  1.00 |  1,395  |  1.00 |    258  |  1.00 |
+| Western larch   |  8 by 16 |  180 |   32  |  51.0 |  25.3 |  3,276  |   .77 |  4,632  |   .64 |  1,272  |   .97 |    298  |  1.11 |
+|                 |  8 by 12 |  180 |   30  |  50.3 |  23.2 |  3,376  |   .79 |  5,286  |   .73 |  1,331  |  1.02 |    254  |   .94 |
+|                 |  5 by  8 |  180 |   14  |  56.0 |  25.6 |  3,528  |   .83 |  5,331  |   .74 |  1,432  |  1.09 |    169  |   .63 |
+|                 |  2 by  2 |   28 |  189  |  46.2 |  26.2 |  4,274  |  1.00 |  7,251  |  1.00 |  1,310  |  1.00 |    269  |  1.00 |
+| Loblolly pine   |  8 by 16 |  180 |   17  |  15.8 |   6.1 |  3,094  |   .75 |  5,394  |   .69 |  1,406  |   .98 |    383  |  1.44 |
+|                 |  5 by 12 |  180 |   94  |  60.9 |   5.9 |  3,030  |   .74 |  5,028  |   .64 |  1,383  |   .96 |    221  |   .83 |
+|                 |  2 by  2 |   30 |   44  |  70.9 |   5.4 |  4,100  |  1.00 |  7,870  |  1.00 |  1,440  |  1.00 |    265  |  1.00 |
+| Tamarack        |  6 by 12 |  162 |   15  |  57.6 |  16.6 |  2,914  |   .75 |  4,500  |   .66 |  1,202  |  1.05 |    255  |  1.11 |
+|                 |  4 by 10 |  162 |   15  |  43.5 |  11.4 |  2,712  |   .70 |  4,611  |   .68 |  1,238  |  1.08 |    209  |   .91 |
+|                 |  2 by  2 |   30 |   82  |  38.8 |  14.0 |  3,875  |  1.00 |  6,820  |  1.00 |  1,141  |  1.00 |    229  |  1.00 |
+| Western hemlock |  8 by 16 |  180 |   39  |  42.5 |  15.6 |  3,516  |   .80 |  5,296  |   .73 |  1,445  |  1.01 |    261  |   .92 |
+|                 |  2 by  2 |   28 |   52  |  51.8 |  12.1 |  4.406  |  1.00 |  7,294  |  1.00 |  1,428  |  1.00 |    284  |  1.00 |
+| Redwood         |  8 by 16 |  180 |   14  |  86.5 |  19.9 |  3,734  |   .79 |  4,492  |   .64 |  1,016  |   .96 |    300  |  1.21 |
+|                 |  6 by 12 |  180 |   14  |  87.3 |  17.8 |  3,787  |   .80 |  4,451  |   .64 |  1,068  |  1.00 |    224  |   .90 |
+|                 |  7 by  9 |  180 |   14  |  79.8 |  16.7 |  4,412  |   .93 |  5,279  |   .76 |  1,324  |  1.25 |    199  |   .80 |
+|                 |  3 by 14 |  180 |   13  |  86.1 |  23.7 |  3,506  |   .74 |  4,364  |   .62 |    947  |   .89 |    255  |  1.03 |
+|                 |  2 by 12 |  180 |   12  |  70.9 |  18.6 |  3,100  |   .65 |  3,753  |   .54 |  1,052  |   .99 |    187  |   .75 |
+|                 |  2 by 10 |  180 |   13  |  55.8 |  20.0 |  3,285  |   .69 |  4,079  |   .58 |  1,107  |  1.04 |    169  |   .68 |
+|                 |  2 by  8 |  180 |   13  |  63.8 |  21.5 |  2,989  |   .63 |  4,063  |   .58 |  1,141  |  1.08 |    134  |   .54 |
+|                 |  2 by  2 |   28 |  157  |  75.5 |  19.1 |  4,750  |  1.00 |  6,980  |  1.00 |  1,061  |  1.00 |    248  |  1.00 |
+| Norway pine     |  6 by 12 |  162 |   15  |  50.3 |  12.5 |  2,305  |   .82 |  3,572  |   .69 |    987  |  1.03 |    201  |  1.17 |
+|                 |  4 by 12 |  162 |   18  |  47.9 |  14.7 |  2,648  |   .94 |  4,107  |   .79 |  1,255  |  1.31 |    238  |  1.38 |
+|                 |  4 by 10 |  162 |   16  |  45.7 |  13.3 |  2,674  |   .95 |  4,205  |   .81 |  1,306  |  1.36 |    198  |  1.15 |
+|                 |  2 by  2 |   30 |  133  |  32.3 |  11.4 |  2,808  |  1.00 |  5,173  |  1.00 |    960  |  1.00 |    172  |  1.00 |
+| Red spruce      |  2 by 10 |  144 |   14  |  32.5 |  21.9 |  2,394  |   .66 |  3,566  |   .60 |  1,180  |  1.02 |    181  |   .80 |
+|                 |  2 by  2 |   26 |   60  |  37.3 |  21.3 |  3,627  |  1.00 |  5,900  |  1.00 |  1,157  |  1.00 |    227  |  1.00 |
+| White spruce    |  2 by 10 |  144 |   16  |  40.7 |   9.3 |  2,239  |   .72 |  3,288  |   .63 |  1,081  |  1.08 |    166  |   .83 |
+|                 |  2 by  2 |   26 |   83  |  58.3 |  10.2 |  3.090  |  1.00 |  5,185  |  1.00 |    998  |  1.00 |    199  |  1.00 |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                           |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                  |
+|    M. of E. = Modulus of elasticity.                                                                                              |
+|    M. of R. = Modulus of rupture.                                                                                                 |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                         |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                      |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XIX                                                         TABLE XIX                                                                   |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                                                 COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL                                                 |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |                     Compression                      |                Compression                |           Shear          |
+|                 |                  parallel to grain                   |          perpendicular  to grain          |                          |
+|                 |------------------------------------------------------+-------------------------------------------+--------------------------|
+|                 |          |       |       | Cr.    |        | Cr.     |        |        |       |       | Cr.     |       |       |          |
+|     Species     |          |       | Per   | str.   | M. of  | str.    |        |        |       | Per   | str.    |       | Per   |          |
+|                 | Size of  |  No.  | cent  | at     | E.     | at max. | Stress |        |  No.  | cent  | at max. |  No.  | cent  |  Shear   |
+|                 | specimen |  of   | of    | E. L.  | per    | ld.,.   | area   | Height |  of   | of    | ld.,    |  of   | of    | strength |
+|                 |          | tests | mois- | per    | square | per     |        |        | tests | mois- | per     | tests | mois- |          |
+|                 |          |       |  ture | square | inch   | square  |        |        |       |  ture | square  |       |  ture |          |
+|                 |          |       |       | inch   |        | inch    |        |        |       |       | inch    |       |       |          |
+|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------|
+|                 |          |       |       |        |  1,000 |         |        |        |       |       |         |       |       |          |
+|                 |  Inches  |       |       |  Lbs.  |  lbs.  |  Lbs.   | Inches | Inches |       |       |   Lbs.  |       |       |   Lbs.   |
+|                 |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+| Longleaf pine   |  4 by 4  |   46  |  26.3 |  3,480 |        |  4,800  | 4 by 4 |     4  |   22  |  25.3 |    568  |   44  |  21.8 |     973  |
+|                 |  2 by 2  |   14  |  34.7 |        |        |  4,400  |        |        |       |       |         |       |       |          |
+| Douglas fir     |  6 by 6  |  515  |  30.7 |  2,780 |  1,181 |  3,500  | 4 by 8 |    16  |  259  |  30.3 |    570  |  531  |  29.7 |     765  |
+|                 |  5 by 6  |  170  |  30.9 |  2,720 |  2,123 |  3,490  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  902  |  29.8 |  3,500 |  1,925 |  4,030  |        |        |       |       |         |       |       |          |
+| Douglas fir     |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+|   (fire-killed) |  6 by 6  |  108  |  34.8 |  2,620 |  1,801 |  3,290  | 6 by 8 |    16  |   24  |  33.7 |    368  |   77  |  35.8 |     631  |
+|                 |  2 by 2  |  204  |  37.9 |        |        |  3,430  |        |        |       |       |         |       |       |          |
+| Shortleaf pine  |  6 by 6  |   95  |  41.2 |  2,514 |  1,565 |  3,436  | 5 by 8 |    16  |   12  |  37.7 |    361  |  179  |  47.0 |     704  |
+|                 |  5 by 8  |   23  |  43.5 |  2,241 |  1,529 |  3,423  | 5 by 8 |    14  |   12  |  42.8 |    366  |       |       |          |
+|                 |  2 by 2  |  281  |  51.4 |        |        |  3,570  | 5 by 8 |    12  |   24  |  53.0 |    325  |       |       |          |
+|                 |          |       |       |        |        |         | 5 by 5 |     8  |   24  |  47.0 |    344  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  277  |  48.5 |    400  |       |       |          |
+| Western larch   |  6 by 6  |  107  |  49.1 |  2,675 |  1,575 |  3,510  | 6 by 8 |    16  |   22  |  43.6 |    417  |  179  |  40.7 |     700  |
+|                 |  2 by 2  |  491  |  50.6 |  3,026 |  1,545 |  3,696  | 6 by 8 |    12  |   20  |  40.2 |    416  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 6 |     6  |   53  |  52.8 |    478  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |   30  |  50.4 |    472  |       |       |          |
+| Loblolly pine   |  8 by 8  |   14  |  63.4 |  1,560 |    365 |  2,140  | 8 by 4 |     8  |   16  |  67.2 |    392  |  121  |  83.2 |     630  |
+|                 |  4 by 8  |   18  |  60.0 |  2,430 |    691 |  3,560  | 4 by 4 |     8  |   38  |  44.6 |    546  |       |       |          |
+|                 |  2 by 2  |   53  |  74.0 |        |        |  3,240  |        |        |       |       |         |       |       |          |
+| Tamarack        |  6 by 7  |    4  |  49.9 |  2,332 |  1,432 |  3,032  |        |        |       |       |         |   24  |  39.2 |     668  |
+|                 |  4 by 7  |    6  |  27.7 |  2,444 |  1,334 |  3,360  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  165  |  36.8 |        |        |  3,190  |        |        |       |       |         |       |       |          |
+| Western hemlock |  6 by 6  |   82  |  46.6 |  2,905 |  1,617 |  3,355  | 6 by 4 |     6  |   30  |  48.7 |    434  |   54  |  65.7 |     630  |
+|                 |  2 by 2  |  131  |  55.6 |  2,938 |  1,737 |  3,392  |        |        |       |       |         |       |       |          |
+| Redwood         |  6 by 6  |   34  |  83.6 |  3,194 |  1,240 |  3,882  | 6 by 8 |    16  |   13  |  86.7 |    473  |  148  |  84.2 |     742  |
+|                 |  2 by 2  |  143  |  36.8 |  3,490 |  1,222 |  3,980  | 6 by 6 |    12  |   14  |  83.0 |    424  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 7 |     9  |   13  |  74.7 |    477  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 3 |    14  |   13  |  75.6 |    411  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |    12  |   12  |  66.5 |    430  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |    10  |   11  |  55.0 |    423  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |     8  |   12  |  56.7 |    396  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  186  |  75.5 |    569  |       |       |          |
+| Norway pine     |  6 by 7  |    5  |  29.0 |  1,928 |    905 |  2,404  |        |        |       |       |         |   20  |  26.7 |     589  |
+|                 |  4 by 7  |    8  |  28.4 |  2,154 |  1,063 |  2,652  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  178  |  26.8 |        |        |  2,504  |        |        |       |       |         |       |       |          |
+| Red spruce      |  2 by 2  |   58  |  35.4 |        |        |  2,750  | 2 by 2 |     2  |   43  |  31.8 |    310  |   30  |  32.0 |     758  |
+| White spruce    |  2 by 2  |   84  |  61.0 |        |        |  2,370  | 2 by 2 |     2  |   46  |  50.4 |    270  |   40  |  58.0 |     651  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                                       |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                              |
+|    M. of E. = Modulus of elasticity.                                                                                                          |
+|    M. of R. = Modulus of rupture.                                                                                                             |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                                     |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XX                                                    TABLE XX                                                              |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                                              BENDING TESTS ON AIR-SEASONED MATERIAL                                               |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                 |      Sizes      |       |       |       |   F.S. at E.L.  |     M. of R.    |     M. of E.    |   Calculated    |
+|                 |-----------------| Num-  | Per   | Rings |                 |                 |                 |      shear      |
+|     Species     |          |      |  ber  | cent  |  per  |-----------------+-----------------+-----------------+-----------------|
+|                 |  Cross   | Span | of    | mois- | inch  | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio |
+|                 | Section  |      | tests |  ture |       | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" |
+|                 |          |      |       |       |       |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |
+|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------|
+|                 |          |      |       |       |       |         |       |         |       |  1,000  |       |         |       |
+|                 |  Inches  | Ins. |       |       |       |   Lbs.  |       |   Lbs.  |       |   lbs.  |       |   Lbs.  |       |
+|                 |          |      |       |       |       |         |       |         |       |         |       |         |       |
+| Longleaf pine   |  8 by 16 |  180 |    5  |  22.2 |  16.0 |  3,390  |  0.50 |  4,274  |  0.37 |  1,747  |  1.00 |    288  |  0.75 |
+|                 |  6 by 16 |  132 |    1  |  23.4 |  17.1 |  3,470  |   .51 |  6,610  |   .57 |  1,501  |   .86 |    388  |  1.01 |
+|                 |  6 by 10 |  177 |    2  |  19.0 |   8.8 |  4,560  |   .68 |  7,880  |   .68 |  1,722  |   .99 |    214  |   .56 |
+|                 |  4 by 11 |  180 |    1  |  18.4 |  23.9 |  3,078  |   .46 |  8,000  |   .69 |  1,660  |   .95 |    251  |   .66 |
+|                 |  6 by  8 |  177 |    6  |  20.0 |  13.7 |  4,227  |   .63 |  8,196  |   .71 |  1,634  |   .94 |    177  |   .46 |
+|                 |  2 by  2 |   30 |   17  |  15.9 |  13.9 |  6,750  |  1.00 | 11,520  |  1.00 |  1,740  |  1.00 |    383  |  1.00 |
+| Douglas fir     |  8 by 16 |  180 |   91  |  20.8 |  13.1 |  4,563  |   .68 |  6,372  |   .61 |  1,549  |   .91 |    269  |   .64 |
+|                 |  5 by  8 |  180 |   30  |  14.9 |  12.2 |  5,065  |   .76 |  6,777  |   .65 |  1,853  |  1.09 |    218  |   .52 |
+|                 |  2 by  2 |   24 |  211  |  19.0 |  16.4 |  6,686  |  1.00 | 10,378  |  1.00 |  1,695  |  1.00 |    419  |  1.00 |
+| Shortleaf pine  |  8 by 16 |  180 |    3  |  17.0 |  12.3 |  4,220  |   .54 |  6,030  |   .50 |  1,517  |   .85 |    398  |   .98 |
+|                 |  8 by 14 |  180 |    3  |  16.0 |  12.3 |  4,253  |   .55 |  5,347  |   .44 |  1,757  |   .98 |    307  |   .76 |
+|                 |  8 by 12 |  180 |    7  |  16.0 |  12.4 |  5,051  |   .65 |  7,331  |   .60 |  1,803  |  1.01 |    361  |   .89 |
+|                 |  5 by  8 |  180 |    6  |  12.2 |  22.5 |  7,123  |   .92 |  9,373  |   .77 |  1,985  |  1.11 |    301  |   .74 |
+|                 |  2 by  2 |   30 |   67  |  14.2 |  13.7 |  7,780  |  1.00 | 12,120  |  1.00 |  1,792  |  1.00 |    404  |  1.00 |
+| Western larch   |  8 by 16 |  180 |   23  |  18.3 |  21.9 |  3,343  |   .57 |  5,440  |   .53 |  1,409  |   .90 |    349  |   .96 |
+|                 |  8 by 12 |  180 |   29  |  17.8 |  23.4 |  3,631  |   .62 |  6,186  |   .60 |  1,549  |   .99 |    295  |   .81 |
+|                 |  5 by  8 |  180 |   10  |  13.6 |  27.6 |  4,730  |   .80 |  7,258  |   .71 |  1,620  |  1.04 |    221  |   .61 |
+|                 |  2 by  2 |   30 |  240  |  16.1 |  26.8 |  5,880  |  1.00 | 10,254  |  1.00 |  1,564  |  1.00 |    364  |  1.00 |
+| Loblolly pine   |  8 by 16 |  180 |   14  |  20.5 |   7.4 |  4,195  |   .81 |  6,734  |   .72 |  1,619  |  1.10 |    462  |  1.45 |
+|                 |  6 by 16 |  126 |    4  |  20.2 |   5.0 |  2,432  |   .47 |  4,295  |   .46 |  1,324  |   .90 |    266  |   .84 |
+|                 |  6 by 10 |  174 |    3  |  21.3 |   4.7 |  3,100  |   .60 |  6,167  |   .66 |  1,449  |   .99 |    173  |   .54 |
+|                 |  4 by 12 |  174 |    4  |  19.8 |   4.7 |  2,713  |   .52 |  5,745  |   .61 |  1,249  |   .85 |    185  |   .58 |
+|                 |  8 by  8 |  180 |    9  |  22.9 |   4.9 |  2,903  |   .56 |  4,557  |   .48 |  1,136  |   .77 |     93  |   .29 |
+|                 |  6 by  7 |  144 |    2  |  21.1 |   5.0 |  2,990  |   .58 |  4,968  |   .53 |  1,286  |   .88 |    116  |   .36 |
+|                 |  4 by  8 |  132 |    8  |  19.5 |   9.1 |  3,384  |   .65 |  6,194  |   .66 |  1,200  |   .82 |    196  |   .62 |
+|                 |  2 by  2 |   30 |  123  |  17.6 |   6.6 |  5,170  |  1.00 |  9,400  |  1.00 |  1,467  |  1.00 |    318  |  1.00 |
+| Tamarack        |  6 by 12 |  162 |    5  |  23.0 |  15.1 |  3,434  |   .45 |  5,640  |   .43 |  1,330  |   .82 |    318  |   .75 |
+|                 |  4 by 10 |  162 |    4  |  14.4 |   9.7 |  4,100  |   .54 |  5,320  |   .41 |  1,386  |   .84 |    252  |   .59 |
+|                 |  2 by  2 |   30 |   47  |  11.3 |  16.2 |  7,630  |  1.00 | 13,080  |  1.00 |  1,620  |  1.00 |    425  |  1.00 |
+| Western hemlock |  8 by 16 |  180 |   44  |  17.7 |  17.8 |  4,398  |   .69 |  6,420  |   .62 |  1,737  |  1.04 |    406  |  1.06 |
+|                 |  2 by  2 |   28 |  311  |  17.9 |  19.4 |  6,333  |  1.00 | 10,369  |  1.00 |  1,666  |  1.00 |    382  |  1.00 |
+| Redwood         |  8 by 16 |  180 |    6  |  26.3 |  22.4 |  3,797  |   .79 |  4,428  |   .57 |  1,107  |   .96 |    294  |  1.05 |
+|                 |  6 by 12 |  180 |    6  |  16.1 |  17.7 |  3,175  |   .66 |  3,353  |   .43 |    728  |   .64 |    167  |   .60 |
+|                 |  7 by  9 |  180 |    6  |  15.9 |  15.2 |  3,280  |   .69 |  4,002  |   .51 |  1,104  |   .96 |    147  |   .53 |
+|                 |  3 by 14 |  180 |    6  |  13.1 |  24.4 |         |       |  5,033  |   .64 |         |       |    291  |  1.04 |
+|                 |  2 by 12 |  180 |    5  |  13.8 |  14.4 |  3,928  |   .82 |  5,336  |   .68 |  1,249  |  1.09 |    260  |   .93 |
+|                 |  2 by 10 |  180 |    5  |  13.8 |  24.8 |  3,757  |   .79 |  4,606  |   .59 |  1,198  |  1.05 |    186  |   .67 |
+|                 |  2 by  8 |  180 |    6  |  13.7 |  20.7 |  4,314  |   .90 |  5,050  |   .65 |  1,313  |  1.15 |    166  |   .60 |
+|                 |  2 by  2 |   28 |  122  |  15.2 |  18.8 |  4,777  |  1.00 |  7,798  |  1.00 |  1,146  |  1.00 |    279  |  1.00 |
+| Norway pine     |  6 by 12 |  162 |    5  |  16.7 |   8.1 |  2,968  |   .56 |  5,204  |   .61 |  1,123  |   .97 |    286  |  1.02 |
+|                 |  4 by 10 |  162 |    5  |  13.7 |  12.0 |  5,170  |   .98 |  6,904  |   .82 |  1,712  |  1.48 |    317  |  1.13 |
+|                 |  2 by  2 |   30 |   60  |  14.9 |  11.2 |  5,280  |  1.00 |  8,470  |  1.00 |  1,158  |  1.00 |    281  |  1.00 |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                           |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                  |
+|    M. of E. = Modulus of elasticity.                                                                                              |
+|    M. of R. = Modulus of rupture.                                                                                                 |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                         |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                      |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XXI                                                         TABLE XXI                                                                   |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                                             COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL                                              |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |                     Compression                      |                Compression                |           Shear          |
+|                 |                  parallel to grain                   |          perpendicular  to grain          |                          |
+|                 |------------------------------------------------------+-------------------------------------------+--------------------------|
+|                 |          |       |       | Cr.    |        | Cr.     |        |        |       |       | Cr.     |       |       |          |
+|     Species     |          |       | Per   | str.   | M. of  | str.    |        |        |       | Per   | str.    |       | Per   |          |
+|                 | Size of  |  No.  | cent  | at     | E.     | at max. | Stress |        |  No.  | cent  | at max. |  No.  | cent  |  Shear   |
+|                 | specimen |  of   | of    | E. L.  | per    | ld.,.   | area   | Height |  of   | of    | ld.,    |  of   | of    | strength |
+|                 |          | tests | mois- | per    | square | per     |        |        | tests | mois- | per     | tests | mois- |          |
+|                 |          |       |  ture | square | inch   | square  |        |        |       |  ture | square  |       |  ture |          |
+|                 |          |       |       | inch   |        | inch    |        |        |       |       | inch    |       |       |          |
+|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------|
+|                 |          |       |       |        |  1,000 |         |        |        |       |       |         |       |       |          |
+|                 |  Inches  |       |       |  Lbs.  |  lbs.  |  Lbs.   | Inches | Inches |       |       |   Lbs.  |       |       |   Lbs.   |
+|                 |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+| Longleaf pine   |  4 by 5  |   46  |  26.3 |  3,480 |        |  4,800  | 4 by 5 |     4  |   22  | 25.1  |    572  |   52  |  20.2 |     984  |
+| Douglas fir     |  6 by 6  |  259  |  20.3 |  3,271 |  1,038 |  4,258  | 4 by 8 |    16  |   44  | 20.8  |    732  |  465  |  22.1 |     822  |
+|                 |  2 by 2  |  247  |  18.7 |  3,842 |  1,084 |  5,002  | 4 by 8 |    10  |   32  | 18.1  |    584  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     8  |   51  | 20.2  |    638  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     6  |   49  | 24.0  |    613  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |   29  | 24.8  |    603  |       |       |          |
+| Shortleaf pine  |  6 by 6  |   29  |  15.7 |  4,070 |  1,951 |  6,030  | 8 by 5 |    16  |    4  | 17.8  |    725  |   85  |       |   1,135  |
+|                 |  2 by 2  |   57  |  14.2 |        |        |  6,380  | 8 by 5 |    14  |    3  | 16.3  |    757  |       |       |          |
+|                 |          |       |       |        |        |         | 8 by 5 |    12  |    5  | 15.1  |    730  |       |       |          |
+|                 |          |       |       |        |        |         | 5 by 5 |     8  |    6  | 13.0  |    918  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |   57  | 13.9  |    926  |       |       |          |
+| Western larch   |  6 by 6  |  112  |  16.0 |        |        |  5,445  | 8 by 6 |    16  |   17  | 18.8  |    491  |  193  |  15.0 |     905  |
+|                 |  4 by 4  |   81  |  14.7 |        |        |  6,161  | 8 by 6 |    12  |   18  | 17.6  |    526  |       |       |          |
+|                 |  2 by 2  |  270  |  14.8 |        |        |  5,934  | 5 by 4 |     8  |   22  | 13.3  |    735  |       |       |          |
+| Loblolly pine   |  6 by 6  |   23  |       |  3,357 |  1,693 |  5,005  | 8 by 5 |    16  |   12  | 19.8  |    602  |  156  |  11.3 |   1,115  |
+|                 |  5 by 5  |   10  |  22.4 |  2,217 |    545 |  2,950  | 8 by 5 |     8  |    7  | 22.9  |    679  |       |       |          |
+|                 |  4 by 8  |    8  |  19.4 |  3,010 |    633 |  3,920  | 4 by 5 |     8  |    8  | 19.5  |    715  |       |       |          |
+|                 |  2 by 2  |   69  |       |        |        |  5,547  |        |        |       |       |         |       |       |          |
+| Tamarack        |  6 by 7  |    3  |  15.7 |  2,257 |  1,042 |  3,323  | 2 by 2 |     2  |   57  | 16.2  |    697  |   60  |  14.0 |     879  |
+|                 |  4 by 7  |    3  |  13.6 |  3,780 |  1,301 |  4,823  |        |        |       |       |         |       |       |          |
+|                 |  4 by 4  |   57  |  14.9 |  3,386 |  1,353 |  4,346  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |   66  |  14.6 |        |        |  4,790  |        |        |       |       |         |       |       |          |
+| Western hemlock |  6 by 6  |  102  |  18.6 |  4,840 |  2,140 |  5,814  | 7 by 6 |    15  |   25  | 18.2  |    514  |  131  |  17.7 |     924  |
+|                 |  2 by 2  |  463  |  17.0 |  4,560 |  1,923 |  5,403  | 6 by 6 |     6  |   26  | 16.8  |    431  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |    6  | 15.9  |    488  |       |       |          |
+| Redwood         |  6 by 6  |   18  |  16.9 |        |        |  4,276  | 8 by 6 |    16  |    5  | 25.4  |    548  |   95  |  12.4 |     671  |
+|                 |  2 by 2  |  115  |  14.6 |        |        |  5,119  | 6 by 6 |    12  |    6  | 14.7  |    610  |       |       |          |
+|                 |          |       |       |        |        |         | 7 by 6 |     9  |    5  | 14.8  |    500  |       |       |          |
+|                 |          |       |       |        |        |         | 3 by 6 |    14  |    2  | 12.6  |    470  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |    12  |    2  | 16.2  |    498  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |    10  |    4  | 14.3  |    511  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |     8  |    2  | 13.2  |    429  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  145  | 13.8  |    564  |       |       |          |
+| Norway pine     |  6 by 7  |    4  |  15.2 |  2,670 |  1,182 |  4,212  | 2 by 2 |     2  |   36  | 10.0  |    924  |   44  |  11.9 |   1,145  |
+|                 |  4 by 7  |    2  |  22.2 |  3,275 |  1,724 |  4,575  |        |        |       |       |         |       |       |          |
+|                 |  4 by 4  |   55  |  16.6 |  3,048 |  1,367 |  4,217  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |   44  |  11.2 |        |        |  7,550  |        |        |       |       |         |       |       |          |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                                       |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                              |
+|    M. of E. = Modulus of elasticity.                                                                                                          |
+|    M. of R. = Modulus of rupture.                                                                                                             |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                                     |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XXII                                                                         TABLE XXII                                                                                    |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                                                                [b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMBER[c]                                                                 |
+|                                                                       EXPRESSED IN POUNDS PER SQUARE INCH                                                                        |
+|                                                      (From Manual of the American Railway Engineering Assn., 1911, p. 153)                                                       |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| NOTE.--The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles the unit-stresses may be increased          |
+| twenty-five (25) per cent. For  buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit-stresses may be   |
+| increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when the load is first applied, only fifty (50) per cent of the  |
+| corresponding modulus of elasticity given in the table is to be employed.                                                                                                        |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |             BENDING             |                 SHEARING                |                      COMPRESSION                                        |          |
+|                 |---------------------------------+-----------------------------------------+-------------------------------------------------------------------------|  Ratio   |
+|                 |      Extreme       | Modulus of |    Parallel to     |    Longitudinal    |   Perpendicular    |    Parallel to     | For       | Formul� for       |    of    |
+|                 |       fibre        | elasticity |     the grain      |    shear in        |    to the grain    |     the grain      | columns   | working stress in |  length  |
+|    KIND OF      |       stress       |            |                    |    beams           |                    |                    | under 15  | long columns over |    of    |
+|    TIMBER       |--------------------+------------+--------------------+--------------------+--------------------+--------------------| diameters | 15 diameters      | stringer |
+|                 | Average  | Working |            | Average  | Working | Elastic  | Working | Elastic  | Working | Average  | Working | working   |                   |    to    |
+|                 | ultimate | stress  |  Average   | ultimate | stress  | limit    | stress  | limit    | stress  | ultimate | stress  | stress    |                   |  depth   |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Douglas fir     |   6100   |   1200  | 1,510,000  |   690    |   170   |    270   |   110   |    630   |   310   |   3600   |   1200  |    900    | 1200(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Longleaf pine   |   6500   |   1300  | 1,610,000  |   720    |   180   |    300   |   120   |    520   |   260   |   3800   |   1300  |    980    | 1300(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Shortleaf pine  |   5600   |   1100  | 1,480,000  |   710    |   170   |    330   |   130   |    340   |   170   |   3400   |   1100  |    830    | 1100(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| White pine      |   4400   |    900  | 1,130,000  |   400    |   100   |    180   |    70   |    290   |   150   |   3000   |   1000  |    750    | 1000(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Spruce          |   4800   |   1000  | 1,310,000  |   600    |   150   |    170   |    70   |    370   |   180   |   3200   |   1100  |    830    | 1100(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Norway pine     |   4200   |    800  | 1,190,000  |   590[d] |   130   |    250   |   100   |          |   150   |  2600[d] |    800  |    600    |  800(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Tamarack        |   4600   |    900  | 1,220,000  |   670    |   170   |    260   |   100   |          |   220   |  3200[d] |   1000  |    750    | 1000(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Western hemlock |   5800   |   1100  | 1,480,000  |   630    |   160   |   270[d] |   100   |    440   |   220   |   3500   |   1200  |    900    | 1200(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Redwood         |   5000   |    900  |   800,000  |   300    |    80   |          |         |    400   |   150   |   3300   |    900  |    680    |  900(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Bald cypress    |   4800   |    900  | 1,150,000  |   500    |   120   |          |         |    340   |   170   |   3900   |   1100  |    830    | 1100(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Red cedar       |   4200   |    800  |   800,000  |          |         |          |         |    470   |   230   |   2800   |    900  |    680    |  900(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| White oak       |   5700   |   1100  | 1,150,000  |   840    |   210   |    270   |   110   |    920   |   450   |   3500   |   1300  |    980    | 1300(1-_l_/60_d_) |    12    |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| These unit-stresses are for a green condition of timber and are                                                                                     _l_ = Length in inches.      |
+| to be used without increasing the live load stresses for impact.                                                                                    _d_ = Least side in inches.  |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.]                                                                                                                          |
+|[Footnote c: Green timber in exposed work.]                                                                                                                                       |
+|[Footnote d: Partially air-dry]                                                                                                                                                   |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+
+
+
+
+BIBLIOGRAPHY
+
+
+
+  Part I: Some general works on mechanics, materials of
+            construction, and testing of materials.
+
+ Part II: Publications and articles on the mechanical properties
+            of wood, and timber testing.
+
+Part III: Publications of the U.S. Government on the mechanical
+            properties of wood, and timber testing.
+
+
+
+PART 1. SOME GENERAL WORKS ON MECHANICS, MATERIALS OF
+          CONSTRUCTION, AND TESTING OF MATERIALS
+
+
+ALLAN, WILLIAM: Strength of beams under transverse loads. New
+York, 1893.
+
+ANDERSON, SIR JOHN: The strength of materials and structures.
+London, 1902.
+
+BARLOW, PETER: Strength of materials, 1st ed. 1817; rev. 1867.
+
+BURR, WILLIAM H.: The elasticity and resistance of the materials
+of engineering. New York, 1911.
+
+CHURCH, IRVING P.: Mechanics of engineering. New York, 1911.
+
+HATFIELD, R.G.: Theory of transverse strain. 1877.
+
+HATT, W.K., and SCOFIELD, H.H.: Laboratory manual of testing
+materials. New York, 1913.
+
+JAMESON, J.M.: Exercises in mechanics. (Wiley technical series.)
+New York, 1913.
+
+JAMIESON, ANDREW: Strength of materials. (Applied mechanics and
+mechanical engineering, Vol. II.) London, 1911.
+
+JOHNSON, J.B.: The materials of construction. New York, 1910.
+
+KENT, WILLIAM: The strength of materials. New York, 1890.
+
+KOTTCAMP, J.P.: Exercises for the applied mechanics laboratory.
+(Wiley technical series.) New York, 1913.
+
+LANZA, GAETANO: Applied mechanics. New York, 1901.
+
+MERRIMAN, MANSFIELD: Mechanics of materials. New York, 1912.
+
+MURDOCK, H.E.: Strength of materials. New York, 1911.
+
+RANKINE, WILLIAM J.M.: A manual of applied mechanics. London,
+1901.
+
+THIL, A.: Conclusion de l'�tude pr�sent�e � la Commission des
+m�thodes d'essai des mat�riaux de construction. Paris, 1900.
+
+THURSTON, ROBERT H.: A treatise on non-metallic materials of
+engineering: stone, timber, fuel, lubricants, etc. (Materials of
+engineering, Part I.) New York, 1899.
+
+UNWIN, WILLIAM C.: The testing of materials of construction.
+London, 1899.
+
+WATERBURY, L.A.: Laboratory manual for testing materials of
+construction. New York, 1912.
+
+WOOD, DEVOLSON: A treatise on the  resistance of materials. New
+York, 1897.
+
+
+
+PART II. PUBLICATIONS AND ARTICLES ON THE MECHANICAL PROPERTIES
+           OF WOOD, AND TIMBER TESTING
+
+
+ABBOT, ARTHUR V.: Testing machines, their history, construction
+and use. Van Nostrand's Eng. Mag., Vol. XXX, 1884, pp. 204-214;
+325-344; 382-397; 477-490.
+
+ADAMS, E.E.: Tests to determine the strength of bolted timber
+joints. Cal. Jour, of Technology, Sept., 1904.
+
+ALVAREZ, ARTHUR C.: The strength of long seasoned Douglas fir
+and redwood. Univ. of Cal. Pub. in Eng., Vol. I, No. 2,
+Berkeley, 1913, pp. 11-20.
+
+BARLOW, PETER: An essay on the strength and stress of timber.
+London, 1817; 3d ed., 1826.
+
+----: Experiments on the strength of different kinds of wood
+made in the carriage department, Royal Arsenal, Woolwich. Jour.
+Franklin Inst., Vol. X, 1832, pp. 49-52. Reprinted from
+Philosophical Mag. and Annals of Philos., No. 63, Mch., 1832.
+
+BATES, ONWARD: Pine stringers and floorbeams for bridges. Trans.
+Am. Soc. C.E., Vol. XXIII.
+
+BAUSCHINGER, JOHANN: Untersuchungen �ber die Elasticit�t und
+Festigkeit von Fichten- und Kiefernbauh�lzern. Mitt. a. d.
+mech.-tech. Laboratorium d. k. techn. Hochschule in M�nchen, 9.
+Hft., M�nchen, 1883.
+
+----: Verhandlungen der M�nchener Conferenz und der von ihr
+gew�hlten st�ndigen Commission zur Vereinbarung einheitlicher
+Pr�fungsmethoden f�r Bau- und Constructions-material. _Ibid._,
+14. Hft., 1886.
+
+----: Untersuchungen �ber die Elasticit�t und Festigkeit
+verschiedener Nadelh�lzer. _Ibid._, 16. Hft., 1887.
+
+BEARE, T. HUDSON: Timber: its strength and how to test it.
+Engineering, London, Dec. 9, 1904.
+
+BEAUVERIE, J.: Le bois. I. Paris, 1905, pp. 105-185.
+
+----: Les bois industriels. Paris, 1910, pp. 55-77. Bending
+tests with wood, executed at the Danish State Testing
+Laboratory, Copenhagen. Proc. Int. Assn. Test. Mat., 1912,
+XXIII_{2}, pp. 17. See also Eng. Record, Vol. LXVI, 1912, p.
+269.
+
+BERG, WALTER G.: Berg's complete timber test record. Chicago,
+1899. Reprint from Am. By. Bridges and Buildings. BOULGER, G.S.:
+Wood. London, 1908, pp. 112-121.
+
+BOUNICEAU,--: Note et exp�riences sur la torsion des bois.
+[N.p., n.d.]
+
+BOVEY, HENRY T.: Results of experiments at McGill University,
+Montreal, on the strength of Canadian Douglas fir, red pine,
+white pine, and spruce. Trans. Can. Soc. C.E., Vol. IX, Part I,
+1895, pp. 69-236.
+
+BREUIL, M. PIERRE: Contribution to the discussion on the testing
+of wood. Proc. Int. Assn. Test. Mat., 1906, Disc, 1_e_, pp. 2.
+
+BROWN, T.S.: An Account of some experiments made by order of
+Col. Totten, at Fort Adams, Newport, R.I., to ascertain the
+relative stiffness and strength of the following kinds of
+timber, viz.: white pine (_Pinus strobus_), spruce (_Abies
+nigra_), and southern pine (_Pinus australis_), also called
+long-leaved pine. Jour. Franklin Inst., Vol. VII (n.s.), 1831,
+pp. 230-238.
+
+BUCHANAN, C.P.: Some tests of old timber. Eng. News, Vol. LXIV,
+No. 23, 1910, p. 67.
+
+BUSGEN, M.: Zur Bestimmung der Holzh�rten. Zeitschrift f. Forst-
+und Jagdwesen. Berlin, 1904, pp. 543-562.
+
+CHEVANDIER, E., et WERTHEIM, G.: M�moire sur les propri�t�s
+m�caniques du bois. Paris, 1846.
+
+CIESLAR, A.: Studien �ber die Qualit�t rasch erwachsenen
+Fichtenholzes. Centralblatt f. d. ges. Forstwesen, Wien, 1902,
+pp. 337-403.
+
+CLINE, McGARVEY: Forest Service investigations of American woods
+with special reference to investigations of mechanical
+properties. Proc. Int. Assn. Test. Mat., 1912, XXIII_{5}, pp.
+17.
+
+----: Forest Service tests to determine the influence of
+different methods and rates of loading on the strength and
+stiffness of timber. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908,
+pp. 535-540.
+
+----: The Forest Products Laboratory: its purpose and work.
+Proc. Am. Soc. Test. Mat., Vol. X, 1910, pp. 477-489.
+
+----: Specifications and grading rules for Douglas fir timber:
+an analysis of Forest Service tests on structural timbers. Proc.
+Am. Soc. Test. Mat., Vol. XI, 1911, pp. 744-766.
+
+Comparative strength and resistance of various tie timbers.
+Elec. Traction Weekly, Chicago, June 15, 1912.
+
+DAY, FRANK M.: Microscopic examination of timber with regard to
+its strength. 1883, pp. 6.
+
+DEWELL, H.D.: Tests of some joints used in heavy timber framing.
+Eng. News, Mch. 19, 1914, pp. 594-598; _et seq._
+
+D�RR, KARL: Die Festigkeit von Fichten- und Kiefernholz.
+Deutsche Bauzeitung, Berlin, Aug. 17, 1910. See also Zeitschrift
+d. ver. deutsch. Ing., Bd. 54, Nr. 36, 1910, p. 1503.
+
+DUPIN, CHARLES: Exp�riences sur la flexibilit�, la force, et
+l'�lasticit� des bois. Jour, de l'�cole Polytechnique, Vol. X,
+1815.
+
+DUPONT, ADOLPHE, et BOUQUET DE LA GRYE: Les bois indig�nes et
+�trangers. Paris, 1875, pp. 273-352.
+
+ESTRADA, ESTEBAN DUQUE: On the strength and other properties of
+Cuban woods. Van Nostrand's Eng. Mag., Vol. XXIX, 1883, pp.
+417-426; 443-449.
+
+EVERETT, W.H.: Memorandum on mechanical tests of some Indian
+timbers. Govt. Bul. No. 6 (o.s.), Calcutta.
+
+EXNER, WILHELM FRANZ: Die mechanische Technologie des Holzes.
+Wien, 1871. (A translation and revision of Chevandier and
+Wertheim's M�moire sur les propri�t�s m�caniques du bois.)
+
+----: Die technischen Eigenschaften der H�lzer. Lorey's Handbuch
+der Forstwissenschaft, II. Bd., 6. Kap., T�bingen, 1903.
+
+FERNOW, B.E.: Scientific timber testing. Digest of Physical
+Tests, Vol. I, No. 2, 1896, pp. 87-95.
+
+FOWKE, FRANCIS: Experiments on British colonial and other woods.
+1867.
+
+GARDNER, ROLAND: I. Mechanical tests, properties, and uses of
+thirty Philippine woods. II. Philippine sawmills, lumber market
+and prices. Bul. 4, Bu. For., P.I., 1906. (2d ed., 1907,
+contains tests of 34 woods.)
+
+GAYER, KARL: Forest utilization. (Vol. V, Schlich's Manual of
+Forestry. Translation of Die Forstbenutzung, Berlin, 1894.)
+London, 1908.
+
+GOLLNER, H.: Ueber die Festigkeit des Schwarzf�hrenholzes. Mitt.
+a. d. forstl. Versuchswesen Oesterreichs. II. Bd., 3. Hft.,
+Wien, 1881.
+
+GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der
+Baumaterialien. 3d ed., Berlin, 1880.
+
+GREEN, A.O.: Tasmanian timbers: their qualities and uses.
+Hobart, Tasmania, 1903, pp. 63.
+
+GREGORY, W.B.: Tests of creosoted timber. Trans. Am. Soc. C.E.,
+Vol. LXXVI, 1913, pp. 1192-1203. See also _ibid._, Vol. LXX, p.
+37.
+
+GRISARD, JULES, et VANDENBERGHE, MAXIMILIEN: Les bois
+industriels, indig�nes et exotiques; synonymie et description
+des esp�ces, propri�t�s physiques des bois, qualit�s, d�fauts,
+usages et emplois. Paris, 189-. From Bul. de la Soci�t�
+nationale d'acclimatation de France, Vols. XXXVIII-XL.
+
+Hardwoods of Western Australia. Engineering, Vol. LXXXIII, Jan.
+11, 1907, pp. 35-37.
+
+HATT, WILLIAM KENDRICK: A Preliminary program for the timber
+test work to be undertaken by the Bureau of Forestry, United
+States Department of Agriculture. Proc. Am. Soc. Test. Mat.,
+Vol. III, 1903, pp. 308-343. Appendix I: Method of determining
+the effect of the rate of application of load on the strength of
+timber, pp. 325-327; App. II: A discussion on the effect of
+moisture on strength and stiffness of timber, together with a
+plan of procedure for future tests, pp. 328-334.
+
+HATT, WILLIAM KENDRICK: Relation of timber tests to forest
+products. Proc. Int. Assn. Test. Mat., 1906, C 2 _e_, pp. 6.
+
+----: Structural timber. Proc. Western Ry. Club, St. Louis, Mch.
+17, 1908.
+
+----: Abstract of report on the present status of timber tests
+in the Forest Service, United States Department of Agriculture.
+Proc. Int. Assn. Test. Mat., 1909, XVL, pp. 10.
+
+---- and TURNER, W.P.: The Purdue University impact machine.
+Proc. Am. Soc. Test. Mat., Vol. VI, 1906, pp. 462-475.
+
+HAUPT, HERMAN: formul� for the strain upon timber. Center of
+gravity of an ungula and semi-cylinder. Jour. Franklin Inst.,
+Vol. XIX, 3d series, 1850, pp. 408-413.
+
+HEARDING, W.H.: Report upon experiments ... upon the compressive
+power of pine and hemlock timber. Washington, 1872, pp. 12.
+
+HOWE, MALVERD A.: Wood in compression; bearing values for
+inclined cuts. Eng. News, Vol. LXVIII, 1912, pp. 190-191.
+
+HOYER, EGBERT: Lehrbuch der vergleichenden mechanischen
+Technologie. 1878.
+
+IHLSENG, MANGUS C.: On the modulus of elasticity of some
+American woods as determined by vibration. Van Nostrand's Eng.
+Mag., Vol. XIX, 1878, pp. 8-9.
+
+----: On a mode of measuring the velocity of sounds in woods.
+Am. Jour. Sci. and Arts, Vol. XVII, 1879.
+
+JACCARD, P.: �tude anatomique des bois comprim�s. Mitt. d. Schw.
+Centralanstalt f. d. forst. Versuchswesen. X. Bd., 1. Hft.,
+Zurich, 1910, pp. 53-101.
+
+JANKA, GABRIEL: Untersuchungen �ber die Elasticit�t und
+Festigkeit der �sterreichischen Bauh�lzer. I. Fichte S�dtirols;
+II. Fichte von Nordtirol vom Wienerwalde und Erzgebirge; III.
+Fichte aus den Karpaten, aus dem B�hmerwalde, Ternovanerwalde
+und den Zentralalpen. Technische Qualit�t des Fichtenholzes im
+allgemeinen; IV. L�rche aus dem Wienerwalde, aus Schlesien,
+Nord- und S�dtirol. Mitt. a. d. forst. Untersuchungswesen
+Oesterreichs, Wien, 1900-13.
+
+----: Untersuchungen �ber Holzqualit�t. Centralblatt f. d. ges.
+Forstwesen. Wien, 1904, pp. 95-115.
+
+----: Ueber neuere holztechnologische Untersuchungen. Oesterr.
+Vierteljahresschrift f�r Forstwesen, Wien, 1906, pp. 248-269.
+
+----: Die H�rte des Holzes. Centralblatt f. d. ges. Forstwesen,
+Wien, 1906, pp. 193-202; 241-260.
+
+JANKA, GABRIEL: Die Einwirkung von S�ss- und Salzw�ssern auf die
+gewerblichen Eigenschaften der Hauptholzarten. I. Teil.
+Untersuchungen u. Ergebnisse in mechanisch-technischer Hinsicht.
+Mitt. a. d. forst. Versuchswesen Oesterreichs, 33. Hft., Wien,
+1907.
+
+----: Results of trials with timber carried out at the Austrian
+forestry testing-station at Mariabrunn. Proc. Int. Assn. Test.
+Mat., 1906, Disc. 2 _e_, pp. 7.
+
+----: Ueber die an der k. k. forstlichen Versuchsanstalt
+Mariabrunnen gewonnenen Resultate der Holzfestigkeitspr�fungen.
+Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Wien, Aug. 9, 1907.
+
+----: Ueber Holzh�rtepruf�ng. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 443-456.
+
+----: Testing the hardness of wood by means of the ball test.
+Proc. Int. Assn. Test. Mat., 1912, XXIII_{3}.
+
+JENNY, K.: Untersuchungen �ber die Festigkeit der H�lzer aus den
+L�ndern der ungarischen Krone. Budapest, 1873.
+
+JOHNSON, J.B.:  Time tests of timber in endwise compression.
+Paper before Section D, Am. Assn. for Adv. of Sci., Aug., 1898.
+
+JOHNSON, WALTER B.: Experiments on the adhesion of iron spikes
+of various forms when driven into different species of timbers.
+Jour. Franklin Inst., Vol. XIX (n.s.), 1837, pp. 281-292.
+
+JULIUS, G.A.: Western Australia timber tests, 1906. The physical
+characteristics of the hardwoods of Western Australia. Perth,
+1906, pp. 36.
+
+----: Supplement to the Western Australia timber tests, 1906.
+The hardwoods of Australia. Perth, 1907, pp. 6.
+
+KARMARSH, CARL: Handbuch der mechanischen Technologie. I. Aufl.,
+1837; V. Aufl., 1875; verm. von H. Fisher, 1888.
+
+KIDDER, F.E.: Experiments on the transverse strength of southern
+and white pine. Van Nostrand's Eng. Mag., Vol. XXII, 1880, pp.
+166-168.
+
+----: Experiments on the strength and stiffness of small spruce
+beams. _Ibid._, Vol. XXIV, 1881, pp. 473-477.
+
+----: Experiments on the fatigue of small spruce beams. Jour.
+Franklin Inst., Vol. CXIV, 1882, pp. 261-279.
+
+KIDWELL, EDGAR: The efficiency of built-up wooden beams. Trans.
+Am. Inst. Min. Eng., Feb., June, 1898.
+
+KIRKALDY, WM. G.: Illustrations of David Kirkaldy's system of
+mechanical testing. London, 1891.
+
+KUMMER, FREDERICK A.: The effects of preservative treatment on
+the strength of timber. Proc. Am. Soc. Test. Mat., Vol. IV,
+1904, pp. 434-438.
+
+LABORD�RE, P., and ANSTETT, F.: Contribution to the study of
+means for improving the strength of wood for pavements. Proc.
+Int. Assn. Test. Mat., 1912, XXIII_{4}, pp. 12.
+
+LANZA, GAETANO: An account of certain tests on the transverse
+strength and stiffness of large spruce beams. Trans. Am. Soc.
+Mech. Eng., Vol. IV, 1882, pp. 119-135. See also Jour. Franklin
+Inst., Vol. XCV, 1883, pp. 81-94.
+
+LASLETT, T.: Properties and characteristics of timber. Chatham,
+1867.
+
+----: Timber and timber trees, native and foreign. (2d ed.
+revised and enlarged by H. Marshall Ward.) London and New York,
+1894.
+
+LEA, W.: Tables of strength and deflection of timber. London,
+1861.
+
+LEDEBUR, A.: Die Verarbeitung des Holzes auf mechanischem Wege.
+1881.
+
+LORENZ, N. VON: Analytische Untersuchung des Begriffes der
+Holzh�rte. Centralblatt f. d. ges. Forstwesen, Wien, 1909, pp.
+348-387.
+
+LUDWIG, PAUL: Die Regelprobe. Ein neues Verfahren zur
+H�rtebestimmung von Materialien. Berlin. 1908.
+
+MACFARLAND, H.B.: Tests of longleaf pine bridge timbers. Bul.
+149, Am. Ry. Eng. Assn., Sept., 1912. See also Eng. News, Dec.
+12, 1912, p. 1035.
+
+McKAY, DONALD: On the weight and strength of American
+ship-timber. Jour. Franklin Inst., Vol. XXXIX (3d series), 1860,
+p. 322.
+
+MALETTE, J.: Essais des bois de construction. Revue Technique,
+Apr. 25, 1905.
+
+MANN, JAMES: Australian timber: its strength, durability, and
+identification. Melbourne, 1900.
+
+MARTIN, CLARENCE A.: Tests on the relation between cross-bending
+and direct compressive strength in timber. Railroad Gazette,
+Mch. 13, 1903.
+
+Methods of testing metals and alloys ... Recommended by the
+Fourth Congress of the International Association for Testing
+Materials, held at Brussels, Sept. 3-6, 1906. London, 1907, pp.
+54. Methods of testing wood, pp. 39-49.
+
+MIKOLASCHEK, CARL: Untersuchungen �ber die Elasticit�t und
+Festigkeit der wichtigsten Bau- und Nutzh�lzer. Mitt. a. d.
+forstl. Versuchswesen Oesterreiches, II. Bd., 1. Hft., Wien,
+1879.
+
+MOELLER, JOSEPH: Die Rohstoffe des Tischler- und
+Drechslergewerbes. I. Theil: Das Holz. Kassel, 1883, pp. 68-122.
+
+MOLESWORTH, G.L.: Graphic diagrams of strength of teak beams.
+Roorke, 1881.
+
+MORGAN, J.J.: Bending strength of yellow pine timber. Eng.
+Record, Vol. LXVII, 1913, pp. 608-609.
+
+MOROTO, K.: Untersuchungen �ber die Biegungselasticit�t und
+-Festigkeit der japanischen Bauh�lzer. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 346-355.
+
+NORDLINGER, H.: Die technischen Eigenschaften der H�lzer f�r
+Forst- und Baubeamte, Technologen und Gewerbetreibende.
+Stuttgart, 1860.
+
+----: Druckfestigkeit des Holzes. 1882.
+
+----: Die gewerblichen Eigenschaften der H�lzer. Stuttgart,
+1890.
+
+NORTH, A.T.: The grading of timber on the strength basis.
+Address before Western Society of Engineers. Lumber World
+Review, May 25, 1914, pp. 27-29.
+
+NORTON, W.A.: Results of experiments on the set of bars of wood,
+iron, and steel, after a transverse stress. Van Nostrand's Eng.
+Mag., Vol. XVII, 1877, pp. 531-535.
+
+PACCINOTTI E PERI: [Investigations into the elasticity of
+timbers.] Il Cimento, Vol. LVIII, 1845.
+
+PALACIO, E.: Tensile tests of timber. La Ingenieria, Buenos
+Aires, May 31, 1903, _et seq._
+
+PARENT,--: Exp�riences sur la r�sistance des bois de ch�ne et de
+sapin. M�moires de l'Acad�mie des Sciences, 1707-08.
+
+Propositions relatives � l'�tablissement d'un prec�d� uniforme
+pour l'essai des qualit�s techniques des bois. Proc. Int. Assn.
+Test. Mat., 1901, Annexe, pp. 13-28.
+
+ROGERS, CHARLES G.: A manual of forest engineering for India.
+Vol. I, Calcutta, 1900, pp. 50-91.
+
+RUDELOFF, M.: Der heutige Stand der Holzuntersuchungen. Mitt. a.
+d. k�niglichen tech. Versuchsanstalt, Berlin, IV, 1899.
+
+----: Principles of a standard method of testing wood. Proc.
+Int. Assn. Test. Mat., 1906, 23 C, pp. 16.
+
+----: Large _vs._ small test-pieces in testing wood. Proc. Int.
+Soc. Test. Mat., 1912, XXIII_{1}, pp. 7.
+
+SARGENT, CHARLES SPRAGUE:  Woods of the United States, with an
+account of their structure, qualities, and uses. New York, 1885.
+
+SCHNEIDER, A.: Zusammengesetzte Tr�ger. Zeitschrift d. Oesterr.
+Ing. u. Arch. Ver., Nov. 24; Dec. 9, 1899.
+
+SCHWAPPACH, A.F.: Beitr�ge zur Kenntniss der Qualit�t des
+Rotbuchenholzes. Zeitschrift f. Forst- und Jagdwesen, Berlin,
+1894, pp. 513-539.
+
+----: Untersuchungen �ber Raumgewicht und Druckfestigkeit des
+Holzes wichtiger Waldb�ume. Berlin, 1897-98.
+
+----: Etablissement de m�thodes uniformes pour l'essai � la
+compression des bois. Proc. Int. Assn. Test. Mat., 1901, Rapport
+23, pp. 28.
+
+SEBERT, H.: Notice sur les bois de la Nouvelle Cal�donie suivie
+de considerations g�nerates sur les propri�t�s m�caniques des
+bois et sur les prec�d�s employ�s pour les mesurer. Paris.
+
+SHERMAN, EDWARD C.: Crushing tests on water-soaked timbers. Eng.
+News, Vol. LXII, 1909, p. 22.
+
+SNOW, CHARLES H.: The principal species of wood: their
+characteristic properties. New York, 1908.
+
+STAUFFER, OTTMAR: Untersuchungen �ber specifisches
+Trockengewicht, sowie anatomisches Verhalten des Holzes der
+Birke. M�nchen, 1892.
+
+STENS, D.: Ueber die Eigenschaftenimpr�gnierter Grubenholzer,
+insbesondere �ber ihre Festigkeit. Gl�ckauf, Essen, Mch. 6,
+1907.
+
+Strength of wood for pavements. Can. Eng., Toronto, Sept. 12,
+1912.
+
+ST�BSCHEN-KISCHNER: Karmarsch-Heerins technisches Wr�terbuch. 3.
+Aufl., 1886.
+
+TALBOT, ARTHUR N.: Tests of timber beams. Bul. 41, Eng. Exp.
+Sta., Univ. of Ill., Urbana, 1910.
+
+Tests of wooden beams made at the Massachusetts Institute of
+Technology on spruce, white pine, yellow pine, and oak beams of
+commercial sizes. Technology Quarterly, Boston, Vol. VII, 1894.
+
+TETMAJER, L. v.: Zur Frage der Knickungsfestigkeit der
+Bauh�lzer. Schweizerische Bauzeitung, Bd. 11, Nr. 17.
+
+----: Methoden und Resultate der Pr�fung der schweizerischen
+Bauh�lzer. Mitt. d. Anstalt z. Pr�fung v. Baumaterialien am
+eidgen�ssischen Polytechnicum in Z�rich. 2. Hft., 1884.
+
+----: Methoden und Resultate der Pr�fung der schweizerischen
+Bauh�lzer. Mitt. d. Materialpr�fungs-Anstalt am Schweiz.
+Polytechnikum in Z�rich. Landesaustellungs-Ausgabe, 2. Hft.,
+Z�rich, 1896.
+
+THELEN, ROLF: The structural timbers  of the Pacific Coast.
+Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 558-567.
+
+THURSTON, R.H.: Torsional resistance of materials determined by
+a new apparatus with automatic registry. Jour. Franklin Inst.,
+Vol. LXV, 1873, pp. 254-260.
+
+----: On the strength of American timber. _Ibid._, Vol. LXXVIII,
+1879, pp. 217-235.
+
+----: Experiments on the strength of yellow pine. _Ibid._, Vol.
+LXXIX, 1880, pp. 157-163.
+
+----: Influence of time on bending strength and elasticity.
+Proc. Am. Assn. for Adv. Sci., 1881. Also Proc. Inst. C.E., Vol.
+LXXI.
+
+----: On the effect of prolonged stress upon the strength and
+elasticity of pine timber. Jour. Franklin Inst., Vol. LXXX,
+1881, pp. 161-169.  THURSTON, R.H.: On Flint's investigations of
+Nicaraguan woods. _Ibid._, Vol. XCIV, 1887, pp. 289-315.
+
+TIEMANN, HARRY DONALD: The effect of moisture and other
+extrinsic factors upon the strength of wood. Proc. Am. Soc.
+Test. Mat., Vol. VII, 1907, pp. 582-594.
+
+----: The effect of the speed of testing upon the strength of
+wood and the standardization of tests for speed. _Ibid._, Vol.
+VIII, 1908, pp. 541-557.
+
+----: The theory of impact and its application to testing
+materials. Jour. Franklin Inst., Vol. CLXVIII, 1909, pp.
+235-259; 336-364.
+
+----: Some results of dead load bending tests of timber by means
+of a recording deflectometer. Proc. Am. Soc. Test. Mat., Vol.
+IX, 1909, pp. 534-548.
+
+TJADEN, M.E.H.: Het Indrukken van Paalkoppen in Kespen. De
+Ingenieur, Sept. 11, 1909.
+
+----: Weerstand van Hout loodrecht op de Vezelrichting. _Ibid._,
+May, 1911.
+
+----: Buigvastheid van Hout. _Ibid._, May 31, 1913.
+
+TRAUTWINE, JOHN C.: Shearing strength of some American woods.
+Jour. Franklin Inst., Vol. CIX, 1880, pp. 105-106.
+
+TREDGOLD, THOMAS: Elementary principles of carpentry. London,
+1870.
+
+TURNBULL, W.: A practical treatise on the strength and stiffness
+of timber. London, 1833.
+
+Untersuchungen �ber den Einfluss des Blauwerdens auf die
+Festigkeit von Kiefernholz. Mitt. a. d. k�nig. techn.
+Versuchsanstalten, I, 1897.
+
+Verfahren zur Pr�fung v. Metallen und Legierungen, von
+hydraulischen Bindemitteln, von Holz, von Ton-, Steinzeug- und
+Zementr�hren. Empfohlen v. dem in Br�ssel v. 3-6, IX, 1906,
+abgeh. IV. Kongress des internationalen Verbandes f. die
+Materialpr�fungen der Technik. Wien, 1907.
+
+WARREN, W.H.: Australian timbers. Sydney, 1892.
+
+----: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Sydney, 1911.
+
+----: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Proc. Int. Assn. Test. Mat., 1912,
+XXIII_{6}, pp. 9.
+
+----: The properties of New South Wales hardwood timbers.
+Builder, London, Nov. 1, 1912.
+
+----: The hardwood timbers of New South Wales, Australia. Jour.
+Soc. of Arts, London, Dec. 6, 1912.
+
+WELLINGTON, A.M.: Experiments on impregnated timber. Railroad
+Gazette, 1880.
+
+WIJKANDER, ----: Untersuchung der Festigkeitseigenschaften
+schwedischer Holzarten in der Materialpr�fungsanstalt des
+Chalmers'schen Institutes ausgef�hrt. 1897.
+
+WING, CHARLES B.: Transverse strength of the Douglas fir. Eng.
+News, Vol. XXXIII, Mch. 14, 1895.
+
+
+
+PART III. PUBLICATIONS OF THE U.S. GOVERNMENT ON THE MECHANICAL
+            PROPERTIES OF WOOD, AND TIMBER TESTING
+
+
+MISCELLANEOUS
+
+
+House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol.
+IX, Tenth Census report.) Report on the forests of North America
+(exclusive of Mexico). Part II, The Woods of the United States.
+
+House Report No. 1442, 53d Cong., 2d sess. Investigations and
+tests of American timber. 1894, pp. 4.
+
+War Dept. Doc. 1. Resolutions of the conventions held at Munich,
+Dresden, Berlin, and Vienna, for the purpose of adopting uniform
+methods for testing construction materials with regard to their
+mechanical properties. By J. Bauschinger. Translated by O.M.
+Carter and E.A. Gieseler. 1896, pp. 44.
+
+War Dept. Doc. 11. On tests of construction materials.
+Translations from the French and from the German. By O.M. Carter
+and E.A. Gieseler. 1896, pp. 84.
+
+House Doc. No. 181, 55th Cong., 3d sess. Report upon the
+forestry investigations of the U.S. Department of Agriculture,
+1877-1898. By B.E. Fernow, 1899, pp. 401. Contains chapter on
+The work in timber physics in the Division of Forestry, by
+Filibert Roth, pp. 330-395.
+
+
+FOREST SERVICE
+
+
+Cir. 7--The Government timber tests [189-], pp. 4.
+
+Cir. 8--Strength of "boxed" or "turpentine" timber. 1892, pp. 4.
+
+Bul. 6--Timber Physics. Pt. I. Preliminary report. 1. Need of
+the investigation. 2. Scope and historical development of the
+science of "timber physics." 3. Organization and methods of
+timber examinations in the Division of Forestry. By B.E. Fernow,
+1892, pp. 57.
+
+Unnumbered Cir.--Instructions for the collection of test pieces
+of pines for timber investigations [1893], pp. 4.
+
+Cir. 9--Effect of turpentine gathering on the timber of longleaf
+pine. By B.E. Fernow [1893], p. 1.
+
+Bul. 8--Timber physics. Pt. II. Progress report. Results of
+investigations on longleaf pine. 1893, pp. 92.
+
+Bul. 10--Timber: an elementary discussion of the characteristics
+and properties of wood. By Filibert Roth. 1895, pp. 88.
+
+Bul. 12--Economical designing of timber trestle bridges. By A.L.
+Johnson, 1896, pp. 57.
+
+Cir. 12--Southern pine, mechanical and physical properties.
+1896, pp. 12.
+
+Cir. 15--Summary of mechanical tests on thirty-two species of
+American woods. 1897, pp. 12.
+
+Cir. 18--Progress in timber physics. 1898, pp. 20.
+
+Cir. 19--Progress in timber physics: Bald cypress (_Taxodium
+distichum_). By Filibert Roth, 1898, pp. 24.
+
+Y.B. Extr. 288--Tests on the physical properties of woods. By
+F.E. Olmstead, 1902, pp. 533-538.
+
+Unnumbered Cir.--Timber tests. [1903], pp. 15.
+
+Unnumbered Cir.--Timber preservation and timber testing at the
+Louisiana Purchase Exposition. 1904, pp. 6.
+
+Cir. 32--Progress report on the strength of structural timber.
+By W.K. Hatt, 1904, pp. 28.
+
+Bul. 58--The red gum. By Alfred Chittenden. Includes a
+discussion of The mechanical properties of red gum wood, by W.K.
+Hatt. 1905, pp. 56.
+
+Cir. 38--Instructions to engineers of timber tests. By W.K.
+Hatt, 1906, pp. 55. Revised edition, 1909, pp. 56.
+
+Cir. 39--Experiments on the strength of treated timber. By W.K.
+Hatt, 1906, pp. 31. Revised edition, 1908.
+
+Bul. 70--Effect of moisture upon the strength and stiffness of
+wood. By H.D. Tiemann, 1906, pp. 144.
+
+Cir. 46--Holding force of railroad spikes in wooden ties. By
+W.K. Hatt, 1906, pp. 7.
+
+Cir. 47--Strength of packing boxes of various woods. By W.K.
+Hatt, 1906, pp. 7.
+
+Cir. 108--The strength of wood as influenced by moisture. By
+H.D. Tiemann, 1907, pp. 42.
+
+Cir. 115--Second progress report on the strength of structural
+timber. By W.K. Hatt, 1907, pp. 39.
+
+Cir. 142--Tests of vehicle and implement woods. By H.B. Holroyd
+and H.S. Betts, 1908, pp. 29.
+
+Cir. 146--Experiments with railway cross-ties. By H.B. Eastman,
+1908, pp. 32.
+
+Cir. 179--Utilization of California eucalypts. By H.S. Betts and
+C. Stowell Smith, 1910, pp. 30.
+
+Bul. 75--California tanbark oak. Part II, Utilization of the
+wood of tanbark oak, by H.S. Betts, 1911, pp. 24-32.
+
+Bul. 88--Properties and uses of Douglas fir. By McGarvey Cline
+and J.B. Knapp, 1911, pp. 75.
+
+Cir. 189--Strength values for structural timbers. By McGarvey
+Cline, 1912, pp. 8.
+
+Cir. 193--Mechanical properties of redwood. By A.L. Heim, 1912,
+pp. 32.
+
+Bul. 108--Tests of structural timbers. By McGarvey Cline and
+A.L. Heim, 1912, pp. 1231.
+
+Bul. 112--Fire-killed Douglas fir: a study of its rate of
+deterioration, usability, and strength. By J.B. Knapp, 1912, pp.
+18.
+
+Bul. 115--Mechanical properties of western hemlock. By O.P.M.
+Goss, 1913, pp. 45.
+
+Bul. 122--Mechanical properties of western larch. By O.P.M.
+Goss, 1913, pp. 45.
+
+Cir. 213--Mechanical properties of woods grown in the United
+States. 1913, pp. 4.
+
+Cir. 214--Tests of packing boxes of various forms. By John A.
+Newlin, 1913, pp. 23.
+
+Review Forest Service Investigations. 1913. [Outline of
+investigations.] Vol. I, pp. 17-21. A microscopic study of the
+mechanical failure of wood, by Warren D. Brush. Vol. II, pp.
+33-38.
+
+Bul. 67, U.S.D.A.--Tests of Rocky Mountain woods for telephone
+poles. By Norman deW. Betts and A.L. Heim, 1914, pp. 28.
+
+Bul. 77, U.S.D.A.--Rocky Mountain mine timbers. By Norman deW.
+Betts, 1914, pp. 34.
+
+Bul. 86, U.S.D.A.--Tests of wooden barrels. By J.A. Newlin,
+1914, pp. 12.
+
+
+REPORTS OF TESTS ON THE STRENGTH OF STRUCTURAL MATERIAL, MADE AT
+THE WATERTOWN ARSENAL, MASS.
+
+
+House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of
+wood grown on the Pacific slope, pp. 19-93.
+
+Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of
+white and yellow pines to forces of compression in the direction
+of the fibers, as used for columns, or posts, pp. 239-395.
+
+Senate Ex. Doc. No. 5, 48th Cong., 1st sess., 1884. Tests of
+California laurel wood by compression, indentation, shearing,
+transverse tension, pp. 223-236. Tests of North American woods
+(under supervision of Prof. C.S. Sargent in charge of the
+forestry division of the Tenth Census), with 16 photographs of
+fractures of American woods, pp. 237-347.
+
+Senate Ex. Doc. No. 35, 49th Cong., 1st sess., 1885. Adhesion of
+nails, spikes, and screws in various woods. Experiments on the
+resistance of cut nails, wire nails (steel), wood screws, lag
+screws in white pine, yellow pine, chestnut, white oak, and
+laurel, pp. 448-471.
+
+House Ex. Doc. No. 14, 51st Cong., 1st sess., 1890. Adhesion of
+spikes and bolts in railroad ties, pp. 595-617.
+
+House Ex. Doc. No. 161, 52d Cong., 1st sess., 1892. Adhesion of
+nails in wood, pp. 744-745.
+
+House Ex. Doc. No. 92, 53d Cong., 3d sess., 1895.
+Woods--compression tests (endwise compression), pp. 471-476.
+
+House Doc. No. 54, 54th Cong., 1st sess., 1896. Compression
+tests on Douglas fir wood, pp. 536-563. Expansion and
+contraction of oak and pine wood, pp. 567-574.
+
+House Doc. No. 164, 55th Cong., 2d sess., 1898. Compression
+tests of timber posts, pp. 405-411. New posts of yellow pine and
+spruce, pp. 413-450; Old yellow pine posts from Boston Fire
+Brick Co. building, No. 394 Federal St., Boston, Mass., pp.
+451-473.
+
+House Doc. No. 143, 55th Cong., 3d sess., 1899. Fire-proofed
+wood (endwise and transverse tests), pp. 676-681.
+
+House Doc. No. 190, 56th Cong., 2d sess., 1901. Cypress wood for
+United States Engineer Corps; compression and transverse tests,
+pp. 1121-1126. Old white pine and red oak from roof trusses of
+Old South Church, Boston, Mass., pp. 1127-1130. Compression of
+rubber, balata, and wood buffers, pp. 1149-1158.
+
+House Doc. No. 335, 57th Cong., 2d sess., 1903. Douglas fir and
+white oak woods. Transverse and shearing tests; also
+observations on heat conductivity of sticks over wood fires and
+a stick exposed to low temperature. Expansion crosswise the
+grain of wood after submersion, pp. 519-561. Adhesion of lag
+screws and bolts in wood, pp. 563-578.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Mechanical Properties of Wood
+by Samuel J. Record
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+  <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
+  <title>The Project Gutenberg eBook of The Mechanical Properties of Wood,
+by Samuel J. Record, M.A., M.F.</title>
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+Project Gutenberg's The Mechanical Properties of Wood, by Samuel J. Record
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
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+Title: The Mechanical Properties of Wood
+       Including a Discussion of the Factors Affecting the Mechanical
+              Properties, and Methods of Timber Testing
+              
+
+Author: Samuel J. Record
+
+Release Date: May 8, 2004 [EBook #12299]
+
+Language: English
+
+Character set encoding: ISO-8859-1
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+*** START OF THIS PROJECT GUTENBERG EBOOK THE MECHANICAL PROPERTIES OF WOOD ***
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+Proofreaders.  Scans provided by Case Western Reserve University's
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+
+
+<h1>THE MECHANICAL PROPERTIES OF WOOD</h1>
+
+<p class="ctr"><img src="images/fig00.jpg" alt="Frontispiece" id="fig00" /></p>
+<p class="imgtitle">Frontispiece.</p>
+<p class="imgtext">Photomicrograph of a small block of western hemlock. At the top
+is the cross section showing to the right the late wood of one
+season's growth, to the left the early wood of the next season.
+The other two sections are longitudinal and show the fibrous
+character of the wood. To the left is the radial section with
+three rays crossing it. To the right is the tangential section
+upon which the rays appear as vertical rows of beads. &times; 35.
+<i>Photo by the author</i>.</p>
+
+<a name="BookTitle"></a><h2>THE MECHANICAL PROPERTIES OF WOOD</h2>
+
+<h4><i>Including a Discussion<br>of the Factors Affecting the Mechanical
+Properties,<br>and Methods of Timber Testing</i></h4><br><br>
+
+<h4>BY</h4>
+<h3>SAMUEL J. RECORD, M.A., M.F.</h3>
+<h4>ASSISTANT PROFESSOR OF FOREST PRODUCTS, YALE UNIVERSITY</h4><br><br>
+
+<h4>FIRST EDITION<br>FIRST THOUSAND<br>1914</h4><br><br>
+
+<h3>BY THE SAME AUTHOR</h3>
+<h4>Identification of the Economic Woods of the United States.<br>
+8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net.</h4><br><br>
+
+<h3>TO THE STAFF OF THE
+<br>FOREST PRODUCTS LABORATORY, AT MADISON, WISCONSIN
+<br>IN APPRECIATION OF THE MANY OPPORTUNITIES
+<br>AFFORDED AND COURTESIES EXTENDED
+<br>THE AUTHOR</h3><br><br>
+
+<a name="PREFACE"></a>
+<h2>PREFACE</h2>
+
+<p>This book was written primarily for students of forestry to whom
+a knowledge of the technical properties of wood is essential.
+The mechanics involved is reduced to the simplest terms and
+without reference to higher mathematics, with which the students
+rarely are familiar. The intention throughout has been to avoid
+all unnecessarily technical language and descriptions, thereby
+making the subject-matter readily available to every one
+interested in wood.</p>
+
+<p>Part I is devoted to a discussion of the mechanical properties
+of wood&mdash;the relation of wood material to stresses and strains.
+Much of the subject-matter is merely elementary mechanics of
+materials in general, though written with reference to wood in
+particular. Numerous tables are included, showing the various
+strength values of many of the more important American woods.</p>
+
+<p>Part II deals with the factors affecting the mechanical
+properties of wood. This is a subject of interest to all who are
+concerned in the rational use of wood, and to the forester it
+also, by retrospection, suggests ways and means of regulating
+his forest product through control of the conditions of
+production. Attempt has been made, in the light of all data at
+hand, to answer many moot questions, such as the effect on the
+quality of wood of rate of growth, season of cutting, heartwood
+and sapwood, locality of growth, weight, water content,
+steaming, and defects.</p>
+
+<p>Part III describes methods of timber testing. They are for the
+most part those followed by the U.S. Forest Service. In schools
+equipped with the necessary machinery the instructions will
+serve to direct the tests; in others a study of the text with
+reference to the illustrations should give an adequate
+conception of the methods employed in this most important line
+of research.</p>
+
+<p>The appendix contains a copy of the working plan followed by the
+U.S. Forest Service in the extensive investigations covering the
+mechanical properties of the woods grown in the United States.
+It contains many valuable suggestions for the independent
+investigator. In addition four tables of strength values for
+structural timbers, both green and air-seasoned, are included.
+The relation of the stresses developed in different structural
+forms to those developed in the small clear specimens is given.</p>
+
+<p>In the bibliography attempt was made to list all of the
+important publications and articles on the mechanical properties
+of wood, and timber testing. While admittedly incomplete, it
+should prove of assistance to the student who desires a fuller
+knowledge of the subject than is presented here.</p>
+
+<p>The writer is indebted to the U.S. Forest Service for nearly all
+of his tables and photographs as well as many of the data upon
+which the book is based, since only the Government is able to
+conduct the extensive investigations essential to a thorough
+understanding of the subject. More than eighty thousand tests
+have been made at the Madison laboratory alone, and the work is
+far from completion.</p>
+
+<p>The writer also acknowledges his indebtedness to Mr. Emanuel
+Fritz, M.E., M.F., for many helpful suggestions in the
+preparation of Part I; and especially to Mr. Harry Donald
+Tiemann, M.E., M.F., engineer in charge of Timber Physics at the
+Government Forest Products Laboratory, Madison, Wisconsin, for
+careful revision of the entire manuscript.</p>
+
+<p>SAMUEL J. RECORD.</p>
+
+<p>YALE FOREST SCHOOL, <i>July 1, 1914.</i></p>
+
+<a name="CONTENTS"></a>
+<h2>CONTENTS</h2>
+
+<ul style="list-style-type: none">
+<li class="hdr"><a href="#PREFACE"><big>PREFACE</big></a></li>
+
+<li class="hdr"><a href="#PART_I"><big>PART I<br>THE MECHANICAL PROPERTIES OF WOOD</big></a></li>
+
+<li><a href="#I_01">Introduction</a></li>
+<li><a href="#I_02">Fundamental considerations and definitions</a></li>
+<li><a href="#I_03">Tensile strength</a></li>
+<li><a href="#I_04">Compressive or crushing strength</a></li>
+<li><a href="#I_05">Shearing strength</a></li>
+<li><a href="#I_06">Transverse or bending strength: Beams</a></li>
+<li><a href="#I_07">Toughness: Torsion</a></li>
+<li><a href="#I_08">Hardness</a></li>
+<li><a href="#I_09">Cleavability</a></li>
+
+<li class="hdr"><a href="#PART_II"><big>PART II<br>FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD</big></a></li>
+
+<li><a href="#II_01">Introduction</a></li>
+<li><a href="#II_02">Rate of growth</a></li>
+<li><a href="#II_03">Heartwood and sapwood</a></li>
+<li><a href="#II_04">Weight, density, and specific gravity</a></li>
+<li><a href="#II_05">Color</a></li>
+<li><a href="#II_06">Cross grain</a></li>
+<li><a href="#II_07">Knots</a></li>
+<li><a href="#II_08">Frost splits</a></li>
+<li><a href="#II_09">Shakes, galls, pitch pockets</a></li>
+<li><a href="#II_10">Insect injuries</a></li>
+<li><a href="#II_11">Marine wood-borer injuries</a></li>
+<li><a href="#II_12">Fungous injuries</a></li>
+<li><a href="#II_13">Parasitic plant injuries</a></li>
+<li><a href="#II_14">Locality of growth</a></li>
+<li><a href="#II_15">Season of cutting</a></li>
+<li><a href="#II_16">Water content</a></li>
+<li><a href="#II_17">Temperature</a></li>
+<li><a href="#II_18">Preservatives</a></li>
+
+<li class="hdr"><a href="#PART_III"><big>PART III<br>TIMBER TESTING</big></a></li>
+
+<li><a href="#III_01">Working plan</a></li>
+<li><a href="#III_02">Forms of material tested</a></li>
+<li><a href="#III_03">Size of test specimens</a></li>
+<li><a href="#III_04">Moisture determination</a></li>
+<li><a href="#III_05">Machine for static tests</a></li>
+<li><a href="#III_06">Speed of testing machine</a></li>
+<li><a href="#III_07">Bending large beams</a></li>
+<li><a href="#III_08">Bending small beams</a></li>
+<li><a href="#III_09">Endwise compression</a></li>
+<li><a href="#III_10">Compression across the grain</a></li>
+<li><a href="#III_11">Shear along the grain</a></li>
+<li><a href="#III_12">Impact test</a></li>
+<li><a href="#III_13">Hardness test: Abrasion and indentation</a></li>
+<li><a href="#III_14">Cleavage test</a></li>
+<li><a href="#III_15">Tension test parallel to the grain</a></li>
+<li><a href="#III_16">Tension test at right angles to the grain</a></li>
+<li><a href="#III_17">Torsion test</a></li>
+<li><a href="#III_18">Special tests</a></li>
+<li><a href="#III_19">Spike pulling test</a></li>
+<li><a href="#III_20">Packing boxes</a></li>
+<li><a href="#III_21">Vehicle and implement woods</a></li>
+<li><a href="#III_22">Cross-arms</a></li>
+<li><a href="#III_23">Other tests</a></li>
+
+<li class="hdr"><a href="#APPENDIX"><big>APPENDIX</big></a></li>
+
+<li><a href="#APPENDIX_1">Sample working plan of United States Forest Service</a></li>
+<li><a href="#APPENDIX_2">Strength values for structural timbers</a></li>
+
+<li class="hdr"><a href="#BIBLIOGRAPHY"><big>BIBLIOGRAPHY</big></a></li>
+
+<li><a href="#BIB_I">Part I: Some general works on mechanics, materials of construction, and testing of materials</a></li>
+<li><a href="#BIB_II">Part II: Publications and articles on the mechanical properties of wood, and timber testing</a></li>
+<li><a href="#BIB_III">Part III: Publications of the United States Government on the mechanical properties of wood, and timber testing</a></li>
+
+<li class="hdr"><big><b>ILLUSTRATIONS</b></big></li>
+
+<li><a href="#fig00"> Frontispiece. Photomicrograph of a small block of western hemlock </a></li>
+<li><a href="#fig01">1. Stress-strain diagrams of two longleaf pine beams </a></li>
+<li><a href="#fig02">2. Compression across the grain </a></li>
+<li><a href="#fig03">3. Side view of failures in compression across the grain </a></li>
+<li><a href="#fig04">4. End view of failures in compression across the grain </a></li>
+<li><a href="#fig05">5. Testing a buggy-spoke in endwise compression </a></li>
+<li><a href="#fig06">6. Unequal distribution of stress in a long column due to lateral bending </a></li>
+<li><a href="#fig07">7. Endwise compression of a short column </a></li>
+<li><a href="#fig08">8. Failures of a short column of green spruce </a></li>
+<li><a href="#fig09">9. Failures of short columns of dry chestnut </a></li>
+<li><a href="#fig10">10. Example of shear along the grain </a></li>
+<li><a href="#fig11">11. Failures of test specimens in shear along the grain </a></li>
+<li><a href="#fig12">12. Horizontal shear in a beam </a></li>
+<li><a href="#fig13">13. Oblique shear in a short column </a></li>
+<li><a href="#fig14">14. Failure of a short column by oblique shear </a></li>
+<li><a href="#fig15">15. Diagram of a simple beam </a></li>
+<li><a href="#fig16">16. Three common forms of beams&mdash;(1) simple, (2) cantilever, (3) continuous </a></li>
+<li><a href="#fig17">17. Characteristic failures of simple beams </a></li>
+<li><a href="#fig18">18. Failure of a large beam by horizontal shear </a></li>
+<li><a href="#fig19">19. Torsion of a shaft </a></li>
+<li><a href="#fig20">20. Effect of torsion on different grades of hickory </a></li>
+<li><a href="#fig21">21. Cleavage of highly elastic wood </a></li>
+<li><a href="#fig22">22. Cross-sections of white ash, red gum, and eastern hemlock </a></li>
+<li><a href="#fig23">23. Cross-section of longleaf pine </a></li>
+<li><a href="#fig24">24. Relation of the moisture content to the various strength values of spruce </a></li>
+<li><a href="#fig25">25. Cross-section of the wood of western larch showing fissures in the thick-walled  cells of the late wood </a></li>
+<li><a href="#fig26">26. Progress of drying throughout the length of a chestnut beam </a></li>
+<li><a href="#fig27">27. Excessive season checking </a></li>
+<li><a href="#fig28">28. Control of season checking by the use of S-irons </a></li>
+<li><a href="#fig29">29. Static bending test on a large beam </a></li>
+<li><a href="#fig30">30. Two methods of loading a beam </a></li>
+<li><a href="#fig31">31. Static bending test on a small beam </a></li>
+<li><a href="#fig32">32. Sample log sheet, giving full details of a transverse bending test on a small pine beam </a></li>
+<li><a href="#fig33">33. Endwise compression test </a></li>
+<li><a href="#fig34">34. Sample log sheet of an endwise compression test on a short pine column </a></li>
+<li><a href="#fig35">35. Compression across the grain </a></li>
+<li><a href="#fig36">36. Vertical section of shearing tool </a></li>
+<li><a href="#fig37">37. Front view of shearing tool </a></li>
+<li><a href="#fig38">38. Two forms of shear test specimens </a></li>
+<li><a href="#fig39">39. Making a shearing test </a></li>
+<li><a href="#fig40">40. Impact testing machine </a></li>
+<li><a href="#fig41">41. Drum record of impact bending test </a></li>
+<li><a href="#fig42">42. Abrasion machine for testing the wearing qualities of woods </a></li>
+<li><a href="#fig43">43. Design of tool for testing the hardness of woods by indentation </a></li>
+<li><a href="#fig44">44. Design of tool for cleavage test </a></li>
+<li><a href="#fig45">45. Design of cleavage test specimen </a></li>
+<li><a href="#fig46">46. Designs of tension test specimens used in United States </a></li>
+<li><a href="#fig47">47. Design of tension test specimen used in New South Wales </a></li>
+<li><a href="#fig48">48. Design of tool and specimen for testing tension at right angles to the grain </a></li>
+<li><a href="#fig49">49. Making a torsion test on hickory </a></li>
+<li><a href="#fig50">50. Method of cutting and marking test specimens </a></li>
+<li><a href="#fig51">51. Diagram of specific gravity apparatus </a></li>
+
+<li class="hdr"><big><b>TABLES</b></big></li>
+
+<li><a href="#TABLE_01">I. Comparative strength of iron, steel, and wood</a></li>
+<li><a href="#TABLE_02">II. Ratio of strength of wood in tension and in compression</a></li>
+<li><a href="#TABLE_03">III. Right-angled tensile strength of small clear pieces of 25 woods in green condition</a></li>
+<li><a href="#TABLE_04">IV. Results of compression tests across the grain on 51 woods in green condition, and comparison with white oak</a></li>
+<li><a href="#TABLE_05">V. Relation of fibre stress at elastic limit in bending to the crushing strength of blocks cut therefrom in pounds per square inch</a></li>
+<li><a href="#TABLE_06">VI. Results of endwise compression tests on small clear pieces of 40 woods in green condition</a></li>
+<li><a href="#TABLE_07">VII. Shearing strength along the grain of small clear pieces of 41 woods in green condition</a></li>
+<li><a href="#TABLE_08">VIII. Shearing strength across the grain of various American woods</a></li>
+<li><a href="#TABLE_09">IX. Results of static bending tests on small clear beams of 49 woods in green condition</a></li>
+<li><a href="#TABLE_10">X. Results of impact bending tests on small clear beams of 34 woods in green condition</a></li>
+<li><a href="#TABLE_11">XI. Manner of first failure of large beams</a></li>
+<li><a href="#TABLE_12">XII. Hardness of 32 woods in green condition, as indicated by the load required to imbed a 0.444-inch steel ball to one-half its diameter</a></li>
+<li><a href="#TABLE_13">XIII. Cleavage strength of small clear pieces of 32 woods in green condition</a></li>
+<li><a href="#TABLE_14">XIV. Specific gravity, and shrinkage of 51 American woods</a></li>
+<li><a href="#TABLE_15">XV. Effect of drying on the mechanical properties of wood, shown in ratio of increase due to reducing moisture content from the green condition to kiln-dry</a></li>
+<li><a href="#TABLE_16">XVI. Effect of steaming on the strength of green loblolly pine</a></li>
+<li><a href="#TABLE_17">XVII. Speed-strength moduli, and relative increase in strength at rates of fibre strain increasing in geometric ratio</a></li>
+<li><a href="#TABLE_18">XVIII. Results of bending tests on green structural timbers</a></li>
+<li><a href="#TABLE_19">XIX. Results of compression and shear tests on green structural timbers</a></li>
+<li><a href="#TABLE_20">XX. Results of bending tests on air-seasoned structural timbers</a></li>
+<li><a href="#TABLE_21">XXI. Results of compression and shear tests on air-seasoned structural timbers</a></li>
+<li><a href="#TABLE_22">XXII. Working unit stresses for structural timber expressed in pounds per square inch</a></li>
+
+<li class="hdr"><a href="#INDEX"><big>INDEX</big></a></li>
+
+<li class="hdr"><a href="#FOOTNOTES"><big>FOOTNOTES</big></a></li>
+</ul>
+
+<a name="pg001"></a>
+<a name="PART_I"></a>
+<h2>PART I<br>THE MECHANICAL PROPERTIES OF WOOD</h2>
+
+<a name="I_01"></a>
+<h3>INTRODUCTION</h3>
+
+<p>The mechanical properties of wood are its fitness and ability to
+resist applied or external forces. By external force is meant
+any force outside of a given piece of material which tends to
+deform it in any manner. It is largely such properties that
+determine the use of wood for structural and building purposes
+and innumerable other uses of which furniture, vehicles,
+implements, and tool handles are a few common examples.</p>
+
+<p>Knowledge of these properties is obtained through
+experimentation either in the employment of the wood in practice
+or by means of special testing apparatus in the laboratory.
+Owing to the wide range of variation in wood it is necessary
+that a great number of tests be made and that so far as possible
+all disturbing factors be eliminated. For comparison of
+different kinds or sizes a standard method of testing is
+necessary and the values must be expressed in some defined
+units. For these reasons laboratory experiments if properly
+conducted have many advantages over any other method.</p>
+
+<p>One object of such investigation is to find unit values for
+strength and stiffness, etc. These, because of the complex
+structure of wood, cannot have a constant value which will be
+exactly repeated in each test, even though no error be made. The
+most that can be accomplished is to find average values, the
+amount of variation above and below, and the laws which govern
+the variation. On account of the great variability in strength
+of different specimens of wood even from the same stick and
+appearing to be alike, it is important to eliminate as far as
+possible all extraneous factors liable to influence the results
+of the tests.</p>
+
+<p>The mechanical properties of wood considered in this book are:
+(1) stiffness and elasticity, (2) tensile strength, (3)
+<a name="pg002"></a>
+compressive or crushing strength, (4) shearing strength, (5)
+transverse or bending strength, (6) toughness, (7) hardness, (8)
+cleavability, (9) resilience. In connection with these,
+associated properties of importance are briefly treated.</p>
+
+<p>In making use of figures indicating the strength or other
+mechanical properties of wood for the purpose of comparing the
+relative merits of different species, the fact should be borne
+in mind that there is a considerable range in variability of
+each individual material and that small differences, such as a
+few hundred pounds in values of 10,000 pounds, cannot be
+considered as a criterion of the quality of the timber. In
+testing material of the same kind and grade, differences of 25
+per cent between individual specimens may be expected in
+conifers and 50 per cent or even more in hardwoods. The figures
+given in the tables should be taken as indications rather than
+fixed values, and as applicable to a large number collectively
+and not to individual pieces.</p>
+
+<a name="I_02"></a>
+<h3>FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS</h3>
+
+<p>Study of the mechanical properties of a material is concerned
+mostly with its behavior in relation to stresses and strains,
+and the factors affecting this behavior. A <b>stress</b> is a
+distributed force and may be defined as the mutual action (1) of
+one body upon another, or (2) of one part of a body upon another
+part. In the first case the stress is <i>external</i>; in the other
+<i>internal</i>. The same stress may be internal from one point of
+view and external from another. An external force is always
+balanced by the internal stresses when the body is in
+equilibrium.</p>
+
+<p>If no external forces act upon a body its particles assume
+certain relative positions, and it has what is called its
+<i>natural shape and size</i>. If sufficient external force is
+applied the natural shape and size will be changed. This
+distortion or deformation of the material is known as the
+<b>strain</b>. Every stress produces a corresponding strain, and
+within a certain limit (see <i>elastic limit</i>, <a href="#pg005">page 5</a>)
+the strain is directly
+proportional to the stress producing it.<a name="fn01r"></a><a href="#fn01"><sup><small>1</small></sup></a> The same intensity
+of stress, however, does not <a name="pg003"></a> produce the same strain in
+different materials or in different qualities of the same
+material. No strain would be produced in a perfectly rigid body,
+but such is not known to exist.</p>
+
+<p>Stress is measured in pounds (or other unit of weight or force).
+A <b>unit stress</b> is the stress on a unit of the sectional
+area.</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td>       <td><br></td><td><i>P</i></td><td rowspan="3"><big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td>Unit stress</td><td>=</td>     <td>---</td></tr>
+<tr>                                                                  <td><br></td>       <td><br></td><td><i>A</i></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>For instance, if a load (<i>P</i>) of one
+hundred pounds is uniformly supported by a vertical post with a
+cross-sectional area (<i>A</i>) of ten square inches, the unit
+compressive stress is ten pounds per square inch.</p>
+
+<p>Strain is measured in inches (or other linear unit). A <b>unit
+strain</b> is the strain per unit of length. Thus if a post 10
+inches long before compression is 9.9 inches long under the
+compressive stress, the total strain is 0.1 inch, and the unit
+strain is</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><i>l</i></td>  <td><br></td> <td>0.1</td> <td><br></td><td><br></td></tr>
+<tr>  <td>---</td>     <td>=</td>   <td>-----</td><td>=</td>   <td>0.01 inch per inch of length.</td></tr>
+<tr><td><i>L</i></td>  <td><br></td> <td>10</td>  <td><br></td><td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>As the stress increases there is a corresponding increase in the
+strain. This ratio may be graphically shown by means of a
+diagram or curve plotted with the increments of load or stress
+as ordinates and the increments of strain as absciss�. This is
+known as the <b>stress-strain diagram</b>. Within the limit mentioned
+above the diagram is a straight line. (<a href="#fig01">See Fig. 1</a>.) If the
+results of similar experiments on different specimens are
+plotted to the same scales, the diagrams furnish a ready means
+for comparison. The greater the resistance a material offers to
+deformation the steeper or nearer the vertical axis will be the
+line.</p>
+
+<p class="ctr"><img src="images/fig01.jpg" alt="Figure 1" id="fig01" /></p>
+<p class="imgtitle">Figure 1</p>
+<p class="imgtext">Stress-strain diagrams of two longleaf
+pine beams. E.L. = elastic limit. The areas of the triangles
+0(EL)A and 0(EL)B represent the elastic resilience of the dry
+and green beams, respectively.</p>
+
+<p>There are three kinds of internal stresses, namely, (1)
+<b>tensile</b>, (2) <b>compressive</b>, and (3) <b>shearing</b>. When external
+forces act upon a bar in a direction away from its ends or a
+direct pull, the stress is a tensile stress; when toward the
+ends or a direct push, compressive stress. In the first instance
+the strain is an <i>elongation</i>; in the second a <i>shortening</i>.
+Whenever the forces tend to cause one portion of the material to
+slide upon another adjacent to it the action is called a
+<i>shear</i>. The action is that of an ordinary pair of shears. When
+riveted plates slide on each other the rivets are sheared off.</p>
+
+<a name="pg004"></a>
+
+<p>These three simple stresses may act together, producing compound
+stresses, as in flexure. When a bow is bent there is a
+compression of the fibres on the inner or concave side and an
+elongation of the fibres on the outer or convex side. There is
+also a tendency of the various fibres to slide past one another
+in a longitudinal direction. If the bow were made of two or more
+separate pieces of equal length it would be noted on bending
+that slipping occurred along the surfaces of contact, and that
+the ends would no longer be even. If these pieces were securely
+glued together they would no longer slip, but the tendency to do
+so would exist just the same. Moreover, it would be found in the
+latter case that the bow would be much harder to bend than where
+the pieces were not glued together&mdash;in other words, the
+<i>stiffness</i> of the bow would be materially increased.</p>
+
+<a name="pg005"></a>
+
+<p><b>Stiffness</b> is the property by means of which a body acted upon
+by external forces tends to retain its natural size and shape,
+or resists deformation. Thus a material that is difficult to
+bend or otherwise deform is stiff; one that is easily bent or
+otherwise deformed is <i>flexible</i>. Flexibility is not the exact
+counterpart of stiffness, as it also involves toughness and
+pliability.</p>
+
+<p>If successively larger loads are applied to a body and then
+removed it will be found that at first the body completely
+regains its original form upon release from the stress&mdash;in other
+words, the body is <b>elastic</b>. No substance known is perfectly
+elastic, though many are practically so under small loads.
+Eventually a point will be reached where the recovery of the
+specimen is incomplete. This point is known as the <b>elastic
+limit</b>, which may be defined as the limit beyond which it is
+impossible to carry the distortion of a body without producing a
+permanent alteration in shape. After this limit has been
+exceeded, the size and shape of the specimen after removal of
+the load will not be the same as before, and the difference or
+amount of change is known as the <b>permanent set</b>.</p>
+
+<p>Elastic limit as measured in tests and used in design may be
+defined as that unit stress at which the deformation begins to
+increase in a faster ratio than the applied load. In practice
+the elastic limit of a material under test is determined from
+the stress-strain diagram. It is that point in the line where
+the diagram begins perceptibly to curve.<a name="fn02r"></a><a href="#fn02"><sup><small>2</small></sup></a> <a href="#fig01">(See Fig. 1</a>.)</p>
+
+<p><b>Resilience</b> is the amount of work done upon a body in deforming
+it. Within the elastic limit it is also a measure of the
+potential energy stored in the material and represents the
+amount of work the material would do upon being released from a
+state of stress. This may be graphically represented by a
+diagram in which the absciss� represent the amount of deflection
+and the ordinates the force acting. The area included between
+the stress-strain curve and the initial line (which is zero)
+represents the work done. (<a href="#fig01">See Fig. 1</a>.) If the unit of space is
+in inches and the unit <a name="pg006"></a> of force is in pounds the result is
+inch-pounds. If the elastic limit is taken as the apex of the
+triangle the area of the triangle will represent the <b>elastic
+resilience</b> of the specimen. This amount of work can be applied
+repeatedly and is perhaps the best measure of the toughness of
+the wood as a working quality, though it is not synonymous with
+toughness.</p>
+
+<p>Permanent set is due to the <b>plasticity</b> of the material. A
+perfectly plastic substance would have no elasticity and the
+smallest forces would cause a set. Lead and moist clay are
+nearly plastic and wood possesses this property to a greater or
+less extent. The plasticity of wood is increased by wetting,
+heating, and especially by steaming and boiling. Were it not for
+this property it would be impossible to dry wood without
+destroying completely its cohesion, due to the irregularity of
+shrinkage.</p>
+
+<p>A substance that can undergo little change in shape without
+breaking or rupturing is <b>brittle</b>. Chalk and glass are common
+examples of brittle materials. Sometimes the word <i>brash</i> is
+used to describe this condition in wood. A brittle wood breaks
+suddenly with a clean instead of a splintery fracture and
+without warning. Such woods are unfitted to resist shock or
+sudden application of load.</p>
+
+<p>The measure of the stiffness of wood is termed the <b>modulus of
+elasticity</b> (or <i>coefficient of elasticity</i>). It is the ratio of
+stress per unit of area to the deformation per unit of
+length.</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td><td><br></td><td>unit stress</td><td rowspan="3"><big><big><big><big>)</big></big></big></big> </td></tr>
+<tr>                                                                  <td>E </td>  <td>= </td> <td>------------- </td></tr>
+<tr>                                                                  <td><br></td><td><br></td><td>unit strain<br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>It is a number indicative of
+stiffness, not of strength, and only applies to conditions
+within the elastic limit. It is nearly the same whether derived
+from compression tests or from tension tests.</p>
+
+<p>A large modulus indicates a stiff material. Thus in green wood
+tested in static bending it varies from 643,000 pounds per
+square inch for arborvit� to 1,662,000 pounds for longleaf pine,
+and 1,769,000 pounds for pignut hickory. (<a href="#TABLE_09">See Table IX</a>.) The
+values derived from tests of small beams of dry material are
+much greater, approaching 3,000,000 for some of our woods. These
+values are small when compared with steel which has a <a name="pg007"></a> modulus of
+elasticity of about 30,000,000 pounds per square inch. (<a href="#TABLE_01">See Table I</a>.)</p>
+
+<center>
+<table id="TABLE_01" summary="TABLE_I" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="6"> TABLE I </th></tr>
+<tr><th colspan="6"> COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD </th></tr>
+<tr><th rowspan="2"> MATERIAL </th>                                            <th rowspan="2"> Sp. gr.,dry </th>
+                                                                                             <th> Modulus of elasticity in bending </th>
+                                                                                                                  <th> Tensile strength </th>
+                                                                                                                                   <th> Crushing strength </th>
+                                                                                                                                                     <th> Modulus of rupture </th></tr>
+<tr>                                                                                         <th><i>Lbs. per sq. in.</i></th>
+                                                                                                                  <th><i>Lbs. per sq. in.</i></th>
+                                                                                                                                   <th><i>Lbs. per sq. in.</i></th>
+                                                                                                                                                     <th><i>Lbs. per sq. in.</i></th></tr>
+<tr><td style="text-align: left;"> Cast iron, cold blast (Hodgkinson)</td>     <td> 7.1 </td><td> 17,270,000 </td><td> 16,700 </td><td> 106,000 </td><td> 38,500 </td></tr>
+<tr><td style="text-align: left;"> Bessenger steel, high grade (Fairbain).</td><td> 7.8 </td><td> 29,215,000 </td><td> 88,400 </td><td> 225,600 </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Longleaf pine, 3.5% moisture (U.S.)</td>    <td> .63 </td><td> 2,800,000 </td> <td><br></td>    <td> 13,000 </td> <td> 21,000 </td></tr>
+<tr><td style="text-align: left;"> Redspruce, 3.5% moisture (U.S.)</td>        <td> .41 </td><td> 1,800,000 </td> <td><br></td>    <td> 8,800 </td>  <td> 14,500 </td></tr>
+<tr><td style="text-align: left;"> Pignut hickory, 3.5% moisture (U.S.)</td>   <td> .86 </td><td> 2,370,000 </td> <td><br></td>    <td> 11,130 </td> <td> 24,000 </td></tr>
+<tr><td style="text-align: left;" colspan="6"> NOTE.&mdash;Great variation may be found in different samples of metals as well as of wood. The examples given represent reasonable values. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="I_03"></a>
+<h3>TENSILE STRENGTH</h3>
+
+<p><b>Tension</b> results when a pulling force is applied to opposite
+ends of a body. This external pull is communicated to the
+interior, so that any portion of the material exerts a pull or
+tensile force upon the remainder, the ability to do so depending
+upon the property of cohesion. The result is an elongation or
+stretching of the material in the direction of the applied
+force. The action is the opposite of compression.</p>
+
+<p>Wood exhibits its greatest strength in tension parallel to the
+grain, and it is very uncommon in practice for a specimen to be
+pulled in two lengthwise. This is due to the difficulty of
+making the end fastenings secure enough for the full tensile
+strength to be brought into play before the fastenings shear off
+longitudinally. This is not the case with metals, and as a
+result they are used in almost all places where tensile strength
+is particularly needed, even though the remainder of the
+structure, such as sills, beams, joists, posts, and flooring,
+may be of wood. Thus in a wooden truss bridge the tension
+members are steel rods.</p>
+
+<a name="pg008"></a>
+
+<p>The tensile strength of wood parallel to the grain depends upon
+the strength of the fibres and is affected not only by the
+nature and dimensions of the wood elements but also by their
+arrangement. It is greatest in straight-grained specimens with
+thick-walled fibres. Cross grain of any kind materially reduces
+the tensile strength of wood, since the tensile strength at
+right angles to the grain is only a small fraction of that
+parallel to the grain.</p>
+
+<center>
+<table id="TABLE_02" summary="TABLE_II" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="4"> TABLE II </th></tr>
+<tr><th colspan="4"> RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION </th></tr>
+<tr><td colspan="4"> (Bul. 10, U. S. Div. of Forestry, p. 44) </td></tr>
+<tr><th rowspan="3"> KIND OF WOOD </th>               <th rowspan="3"> Ratio:<br>R =<br>Tensile strength<br>---------------------<br>compressive&nbsp;strength </th>
+                                                                    <th colspan="2"> A stick 1 square inch in cross section.<br></th></tr>
+<tr>                                                                <th colspan="2"> Weight required to&mdash; </th></tr>
+<tr>                                                                <th> Pull apart</th>
+                                                                                     <th> Crush endwise</th></tr>
+<tr><td style="text-align: left;"> Hickory </td>      <td> 3.7 </td><td> 32,000 </td><td> 8,500 </td></tr>
+<tr><td style="text-align: left;"> Elm </td>          <td> 3.8 </td><td> 29,000 </td><td> 7,500 </td></tr>
+<tr><td style="text-align: left;"> Larch </td>        <td> 2.3 </td><td> 19,400 </td><td> 8,600 </td></tr>
+<tr><td style="text-align: left;"> Longleaf Pine </td><td> 2.2 </td><td> 17,300 </td><td> 7,400 </td></tr>
+<tr><td style="text-align: left;" colspan="4"> NOTE.&mdash;Moisture condition not given. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>Failure of wood in tension parallel to the grain occurs
+sometimes in flexure, especially with dry material. The tension
+portion of the fracture is nearly the same as though the piece
+were pulled in two lengthwise. The fibre walls are torn across
+obliquely and usually in a spiral direction. There is
+practically no pulling apart of the fibres, that is, no
+separation of the fibres along their walls, regardless of their
+thickness. The nature of tension failure is apparently not
+affected by the moisture condition of the specimen, at least not
+so much so as the other strength values.<a name="fn03r"></a><a href="#fn03"><sup><small>3</small></sup></a></p>
+
+<p>Tension at right angles to the grain is closely related to
+cleavability. When wood fails in this manner the thin fibre
+walls are torn in two lengthwise while the thick-walled fibres
+are usually pulled apart along the primary wall.</p>
+
+<a name="pg009"></a>
+
+<center>
+<table id="TABLE_03" summary="TABLE_III" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="3"> TABLE III </th></tr>
+<tr><th colspan="3"> TENSILE STRENGTH AT RIGHT ANGLES TO THE GRAIN OF SMALL CLEAR PIECES OF 25 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="3"> (Forest Service Cir. 213)</td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES</th>                   <th> When surface of failure is radial </th>
+                                                                                  <th> When surface of failure is tangential </th></tr>
+<tr>                                                               <th><i>Lbs. per sq. inch</i></th>
+                                                                                   <th><i>Lbs. per sq. inch</i></th></tr>
+<tr><th> <i>Hardwoods </i></th>                                    <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, white </td>                <td> 645 </td>  <td> 671 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>                  <td> 226 </td>  <td> 303 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                     <td> 633 </td>  <td> 969 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>             <td> 446 </td>  <td> 526 </td></tr>
+<tr><td style="text-align: left;"> Elm, slippery </td>             <td> 765 </td>  <td> 832 </td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>                 <td> 661 </td>  <td> 786 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>             <td> 1,133 </td><td> 1,445 </td></tr>
+<tr><td style="text-align: left;"> Maple, sugar </td>              <td> 610 </td>  <td> 864 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>                 <td> 714 </td>  <td> 924 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 639 </td>  <td> 874 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td>   <td> 757 </td>  <td> 909 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 622 </td>  <td> 749 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>        <td> 728 </td>  <td> 929 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                  <td> 540 </td>  <td> 781 </td></tr>
+<tr><td style="text-align: left;"> Tupelo</td>                     <td> 472 </td>  <td> 796 </td></tr>
+<tr><th> <i>Conifers </i></th>                                     <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>           <td> 241 </td>  <td> 235 </td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>             <td> 242 </td>  <td> 251 </td></tr>
+<tr><td style="text-align: left;"> Fir, white </td>                <td> 213 </td>  <td> 304 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>                   <td> 271 </td>  <td> 323 </td></tr>
+<tr><td style="text-align: left;"> Pine, longleaf </td>            <td> 240 </td>  <td> 298 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 179 </td>  <td> 205 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 239 </td>  <td> 304 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; western yellow </td><td> 230 </td>  <td> 252 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 225 </td>  <td> 285 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                  <td> 236 </td>  <td> 274 </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="I_04"></a>
+<h3>COMPRESSIVE OR CRUSHING STRENGTH</h3>
+
+<p><b>Compression across the grain</b> is very closely related to
+hardness and transverse shear. There are two ways in which wood
+is subjected to stress of this kind, namely, (1) with the load
+acting over the entire area of the specimen, and (2) with a load
+concentrated over a portion of the area. (<a href="#fig02">See Fig. 2</a>.) The
+latter is the condition more commonly met with in practice, as,
+for example, where a post rests on a horizontal sill, or a rail
+rests on a cross-tie. The former condition, however, gives the
+true resistance of the grain to simple crushing.]</p>
+
+<p class="ctr"><img src="images/fig02.jpg" alt="Figure 2" id="fig02" /></p>
+<p class="imgtitle">Figure 2</p>
+<p class="imgtext">Compression across the grain.</p>
+
+<a name="pg010"></a>
+
+<p>The first effect of compression across the grain is to compact
+the fibres, the load gradually but irregularly increasing as the
+density of the material is increased. If the specimen lies on a
+flat surface and the load is applied to only a portion of the
+upper area, the bearing plate indents the wood, crushing the
+upper fibres without affecting the lower part. (<a href="#fig03">See Fig. 3</a>.) As
+the load increases the projecting ends sometimes split
+horizontally. (<a href="#fig04">See Fig. 4</a>.) The irregularities in the load are
+due to the fact that the fibres collapse a few at a time,
+beginning with those with the thinnest walls. The projection <a name="pg012"></a> of
+the ends increases the strength of the material directly beneath
+the compressing weight by introducing a beam action which helps
+support the load. This influence is exerted for a short distance
+only.</p>
+
+<p class="ctr"><img src="images/fig03.jpg" alt="Figure 3" id="fig03" /></p>
+<p class="imgtitle">Figure 3</p>
+<p class="imgtext">Side view of failures in compression
+across the grain, showing crushing of blocks under bearing
+plate. Specimen at right shows splitting at ends.</p>
+
+<p class="ctr"><img src="images/fig04.jpg" alt="Figure 4" id="fig04" /></p>
+<p class="imgtitle">Figure 4</p>
+<p class="imgtext">End view of failures in compression
+across the grain, showing splitting of the ends of the test
+specimens.</p>
+
+<a name="pg013"></a>
+
+<center>
+<table id="TABLE_04" summary="TABLE_IV" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="3"> TABLE IV </th></tr>
+<tr><th colspan="3"> RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON 51 WOODS IN GREEN CONDITION, AND COMPARISON WITH WHITE OAK </th></tr>
+<tr><th colspan="3"> (U. S. Forest Service) </th></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>
+                                                              <th> Fibre stress at elastic limit perpendicular to grain </th>
+                                                                              <th> Fiber stress in per cent of white oak, or 853 pounds per sq. in. </th></tr>
+<tr>                                                          <th><i>Lbs. per sq. inch</i></th>
+                                                                              <th><i>Per cent</i></th></tr>
+<tr><td style="text-align: left;"> Osage orange </td>         <td> 2,260 </td><td> 265.0 </td></tr>
+<tr><td style="text-align: left;"> Honey locust </td>         <td> 1,684 </td><td> 197.5 </td></tr>
+<tr><td style="text-align: left;"> Black locust </td>         <td> 1,426 </td><td> 167.2 </td></tr>
+<tr><td style="text-align: left;"> Post oak </td>             <td> 1,148 </td><td> 134.6 </td></tr>
+<tr><td style="text-align: left;"> Pignut hickory </td>       <td> 1,142 </td><td> 133.9 </td></tr>
+<tr><td style="text-align: left;"> Water hickory </td>        <td> 1,088 </td><td> 127.5 </td></tr>
+<tr><td style="text-align: left;"> Shagbark hickory </td>     <td> 1,070 </td><td> 125.5 </td></tr>
+<tr><td style="text-align: left;"> Mockernut hickory </td>    <td> 1,012 </td><td> 118.6 </td></tr>
+<tr><td style="text-align: left;"> Big shellbark hickory </td><td> 997 </td>  <td> 116.9 </td></tr>
+<tr><td style="text-align: left;"> Bitternut hickory </td>    <td> 986 </td>  <td> 115.7 </td></tr>
+<tr><td style="text-align: left;"> Nutmeg hickory </td>       <td> 938 </td>  <td> 110.0 </td></tr>
+<tr><td style="text-align: left;"> Yellow oak </td>           <td> 857 </td>  <td> 100.5 </td></tr>
+<tr><td style="text-align: left;"> White oak </td>            <td> 853 </td>  <td> 100.0 </td></tr>
+<tr><td style="text-align: left;"> Bur oak </td>              <td> 836 </td>  <td> 98.0 </td></tr>
+<tr><td style="text-align: left;"> White ash </td>            <td> 828 </td>  <td> 97.1 </td></tr>
+<tr><td style="text-align: left;"> Red oak </td>              <td> 778 </td>  <td> 91.2 </td></tr>
+<tr><td style="text-align: left;"> Sugar maple </td>          <td> 742 </td>  <td> 87.0 </td></tr>
+<tr><td style="text-align: left;"> Rock elm </td>             <td> 696 </td>  <td> 81.6 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                <td> 607 </td>  <td> 71.2 </td></tr>
+<tr><td style="text-align: left;"> Slippery elm </td>         <td> 599 </td>  <td> 70.2 </td></tr>
+<tr><td style="text-align: left;"> Redwood </td>              <td> 578 </td>  <td> 67.8 </td></tr>
+<tr><td style="text-align: left;"> Bald cypress </td>         <td> 548 </td>  <td> 64.3 </td></tr>
+<tr><td style="text-align: left;"> Red maple </td>            <td> 531 </td>  <td> 62.3 </td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>            <td> 525 </td>  <td> 61.6 </td></tr>
+<tr><td style="text-align: left;"> Incense cedar </td>        <td> 518 </td>  <td> 60.8 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>              <td> 497 </td>  <td> 58.3 </td></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>        <td> 491 </td>  <td> 57.6 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>             <td> 480 </td>  <td> 56.3 </td></tr>
+<tr><td style="text-align: left;"> Silver maple </td>         <td> 456 </td>  <td> 53.5 </td></tr>
+<tr><td style="text-align: left;"> Yellow birch </td>         <td> 454 </td>  <td> 53.2 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>               <td> 451 </td>  <td> 52.9 </td></tr>
+<tr><td style="text-align: left;"> Black cherry </td>         <td> 444 </td>  <td> 52.1 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>             <td> 433 </td>  <td> 50.8 </td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>          <td> 427 </td>  <td> 50.1 </td></tr>
+<tr><td style="text-align: left;"> Cucumber tree </td>        <td> 408 </td>  <td> 47.8 </td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td>       <td> 400 </td>  <td> 46.9 </td></tr>
+<tr><td style="text-align: left;"> Red pine </td>             <td> 358 </td>  <td> 42.0 </td></tr>
+<tr><td style="text-align: left;"> Sugar pine </td>           <td> 353 </td>  <td> 41.1 </td></tr>
+<tr><td style="text-align: left;"> White elm</td>             <td> 351 </td>  <td> 41.2 </td></tr>
+<tr><td style="text-align: left;"> Western yellow pine </td>  <td> 348 </td>  <td> 40.8 </td></tr>
+<tr><td style="text-align: left;"> Lodgepole pine </td>       <td> 348 </td>  <td> 40.8 </td></tr>
+<tr><td style="text-align: left;"> Red spruce </td>           <td> 345 </td>  <td> 40.5 </td></tr>
+<tr><td style="text-align: left;"> White pine </td>           <td> 314 </td>  <td> 36.8 </td></tr>
+<tr><td style="text-align: left;"> Engelman spruce </td>      <td> 290 </td>  <td> 34.0 </td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>      <td> 288 </td>  <td> 33.8 </td></tr>
+<tr><td style="text-align: left;"> Largetooth aspen </td>     <td> 269 </td>  <td> 31.5 </td></tr>
+<tr><td style="text-align: left;"> White spruce </td>         <td> 262 </td>  <td> 30.7 </td></tr>
+<tr><td style="text-align: left;"> Butternut </td>            <td> 258 </td>  <td> 30.3 </td></tr>
+<tr><td style="text-align: left;"> Buckeye (yellow) </td>     <td> 210 </td>  <td> 24.6 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>             <td> 209 </td>  <td> 24.5 </td></tr>
+<tr><td style="text-align: left;"> Black willow </td>         <td> 193 </td>  <td> 22.6 </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>When wood is used for columns, props, posts, and spokes, the
+weight of the load tends to shorten the material endwise. This
+is <b>endwise compression</b>, or compression parallel to the grain.
+In the case of long columns, that is, pieces in which the length
+is very great compared with their diameter, the failure is by
+sidewise bending or flexure, instead of by crushing or
+splitting. (<a href="#fig05">See Fig. 5</a>.) A familiar instance of this action is
+afforded by a flexible walking-stick. If downward pressure is
+exerted with the hand on the upper end of the stick placed
+vertically on the floor, it will be noted that a definite amount
+of force must be applied in each instance before decided flexure
+takes place. After this point is reached a very slight increase
+of pressure very largely increases the deflection, thus
+obtaining so great a leverage about the middle section as to
+cause rupture.</p>
+
+<a name="pg011"></a>
+
+<p class="ctr"><img src="images/fig05.jpg" alt="Figure 5" id="fig05" /></p>
+<p class="imgtitle">Figure 5</p>
+<p class="imgtext"> Testing a buggy spoke in endwise
+compression, illustrating the failure by sidewise bending of a
+long column fixed only at the lower end. <i>Photo by U. S. Forest
+Service</i></p>
+
+<p>The lateral bending of a column produces a combination of
+bending with compressive stress over the section, the
+compressive stress being maximum at the section of greatest
+deflection on the concave side. The convex surface is under
+tension, as in an ordinary beam test. (<a href="#fig06">See Fig. 6</a>.) If the same
+stick is braced in such a way that flexure is prevented, its
+supporting strength is increased enormously, since the
+compressive stress acts uniformly over the section, and failure
+is by crushing or splitting, as in small blocks. In all columns
+free to bend in any direction the deflection will be seen in the
+direction in which the column is least stiff. This sidewise
+bending can be overcome by making pillars and columns thicker in
+the middle than at the ends, and by bracing studding, props, and
+compression <a name="pg014"></a> members of trusses. The strength of a column also
+depends to a considerable extent upon whether the ends are free
+to turn or are fixed.</p>
+
+<p class="ctr"><img src="images/fig06.jpg" alt="Figure 6" id="fig06" /></p>
+<p class="imgtitle">Figure 6</p>
+<p class="imgtext"> Unequal distribution of stress in a long
+column due to lateral bending.</p>
+
+<p>The complexity of the computations depends upon the way in which
+the stress is applied and the manner in which the stick bends.
+Ordinarily where the length of the test specimen is not greater
+than four diameters and the ends are squarely faced (<a href="#fig07">See Fig. 7</a>.),
+the force acts uniformly over each square inch of area and
+the crushing strength is equal to the maximum load (<i>P</i>) divided
+by the area of the cross-section (<i>A</i>).</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td>    <td><br></td><td><i>P</i></td><td rowspan="3"><big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td><i>C</i></td><td>=</td>     <td>---</td></tr>
+<tr>                                                                  <td><br></td>    <td><br></td><td><i>A</i></td></tr>
+</tbody>
+</table>
+</center>
+
+<p class="ctr"><img src="images/fig07.jpg" alt="Figure 7" id="fig07" /></p>
+<p class="imgtitle">Figure 7</p>
+<p class="imgtext"> Endwise compression of a short column.</p>
+
+<p>It has been demonstrated<a name="fn04r"></a><a href="#fn04"><sup><small>4</small></sup></a> that the ultimate strength in
+compression parallel to the grain is very nearly the same as the
+extreme <a name="pg015"></a> fibre stress at the elastic limit in bending. (<a href="#TABLE_05">See Table 5</a>.)
+In other words, the transverse strength of beams at elastic
+limit is practically equal to the compressive strength of the
+same material in short columns. It is accordingly possible to
+calculate the approximate breaking strength of beams from the
+compressive strength of short columns except when the wood is
+brittle. Since tests on endwise compression are simpler, easier
+to make, and less expensive than transverse bending tests, the
+importance of this relation is obvious, though it does not do
+away with the necessity of making beam tests.</p>
+
+<center>
+<table id="TABLE_05" summary="TABLE_V" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="7"> TABLE V </th></tr>
+<tr><th colspan="7"> RELATION OF FIBRE STRESS AT ELASTIC LIMIT (<i>r</i>) IN BENDING TO THE CRUSHING STRENGTH (<i>C</i>) OF BLOCKS CUT THEREFROM, IN POUNDS PER SQUARE INCH </th></tr>
+<tr><td colspan="7"> (Forest Service Bul. 70, p. 90) </td></tr>
+<tr><th colspan="7"> LONGLEAF PINE </th></tr>
+<tr><th> MOISTURE CONDITION </th>                                    <th> Soaked 50 per cent </th><th> Green 23 per cent </th> <th> 14 per cent </th>      <th> 11.5 per cent </th><th> 9.5 per cent </th><th> Kiln-dry 6.2 per cent </th></tr>
+<tr><td> Number of tests averaged </td>                              <td> 5 </td>                 <td> 5 </td>                 <td> 5 </td>                <td> 5 </td>            <td> 4 </td>           <td> 5 </td></tr>
+<tr><td> <i>r</i> in bending </td>                                   <td> 4,920 </td>             <td> 5,944 </td>             <td> 6,924 </td>            <td> 7,852 </td>        <td> 9,280 </td>       <td> 11,550 </td></tr>
+<tr><td> <i>C</i> in compression </td>                               <td> 4,668 </td>             <td> 5,100 </td>             <td> 6,466 </td>            <td> 7,466 </td>        <td> 8,985 </td>       <td> 10,910 </td></tr>
+<tr><td> Per cent <i>r</i> is in excess of <i>C</i> </td>            <td> 5.5 </td>               <td> 16.5 </td>              <td> 7.1 </td>              <td> 5.2 </td>          <td> 3.3</td>          <td> 5.9 </td></tr>
+<tr><th colspan="7"> SPRUCE </th></tr>
+<tr><th colspan="2"> MOISTURE CONDITION </th>                                                     <th> Soaked 30 per cent </th><th> Green 30 per cent </th><th> 10 per cent </th>  <th> 8.1 per cent </th><th> Kiln-dry 3.9 per cent </th></tr>
+<tr><td colspan="2"> Number of tests averaged </td>                                               <td> 5 </td>                 <td> 4 </td>                <td> 5 </td>            <td> 3 </td>           <td> 4 </td></tr>
+<tr><td colspan="2"><i>r</i> in bending </td>                                                     <td> 3,002 </td>             <td> 3,362 </td>            <td> 6,458 </td>        <td> 8,400 </td>       <td> 10,170 </td></tr>
+<tr><td colspan="2"><i>C</i> in compression </td>                                                 <td> 2,680 </td>             <td> 3,025 </td>            <td> 6,120 </td>        <td> 7,610 </td>       <td> 9,335 </td></tr>
+<tr><td colspan="2"> Per cent <i>r</i> is in excess of <i>C</i> </td>                             <td> 12.0 </td>              <td> 11.1</td>              <td> 5.5 </td>          <td> 10.4 </td>        <td> 9.0 </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>When a short column is compressed until it breaks, the manner of
+failure depends partly upon the anatomical structure and partly
+upon the degree of humidity of the wood. The fibres (tracheids
+in conifers) act as hollow tubes bound closely together, and in
+giving way they either (1) buckle, or (2) bend.<a name="fn05r"></a><a href="#fn05"><sup><small>5</small></sup></a></p>
+
+<p>The first is typical of any dry thin-walled cells, as is usually
+the case in seasoned white pine and spruce, and in the early
+wood of hard pines, hemlock, and other species with decided
+contrast between the two portions of the growth ring. As a rule
+buckling of a tracheid begins at the bordered pits which form
+places of least resistance in the walls. In hardwoods such as
+oak, chestnut, ash, etc., buckling occurs only in the
+thinnest-walled elements, such as the vessels, and not in the
+true fibres.</p>
+
+<p>According to Jaccard<a name="fn06r"></a><a href="#fn06"><sup><small>6</small></sup></a> the folding of the cells is accompanied
+by characteristic alterations of their walls which seem to split
+them into extremely thin layers. When greatly magnified, these
+layers appear in longitudinal sections as delicate threads
+without <a name="pg017"></a> any definite arrangements, while on cross section they
+appear as numerous concentric strata. This may be explained on
+the ground that the growth of a fibre is by successive layers
+which, under the influence of compression, are sheared apart.
+This is particularly the case with thick-walled cells such as
+are found in late wood.</p>
+
+<a name="pg016"></a>
+
+<center>
+<table id="TABLE_06" summary="TABLE_VI" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="4"> TABLE VI </th></tr>
+<tr><th colspan="4"> RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR PIECES OF 40 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="4"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>                  <th> Fibre stress at elastic limit </th>
+                                                                                   <th> Crushing strength</th>
+                                                                                                   <th> Modulus of elasticity </th></tr>
+<tr>                                                               <th><i>Lbs. per sq. inch</i></th>
+                                                                                   <th><i>Lbs. per sq. inch</i></th>
+                                                                                                   <th><i>Lbs. per sq. inch</i></th></tr>
+<tr><th> <i>Hardwoods</i> </th>                                    <td><br></td>   <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, white </td>                <td> 3,510 </td><td> 4,220 </td><td> 1,531,000 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>                  <td> 780 </td>  <td> 1,820 </td><td> 1,016,000 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                     <td> 2,770 </td><td> 3,480 </td><td> 1,412,000 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>             <td> 2,570 </td><td> 3,400 </td><td> 1,915,000 </td></tr>
+<tr><td style="text-align: left;"> Elm, slippery </td>             <td> 3,410 </td><td> 3,990 </td><td> 1,453,000 </td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>                 <td> 2,730 </td><td> 3,310 </td><td> 1,068,000 </td></tr>
+<tr><td style="text-align: left;"> Hickory, big shellbark </td>    <td> 3,570 </td><td> 4,520 </td><td> 1,658,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; bitternut </td>     <td> 4,330 </td><td> 4,570 </td><td> 1,616,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; mockernut </td>     <td> 3,990 </td><td> 4,320 </td><td> 1,359,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; nutmeg </td>        <td> 3,620 </td><td> 3,980 </td><td> 1,411,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; pignut </td>        <td> 3,520 </td><td> 4,820 </td><td> 1,980,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shagbark </td>      <td> 3,730 </td><td> 4,600 </td><td> 1,943,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; water </td>         <td> 3,240 </td><td> 4,660 </td><td> 1,926,000 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>             <td> 4,300 </td><td> 4,970 </td><td> 1,536,000 </td></tr>
+<tr><td style="text-align: left;"> Maple, sugar </td>              <td> 3,040 </td><td> 3,670 </td><td> 1,463,000 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>                 <td> 2,780 </td><td> 3,330 </td><td> 1,062,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 2,290 </td><td> 3,210 </td><td> 1,295,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td>   <td> 3,470 </td><td> 4,360 </td><td> 1,489,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 2,400 </td><td> 3,520 </td><td> 946,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>        <td> 2,870 </td><td> 3,700 </td><td> 1,465,000 </td></tr>
+<tr><td style="text-align: left;"> Osage orange </td>              <td> 3,980 </td><td> 5,810 </td><td> 1,331,000 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                  <td> 2,320 </td><td> 2,790 </td><td> 1,073,000 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                    <td> 2,280 </td><td> 3,550 </td><td> 1,280,000 </td></tr>
+<tr><th> <i>Conifers</i> </th>                                     <td><br></td>   <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>           <td> 1,420 </td><td> 1,990 </td><td> 754,000 </td></tr>
+<tr><td style="text-align: left;"> Cedar, incense </td>            <td> 2,710 </td><td> 3,030 </td><td> 868,000 </td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>             <td> 3,560 </td><td> 3,960 </td><td> 1,738,000 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine </td>               <td> 1,660 </td><td> 2,060 </td><td> 882,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; amabilis </td>      <td> 2,763 </td><td> 3,040 </td><td> 1,579,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas </td>       <td> 2,390 </td><td> 2,920 </td><td> 1,440,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 2,610 </td><td> 2,800 </td><td> 1,332,000 </td></tr>
+<tr><td style="text-align: left;"> Hemlock</td>                    <td> 2,110 </td><td> 2,750 </td><td> 1,054,000 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole</td>            <td> 2,290 </td><td> 2,530 </td><td> 1,219,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf </td>      <td> 3,420 </td><td> 4,280 </td><td> 1,890,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 2,470 </td><td> 3,080 </td><td> 1,646,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 2,340 </td><td> 2,600 </td><td> 1,029,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; western yellow </td><td> 2,100 </td><td> 2,420 </td><td> 1,271,000 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 2,370 </td><td> 2,720 </td><td> 1,318,000 </td></tr>
+<tr><td style="text-align: left;"> Redwood </td>                   <td> 3,420 </td><td> 3,820 </td><td> 1,175,000 </td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>         <td> 1,880 </td><td> 2,170 </td><td> 1,021,000 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                  <td> 3,010 </td><td> 3,480 </td><td> 1,596,000 </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>The second case, where the fibres bend with more or less regular
+curves instead of buckling, is characteristic of any green or
+wet wood, and in dry woods where the fibres are thick-walled. In
+woods in which the fibre walls show all gradations of
+thickness&mdash;in other words, where the transition from the
+thin-walled cells of the early wood to the thick-walled cells of
+the late wood is gradual&mdash;the two kinds of failure, namely,
+buckling and bending, grade into each other. In woods with very
+decided contrast between early and late wood the two forms are
+usually distinct. Except in the case of complete failure the
+cavity of the deformed cells remains open, and in hardwoods this
+is true not only of the wood fibres but also of the tube-like
+vessels. In many cases longitudinal splits occur which isolate
+bundles of elements by greater or less intervals. The splitting
+occurs by a tearing of the fibres or rays and not by the
+separation of the rays from the adjacent elements.</p>
+
+<p class="ctr"><img src="images/fig08.jpg" alt="Figure 8" id="fig08" /></p>
+<p class="imgtitle">Figure 8</p>
+<p class="imgtext"> Failures of short columns of green spruce.</p>
+
+<p class="ctr"><img src="images/fig09.jpg" alt="Figure 9" id="fig09" /></p>
+<p class="imgtitle">Figure 9</p>
+<p class="imgtext"> Failures of short columns of dry chestnut.</p>
+
+<p>Moisture in wood decreases the stiffness of the fibre walls and
+enlarges the region of failure. The curve which the fibre walls
+<a name="pg018"></a>
+make in the region of failure is more gradual and also more
+irregular than in dry wood, and the fibres are more likely to be
+separated.</p>
+
+<p>In examining the lines of rupture in compression parallel to the
+grain it appears that there does not exist any specific type,
+that is, one that is characteristic of all woods. Test blocks
+taken from different parts of the same log may show very decided
+differences in the manner of failure, while blocks that are much
+alike in the size, number, and distribution of the elements of
+unequal resistance may behave very similarly. The direction of
+rupture is, according to Jaccard, not influenced by the
+distribution of the medullary rays.<a name="fn07r"></a><a href="#fn07"><sup><small>7</small></sup></a> These are curved with the
+bundles of fibres to which they are attached. In any case the
+failure starts at <a name="pg019"></a> the weakest points and follows the lines of
+least resistance. The plane of failure, as visible on radial
+surfaces, is horizontal, and on the tangential surface it is
+diagonal.</p>
+
+<a name="I_05"></a>
+<h3>SHEARING STRENGTH</h3>
+
+<p>Whenever forces act upon a body in such a way that one portion
+tends to slide upon another adjacent to it the action is called
+a <b>shear</b>.<a name="fn08r"></a><a href="#fn08"><sup><small>8</small></sup></a> In wood this shearing action may be (1) <b>along the
+grain</b>, or (2) <b>across the grain</b>. A tenon breaking out its
+mortise is a familiar example of shear along the grain, while
+the shoving off of the tenon itself would be shear across the
+grain. The use of wood for pins or tree-nails involves
+resistance to shear across the grain. Another common instance of
+the latter is where the steel edge of the eye of an axe or
+hammer tends to cut off the handle. In <a href="#fig10">Fig. 10</a> the action of the
+wooden strut tends to shear off along the grain the portion <i>AB</i>
+of the wooden tie rod, and it is essential that the length of
+this portion be great enough to guard against it. <a href="#fig11">Fig. 11</a> shows
+characteristic failures in shear along the grain.</p>
+
+<p class="ctr"><img src="images/fig10.jpg" alt="Figure 10" id="fig10" /></p>
+<p class="imgtitle">Figure 10</p>
+<p class="imgtext"> Example of shear along the grain.</p>
+
+<p class="ctr"><img src="images/fig11.jpg" alt="Figure 11" id="fig11" /></p>
+<p class="imgtitle">Figure 11</p>
+<p class="imgtext"> Failures of test specimens in shear
+along the grain. In the block at the left the surface of failure
+is radial; in the one at the right, tangential.</p>
+
+<a name="pg020"></a>
+
+<center>
+<table id="TABLE_07" summary="TABLE_VII" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="3"> TABLE VII </th></tr>
+<tr><th colspan="3"> SHEARING STRENGTH ALONG THE GRAIN OF SMALL CLEAR PIECES OF 41 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="3"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>                  <th> When surface of failure is radial </th>
+                                                                                   <th> When surface of failure is tangential </th></tr>
+<tr>                                                               <th><i>Lbs. per sq. inch</i></th>
+                                                                                   <th><i>Lbs. per sq. inch</i></th></tr>
+<tr><th> <i>Hardwoods </i></th>                                    <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, black </td>                <td> 876 </td>  <td> 832 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 1,360 </td><td> 1,312 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>                  <td> 560 </td>  <td> 617 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                     <td> 1,154 </td><td> 1,375 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>             <td> 1,103 </td><td> 1,188 </td></tr>
+<tr><td style="text-align: left;"> Elm, slippery </td>             <td> 1,197 </td><td> 1,174 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 778 </td>  <td> 872 </td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>                 <td> 1,095 </td><td> 1,161 </td></tr>
+<tr><td style="text-align: left;"> Hickory, big shellbark </td>    <td> 1,134 </td><td> 1,191 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; bitternut </td>     <td> 1,134 </td><td> 1,348 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; mockernut </td>     <td> 1,251 </td><td> 1,313 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; nutmeg </td>        <td> 1,010 </td><td> 1,053 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; pignut </td>        <td> 1,334 </td><td> 1,457 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shagbark </td>      <td> 1,230 </td><td> 1,297 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; water </td>         <td> 1,390 </td><td> 1,490 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>             <td> 1,885 </td><td> 2,096 </td></tr>
+<tr><td style="text-align: left;"> Maple, red </td>                <td> 1,130 </td><td> 1,330 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 1,193 </td><td> 1,455 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>                 <td> 1,196 </td><td> 1,402 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 1,132 </td><td> 1,195 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td>   <td> 1,198 </td><td> 1,394 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 1,096 </td><td> 1,292 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>        <td> 1,162 </td><td> 1,196 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                  <td> 900 </td>  <td> 1,102 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                    <td> 978 </td>  <td> 1,084 </td></tr>
+<tr><th> <i>Conifers</i> </th>                                     <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>           <td> 617 </td>  <td> 614 </td></tr>
+<tr><td style="text-align: left;"> Cedar, incense </td>            <td> 613 </td>  <td> 662 </td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>             <td> 836 </td>  <td> 800 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine </td>               <td> 573 </td>  <td> 654 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; amabilis </td>      <td> 517 </td>  <td> 639 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas </td>       <td> 853 </td>  <td> 858 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 742 </td>  <td> 723 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>                   <td> 790 </td>  <td> 813 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole </td>           <td> 672 </td>  <td> 747 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf </td>      <td> 1,060 </td><td> 953 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 812 </td>  <td> 741 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 702 </td>  <td> 714 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; western yellow </td><td> 686 </td>  <td> 706 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 649 </td>  <td> 639 </td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>         <td> 607 </td>  <td> 624 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                  <td> 883 </td>  <td> 843 </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg021"></a>
+
+<p>Both shearing stresses may act at the same time. Thus the weight
+carried by a beam tends to shear it off at right angles to the
+axis; this stress is equal to the resultant force acting
+perpendicularly at any point, and in a beam uniformly loaded and
+supported at either end is maximum at the points of support and
+zero at the centre. In addition there is a shearing force
+tending to move the fibres of the beam past each other in a
+longitudinal direction. (<a href="#fig12">See Fig. 12</a>.) This longitudinal shear
+is maximum at the neutral plane and decreases toward the upper
+and lower surfaces.</p>
+
+<p class="ctr"><img src="images/fig12.jpg" alt="Figure 12" id="fig12" /></p>
+<p class="imgtitle">Figure 12</p>
+<p class="imgtext"> Horizontal shear in a beam.</p>
+
+<p>Shearing across the grain is so closely related to compression
+at right angles to the grain and to hardness that there is
+little to be gained by making separate tests upon it. Knowledge
+of shear parallel to the grain is important, since wood
+frequently fails in that way. The value of shearing stress
+parallel to the grain is found by dividing the maximum load in
+pounds (<i>P</i>) by the area of the cross section in inches (<i>A</i>).</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td> <td><br></td><td><i>P</i></td><td rowspan="3"><big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td>Shear</td><td>=</td>     <td>---</td></tr>
+<tr>                                                                  <td><br></td> <td><br></td><td><i>A</i></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>Oblique shearing stresses are developed in a bar when it is
+
+subjected to direct tension or compression. The maximum <a name="pg022"></a> shearing
+stress occurs along a plane when it makes an angle of 45 degrees
+with the axis of the specimen. In this case,</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td> <td><br></td> <td><i>P</i></td></tr>
+<tr><td>shear</td><td>=</td>     <td>-----.</td></tr>
+<tr><td><br></td> <td><br></td> <td>2&nbsp;<i>A</i></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>When the value of the angle <i>&theta;</i> is less than 45 degrees,</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td>                     <td><br></td> <td><i>P</i></td><td><br></td></tr>
+<tr><td>the shear along the plane</td><td>=</td>      <td>---</td>   <td>sin <i>&theta;</i> cos <i>&theta;</i>.</td></tr>
+<tr><td><br></td>                     <td><br></td> <td><i>A</i></td><td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>(<a href="#fig13">See Fig. 13</a>.) The effect of oblique shear is often
+visible in the failures of short columns. (<a href="#fig14">See Fig. 14</a>.)</p>
+
+<p class="ctr"><img src="images/fig13.jpg" alt="Figure 13" id="fig13" /></p>
+<p class="imgtitle">Figure 13</p>
+<p class="imgtext"> Oblique shear in a short column.</p>
+
+<p class="ctr"><img src="images/fig14.jpg" alt="Figure 14" id="fig14" /></p>
+<p class="imgtitle">Figure 14</p>
+<p class="imgtext"> Failure of short column by oblique shear.</p>
+
+<center>
+<table id="TABLE_08" summary="TABLE_VIII" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="4"> TABLE VIII </th></tr>
+<tr><th colspan="4"> SHEARING STRENGTH ACROSS THE GRAIN OF VARIOUS AMERICAN WOODS </th></tr>
+<tr><td colspan="4">(J.C. Trautwine. Jour. Franklin Institute. Vol. 109, 1880, pp. 105-106)</td></tr>
+<tr><th> KIND OF WOOD </th>                                   <th><i>Lbs. per sq. inch</i></th>
+                                                                              <th> KIND OF WOOD </th>                                         <th><i>Lbs. per sq. inch</i></th></tr>
+<tr><td style="text-align: left;"> Ash </td>                  <td> 6,280 </td><td style="text-align: left;"> Hickory </td>                    <td> 7,285 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                <td> 5,223 </td><td style="text-align: left;"> Locust </td>                     <td> 7,176 </td></tr>
+<tr><td style="text-align: left;"> Birch </td>                <td> 5,595 </td><td style="text-align: left;"> Maple </td>                      <td> 6,355 </td></tr>
+<tr><td style="text-align: left;"> Cedar (white) </td>        <td> 1,372 </td><td style="text-align: left;"> Oak </td>                        <td> 4,425 </td></tr>
+<tr><td style="text-align: left;"> Cedar (white) </td>        <td> 1,519 </td><td style="text-align: left;"> Oak (live) </td>                 <td> 8,480 </td></tr>
+<tr><td style="text-align: left;"> Cedar (Central Amer.) </td><td> 3,410 </td><td style="text-align: left;"> Pine (white )</td>               <td> 2,480 </td></tr>
+<tr><td style="text-align: left;"> Cherry </td>               <td> 2,945 </td><td style="text-align: left;"> Pine (northern yellow) </td>     <td> 4,340 </td></tr>
+<tr><td style="text-align: left;"> Chestnut </td>             <td> 1,536 </td><td style="text-align: left;"> Pine (southernyellow) </td>      <td> 5,735 </td></tr>
+<tr><td style="text-align: left;"> Dogwood </td>              <td> 6,510 </td><td style="text-align: left;"> Pine (very resinous yellow) </td><td> 5,053 </td></tr>
+<tr><td style="text-align: left;"> Ebony </td>                <td> 7,750 </td><td style="text-align: left;"> Poplar </td>                     <td> 4,418 </td></tr>
+<tr><td style="text-align: left;"> Gum </td>                  <td> 5,890 </td><td style="text-align: left;"> Spruce </td>                     <td> 3,255 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>              <td> 2,750 </td><td style="text-align: left;"> Walnut (black)</td>              <td> 4,728 </td></tr>
+<tr><td style="text-align: left;"> Hickory </td>              <td> 6,045 </td><td style="text-align: left;"> Walnut (common)</td>             <td> 2,830 </td></tr>
+<tr><td style="text-align: left;" colspan="4"> NOTE.&mdash;Two specimens of each were tested. All were fairly seasoned and without defects. The piece sheared off was 5/8 in. The single circular area of each pin was 0.322 sq. in. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="I_06"></a>
+<h3>TRANSVERSE OR BENDING STRENGTH: BEAMS</h3>
+
+<p>When external forces acting in the same plane are applied at
+right angles to the axis of a bar so as to cause it to bend,
+they occasion a shortening of the longitudinal fibres on the
+concave side and an elongation of those on the convex side.
+Within the elastic limit the relative stretching and contraction
+of the fibres is directly<a name="fn09r"></a><a href="#fn09"><sup><small>9</small></sup></a>] proportional to their distances
+from a plane <a name="pg023"></a> intermediate between them&mdash;the <b>neutral plane</b>.
+(<i>N</i><sub><small>1</small></sub><i>P</i> in <a href="#fig15">Fig. 15</a>.) Thus the fibres half-way between the
+neutral plane and the outer surface experience only half as much
+shortening or elongation as the outermost or extreme fibres.
+Similarly for other distances. The elements along the neutral
+plane experience no tension or compression in an axial
+direction. The line of intersection of this plane and the plane
+of section is known as the <b>neutral axis</b> (<i>N A</i> in <a href="#fig15">Fig. 15</a>.) of
+the section.</p>
+
+<p class="ctr"><img src="images/fig15.jpg" alt="Figure 15" id="fig15" /></p>
+<p class="imgtitle">Figure 15</p>
+<p class="imgtext"> Diagram of a simple beam. <i>N</i><sub><small>1</small></sub><i>P</i> =
+neutral plane, <i>N A</i> = neutral axis of section <i>R S</i>.</p>
+
+<p>If the bar is symmetrical and homogeneous the neutral plane is
+located half-way between the upper and lower surfaces, so long
+as the deflection does not exceed the elastic limit of the
+material. Owing to the fact that the tensile strength of wood is
+from two to nearly four times the compressive strength, it
+follows that at rupture the neutral plane is much nearer the
+convex than the concave side of the bar or beam, since the sum
+of all the compressive stresses on the concave portion must
+always equal the sum of the tensile stresses on the convex
+portion. The neutral plane begins to change from its central
+position as soon as the elastic limit has been passed. Its
+location at any time is very uncertain.</p>
+
+<p>The external forces acting to bend the bar also tend to rupture
+it at right angles to the neutral plane by causing one
+transverse section to slip past another. This stress at any
+point is equal to the resultant perpendicular to the axis of the
+forces acting at this point, and is termed the <b>transverse
+shear</b> (or in the case of beams, <b>vertical shear</b>).</p>
+
+<a name="pg024"></a>
+
+<p>In addition to this there is a shearing stress, tending to move
+the fibres past one another in an axial direction, which is
+called <b>longitudinal shear</b> (or in the case of beams,
+<b>horizontal shear</b>). This stress must be taken into
+consideration in the design of timber structures. It is maximum
+at the neutral plane and decreases to zero at the outer elements
+of the section. The shorter the span of a beam in proportion to
+its height, the greater is the liability of failure in
+horizontal shear before the ultimate strength of the beam is
+reached.</p>
+
+<h4><i>Beams</i></h4>
+
+<p>There are three common forms of beams, as follows:</p>
+
+<p>(1) <b>Simple beam</b>&mdash;a bar resting upon two supports, one near
+each end. (<a href="#fig16">See Fig. 16</a>, No. 1.)</p>
+
+<p>(2) <b>Cantilever beam</b>&mdash;a bar resting upon one support or
+fulcrum, or that portion of any beam projecting out of a wall or
+beyond a support. (<a href="#fig16">See Fig. 16</a>, No. 2.)</p>
+
+<p>(3) <b>Continuous beam</b>&mdash;a bar resting upon more than two
+supports. (<a href="#fig16">See Fig. 16</a>, No. 3.)</p>
+
+<p class="ctr"><img src="images/fig16.jpg" alt="Figure 16" id="fig16" /></p>
+<p class="imgtitle">Figure 16</p>
+<p class="imgtext"> Three common forms of beams. 1. Simple. 2. Cantilever. 3. Continuous.</p>
+
+<a name="pg025"></a>
+
+<h4><i>Stiffness of Beams</i></h4>
+
+<p>The two main requirements of a beam are stiffness and strength.
+The formul� for the <i>modulus of elasticity (E)</i> or measure of
+stiffness of a rectangular prismatic simple beam loaded at the
+centre and resting freely on supports at either end is:<a name="fn10r"></a><a href="#fn10"><sup><small>10</small></sup></a></p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td>     <td><br></td><td><i>P' l</i><sup><small>3</small></sup></td></tr>
+<tr><td><i>E</i></td> <td>=</td>   <td>-----------</td></tr>
+<tr><td><br></td>     <td><br></td><td><i>4 D b h</i><sup><small>3</small></sup></td></tr>
+<tr><td><br></td>     <td><br></td><td><br></td></tr>
+<tr><td><i>b</i></td> <td>=</td>   <td style="text-align: left;"> breadth or width of beam, inches. </td></tr>
+<tr><td><i>h</i></td> <td>=</td>   <td style="text-align: left;"> height or depth of beam, inches. </td></tr>
+<tr><td><i>l</i></td> <td>=</td>   <td style="text-align: left;"> span (length between points of supports) of beam, inches. </td></tr>
+<tr><td><i>D</i></td> <td>=</td>   <td style="text-align: left;"> deflection produced by load <i>P'</i>, inches. </td></tr>
+<tr><td><i>P'</i></td><td>=</td>   <td style="text-align: left;"> load at or below elastic limit, pounds. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>From this formul� it is evident that for rectangular beams of
+the same material, mode of support, and loading, the deflection
+is affected as follows:</p>
+
+<p>(1) It is inversely proportional to the width for beams of the
+same length and depth. If the width is tripled the deflection is
+one-third as great.</p>
+
+<p>(2) It is inversely proportional to the cube of the depth for
+beams of the same length and breadth. If the depth is tripled
+the deflection is one twenty-seventh as great.</p>
+
+<p>(3) It is directly proportional to the cube of the span for
+beams of the same breadth and depth. Tripling the span gives
+twenty-seven times the deflection.</p>
+
+<p>The number of pounds which concentrated at the centre will
+deflect a rectangular prismatic simple beam one inch may be
+found from the preceding formul� by substituting <i>D</i> = 1" and
+solving for <i>P'</i>. The formul� then becomes:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td>                        <td><br></td> <td>4 <i>E b h</i><sup><small>3</small></sup></td></tr>
+<tr><td>Necessary weight (<i>P'</i>)</td><td>=</td>    <td>----------</td></tr>
+<tr><td><br></td>                        <td><br></td> <td><i>l</i><sup><small>3</small></sup></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>In this case the values for <i>E</i> are read from tables prepared from
+data obtained by experimentation on the given material.
+
+<a name="pg026"></a>
+
+<h4><i>Strength of Beams</i></h4>
+
+<p>The measure of the breaking strength of a beam is expressed in
+terms of unit stress by a <i>modulus of rupture</i>, which is a
+purely hypothetical expression for points beyond the elastic
+limit. The formul� used in computing this modulus is as follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td>              <td><br></td> <td> 1.5 <i>P l</i></td></tr>
+<tr><td> <i>R</i> </td>        <td>=</td>    <td>---------</td></tr>
+<tr><td><br></td>              <td><br></td> <td> <i>b h</i><sup><small>2</small></sup></td></tr>
+<tr><td><br></td>              <td><br></td><td><br></td></tr>
+<tr><td><i>b, h, l</i></td>    <td>=</td>    <td style="text-align: left;"> breadth, height, and span, respectively, as in preceding formul�. </td></tr>
+<tr><td><i>R</i></td>          <td>=</td>    <td style="text-align: left;"> modulus of rupture, pounds per square inch. </td></tr>
+<tr><td><i>P</i></td>          <td>=</td>    <td style="text-align: left;"> maximum load, pounds. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>In calculating the fibre stress at the elastic limit the same
+formul� is used except that the load at elastic limit (<i>P</i><sub><small>1</small></sub>) is
+substituted for the maximum load (<i>P</i>).</p>
+
+<p>From this formul� it is evident that for rectangular prismatic
+beams of the same material, mode of support, and loading, the
+load which a given beam can support varies as follows:</p>
+
+<p>(1) It is directly proportional to the breadth for beams of the
+same length and depth, as is the case with stiffness.</p>
+
+<p>(2) It is directly proportional to the square of the height for
+beams of the same length and breadth, instead of as the cube of
+this dimension as in stiffness.</p>
+
+<p>(3) It is inversely proportional to the span for beams of the
+same breadth and depth and not to the cube of this dimension as
+in stiffness.</p>
+
+<p>The fact that the strength varies as the <i>square</i> of the height
+and the stiffness as the <i>cube</i> explains the relationship of
+bending to thickness. Were the law the same for strength and
+stiffness a thin piece of material such as a sheet of paper
+could not be bent any further without breaking than a thick
+piece, say an inch board.</p>
+
+<a name="pg027"></a>
+
+<center>
+<table id="TABLE_09" summary="TABLE_IX" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="7"> TABLE IX </th></tr>
+<tr><th colspan="7"> RESULTS OF STATIC BENDING TESTS ON SMALL CLEAR BEAMS OF 49 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="7"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="3"> COMMON NAME OF SPECIES </th>                <th rowspan="2"> Fibre stress at elastic limit </th>
+                                                                                 <th rowspan="2"> Modulus of rupture </th>
+                                                                                                  <th rowspan="2"> Modulus of elasticity </th>
+                                                                                                                      <th colspan="3"> Work in Bending </th></tr>
+<tr>                                                                                                                  <th> To elastic limit </th>
+                                                                                                                                     <th> To maximum load </th>
+                                                                                                                                                    <th> Total </th></tr>
+<tr>                                                             <th><i>Lbs. per sq. in.</i></th>
+                                                                                 <th><i>Lbs. per sq. in.</i></th>
+                                                                                                  <th><i>Lbs. per sq. in.</i></th>
+                                                                                                                      <th><i>In.-lbs. per cu. inch</i></th>
+                                                                                                                                     <th><i>In.-lbs. per cu. inch</i></th>
+                                                                                                                                                    <th><i>In.-lbs. per cu. inch</i></th></tr>
+<tr><th> <i>Hardwoods</i> </th>                                         <td><br></td>   <td><br></td>    <td><br></td>       <td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, black </td>              <td> 2,580 </td><td> 6,000 </td> <td> 960,000 </td>  <td> 0.41 </td><td> 13.1 </td><td> 38.9 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>       <td> 5,180 </td><td> 9,920 </td> <td> 1,416,000 </td><td> 1.10 </td><td> 20.0 </td><td> 43.7 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>                <td> 2,480 </td><td> 4,450 </td> <td> 842,000 </td>  <td> .45 </td> <td> 5.8 </td> <td> 8.9 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                   <td> 4,490 </td><td> 8,610 </td> <td> 1,353,000 </td><td> .96 </td> <td> 14.1 </td><td> 31.4 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>           <td> 4,190 </td><td> 8,390 </td> <td> 1,597,000 </td><td> .62 </td> <td> 14.2 </td><td> 31.5 </td></tr>
+<tr><td style="text-align: left;"> Elm, rock </td>               <td> 4,290 </td><td> 9,430 </td> <td> 1,222,000 </td><td> .90 </td> <td> 19.4 </td><td> 47.4 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; slippery </td>    <td> 5,560 </td><td> 9,510 </td> <td> 1,314,000 </td><td> 1.32 </td><td> 11.7 </td><td> 44.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>       <td> 2,850 </td><td> 6,940 </td> <td> 1,052,000 </td><td> .44 </td> <td> 11.8 </td><td> 27.4 </td></tr>
+<tr><td style="text-align: left;"> Gum, red </td>                <td> 3,460 </td><td> 6,450 </td> <td> 1,138,000 </td><td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>               <td> 3,320 </td><td> 7,800 </td> <td> 1,170,000 </td><td> .56 </td> <td> 19.6 </td><td> 52.9 </td></tr>
+<tr><td style="text-align: left;"> Hickory, big shellbark </td>  <td> 6,370 </td><td> 11,110 </td><td> 1,562,000 </td><td> 1.47 </td><td> 24.3 </td><td> 78.0 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; bitternut </td>   <td> 5,470 </td><td> 10,280 </td><td> 1,399,000 </td><td> 1.22 </td><td> 20.0 </td><td> 75.5 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; mockernut </td>   <td> 6,550 </td><td> 11,110 </td><td> 1,508,000 </td><td> 1.50 </td><td> 31.7 </td><td> 84.4 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; nutmeg </td>      <td> 4,860 </td><td> 9,060 </td> <td> 1,289,000 </td><td> 1.06 </td><td> 22.8 </td><td> 58.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; pignut </td>      <td> 5,860 </td><td> 11,810 </td><td> 1,769,000 </td><td> 1.12 </td><td> 30.6 </td><td> 86.7 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shagbark </td>    <td> 6,120 </td><td> 11,000 </td><td> 1,752,000 </td><td> 1.22 </td><td> 18.3 </td><td> 72.3 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; water </td>       <td> 5,980 </td><td> 10,740 </td><td> 1,563,000 </td><td> 1.29 </td><td> 18.8 </td><td> 52.9 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>           <td> 6,020 </td><td> 12,360 </td><td> 1,732,000 </td><td> 1.28 </td><td> 17.3 </td><td> 64.4 </td></tr>
+<tr><td style="text-align: left;"> Maple, red </td>              <td> 4,450 </td><td> 8,310 </td> <td> 1,445,000 </td><td> .78 </td> <td> 9.8 </td> <td> 17.1 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>       <td> 4,630 </td><td> 8,860 </td> <td> 1,462,000 </td><td> .88 </td> <td> 12.7 </td><td> 32.0 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>               <td> 4,720 </td><td> 7,380 </td> <td> 913,000 </td>  <td> 1.39 </td><td> 9.1 </td> <td> 17.4 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>         <td> 3,490 </td><td> 7,780 </td> <td> 1,268,000 </td><td> .60 </td> <td> 11.4 </td><td> 26.0 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td> <td> 5,380 </td><td> 9,860 </td> <td> 1,593,000 </td><td> 1.05 </td><td> 14.5 </td><td> 37.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; tanbark </td>     <td> 6,580 </td><td> 10,710 </td><td> 1,678,000 </td><td> 1.49 </td><td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>       <td> 4,320 </td><td> 8,090 </td> <td> 1,137,000 </td><td> .95 </td> <td> 12.1 </td><td> 36.7 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>      <td> 5,060 </td><td> 8,570 </td> <td> 1,219,000 </td><td> 1.20 </td><td> 11.7 </td><td> 30.7 </td></tr>
+<tr><td style="text-align: left;"> Osage orange </td>            <td> 7,760 </td><td> 13,660 </td><td> 1,329,000 </td><td> 2.53 </td><td> 37.9 </td><td> 101.7 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                <td> 2,820 </td><td> 6,300 </td> <td> 961,000 </td>  <td> .51 </td> <td> 7.1 </td> <td> 13.6 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                  <td> 4,300 </td><td> 7,380 </td> <td> 1,045,000 </td><td> 1.00 </td><td> 7.8 </td> <td> 20.9 </td></tr>
+<tr><th> <i>Conifers</i></th>                                    <td><br></td>   <td><br></td>    <td><br></td>       <td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>         <td> 2,600 </td><td> 4,250 </td> <td> 643,000 </td>  <td> .60 </td> <td> 5.7 </td> <td> 9.5 </td></tr>
+<tr><td style="text-align: left;"> Cedar, incense </td>          <td> 3,950 </td><td> 6,040 </td> <td> 754,000 </td>  <td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>           <td> 4,430 </td><td> 7,110 </td> <td> 1,378,000 </td><td> .96 </td> <td> 5.1 </td> <td> 15.4 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine </td>             <td> 2,366 </td><td> 4,450 </td> <td> 861,000 </td>  <td> .66 </td> <td> 4.4 </td> <td> 7.4 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; amabilis </td>    <td> 4,060 </td><td> 6,570 </td> <td> 1,323,000 </td><td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas </td>     <td> 3,570 </td><td> 6,340 </td> <td> 1,242,000 </td><td> .59 </td> <td> 6.6 </td> <td> 13.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>       <td> 3,880 </td><td> 5,970 </td> <td> 1,131,000 </td><td> .77 </td> <td> 5.2 </td> <td> 14.9 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>                 <td> 3,410 </td><td> 5,770 </td> <td> 917,000 </td>  <td> .73 </td> <td> 6.6 </td> <td> 12.9 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole </td>         <td> 3,080 </td><td> 5,130 </td> <td> 1,015,000 </td><td> .54 </td> <td> 5.1 </td> <td> 7.4 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf </td>    <td> 5,090 </td><td> 8,630 </td> <td> 1,662,000 </td><td> .88 </td> <td> 8.1 </td> <td> 34.8 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>         <td> 3,740 </td><td> 6,430 </td> <td> 1,384,000 </td><td> .59 </td> <td> 5.8 </td> <td> 28.0 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shortleaf </td>   <td> 4,360 </td><td> 7,710 </td> <td> 1,395,000 </td><td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>       <td> 3,330 </td><td> 5,270 </td> <td> 966,000 </td>  <td> .66 </td> <td> 5.0 </td> <td> 11.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; west, yellow </td><td> 3,180 </td><td> 5,180 </td> <td> 1,111,000 </td><td> .52 </td> <td> 4.3 </td> <td> 15.6 </td></tr>
+<tr><td style="text-align: left;"> White </td>                   <td> 3,410 </td><td> 5,310 </td> <td> 1,073,000 </td><td> .62 </td> <td> 5.9 </td> <td> 13.3 </td></tr>
+<tr><td style="text-align: left;"> Redwood </td>                 <td> 4,530 </td><td> 6,560 </td> <td> 1,024,000 </td><td><br></td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>       <td> 2,740 </td><td> 4,550 </td> <td> 866,000 </td>  <td> .50 </td> <td> 4.8 </td> <td> 6.1 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>         <td> 3,440 </td><td> 5,820 </td> <td> 1,143,000 </td><td> .62 </td> <td> 6.0 </td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>       <td> 3,160 </td><td> 5,200 </td> <td> 968,000 </td>  <td> .58 </td> <td> 6.6 </td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                <td> 4,200 </td><td> 7,170 </td> <td> 1,236,000 </td><td> .84 </td> <td> 7.2 </td> <td> 30.0 </td></tr>
+</tbody>
+</table>
+</center>
+
+<h4><i>Kinds of Loads</i></h4>
+
+<p>There are various ways in which beams are loaded, of which the
+following are the most important:</p>
+
+<p>(1) <b>Uniform load</b> occurs where the load is spread evenly over
+the beam.</p>
+
+<a name="pg028"></a>
+
+<p>(2) <b>Concentrated load</b> occurs where the load is applied at
+single point or points.</p>
+
+<p>(3) <b>Live</b> or <b>immediate load</b> is one of momentary or short
+duration at any one point, such as occurs in crossing a bridge.</p>
+
+<p>(4) <b>Dead</b> or <b>permanent load</b> is one of constant and
+indeterminate duration, as books on a shelf. In the case of a
+bridge the weight of the structure itself is the dead load. All
+large beams support a uniform dead load consisting of their own
+weight.</p>
+
+<p>The effect of dead load on a wooden beam may be two or more
+times that produced by an immediate load of the same weight.
+Loads greater than the elastic limit are unsafe and will
+generally result in rupture if continued long enough. A beam may
+be considered safe under permanent load when the deflections
+diminish during equal successive periods of time. A continual
+increase in deflection indicates an unsafe load which is almost
+certain to rupture the beam eventually.</p>
+
+<p>Variations in the humidity of the surrounding air influence the
+deflection of dry wood under dead load, and increased
+deflections during damp weather are cumulative and not recovered
+by subsequent drying. In the case of longleaf pine, dry beams
+may with safety be loaded permanently to within three-fourths of
+their elastic limit as determined from ordinary static tests.
+Increased moisture content, due to greater humidity of the air,
+lowers the elastic limit of wood so that what was a safe load
+for the dry material may become unsafe.</p>
+
+<p>When a dead load not great enough to rupture a beam has been
+removed, the beam tends gradually to recover its former shape,
+but the recovery is not always complete. If specimens from such
+a beam are tested in the ordinary testing machine it will be
+found that the application of the dead load did not affect the
+stiffness, ultimate strength, or elastic limit of the material.
+In other words, the deflections and recoveries produced by live
+loads are the same as would have been produced had not the beam
+previously been subjected to a dead load.<a name="fn11r"></a><a href="#fn11"><sup><small>11</small></sup></a></p>
+
+<a name="pg029"></a>
+
+<p><b>Maximum load</b> is the greatest load a material will support and
+is usually greater than the load at rupture.</p>
+
+<p><b>Safe load</b> is the load considered safe for a material to
+support in actual practice. It is always less than the load at
+elastic limit and is usually taken as a certain proportion of
+the ultimate or breaking load.</p>
+
+<p>The ratio of the breaking to the safe load is called the factor
+of safety.</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td>                <td><br></td><td>ultimate strength</td><td rowspan="3"><big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td>Factor of safety<br></td><td>=</td>  <td>-------------------<br></td></tr>
+<tr>                                                                  <td><br></td>                <td><br></td>     <td>safe load<br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>In
+order to make due allowance for the natural variations and
+imperfections in wood and in the aggregate structure, as well as
+for variations in the load, the factor of safety is usually as
+high as 6 or 10, especially if the safety of human life depends
+upon the structure. This means that only from one-sixth to
+one-tenth of the computed strength values is considered safe to
+use. If the depth of timbers exceeds four times their thickness
+there is a great tendency for the material to twist when loaded.
+It is to overcome this tendency that floor joists are braced at
+frequent intervals. Short deep pieces shear out or split before
+their strength in bending can fully come into play.
+
+<h4><i>Application of Loads</i></h4>
+
+<p>There are three<a name="fn12r"></a><a href="#fn12"><sup><small>12</small></sup></a> general methods in which loads may be
+applied to beams, namely:</p>
+
+<p>(1) <b>Static loading</b> or the gradual imposition of load so that
+the moving parts acquire no appreciable momentum. Loads are so
+applied in the ordinary testing machine.</p>
+
+<p>(2) <b>Sudden imposition of load without initial velocity.</b> "Thus
+in the case of placing a load on a beam, if the load be brought
+into contact with the beam, but its weight sustained by external
+means, as by a cord, and then this external support be
+<i>suddenly</i> (instantaneously) removed, as by quickly cutting the
+cord, then, although the load is already touching the beam (and
+hence there is no real impact), yet the beam is at first
+offering no resistance, as it has yet suffered no deformation.
+Furthermore, as the beam deflects the <a name="pg030"></a> resistance increases, but
+does not come to be equal to the load until it has attained its
+normal deflection. In the meantime there has been an unbalanced
+force of gravity acting, of a constantly diminishing amount,
+equal at first to the entire load, at the normal deflection. But
+at this instant the load and the beam are in motion, the
+hitherto unbalanced force having produced an accelerated
+velocity, and this velocity of the weight and beam gives to them
+an energy, or <i>vis viva</i>, which must now spend itself in
+overcoming an <i>excess</i> of resistance over and above the imposed
+load, and the whole mass will not stop until the deflection (as
+well as the resistance) has come to be equal to <i>twice</i> that
+corresponding to the static load imposed. Hence we say the
+effect of a suddenly imposed load is to produce twice the
+deflection and stress of the same load statically applied. It
+must be evident, however, that this case has nothing in common
+with either the ordinary 'static' tests of structural materials
+in testing-machines, or with impact tests."<a name="fn13r"></a><a href="#fn13"><sup><small>13</small></sup></a></p>
+
+<p>(3) <b>Impact, shock,</b> or <b>blow.</b><a name="fn14r"></a><a href="#fn14"><sup><small>14</small></sup></a> There are various common
+uses of wood where the material is subjected to sudden shocks
+and jars or impact. Such is the action on the felloes and spokes
+of a wagon wheel passing over a rough road; on a hammer handle
+when a blow is struck; on a maul when it strikes a wedge.</p>
+
+<p>Resistance to impact is resistance to energy which is measured
+by the product of the force into the space through which it
+moves, or by the product of one-half the moving mass which
+causes the shock into the square of its velocity. The work done
+upon the piece at the instant the velocity is entirely removed
+from the striking body is equal to the total energy of that
+body. It is impossible, however, to get all of the energy of the
+striking body stored in the specimen, though the greater the
+mass and the shorter the space through which it moves, or, in
+other words, the greater the proportion of weight and the
+smaller the proportion of velocity making up the energy of the
+striking body, the more energy the specimen will absorb. The
+rest is lost in friction, vibrations, heat, and motion of the
+anvil.</p>
+
+<a name="pg031"></a>
+
+<p>In impact the stresses produced become very complex and
+difficult to measure, especially if the velocity is high, or the
+mass of the beam itself is large compared to that of the weight.</p>
+
+<p>The difficulties attending the measurement of the stresses
+beyond the elastic limit are so great that commonly they are not
+reckoned. Within the elastic limit the formul� for calculating
+the stresses are based on the assumption that the deflection is
+proportional to the stress in this case as in static tests.</p>
+
+<p>A common method of making tests upon the resistance of wood to
+shock is to support a small beam at the ends and drop a heavy
+weight upon it in the middle. (<a href="#fig40">See Fig. 40</a>.) The height of the
+weight is increased after each drop and records of the
+deflection taken until failure. The total work done upon the
+specimen is equal to the area of the stress-strain diagram plus
+the effect of local inertia of the molecules at point of
+contact.</p>
+
+<p>The stresses involved in impact are complicated by the fact that
+there are various ways in which the energy of the striking body
+may be spent:</p>
+
+<p>(<i>a</i>) It produces a local deformation of both bodies at the
+surface of contact, within or beyond the elastic limit. In
+testing wood the compression of the substance of the steel
+striking-weight may be neglected, since the steel is very hard
+in comparison with the wood. In addition to the compression of
+the fibres at the surface of contact resistance is also offered
+by the inertia of the particles there, the combined effect of
+which is a stress at the surface of contact often entirely out
+of proportion to the compression which would result from the
+action of a static force of the same magnitude. It frequently
+exceeds the crushing strength at the extreme surface of contact,
+as in the case of the swaging action of a hammer on the head of
+an iron spike, or of a locomotive wheel on the steel rail. This
+is also the case when a bullet is shot through a board or a pane
+of glass without breaking it as a whole.</p>
+
+<p>(<i>b</i>) It may move the struck body as a whole with an accelerated
+velocity, the resistance consisting of the inertia of the body.
+This effect is seen when a croquet ball is struck with a mallet.</p>
+
+<p>(<i>c</i>) It may deform a fixed body against its external supports
+and resistances. In making impact tests in the laboratory the
+test specimen is in reality in the nature of a cushion between
+two <a name="pg032"></a> impacting bodies, namely, the striking weight and the base
+of the machine. It is important that the mass of this base be
+sufficiently great that its relative velocity to that of the
+common centre of gravity of itself and the striking weight may
+be disregarded.</p>
+
+<p>(<i>d</i>) It may deform the struck body as a whole against the
+resisting stresses developed by its own inertia, as, for
+example, when a baseball bat is broken by striking the ball.</p>
+
+<center>
+<table id="TABLE_10" summary="TABLE_X" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="4"> TABLE X </th></tr>
+<tr><th colspan="4"> RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS OF 34 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="4"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>                  <th> Fibre stress at elastic limit </th>
+                                                                                    <th> Modulus of elasticity </th>
+                                                                                                        <th> Work in bending to elastic limit </th></tr>
+<tr>                                                               <th> <i>Lbs. per sq. in.</i></th>
+                                                                                    <th><i>Lbs. per sq. in.</i></th>
+                                                                                                        <th><i>In.-lbs. per cu. inch</i></th></tr>
+<tr><th> <i>Hardwoods</i> </th>                                    <td><br></td>    <td><br></td>       <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, black</td>                <td> 7,840 </td> <td> 955,000 </td>  <td> 3.69 </td></tr>
+<tr><td style="text-align: left;"> white </td>                     <td> 11,710 </td><td> 1,564,000 </td><td> 4.93 </td></tr>
+<tr><td style="text-align: left;"> Basswood </td>                  <td> 5,480 </td> <td> 917,000 </td>  <td> 1.84 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                     <td> 11,760 </td><td> 1,501,000 </td><td> 5.10 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>             <td> 11,080 </td><td> 1,812,000 </td><td> 3.79 </td></tr>
+<tr><td style="text-align: left;"> Elm, rock </td>                 <td> 12,090 </td><td> 1,367,000 </td><td> 6.52 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; slippery </td>      <td> 11,700 </td><td> 1,569,000 </td><td> 4.86 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 9,910 </td> <td> 1,138,000 </td><td> 4.82 </td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>                 <td> 10,420 </td><td> 1,398,000 </td><td> 4.48 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>             <td> 13,460 </td><td> 2,114,000 </td><td> 4.76 </td></tr>
+<tr><td style="text-align: left;"> Maple, red </td>                <td> 11,670 </td><td> 1,411,000 </td><td> 5.45 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 11,680 </td><td> 1,680,000 </td><td> 4.55 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>                 <td> 11,260 </td><td> 1,596,000 </td><td> 4.41 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 10,580 </td><td> 1,506,000 </td><td> 4.16 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td>   <td> 13,280 </td><td> 2,048,000 </td><td> 4.79 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 9,860 </td> <td> 1,414,000 </td><td> 3.84 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>        <td> 10,840 </td><td> 1,479,000 </td><td> 4.44 </td></tr>
+<tr><td style="text-align: left;"> Osage orange </td>              <td> 15,520 </td><td> 1,498,000 </td><td> 8.92 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                  <td> 8,180 </td> <td> 1,165,000 </td><td> 3.22 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                    <td> 7,650 </td> <td> 1,310,000 </td><td> 2.49 </td></tr>
+<tr><th> <i>Conifers</i> </th>                                     <td><br></td>    <td><br></td>       <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>           <td> 5,290 </td> <td> 778,000 </td>  <td> 2.04 </td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>             <td> 8,290 </td> <td> 1,431,000 </td><td> 2.71 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine </td>               <td> 5,280 </td> <td> 980,000 </td>  <td> 1.59 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas </td>       <td> 8,870 </td> <td> 1,579,000 </td><td> 2.79 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 7,230 </td> <td> 1,326,000 </td><td> 2.21 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>                   <td> 6,330 </td> <td> 1,025,000 </td><td> 2.19 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole </td>           <td> 6,870 </td> <td> 1,142,000 </td><td> 2.31 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf </td>      <td> 9,680 </td> <td> 1,739,000 </td><td> 3.02 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 7,480 </td> <td> 1,438,000 </td><td> 2.18 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 6,740 </td> <td> 1,083,000 </td><td> 2.34 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; western yellow </td><td> 7,070 </td> <td> 1,115,000 </td><td> 2.51 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 6,490 </td> <td> 1,156,000 </td><td> 2.06 </td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>         <td> 6,300 </td> <td> 1,076,000 </td><td> 2.09 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                  <td> 7,750 </td> <td> 1,263,000 </td><td> 2.67 </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg033"></a>
+
+<p>Impact testing is difficult to conduct satisfactorily and the
+data obtained are of chief value in a relative sense, that is,
+for comparing the shock-resisting ability of woods of which like
+specimens have been subjected to exactly identical treatment.
+Yet this test is one of the most important made on wood, as it
+brings out properties not evident from other tests. Defects and
+brittleness are revealed by impact better than by any other kind
+of test. In common practice nearly all external stresses are of
+the nature of impact. In fact, no two moving bodies can come
+together without impact stress. Impact is therefore the
+commonest form of applied stress, although the most difficult to
+measure.
+
+<h4><i>Failures in Timber Beams</i></h4>
+
+<p>If a beam is loaded too heavily it will break or fail in some
+characteristic manner. These failures may be classified
+according to the way in which they develop, as tension,
+compression, and horizontal shear; and according to the
+appearance of the broken surface, as brash, and fibrous. A
+number of forms may develop if the beam is completely ruptured.</p>
+
+<p>Since the tensile strength of wood is on the average about three
+times as great as the compressive strength, a beam should,
+therefore, be expected to fail by the formation in the first
+place of a fold on the compression side due to the crushing
+action, followed by failure on the tension side. This is usually
+the case in green or moist wood. In dry material the first
+visible failure is not infrequently on the lower or tension
+side, and various attempts have been made to explain why such is
+the case.<a name="fn15r"></a><a href="#fn15"><sup><small>15</small></sup></a></p>
+
+<p>Within the elastic limit the elongations and shortenings are
+equal, and the neutral plane lies in the middle of the beam.
+(<a href="#pg023">See page 23</a>.) Later the
+top layer of fibres on the upper or compression side fail, and
+on the load increasing, the next layer of fibres fail, and so
+on, even though this failure may not be visible. As a result the
+shortenings on the upper side of the beam become considerably
+greater than the elongations on the lower side. The neutral
+plane must be presumed to sink gradually toward the tension
+side, and when the stresses on the outer fibres at the bottom
+<a name="pg034"></a>
+have become sufficiently great, the fibres are pulled in two,
+the tension area being much smaller than the compression area.
+The rupture is often irregular, as in direct tension tests.
+Failure may occur partially in single bundles of fibres some
+time before the final failure takes place. One reason why the
+failure of a dry beam is different from one that is moist, is
+that drying increases the stiffness of the fibres so that they
+offer more resistance to crushing, while it has much less effect
+upon the tensile strength.</p>
+
+<p>There is considerable variation in tension failures depending
+upon the toughness or the brittleness of the wood, the
+arrangement of the grain, defects, etc., making further
+classification desirable. The four most common forms are:</p>
+
+<p>(1) <b>Simple tension,</b> in which there is a direct pulling in two
+of the wood on the under side of the beam due to a tensile
+stress parallel to the grain, (<a href="#fig17">See Fig. 17</a>, No. 1.) This is
+common in straight-grained beams, particularly when the wood is
+seasoned.</p>
+
+<p>(2) <b>Cross-grained tension,</b> in which the fracture is caused by a
+tensile force acting oblique to the grain. (<a href="#fig17">See Fig. 17</a>, No. 2.)
+This is a common form of failure where the beam has diagonal,
+spiral or other form of cross grain on its lower side. Since the
+tensile strength of wood across the grain is only a small
+fraction of that with the grain it is easy to see why a
+cross-grained timber would fail in this manner.</p>
+
+<p>(3) <b>Splintering tension,</b> in which the failure consists of a
+considerable number of slight tension failures, producing a
+ragged or splintery break on the under surface of the beam. (<a href="#fig17">See
+Fig. 17</a>, No. 3.) This is common in tough woods. In this case the
+surface of fracture is fibrous.</p>
+
+<p>(4) <b>Brittle tension,</b> in which the beam fails by a clean break
+extending entirely through it. (<a href="#fig17">See Fig. 17</a>, No. 4.) It is
+characteristic of a brittle wood which gives way suddenly
+without warning, like a piece of chalk. In this case the surface
+of fracture is described as brash.</p>
+
+<p><b>Compression failure</b> (<a href="#fig17">see Fig. 17</a>, No. 5) has few variations
+except that it appears at various distances from the neutral
+plane of the beam. It is very common in green timbers. The
+compressive stress parallel to the fibres causes them to buckle
+or bend as in an endwise compressive test. This action usually
+<a name="pg035"></a>
+begins on the top side shortly after the elastic limit is
+reached and extends downward, sometimes almost reaching the
+neutral plane before complete failure occurs. Frequently two or
+more failures develop at about the same time.</p>
+
+<p class="ctr"><img src="images/fig17.jpg" alt="Figure 17" id="fig17" /></p>
+<p class="imgtitle">Figure 17</p>
+<p class="imgtext"> Characteristic failures of simple beams.</p>
+
+<p><b>Horizontal shear failure,</b> in which the upper and lower
+portions of the beam slide along each other for a portion of
+their length either at one or at both ends (<a href="#fig17">see Fig. 17</a>, No. 6),
+is fairly common in air-dry material and in green material when
+the ratio of the height of the beam to the span is relatively
+large. It is not common in small clear specimens. It is often
+due to shake or season checks, common in large timbers, which
+<a name="pg036"></a>
+reduce the actual area resisting the shearing action
+considerably below the calculated area used in the formul� for
+horizontal shear. (<a href="#pg098">See page 98</a> for this formul�.) For this
+reason it is unsafe, in designing large timber beams, to use
+shearing stresses higher than those calculated for beams that
+failed in horizontal shear. The effect of a failure in
+horizontal shear is to divide the beam into two or more beams
+the combined strength of which is much less than that of the
+original beam. <a href="#fig18">Fig. 18</a> shows a large beam in which two failures
+<a name="pg037"></a>
+in horizontal shear occurred at the same end. That the parts
+behave independently is shown by the compression failure below
+the original location of the neutral plane.</p>
+
+<p class="ctr"><img src="images/fig18.jpg" alt="Figure 18" id="fig18" /></p>
+<p class="imgtitle">Figure 18</p>
+<p class="imgtext"> Failure of a large beam by horizontal shear. <i>Photo by U. S, Forest Service</i>.</p>
+
+<p>Table XI gives an analysis of the causes of first failure in 840
+large timber beams of nine different species of conifers. Of the
+total number tested 165 were air-seasoned, the remainder green.
+The failure occurring first signifies the point of greatest
+weakness in the specimen under the particular conditions of
+loading employed (in this case, third-point static loading).</p>
+
+<center>
+<table id="TABLE_11" summary="TABLE_XI" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="5"> TABLE XI </th></tr>
+<tr><th colspan="5"> MANNER OF FIRST FAILURE OF LARGE BEAMS </th></tr>
+<tr><td colspan="5"> (Forest Service Bul. 108, p. 56) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>        <th rowspan="2"> Total number of tests </th>
+                                                                       <th colspan="3"> Per cent of total failing by </th></tr>
+                                                                       <tr><th> Tension </th>
+                                                                                    <th> Compression </th>
+                                                                                                 <th> Shear </th></tr>
+<tr><th style="text-align: left;"> Longleaf pine: </th>  <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 17 </td> <td> 18 </td><td> 24 </td><td> 58 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 9 </td>  <td> 22 </td><td> 22 </td><td> 56 </td></tr>
+<tr><th style="text-align: left;"> Douglas fir: </th>    <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 191 </td><td> 27 </td><td> 72 </td><td> 1 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 91 </td> <td> 19 </td><td> 76 </td><td> 5 </td></tr>
+<tr><th style="text-align: left;"> Shortleaf pine: </th> <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 48 </td> <td> 27 </td><td> 56 </td><td> 17 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 13 </td> <td> 54 </td><td><br></td><td> 46 </td></tr>
+<tr><th style="text-align: left;"> Western larch: </th>  <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 62 </td> <td> 23 </td><td> 71 </td><td> 6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 52 </td> <td> 54 </td><td> 19 </td><td> 27 </td></tr>
+<tr><th style="text-align: left;"> Loblolly pine: </th>  <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 111 </td><td> 40 </td><td> 53 </td><td> 7 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 25 </td> <td> 60 </td><td> 12 </td><td> 28 </td></tr>
+<tr><th style="text-align: left;"> Tamarack: </th>       <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 30 </td> <td> 37 </td><td> 53 </td><td> 10 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 9 </td>  <td> 45 </td><td> 22 </td><td> 33 </td></tr>
+<tr><th style="text-align: left;"> Western hemlock: </th><td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 39 </td> <td> 21 </td><td> 74 </td><td> 5 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 44 </td> <td> 11 </td><td> 66 </td><td> 23 </td></tr>
+<tr><th style="text-align: left;"> Redwood: </th>        <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 28 </td> <td> 43 </td><td> 50 </td><td> 7 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 12 </td> <td> 83 </td><td> 17 </td><td><br></td></tr>
+<tr><th style="text-align: left;"> Norway pine: </th>    <td><br></td> <td><br></td><td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; green</td><td> 49 </td> <td> 18 </td><td> 76 </td><td> 6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; dry</td>  <td> 10 </td> <td> 30 </td><td> 60 </td><td> 10 </td></tr>
+<tr><td style="text-align: left;" colspan="5"> NOTE.&mdash;These tests were made on timbers ranging in cross section from 4" &times; 10" to 8" &times; 16", and with a span of 15 feet. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="I_07"></a>
+<h3>TOUGHNESS: TORSION</h3>
+
+<p>Toughness is a term applied to more than one property of wood.
+Thus wood that is difficult to split is said to be tough. Again,
+a tough wood is one that will not rupture until it has deformed
+considerably under loads at or near its maximum strength, or one
+which still hangs together after it has been ruptured and may be
+bent back and forth without breaking apart. Toughness includes
+flexibility and is the reverse of brittleness, in that tough
+woods <a name="pg038"></a> break gradually and give warning of failure. Tough woods
+offer great resistance to impact and will permit rougher
+treatment in manipulations attending manufacture and use.
+Toughness is dependent upon the strength, cohesion, quality,
+length, and arrangement of fibre, and the pliability of the
+wood. Coniferous woods as a rule are not as tough as hardwoods,
+of which hickory and elm are the best examples.</p>
+
+<p class="ctr"><img src="images/fig19.jpg" alt="Figure 19" id="fig19" /></p>
+<p class="imgtitle">Figure 19</p>
+<p class="imgtext"> Torsion of a shaft.</p>
+
+<p>The torsion or twisting test is useful in determining the
+toughness of wood. If the ends of a shaft are turned in opposite
+directions, or one end is turned and the other is fixed, all of
+the fibres except those at the axis tend to assume the form of
+helices. (<a href="#fig19">See Fig. 19</a>.) The strain produced by torsion or
+twisting is essentially shear transverse and parallel to the
+fibres, combined with longitudinal tension and transverse
+compression. Within the elastic limit the strains increase
+directly as the distance from the axis of the specimen. The
+outer elements are subjected to tensile stresses, and as they
+become twisted tend to compress those near the axis. The
+elongated elements also contract laterally. Cross sections which
+were originally plane become warped. With increasing strain the
+lateral adhesion of the outer fibres is destroyed, allowing them
+to slide past each other, and reducing greatly their power of
+resistance. In this way the strains on the fibres nearer the
+axis are progressively increased until finally all of the
+elements are sheared apart. It is only in the toughest materials
+that the full effect of this action can be observed. (<a href="#fig20">See Fig.
+20</a>.) Brittle woods snap off suddenly with only a small amount of
+torsion, and their fracture is irregular and oblique <a name="pg039"></a> to the axis
+of the piece instead of frayed out and more nearly perpendicular
+to the axis as is the case with tough woods.</p>
+
+<p class="ctr"><img src="images/fig20.jpg" alt="Figure 20" id="fig20" /></p>
+<p class="imgtitle">Figure 20</p>
+<p class="imgtext"> Effect of torsion on different grades of hickory. <i>Photo by U. S. Forest Service.</i></p>
+
+<a name="I_08"></a>
+<h3>HARDNESS</h3>
+
+<p>The term <i>hardness</i> is used in two senses, namely: (1)
+resistance to indentation, and (2) resistance to abrasion or
+scratching. In the latter sense hardness combined with toughness
+is a measure of the wearing ability of wood and is an important
+consideration in the use of wood for floors, paving blocks,
+bearings, and rollers. While resistance to indentation is
+dependent mostly upon the density of the wood, the wearing
+qualities may be governed by other factors such as toughness,
+and the size, cohesion, and arrangement of the fibres. In use
+for floors, some woods tend to compact and wear smooth, while
+others become splintery and rough. This feature is affected to
+some extent by the manner in which the wood is sawed; thus
+edge-grain pine flooring is much better than flat-sawn for
+uniformity of wear.</p>
+
+<a name="pg040"></a>
+
+<center>
+<table id="TABLE_12" summary="TABLE_XII" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="5"> TABLE XII </th></tr>
+<tr><th colspan="5"> HARDNESS OF 32 WOODS IN GREEN CONDITION, AS INDICATED BY THE LOAD REQUIRED TO IMBED A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER </th></tr>
+<tr><td colspan="5"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>            <th> Average </th>
+                                                                             <th> End surface </th>
+                                                                                             <th> Radial surface </th>
+                                                                                                             <th> Tangential surface </th></tr>
+                                                             <tr><th><i>Pounds</i></th>
+                                                                             <th><i>Pounds</i></th>
+                                                                                             <th><i>Pounds</i></th>
+                                                                                                             <th><i>Pounds</i></th></tr>
+<tr><th> <i>Hardwoods</i> </th>                              <td><br></td>   <td><br></td>   <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> 1 Osage orange</td>       <td> 1,971 </td><td> 1,838 </td><td> 2,312 </td><td> 1,762 </td></tr>
+<tr><td style="text-align: left;"> 2 Honey locust</td>       <td> 1,851 </td><td> 1,862 </td><td> 1,860 </td><td> 1,832 </td></tr>
+<tr><td style="text-align: left;"> 3 Swamp white oak</td>    <td> 1,174 </td><td> 1,205 </td><td> 1,217 </td><td> 1,099 </td></tr>
+<tr><td style="text-align: left;"> 4 White oak</td>          <td> 1,164 </td><td> 1,183 </td><td> 1,163 </td><td> 1,147 </td></tr>
+<tr><td style="text-align: left;"> 5 Post oak</td>           <td> 1,099 </td><td> 1,139 </td><td> 1,068 </td><td> 1,081 </td></tr>
+<tr><td style="text-align: left;"> 6 Black oak</td>          <td> 1,069 </td><td> 1,093 </td><td> 1,083 </td><td> 1,031 </td></tr>
+<tr><td style="text-align: left;"> 7 Red oak</td>            <td> 1,043 </td><td> 1,107 </td><td> 1,020 </td><td> 1,002 </td></tr>
+<tr><td style="text-align: left;"> 8 White ash</td>          <td> 1,046 </td><td> 1,121 </td><td> 1,000 </td><td> 1,017 </td></tr>
+<tr><td style="text-align: left;"> 9 Beech</td>              <td> 942 </td>  <td> 1,012 </td><td> 897 </td>  <td> 918 </td></tr>
+<tr><td style="text-align: left;"> 10 Sugar maple</td>       <td> 937 </td>  <td> 992 </td>  <td> 918 </td>  <td> 901 </td></tr>
+<tr><td style="text-align: left;"> 11 Rock elm</td>          <td> 910 </td>  <td> 954 </td>  <td> 883 </td>  <td> 893 </td></tr>
+<tr><td style="text-align: left;"> 12 Hackberry</td>         <td> 799 </td>  <td> 829 </td>  <td> 795 </td>  <td> 773 </td></tr>
+<tr><td style="text-align: left;"> 13 Slippery elm</td>      <td> 788 </td>  <td> 919 </td>  <td> 757 </td>  <td> 687 </td></tr>
+<tr><td style="text-align: left;"> 14 Yellow birch</td>      <td> 778 </td>  <td> 827 </td>  <td> 768 </td>  <td> 739 </td></tr>
+<tr><td style="text-align: left;"> 15 Tupelo</td>            <td> 738 </td>  <td> 814 </td>  <td> 666 </td>  <td> 733 </td></tr>
+<tr><td style="text-align: left;"> 16 Red maple</td>         <td> 671 </td>  <td> 766 </td>  <td> 621 </td>  <td> 626 </td></tr>
+<tr><td style="text-align: left;"> 17 Sycamore</td>          <td> 608 </td>  <td> 664 </td>  <td> 560 </td>  <td> 599 </td></tr>
+<tr><td style="text-align: left;"> 18 Black ash</td>         <td> 551 </td>  <td> 565 </td>  <td> 542 </td>  <td> 546 </td></tr>
+<tr><td style="text-align: left;"> 19 White elm</td>         <td> 496 </td>  <td> 536 </td>  <td> 456 </td>  <td> 497 </td></tr>
+<tr><td style="text-align: left;"> 20 Basswood</td>          <td> 239 </td>  <td> 273 </td>  <td> 226 </td>  <td> 217 </td></tr>
+<tr><th> <i>Conifers</i> </th>                               <td><br></td>   <td><br></td>   <td><br></td>   <td><br></td></tr>
+<tr><td style="text-align: left;"> 1 Longleaf pine</td>      <td> 532 </td>  <td> 574 </td>  <td> 502 </td>  <td> 521 </td></tr>
+<tr><td style="text-align: left;"> 2 Douglas fir</td>        <td> 410 </td>  <td> 415 </td>  <td> 399 </td>  <td> 416 </td></tr>
+<tr><td style="text-align: left;"> 3 Bald cypress</td>       <td> 390 </td>  <td> 460 </td>  <td> 355 </td>  <td> 354 </td></tr>
+<tr><td style="text-align: left;"> 4 Hemlock</td>            <td> 384 </td>  <td> 463 </td>  <td> 354 </td>  <td> 334 </td></tr>
+<tr><td style="text-align: left;"> 5 Tamarack</td>           <td> 384 </td>  <td> 401 </td>  <td> 380 </td>  <td> 370 </td></tr>
+<tr><td style="text-align: left;"> 6 Red pine</td>           <td> 347 </td>  <td> 355 </td>  <td> 345 </td>  <td> 340 </td></tr>
+<tr><td style="text-align: left;"> 7 White fir</td>          <td> 346 </td>  <td> 381 </td>  <td> 322 </td>  <td> 334 </td></tr>
+<tr><td style="text-align: left;"> 8 Western yellow pine</td><td> 328 </td>  <td> 334 </td>  <td> 307 </td>  <td> 342 </td></tr>
+<tr><td style="text-align: left;"> 9 Lodgepole pine</td>     <td> 318 </td>  <td> 316 </td>  <td> 318 </td>  <td> 319 </td></tr>
+<tr><td style="text-align: left;"> 10 White pine</td>        <td> 299 </td>  <td> 304 </td>  <td> 294 </td>  <td> 299 </td></tr>
+<tr><td style="text-align: left;"> 11 Engelmann pine</td>    <td> 266 </td>  <td> 272 </td>  <td> 253 </td>  <td> 274 </td></tr>
+<tr><td style="text-align: left;"> 12 Alpine fir</td>        <td> 241 </td>  <td> 284 </td>  <td> 203 </td>  <td> 235 </td></tr>
+<tr><td style="text-align: left;" colspan="5"> NOTE.&mdash;Black locust and hickory are not included in this table, but their position would be near the head of the list. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg041"></a>
+
+<p>Tests for either form of hardness are of comparative value only.
+Tests for indentation are commonly made by penetrations of the
+material with a steel punch or ball.<a name="fn16r"></a><a href="#fn16"><sup><small>16</small></sup></a> Tests for abrasion are
+made by wearing down wood with sandpaper or by means of a sand
+blast.</p>
+
+<a name="I_09"></a>
+<h3>CLEAVABILITY</h3>
+
+<p><i>Cleavability</i> is the term used to denote the facility with
+which wood is split. A splitting stress is one in which the
+forces act normally like a wedge. (<a href="#fig21">See Fig. 21</a>.) The plane of
+cleavage is parallel to the grain, either radially or
+tangentially.</p>
+
+<p class="ctr"><img src="images/fig21.jpg" alt="Figure 21" id="fig21" /></p>
+<p class="imgtitle">Figure 21</p>
+<p class="imgtext"> Cleavage of highly elastic wood. The cleft runs far ahead of the wedge.</p>
+
+<p>This property of wood is very important in certain uses such as
+firewood, fence rails, billets, and squares. Resistance to
+splitting or low cleavability is desirable where wood must hold
+nails or screws, as in box-making. Wood usually splits more
+readily along the radius than parallel to the growth rings
+though exceptions occur, as in the case of cross grain.</p>
+
+<p>Splitting involves transverse tension, but only a portion of the
+fibres are under stress at a time. A wood of little stiffness
+and strong cohesion across the grain is difficult to split,
+while one with great stiffness, such as longleaf pine, is easily
+split. The form of the grain and the presence of knots greatly
+affect this quality.</p>
+
+<a name="pg042"></a>
+
+<center>
+<table id="TABLE_13" summary="TABLE_XIII" border="3" cellspacing="0" cellpadding="1" width="60%">
+<tbody>
+<tr><th colspan="3"> TABLE XIII </th></tr>
+<tr><th colspan="3"> CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF 32 WOODS IN GREEN CONDITION </th></tr>
+<tr><td colspan="3"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="2"> COMMON NAME OF SPECIES </th>                  <th> When surface of failure is radial </th>
+                                                                                 <th> When surface of failure is tangential </th></tr>
+<tr>                                                               <th><i>Lbs. per sq. inch</i></th>
+                                                                                 <th><i>Lbs. per sq. inch</i></th></tr>
+<tr><th> <i>Hardwoods</i> </th>                                    <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, black</td>                 <td> 275 </td><td> 260 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 333 </td><td> 346 </td></tr>
+<tr><td style="text-align: left;"> Bashwood </td>                  <td> 130 </td><td> 168 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                     <td> 339 </td><td> 527 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>             <td> 294 </td><td> 287 </td></tr>
+<tr><td style="text-align: left;"> Elm, slippery </td>             <td> 401 </td><td> 424 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 210 </td><td> 270 </td></tr>
+<tr><td style="text-align: left;"> Hackberr </td>                  <td> 422 </td><td> 436 </td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>             <td> 552 </td><td> 610 </td></tr>
+<tr><td style="text-align: left;"> Maple, red </td>                <td> 297 </td><td> 330 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 376 </td><td> 513 </td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>                 <td> 354 </td><td> 487 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 380 </td><td> 470 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td>   <td> 428 </td><td> 536 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 382 </td><td> 457 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>        <td> 379 </td><td> 470 </td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>                  <td> 265 </td><td> 425 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                    <td> 277 </td><td> 380 </td></tr>
+<tr><th> <i>Conifers</i> </th>                                     <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig; </td>           <td> 148 </td><td> 139 </td></tr>
+<tr><td style="text-align: left;"> Cypress, bald </td>             <td> 167 </td><td> 154 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine </td>               <td> 130 </td><td> 133 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas </td>       <td> 139 </td><td> 127 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 145 </td><td> 187 </td></tr>
+<tr><td style="text-align: left;"> Hemlock </td>                   <td> 168 </td><td> 151 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole </td>           <td> 142 </td><td> 140 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf </td>      <td> 187 </td><td> 180 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>           <td> 161 </td><td> 154 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>         <td> 168 </td><td> 189 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; western yellow </td><td> 162 </td><td> 187 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>         <td> 144 </td><td> 160 </td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>         <td> 110 </td><td> 135 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                  <td> 167 </td><td> 159 </td></tr>
+  </tbody>
+</table>
+</center>
+
+<a name="pg043"></a>
+<a name="PART_II"></a>
+<h2>PART II<br>FACTORS AFFECTING THE MECHANICAL PROPERTIES OF WOOD</h2>
+
+<a name="II_01"></a>
+<h3>INTRODUCTION</h3>
+
+<p>Wood is an organic product&mdash;a structure of infinite variation of
+detail and design.<a name="fn17r"></a><a href="#fn17"><sup><small>17</small></sup></a> It is on this account that no two woods
+are alike&mdash;in reality no two specimens from the same log are
+identical. There are certain properties that characterize each
+species, but they are subject to considerable variation. Oak,
+for example, is considered hard, heavy, and strong, but some
+pieces, even of the same species of oak, are much harder,
+heavier, and stronger than others. With hickory are associated
+the properties of great strength, toughness, and resilience, but
+some pieces are comparatively weak and brash and ill-suited for
+the exacting demands for which good hickory is peculiarly
+adapted.</p>
+
+<p>It follows that no definite value can be assigned to the
+properties of any wood and that tables giving average results of
+tests may not be directly applicable to any individual stick.
+With sufficient knowledge of the intrinsic factors affecting the
+results it becomes possible to infer from the appearance of
+material its probable variation from the average. As yet too
+little is known of the relation of structure and chemical
+composition to the mechanical and physical properties to permit
+more than general conclusions.
+
+<a name="II_02"></a>
+<h3>RATE OF GROWTH</h3>
+
+<p>To understand the effect of variations in the rate of growth it
+is first necessary to know how wood is formed. A tree increases
+in diameter by the formation, between the old wood and the inner
+bark, of new woody layers which envelop the entire stem, living
+<a name="pg044"></a>
+branches, and roots. Under ordinary conditions one layer is
+formed each year and in cross section as on the end of a log
+they appear as rings&mdash;often spoken of as <i>annual rings</i>. These
+growth layers are made up of wood cells of various kinds, but
+for the most part fibrous. In timbers like pine, spruce,
+hemlock, and other coniferous or softwood species the wood cells
+are mostly of one kind, and as a result the material is much
+more uniform in structure than that of most hardwoods. (<a href="#fig00">See
+Frontispiece</a>.) There are no vessels or pores in coniferous wood
+such as one sees so prominently in oak and ash, for example.
+(<a href="#fig22">See Fig. 22</a>.)</p>
+
+<p class="ctr"><img src="images/fig22.jpg" alt="Figure 22" id="fig22" /></p>
+<p class="imgtitle">Figure 22</p>
+<p class="imgtext"> Cross sections of a ring-porous
+hardwood (white ash), a diffuse-porous hardwood (red gum), and a
+non-porous or coniferous wood (eastern hemlock). &times; 30.
+<i>Photomicrographs by the author.</i></p>
+
+<p>The structure of the hardwoods is more complex. They are more or
+less filled with vessels, in some cases (oak, chestnut, ash)
+quite large and distinct, in others (buckeye, poplar, gum) too
+small to be seen plainly without a small hand lens. In
+discussing such woods it is customary to divide them into two
+large classes&mdash;<i>ring-porous</i> and <i>diffuse-porous</i>. (<a href="#fig22">See Fig.
+22</a>.) In ring-porous species, such as oak, chestnut, ash, black
+locust, catalpa, mulberry, hickory, and elm, the larger vessels
+or pores (as cross sections of vessels are called) become
+localized in one part of the growth ring, thus forming a region
+of more or less open and porous tissue. The rest of the ring is
+made up of smaller vessels and a much greater proportion of wood
+fibres. These fibres are the elements which give strength and
+toughness to wood, while the vessels are a source of weakness.</p>
+
+<p>In diffuse-porous woods the pores are scattered throughout the
+growth ring instead of being collected in a band or row.
+Examples of this kind of wood are gum, yellow poplar, birch,
+maple, cottonwood, basswood, buckeye, and willow. Some species,
+such as walnut and cherry, are on the border between the two
+classes, forming a sort of intermediate group.</p>
+
+<p>If one examines the smoothly cut end of a stick of almost any
+kind of wood, he will note that each growth ring is made up of
+two more or less well-defined parts. That originally nearest the
+centre of the tree is more open textured and almost invariably
+lighter in color than that near the outer portion of the ring.
+The inner portion was formed early in the season, when growth
+was comparatively rapid and is known as <i>early wood</i> (also
+spring wood); the outer portion is the <i>late wood</i>, being
+produced in the <a name="pg045"></a> summer or early fall. In soft pines there is not
+much contrast in the different parts of the ring, and as a
+result the wood is very uniform in texture and is easy to work.
+In hard pine, on the other hand, the late wood is very dense and
+is deep-colored, presenting a very decided contrast to the soft,
+straw-colored early wood. (<a href="#fig23">See Fig. 23</a>.) In ring-porous woods
+each season's growth is always well defined, because the large
+pores of the spring abut on the denser tissue of the fall
+before. In the diffuse-porous, the demarcation between rings is
+not always so clear and in not a few <a name="pg046"></a> cases is almost, if not
+entirely, invisible to the unaided eye. (<a href="#fig22">See Fig. 22</a>.)</p>
+
+<p class="ctr"><img src="images/fig23.jpg" alt="Figure 23" id="fig23" /></p>
+<p class="imgtitle">Figure 23</p>
+<p class="imgtext"> Cross section of longleaf pine showing
+several growth rings with variations in the width of the
+dark-colored late wood. Seven resin ducts are visible. &times; 33.
+<i>Photomicrograph by U.S. Forest Service</i>.</p>
+
+<p>If one compares a heavy piece of pine with a light specimen it
+will be seen at once that the heavier one contains a larger
+proportion of late wood than the other, and is therefore
+considerably darker. The late wood of all species is denser than
+that formed early in the season, hence the greater the
+<a name="pg047"></a>
+proportion of late wood the greater the density and strength.
+When examined under a microscope the cells of the late wood are
+seen to be very thick-walled and with very small cavities, while
+those formed first in the season have thin walls and large
+cavities. The strength is in the walls, not the cavities. In
+choosing a piece of pine where strength or stiffness is the
+important consideration, the principal thing to observe is the
+comparative amounts of early and late wood. The width of ring,
+that is, the number per inch, is not nearly so important as the
+proportion of the late wood in the ring.</p>
+
+<p>It is not only the proportion of late wood, but also its
+quality, that counts. In specimens that show a very large
+proportion of late wood it may be noticeably more porous and
+weigh considerably less than the late wood in pieces that
+contain but little. One can judge comparative density, and
+therefore to some extent weight and strength, by visual
+inspection.</p>
+
+<p>The conclusions of the U.S. Forest Service regarding the effect
+of rate of growth on the properties of Douglas fir are
+summarized as follows:</p>
+
+<p>"1. In general, rapidly grown wood (less than eight rings per
+inch) is relatively weak. A study of the individual tests upon
+which the average points are based shows, however, that when it
+is not associated with light weight and a small proportion of
+summer wood, rapid growth is not indicative of weak wood.</p>
+
+<p>"2. An average rate of growth, indicated by from 12 to 16 rings
+per inch, seems to produce the best material.</p>
+
+<p>"3. In rates of growths lower than 16 rings per inch, the
+average strength of the material decreases, apparently
+approaching a uniform condition above 24 rings per inch. In such
+slow rates of growth the texture of the wood is very uniform,
+and naturally there is little variation in weight or strength.</p>
+
+<p>"An analysis of tests on large beams was made to ascertain if
+average rate of growth has any relation to the mechanical
+properties of the beams. The analysis indicated conclusively
+that there was no such relation. Average rate of growth [without
+consideration also of density], therefore, has little
+significance in <a name="pg048"></a> grading structural timber."<a name="fn18r"></a><a href="#fn18"><sup><small>18</small></sup></a> This is because
+of the wide variation in the percentage of late wood in
+different parts of the cross section.</p>
+
+<p>Experiments seem to indicate that for most species there is a
+rate of growth which, in general, is associated with the
+greatest strength, especially in small specimens. For eight
+conifers it is as follows:<a name="fn19r"></a><a href="#fn19"><sup><small>19</small></sup></a></p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td>                                     <td>Rings per inch</td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>    <td>24</td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td> <td>12</td></tr>
+<tr><td style="text-align: left;"> Loblolly pine </td>  <td>6</td></tr>
+<tr><td style="text-align: left;"> Western larch </td>  <td>18</td></tr>
+<tr><td style="text-align: left;"> Western hemlock </td><td>14</td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>       <td>20</td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>    <td>18</td></tr>
+<tr><td style="text-align: left;"> Redwood </td>        <td>30</td></tr>
+</tbody>
+</table>
+</center>
+
+<p>No satisfactory explanation can as yet be given for the real
+causes underlying the formation of early and late wood. Several
+factors may be involved. In conifers, at least, rate of growth
+alone does not determine the proportion of the two portions of
+the ring, for in some cases the wood of slow growth is very hard
+and heavy, while in others the opposite is true. The quality of
+the site where the tree grows undoubtedly affects the character
+of the wood formed, though it is not possible to formulate a
+rule governing it. In general, however, it may be said that
+where strength or ease of working is essential, woods of
+moderate to slow growth should be chosen. But in choosing a
+particular specimen it is not the width of ring, but the
+proportion and character of the late wood which should govern.</p>
+
+<p>In the case of the ring-porous hardwoods there seems to exist a
+pretty definite relation between the rate of growth of timber
+and its properties. This may be briefly summed up in the general
+statement that the more rapid the growth or the wider the rings
+of growth, the heavier, harder, stronger, and stiffer the wood.
+This, it must be remembered, applies only to ring-porous woods
+such as oak, ash, hickory, and others of the same group, and is,
+of course, subject to some exceptions and limitations.</p>
+
+<p>In ring-porous woods of good growth it is usually the middle
+<a name="pg049"></a>
+portion of the ring in which the thick-walled, strength-giving
+fibres are most abundant. As the breadth of ring diminishes,
+this middle portion is reduced so that very slow growth produces
+comparatively light, porous wood composed of thin-walled vessels
+and wood parenchyma. In good oak these large vessels of the
+early wood occupy from 6 to 10 per cent of the volume of the
+log, while in inferior material they may make up 25 per cent or
+more. The late wood of good oak, except for radial grayish
+patches of small pores, is dark colored and firm, and consists
+of thick-walled fibres which form one-half or more of the wood.
+In inferior oak, such fibre areas are much reduced both in
+quantity and quality. Such variation is very largely the result
+of rate of growth.</p>
+
+<p>Wide-ringed wood is often called "second-growth," because the
+growth of the young timber in open stands after the old trees
+have been removed is more rapid than in trees in the forest, and
+in the manufacture of articles where strength is an important
+consideration such "second-growth" hardwood material is
+preferred. This is particularly the case in the choice of
+hickory for handles and spokes. Here not only strength, but
+toughness and resilience are important. The results of a series
+of tests on hickory by the U.S. Forest Service show that "the
+work or shock-resisting ability is greatest in wide-ringed wood
+that has from 5 to 14 rings per inch, is fairly constant from 14
+to 38 rings, and decreases rapidly from 38 to 47 rings. The
+strength at maximum load is not so great with the most
+rapid-growing wood; it is maximum with from 14 to 20 rings per
+inch, and again becomes less as the wood becomes more closely
+ringed. The natural deduction is that wood of first-class
+mechanical value shows from 5 to 20 rings per inch and that
+slower growth yields poorer stock. Thus the inspector or buyer
+of hickory should discriminate against timber that has more than
+20 rings per inch. Exceptions exist, however, in the case of
+normal growth upon dry situations, in which the slow-growing
+material may be strong and tough."<a name="fn20r"></a><a href="#fn20"><sup><small>20</small></sup></a></p>
+
+<p>The effect of rate of growth on the qualities of chestnut wood
+is summarized by the same authority as follows: "When the rings
+are wide, the transition from spring wood to summer wood <a name="pg050"></a> is
+gradual, while in the narrow rings the spring wood passes into
+summer wood abruptly. The width of the spring wood changes but
+little with the width of the annual ring, so that the narrowing
+or broadening of the annual ring is always at the expense of the
+summer wood. The narrow vessels of the summer wood make it
+richer in wood substance than the spring wood composed of wide
+vessels. Therefore, rapid-growing specimens with wide rings have
+more wood substance than slow-growing trees with narrow rings.
+Since the more the wood substance the greater the weight, and
+the greater the weight the stronger the wood, chestnuts with
+wide rings must have stronger wood than chestnuts with narrow
+rings. This agrees with the accepted view that sprouts (which
+always have wide rings) yield better and stronger wood than
+seedling chestnuts, which grow more slowly in diameter."<a name="fn21r"></a><a href="#fn21"><sup><small>21</small></sup></a></p>
+
+<p>In diffuse-porous woods, as has been stated, the vessels or
+pores are scattered throughout the ring instead of collected in
+the early wood. The effect of rate of growth is, therefore, not
+the same as in the ring-porous woods, approaching more nearly
+the conditions in the conifers. In general it may be stated that
+such woods of medium growth afford stronger material than when
+very rapidly or very slowly grown. In many uses of wood,
+strength is not the main consideration. If ease of working is
+prized, wood should be chosen with regard to its uniformity of
+texture and straightness of grain, which will in most cases
+occur when there is little contrast between the late wood of one
+season's growth and the early wood of the next.</p>
+
+<a name="II_03"></a>
+<h3>HEARTWOOD AND SAPWOOD</h3>
+
+<p>Examination of the end of a log of many species reveals a
+darker-colored inner portion&mdash;the <i>heartwood</i>, surrounded by a
+lighter-colored zone&mdash;the <i>sapwood</i>. In some instances this
+distinction in color is very marked; in others, the contrast is
+slight, so that it is not always easy to tell where one leaves
+off and the other begins. The color of fresh sapwood is always
+light, sometimes pure white, but more often with a decided tinge
+of green or brown.</p>
+
+<a name="pg051"></a>
+
+<p>Sapwood is comparatively new wood. There is a time in the early
+history of every tree when its wood is all sapwood. Its
+principal functions are to conduct water from the roots to the
+leaves and to store up and give back according to the season the
+food prepared in the leaves. The more leaves a tree bears and
+the more thrifty its growth, the larger the volume of sapwood
+required, hence trees making rapid growth in the open have
+thicker sapwood for their size than trees of the same species
+growing in dense forests. Sometimes trees grown in the open may
+become of considerable size, a foot or more in diameter, before
+any heartwood begins to form, for example, in second-growth
+hickory, or field-grown white and loblolly pines.</p>
+
+<p>As a tree increases in age and diameter an inner portion of the
+sapwood becomes inactive and finally ceases to function. This
+inert or dead portion is called heartwood, deriving its name
+solely from its position and not from any vital importance to
+the tree, as is shown by the fact that a tree can thrive with
+its heart completely decayed. Some, species begin to form
+heartwood very early in life, while in others the change comes
+slowly. Thin sapwood is characteristic of such trees as
+chestnut, black locust, mulberry, Osage orange, and sassafras,
+while in maple, ash, gum, hickory, hackberry, beech, and
+loblolly pine, thick sapwood is the rule.</p>
+
+<p>There is no definite relation between the annual rings of growth
+and the amount of sapwood. Within the same species the
+cross-sectional area of the sapwood is roughly proportional to
+the size of the crown of the tree. If the rings are narrow, more
+of them are required than where they are wide. As the tree gets
+larger, the sapwood must necessarily become thinner or increase
+materially in volume. Sapwood is thicker in the upper portion of
+the trunk of a tree than near the base, because the age and the
+diameter of the upper sections are less.</p>
+
+<p>When a tree is very young it is covered with limbs almost, if
+not entirely, to the ground, but as it grows older some or all
+of them will eventually die and be broken off. Subsequent growth
+of wood may completely conceal the stubs which, however, will
+remain as knots. No matter how smooth and clear a log is on the
+outside, it is more or less knotty near the middle. Consequently
+the sapwood of an old tree, and particularly of a <a name="pg052"></a> forest-grown
+tree, will be freer from knots than the heartwood. Since in most
+uses of wood, knots are defects that weaken the timber and
+interfere with its ease of working and other properties, it
+follows that sapwood, because of its position in the tree, may
+have certain advantages over heartwood.</p>
+
+<p>It is really remarkable that the inner heartwood of old trees
+remains as sound as it usually does, since in many cases it is
+hundreds of years, and in a few instances thousands of years,
+old. Every broken limb or root, or deep wound from fire,
+insects, or falling timber, may afford an entrance for decay,
+which, once started, may penetrate to all parts of the trunk.
+The larv� of many insects bore into the trees and their tunnels
+remain indefinitely as sources of weakness. Whatever advantages,
+however, that sapwood may have in this connection are due solely
+to its relative age and position.</p>
+
+<p>If a tree grows all its life in the open and the conditions of
+soil and site remain unchanged, it will make its most rapid
+growth in youth, and gradually decline. The annual rings of
+growth are for many years quite wide, but later they become
+narrower and narrower. Since each succeeding ring is laid down
+on the outside of the wood previously formed, it follows that
+unless a tree materially increases its production of wood from
+year to year, the rings must necessarily become thinner. As a
+tree reaches maturity its crown becomes more open and the annual
+wood production is lessened, thereby reducing still more the
+width of the growth rings. In the case of forest-grown trees so
+much depends upon the competition of the trees in their struggle
+for light and nourishment that periods of rapid and slow growth
+may alternate. Some trees, such as southern oaks, maintain the
+same width of ring for hundreds of years. Upon the whole,
+however, as a tree gets larger in diameter the width of the
+growth rings decreases.</p>
+
+<p>It is evident that there may be decided differences in the grain
+of heartwood and sapwood cut from a large tree, particularly one
+that is overmature. The relationship between width of growth
+rings and the mechanical properties of wood is discussed under
+<a href="#II_02">Rate of Growth</a>. In this connection, however, it may be stated
+that as a general rule the wood laid on late in the life of a
+tree is softer, lighter, weaker, and more even-textured than
+that <a name="pg053"></a> produced earlier. It follows that in a large log the
+sapwood, because of the time in the life of the tree when it was
+grown, may be inferior in hardness, strength, and toughness to
+equally sound heartwood from the same log.</p>
+
+<p>After exhaustive tests on a number of different woods the U.S.
+Forest Service concludes as follows: "Sapwood, except that from
+old, overmature trees, is as strong as heartwood, other things
+being equal, and so far as the mechanical properties go should
+not be regarded as a defect."<a name="fn22r"></a><a href="#fn22"><sup><small>22</small></sup></a> Careful inspection of the
+individual tests made in the investigation fails to reveal any
+relation between the proportion of sapwood and the breaking
+strength of timber.</p>
+
+<p>In the study of the hickories the conclusion was: "There is an
+unfounded prejudice against the heartwood. Specifications place
+white hickory, or sapwood, in a higher grade than red hickory,
+or heartwood, though there is no inherent difference in
+strength. In fact, in the case of large and old hickory trees,
+the sapwood nearest the bark is comparatively weak, and the best
+wood is in the heart, though in young trees of thrifty growth
+the best wood is in the sap."<a name="fn23r"></a><a href="#fn23"><sup><small>23</small></sup></a> The results of tests from
+selected pieces lying side by side in the same tree, and also
+the average values for heartwood and sapwood in shipments of the
+commercial hickories without selection, show conclusively that
+"the transformation of sapwood into heartwood does not affect
+either the strength or toughness of the wood.... It is true,
+however, that sapwood is usually more free from latent defects
+than heartwood."<a name="fn24r"></a><a href="#fn24"><sup><small>24</small></sup></a></p>
+
+<p>Specifications for paving blocks often require that longleaf
+pine be 90 per cent heart. This is on the belief that sapwood is
+not only more subject to decay, but is also weaker than
+heartwood. In reality there is no sound basis for discrimination
+against sapwood on account of strength, provided other
+conditions are equal. It is true that sapwood will not resist
+decay as long as heartwood, if both are untreated with
+preservatives. It is especially so of woods with deep-colored
+heartwood, and is due to infiltrations of tannins, oils, and
+resins, which make the wood more or <a name="pg054"></a> less obnoxious to
+decay-producing fungi. If, however, the timbers are to be
+treated, sapwood is not a defect; in fact, because of the
+relative ease with which it can be impregnated with
+preservatives it may be made more desirable than heartwood.<a name="fn25r"></a><a href="#fn25"><sup><small>25</small></sup></a></p>
+
+<p>In specifications for structural timbers reference is sometimes
+made to "boxheart," meaning the inclusion of the pith or centre
+of the tree within a cross section of the timber. From numerous
+experiments it appears that the position of the pith does not
+bear any relation to the strength of the material. Since most
+season checks, however, are radial, the position of the pith may
+influence the resistance of a seasoned beam to horizontal shear,
+being greatest when the pith is located in the middle half of
+the section.<a name="fn26r"></a><a href="#fn26"><sup><small>26</small></sup></a></p>
+
+<a name="II_04"></a>
+<h3>WEIGHT, DENSITY, AND SPECIFIC GRAVITY</h3>
+
+<p>From data obtained from a large number of tests on the strength
+of different woods it appears that, other things being equal,
+the crushing strength parallel to the grain, fibre stress at
+elastic limit in bending, and shearing strength along the grain
+of wood vary in direct proportion to the weight of dry wood per
+unit of volume when green. Other strength values follow
+different laws. The hardness varies in a slightly greater ratio
+than the square of the density. The work to the breaking point
+increases even more rapidly than the cube of density. The
+modulus of rupture in bending lies between the first power and
+the square of the density. This, of course, is true only in case
+the greater weight is due to increase in the amount of wood
+substance. A <a name="pg055"></a> wood heavy with resin or other infiltrated
+substance is not necessarily stronger than a similar specimen
+free from such materials. If differences in weight are due to
+degree of seasoning, in other words, to the relative amounts of
+water contained, the rules given above will of course not hold,
+since strength increases with dryness. But of given specimens of
+pine or of oak, for example, in the green condition, the
+comparative strength may be inferred from the weight. It is not
+permissible, however, to compare such widely different woods as
+oak and pine on a basis of their weights.<a name="fn27r"></a><a href="#fn27"><sup><small>27</small></sup></a></p>
+
+<p>The weight of wood substance, that is, the material which
+composes the walls of the fibres and other cells, is practically
+the same in all species, whether pine, hickory, or cottonwood,
+being a little greater than half again as heavy as water. It
+varies slightly from beech sapwood, 1.50, to Douglas fir
+heartwood, 1.57, averaging about 1.55 at 30&deg; to 35&deg; C., in terms
+of water at its greatest density 4&deg; C. The reason any wood
+floats is that the air imprisoned in its cavities buoys it up.
+When this is displaced by water the wood becomes water-logged
+and sinks. Leaving out of consideration infiltrated substances,
+the reason a cubic foot of one kind of dry wood is heavier than
+that of another is because it contains a greater amount of wood
+substance.  <b>Density</b> is merely the weight of a unit of volume,
+as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.
+<b>Specific gravity</b> or relative density is the ratio of the
+density of any material to the density of distilled water at 4&deg;
+C. (39.2&deg; F.). A cubic foot of distilled water at 4&deg; C. weighs
+62.43 pounds. Hence the specific gravity of a piece of wood with
+a density of 35 pounds is</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr>  <td>35<br></td>   <td><br></td> <td><br></td>     <td><br></td></tr>
+<tr><td>-------<br></td><td>=<br></td><td>0.561<br></td><td>.<br></td></tr>
+<tr> <td>62.43<br></td> <td><br></td> <td><br></td>     <td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>To find the weight
+per cubic foot when the specific gravity is given, simply
+multiply by 62.43. Thus, 0.561 &times; 62.43 = 35. In the metric
+system, since the weight of a cubic centimetre of pure water is
+one gram, the density in grams per cubic centimetre has the same
+numerical value as the specific gravity.</p>
+
+<p>Since the amount of water in wood is extremely variable it
+usually is not satisfactory to refer to the density of green
+wood. <a name="pg057"></a> For scientific purposes the density of "oven-dry" wood is
+used; that is, the wood is dried in an oven at a temperature of
+100&deg;C. (212&deg;F.) until a constant weight is attained. For
+commercial purposes the weight or density of air-dry or
+"shipping-dry" wood is used. This is usually expressed in pounds
+per thousand board feet, a board foot being considered as
+one-twelfth of a cubic foot.</p>
+
+<p>Wood shrinks greatly in drying from the green to the oven-dry
+condition. (<a href="#TABLE_14">See Table XIV</a>.) Consequently a block of wood
+measuring a cubic foot when green will measure considerably less
+when oven-dry. It follows that the density of oven-dry wood does
+not represent the weight of the dry wood substance in a cubic
+foot of green wood. In other words, it is not the weight of a
+cubic foot <a name="pg058"></a> of green wood minus the weight of the water which it
+contains. Since the latter is often a more convenient figure to
+use and much easier to obtain than the weight of oven-dry wood,
+it is commonly expressed in tables of "specific gravity or
+density of dry wood."</p>
+
+<a name="pg056"></a>
+
+<center>
+<table id="TABLE_14" summary="TABLE_XIV" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="7"> TABLE XIV </th></tr>
+<tr><th colspan="7"> SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS </th></tr>
+<tr><td colspan="7"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="3"> COMMON&nbsp;NAME OF SPECIES </th>
+                                                                <th rowspan="2"> Moisture content </th>
+                                                                              <th colspan="2"> Specific gravity oven-dry, based on </th>
+                                                                                                              <th colspan="3"> Shrinkage from green to oven-dry condition </th></tr>
+<tr>                                                                          <th> Volume when green </th>
+                                                                                              <th> Volume when oven-dry </th>
+                                                                                                              <th> In volume </th>
+                                                                                                                             <th> Radial </th>
+                                                                                                                                           <th> Tangential </th></tr>
+<tr>                                                            <th> <i>Per cent</i> </th>
+                                                                              <td><br></td>   <td><br></td>   <th> <i>Per cent</i> </th>
+                                                                                                                             <th> <i>Per cent</i> </th>
+                                                                                                                                           <th> <i>Per cent</i> </th></tr>
+<tr><th> <i>Hardwoods</i></th>                                  <td><br></td> <td><br></td>   <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Ash, black </td>             <td> 77 </td> <td> 0.466 </td><td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>      <td> 38 </td> <td> .550 </td> <td> 0.640 </td><td> 12.6 </td><td> 4.3 </td><td> 6.4 </td></tr>
+<tr><td> " </td>                                                <td> 47 </td> <td> .516 </td> <td> .590 </td> <td> 11.7 </td><td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Basswood </td>               <td> 110 </td><td> .315 </td> <td> .374 </td> <td> 14.5 </td><td> 6.2 </td><td> 8.4 </td></tr>
+<tr><td style="text-align: left;"> Beech </td>                  <td> 61 </td> <td> .556 </td> <td> .669 </td> <td> 16.5 </td><td> 4.6 </td><td> 10.5 </td></tr>
+<tr><td style="text-align: left;"> Birch, yellow </td>          <td> 72 </td> <td> .545 </td> <td> .661 </td> <td> 17.0 </td><td> 7.9 </td><td> 9.0 </td></tr>
+<tr><td style="text-align: left;"> Elm, rock </td>              <td> 46 </td> <td> .578 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; slippery </td>   <td> 57 </td> <td> .541 </td> <td> .639 </td> <td> 15.5 </td><td> 5.1 </td><td> 9.9 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>      <td> 66 </td> <td> .430 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Gum, red </td>               <td> 71 </td> <td> .434 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Hackberry </td>              <td> 50 </td> <td> .504 </td> <td> .576 </td> <td> 14.0 </td><td> 4.2 </td><td> 8.9 </td></tr>
+<tr><td style="text-align: left;"> Hickory, big shellbark </td> <td> 64 </td> <td> .601 </td> <td><br></td>   <td> 17.6 </td><td> 7.4 </td><td> 11.2 </td></tr>
+<tr><td> " </td>                                                <td> 55 </td> <td> .666 </td> <td><br></td>   <td> 20.9 </td><td> 7.9 </td><td> 14.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; bitternut </td>  <td> 65 </td> <td> .624 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; mockernut </td>  <td> 64 </td> <td> .606 </td> <td><br></td>   <td> 16.5 </td><td> 6.9 </td><td> 10.4 </td></tr>
+<tr><td> " </td>                                                <td> 57 </td> <td> .662 </td> <td><br></td>   <td> 18.9 </td><td> 8.4 </td><td> 11.4 </td></tr>
+<tr><td> " </td>                                                <td> 48 </td> <td> .666 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; nutmeg </td>     <td> 76 </td> <td> .558 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; pignut </td>     <td> 59 </td> <td> .627 </td> <td><br></td>   <td> 15.0 </td><td> 5.6 </td><td> 9.8 </td></tr>
+<tr><td> " </td>                                                <td> 54 </td> <td> .667 </td> <td><br></td>   <td> 15.3 </td><td> 6.3 </td><td> 9.5 </td></tr>
+<tr><td> " </td>                                                <td> 55 </td> <td> .667 </td> <td><br></td>   <td> 16.9 </td><td> 6.8 </td><td> 10.9 </td></tr>
+<tr><td> " </td>                                                <td> 52 </td> <td> .667 </td> <td><br></td>   <td> 21.2 </td><td> 8.5 </td><td> 13.8 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shagbark </td>   <td> 65 </td> <td> .608 </td> <td><br></td>   <td> 16.0 </td><td> 6.5 </td><td> 10.2 </td></tr>
+<tr><td> " </td>                                                <td> 58 </td> <td> .646 </td> <td><br></td>   <td> 18.4 </td><td> 7.9 </td><td> 11.4 </td></tr>
+<tr><td> " </td>                                                <td> 64 </td> <td> .617 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td> " </td>                                                <td> 60 </td> <td> .653 </td> <td><br></td>   <td> 15.5 </td><td> 6.5 </td><td> 9.7 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; water </td>      <td> 74 </td> <td> .630 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Locust, honey </td>          <td> 53 </td> <td> .695 </td> <td> .759 </td> <td> 8.6 </td> <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Maple, red </td>             <td> 69 </td> <td> .512 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar </td>      <td> 57 </td> <td> .546 </td> <td> .643 </td> <td> 14.3 </td><td> 4.9 </td><td> 9.1 </td></tr>
+<tr><td> " </td>                                                <td> 56 </td> <td> .577 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Oak, post </td>              <td> 64 </td> <td> .590 </td> <td> .732 </td> <td> 16.0 </td><td> 5.7 </td><td> 10.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red </td>        <td> 80 </td> <td> .568 </td> <td> .660 </td> <td> 13.1 </td><td> 3.7 </td><td> 8.3 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; swamp white </td><td> 74 </td> <td> .637 </td> <td> .792 </td> <td> 17.7 </td><td> 5.5 </td><td> 10.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; tanbark </td>    <td> 88 </td> <td> .585 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white </td>      <td> 58 </td> <td> .594 </td> <td> .704 </td> <td> 15.8 </td><td> 6.2 </td><td> 8.3 </td></tr>
+<tr><td> " </td>                                                <td> 62 </td> <td> .603 </td> <td> .696 </td> <td> 14.3 </td><td> 4.9 </td><td> 9.0 </td></tr>
+<tr><td> " </td>                                                <td> 78 </td> <td> .600 </td> <td> .708 </td> <td> 16.0 </td><td> 4.8 </td><td> 9.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; yellow </td>     <td> 77 </td> <td> .573 </td> <td> .669 </td> <td> 14.2 </td><td> 4.5 </td><td> 9.7 </td></tr>
+<tr><td> " </td>                                                <td> 80 </td> <td> .550 </td> <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Osage orange </td>           <td> 31 </td> <td> .761 </td> <td> .838 </td> <td> 8.9 </td> <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Sycamore </td>               <td> 81 </td> <td> .454 </td> <td> .526 </td> <td> 13.5 </td><td> 5.0 </td><td> 7.3 </td></tr>
+<tr><td style="text-align: left;"> Tupelo </td>                 <td> 121 </td><td> .475 </td> <td> .545 </td> <td> 12.4 </td><td> 4.4 </td><td> 7.9 </td></tr>
+</tbody>
+</table>
+</center>
+
+<center>
+<table id="TABLE_14_cont" summary="TABLE_XIV_CONT" border="3" cellspacing="0" cellpadding="1" width="40%">
+<tbody>
+<tr><th colspan="7"> TABLE XIV (CONT.) </th></tr>
+<tr><th colspan="7"> SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS </th></tr>
+<tr><td colspan="7"> (Forest Service Cir. 213) </td></tr>
+<tr><th rowspan="3"> COMMON&nbsp;NAME OF SPECIES </th>
+                                                                <th rowspan="2"> Moisture content </th>
+                                                                               <th colspan="2"> Specific gravity oven-dry, based on </th>
+                                                                                                              <th colspan="3"> Shrinkage from green to oven-dry condition </th></tr>
+<tr>                                                                           <th> Volume when green </th>
+                                                                                              <th> Volume when oven-dry </th>
+                                                                                                              <th> In volume </th>
+                                                                                                                             <th> Radial </th>
+                                                                                                                                           <th> Tangential </th></tr>
+<tr>                                                            <th> <i>Per cent</i> </th>
+                                                                               <td><br></td>  <td><br></td>   <th> <i>Per cent</i> </th>
+                                                                                                                             <th> <i>Per cent</i> </th>
+                                                                                                                                           <th> <i>Per cent</i> </th></tr>
+<tr><th><i> Conifers </i></th>                                  <td><br></td>  <td><br></td>  <td><br></td>   <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td style="text-align: left;"> Arborvit&aelig;</td>         <td> 55 </td>  <td> .293 </td><td> .315 </td> <td> 7.0 </td> <td> 2.1 </td><td> 4.9 </td></tr>
+<tr><td style="text-align: left;"> Cedar, incense</td>          <td> 80 </td>  <td> .363 </td><td><br></td>  <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;"> Cypress, bald</td>           <td> 79 </td>  <td> .452 </td><td> .513 </td> <td> 11.5 </td><td> 3.8 </td><td> 6.0 </td></tr>
+<tr><td style="text-align: left;"> Fir, alpine</td>             <td> 47 </td>  <td> .306 </td><td> .321 </td> <td> 9.0 </td> <td> 2.5 </td><td> 7.1 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; amabilis</td>    <td> 117 </td> <td> .383 </td><td><br></td>  <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; Douglas</td>     <td> 32 </td>  <td> .418 </td><td> .458 </td> <td> 10.9 </td><td> 3.7 </td><td> 6.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white</td>       <td> 156 </td> <td> .350 </td><td> .437 </td> <td> 10.2 </td><td> 3.4 </td><td> 7.0 </td></tr>
+<tr><td style="text-align: left;"> Hemlock (east.)</td>         <td> 129 </td> <td> .340 </td><td> .394 </td> <td> 9.2 </td> <td> 2.3 </td><td> 5.0 </td></tr>
+<tr><td style="text-align: left;"> Pine, lodgepole</td>         <td> 44 </td>  <td> .370 </td><td> .415 </td> <td> 11.3 </td><td> 4.2 </td><td> 7.1 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; lodgepole</td>   <td> 58 </td>  <td> .371 </td><td> .407 </td> <td> 10.1 </td><td> 3.6 </td><td> 5.9 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; longleaf</td>    <td> 63 </td>  <td> .528 </td><td> .599 </td> <td> 12.8 </td><td> 6.0 </td><td> 7.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red or Nor</td>  <td> 54 </td>  <td> .440 </td><td> .507 </td> <td> 11.5 </td><td> 4.5 </td><td> 7.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; shortleaf</td>   <td> 52 </td>  <td> .447 </td><td><br></td>  <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; sugar</td>       <td> 123 </td> <td> .360 </td><td> .386 </td> <td> 8.4 </td> <td> 2.9 </td><td> 5.6 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; west yellow</td> <td> 98 </td>  <td> .353 </td><td> .395 </td> <td> 9.2 </td> <td> 4.1 </td><td> 6.4 </td></tr>
+<tr><td> " </td>                                                <td> 125 </td> <td> .377 </td><td> .433 </td> <td> 11.5 </td><td> 4.3 </td><td> 7.3 </td></tr>
+<tr><td> " </td>                                                <td> 93 </td>  <td> .391 </td><td> .435 </td> <td> 9.9 </td> <td> 3.8 </td><td> 5.8 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white</td>       <td> 74 </td>  <td> .363 </td><td> .391 </td> <td> 7.8 </td> <td> 2.2 </td><td> 5.9 </td></tr>
+<tr><td style="text-align: left;"> Redwood</td><td> 81 </td>    <td> .334 </td><td><br></td>  <td><br></td>  <td><br></td> <td><br></td></tr>
+<tr><td> " </td>                                                <td> 69 </td>  <td> .366 </td><td><br></td>   <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;"> Spruce, Engelmann </td>      <td> 45 </td>  <td> .325 </td><td> .359 </td> <td> 10.5 </td><td> 3.7 </td><td> 6.9 </td></tr>
+<tr><td> " </td>                                                <td> 156 </td> <td> .299 </td><td> .335 </td> <td> 10.3 </td><td> 3.0 </td><td> 6.2 </td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; red</td>         <td> 31 </td>  <td> .396 </td><td><br></td>  <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;">&nbsp;&nbsp; white</td>       <td> 41 </td>  <td> .318 </td><td><br></td>  <td><br></td> <td><br></td><td><br></td></tr>
+<tr><td style="text-align: left;"> Tamarack</td><td> 52 </td>   <td> .491 </td><td> .558 </td><td> 13.6 </td> <td> 3.7 </td> <td> 7.4 </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>This weight divided by 62.43 gives the specific gravity per
+green volume. It is purely a fictitious quantity. To convert
+this figure into actual density or specific gravity of the dry
+wood, it is necessary to know the amount of shrinkage in volume.
+If <i>S</i> is the percentage of shrinkage from the green to the
+oven-dry condition, based on the green volume; <i>D</i>, the density of
+the dry wood per cubic foot while green; and <i>d</i> the actual
+density of oven-dry wood, then</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr>      <td><i>D</i></td>          <td><br></td> <td><br></td></tr>
+<tr>     <td>----------</td>         <td>=</td>    <td><i>d</i>.</td></tr>
+<tr><td><i>1</i> - .0 <i>S</i></td>  <td><br></td> <td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>This relation becomes clearer from the following analysis:
+Taking <i>V</i> and <i>W</i> as the volume and weight, respectively, when
+green, and <i>v</i> and <i>w</i> as the corresponding volume and weight when
+oven-dry, then,</p>
+
+<! Sorry, the following is just not very conducive to proofreading... /!>
+
+<center>
+<table summary="text">
+<tbody style="text-align: center; vertical-align: middle; white-space: nowrap;">
+<tr><td><br></td><td><br></td><td><i>w</i></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>W</i></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>V</i> - <i>v</i></td><td><br></td><td><br></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>V</i> - <i>v</i></td><td><br></td><td><br></td><td><br></td><td><br></td></tr>
+<tr><td><i>d</i></td><td>=</td><td>---</td><td style="width: 40px;"> ; </td><td><i>D</i></td><td>=</td><td>---</td><td style="width: 40px;">;</td><td><i>S</i></td><td>=</td><td>-------</td><td>&times;</td><td>100</td><td style="width: 40px;">, and</td><td><i>s</i></td><td>= </td><td>-------</td><td>&times;</td><td>100</td><td>,</td><td><br></td></tr>
+<tr><td><br></td><td><br></td><td><i>v</i></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>V</i></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>V</i></td><td><br></td><td><br></td><td style="width: 40px;"><br></td><td><br></td><td><br></td><td><i>v</i></td><td><br></td><td><br></td><td><br></td><td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>in which <i>S</i> is the percentage of shrinkage
+from the green to the oven-dry condition, based on the green
+volume, and <i>s</i> the same based on the oven-dry volume.</p>
+
+<p>In tables of specific gravity or density of wood it should
+always be stated whether the dry weight per unit of volume when
+green or the dry weight per unit of volume when dry is intended,
+since the shrinkage in volume may vary from 6 to 50 per cent,
+though in conifers it is usually about 10 per cent, and in
+hardwoods nearer 15 per cent. (<a href="#TABLE_14">See Table XIV</a>.)</p>
+
+<a name="II_05"></a>
+<h3>COLOR</h3>
+
+<p>In species which show a distinct difference between heartwood
+and sapwood the natural color of heartwood is invariably darker
+than that of the sapwood, and very frequently the contrast is
+conspicuous. This is produced by deposits in the heartwood of
+various materials resulting from the process of growth,
+increased possibly by oxidation and other chemical changes,
+which usually have little or no appreciable effect on the
+mechanical properties of <a name="pg059"></a> the wood. (See <a href="#II_03">Heartwood and Sapwood</a>.)
+Some experiments<a name="fn28r"></a><a href="#fn28"><sup><small>28</small></sup></a> on very resinous longleaf pine
+specimens, however, indicate an increase in strength. This is
+due to the resin which increases the strength when dry. Spruce
+impregnated with crude resin and dried is greatly increased in
+strength thereby.</p>
+
+<p>Since the late wood of a growth ring is usually darker in color
+than the early wood, this fact may be used in judging the
+density, and therefore the hardness and strength of the
+material. This is particularly the case with coniferous woods.
+In ring-porous woods the vessels of the early wood not
+infrequently appear on a finished surface as darker than the
+denser late wood, though on cross sections of heartwood the
+reverse is commonly true. Except in the manner just stated the
+color of wood is no indication of strength.</p>
+
+<p>Abnormal discoloration of wood often denotes a diseased
+condition, indicating unsoundness. The black check in western
+hemlock is the result of insect attacks.<a name="fn29r"></a><a href="#fn29"><sup><small>29</small></sup></a> The reddish-brown
+streaks so common in hickory and certain other woods are mostly
+the result of injury by birds.<a name="fn30r"></a><a href="#fn30"><sup><small>30</small></sup></a> The discoloration is merely
+an indication of an injury, and in all probability does not of
+itself affect the properties of the wood. Certain rot-producing
+fungi impart to wood characteristic colors which thus become
+criterions of weakness. Ordinary sap-staining is due to fungous
+growth, but does not necessarily produce a weakening effect.<a name="fn31r"></a><a href="#fn31"><sup><small>31</small></sup></a></p>
+
+<a name="II_06"></a>
+<h3>CROSS GRAIN</h3>
+
+<p><i>Cross grain</i> is a very common defect in timber. One form of it
+is produced in lumber by the method of sawing and has no
+reference to the natural arrangement of the wood elements. Thus
+<a name="pg060"></a>
+if the plane of the saw is not approximately parallel to the
+axis of the log the grain of the lumber cut is not parallel to
+the edges and is termed diagonal. This is likely to occur where
+the logs have considerable taper, and in this case may be
+produced if sawed parallel to the axis of growth instead of
+parallel to the growth rings.</p>
+
+<p>Lumber and timber with diagonal grain is always weaker than
+straight-grained material, the extent of the defect varying with
+the degree of the angle the fibres make with the axis of the
+stick. In the vicinity of large knots the grain is likely to be
+cross. The defect is most serious where wood is subjected to
+flexure, as in beams.</p>
+
+<p><i>Spiral grain</i> is a very common defect in a tree, and when
+excessive renders the timber valueless for use except in the
+round. It is produced by the arrangement of the wood fibres in a
+spiral direction about the axis instead of exactly vertical.
+Timber with spiral grain is also known as "torse wood." Spiral
+grain usually cannot be detected by casual inspection of a
+stick, since it does not show in the so-called visible grain of
+the wood, by which is commonly meant a sectional view of the
+annual rings of growth cut longitudinally. It is accordingly
+very easy to allow spiral-grained material to pass inspection,
+thereby introducing an element of weakness in a structure.</p>
+
+<p>There are methods for readily detecting spiral grain. The
+simplest is that of splitting a small piece radially. It is
+necessary, of course, that the split be radial, that is, in a
+plane passing through the axis of the log, and not tangentially.
+In the latter case it is quite probable that the wood would
+split straight, the line of cleavage being between the growth
+rings.</p>
+
+<p>In inspection, the elements to examine are the rays. In the case
+of oak and certain other hardwoods these rays are so large that
+they are readily seen not only on a radial surface, but on the
+tangential as well. On the former they appear as flakes, on the
+latter as short lines. Since these rays are between the fibres
+it naturally follows that they will be vertical or inclined
+according as the tree is straight-grained or spiral-grained.
+While they are not conspicuous in the softwoods, they can be
+seen upon close scrutiny, and particularly so if a small hand
+magnifier is used.</p>
+
+<p>When wood has begun to dry and check it is very easy to see
+<a name="pg061"></a>
+whether or not it is straight- or spiral-grained, since the
+checks will for the most part follow along the rays. If one
+examines a row of telephone poles, for example, he will probably
+find that most of them have checks running spirally around them.
+If boards were sawed from such a pole after it was badly checked
+they would fall to pieces of their own weight. The only way to
+get straight material would be to split it out.</p>
+
+<p>It is for this reason that split billets and squares are
+stronger than most sawed material. The presence of the spiral
+grain has little, if any, effect on the timber when it is used
+in the round, but in sawed material the greater the pitch of the
+spiral the greater is the defect.
+
+<a name="II_07"></a>
+<h3>KNOTS</h3>
+
+<p><i>Knots</i> are portions of branches included in the wood of the
+stem or larger branch. Branches originate as a rule from the
+central axis of a stem, and while living increase in size by the
+addition of annual woody layers which are a continuation of
+those of the stem. The included portion is irregularly conical
+in shape with the tip at the pith. The direction of the fibre is
+at right angles or oblique to the grain of the stem, thus
+producing local cross grain.</p>
+
+<p>During the development of a tree most of the limbs, especially
+the lower ones, die, but persist for a time&mdash;often for years.
+Subsequent layers of growth of the stem are no longer intimately
+joined with the dead limb, but are laid around it. Hence dead
+branches produce knots which are nothing more than pegs in a
+hole, and likely to drop out after the tree has been sawed into
+lumber. In grading lumber and structural timber, knots are
+classified according to their form, size, soundness, and the
+firmness with which they are held in place.<a name="fn32r"></a><a href="#fn32"><sup><small>32</small></sup></a></p>
+
+<p>Knots materially affect checking and warping, ease in working,
+and cleavability of timber. They are defects which weaken timber
+and depreciate its value for structural purposes where strength
+is an important consideration. The weakening effect is much more
+<a name="pg062"></a>
+serious where timber is subjected to bending and tension than
+where under compression. The extent to which knots affect the
+strength of a beam depends upon their position, size, number,
+direction of fibre, and condition. A knot on the upper side is
+compressed, while one on the lower side is subjected to tension.
+The knot, especially (as is often the case) if there is a season
+check in it, offers little resistance to this tensile stress.
+Small, knots, however, may be so located in a beam along the
+neutral plane as actually to increase the strength by tending to
+prevent longitudinal shearing. Knots in a board or plank are
+least injurious when they extend through it at right angles to
+its broadest surface. Knots which occur near the ends of a beam
+do not weaken it. Sound knots which occur in the central portion
+one-fourth the height of the beam from either edge are not
+serious defects.</p>
+
+<p>Extensive experiments by the U.S. Forest Service<a name="fn33r"></a><a href="#fn33"><sup><small>33</small></sup></a> indicate
+the following effects of knots on structural timbers:</p>
+
+<p>(1) Knots do not materially influence the stiffness of
+structural timber.</p>
+
+<p>(2) Only defects of the most serious character affect the
+elastic limit of beams. Stiffness and elastic strength are more
+dependent upon the quality of the wood fibre than upon defects
+in the beam.</p>
+
+<p>(3) The effect of knots is to reduce the difference between the
+fibre stress at elastic limit and the modulus of rupture of
+beams. The breaking strength is very susceptible to defects.</p>
+
+<p>(4) Sound knots do not weaken wood when subject to compression
+parallel to the grain.<a name="fn34r"></a><a href="#fn34"><sup><small>34</small></sup></a></p>
+
+<a name="II_08"></a>
+<h3>FROST SPLITS</h3>
+
+<p>A common defect in standing timber results from radial splits
+which extend inward from the periphery of the tree, and almost,
+if not always, near the base. It is most common in trees which
+split readily, and those with large rays and thin bark. The
+<a name="pg063"></a>
+primary cause of the splitting is frost, and various theories
+have been advanced to explain the action.</p>
+
+<p>R. Hartig<a name="fn35r"></a><a href="#fn35"><sup><small>35</small></sup></a> believes that freezing forces out a part of the
+imbibition water of the cell walls, thereby causing the wood to
+shrink, and if the interior layers have not yet been cooled,
+tangential strains arise which finally produce radial clefts.</p>
+
+<p>Another theory holds that the water is not driven out of the
+cell walls, but that difference in temperature conditions of
+inner and outer layers is itself sufficient to set up the
+strains, resulting in splitting. An air temperature of 14&deg;F. or
+less is considered necessary to produce frost splits.</p>
+
+<p>A still more recent theory is that of Busse<a name="fn36r"></a><a href="#fn36"><sup><small>36</small></sup></a> who considers
+the mechanical action of the wind a very important factor. He
+observed: (<i>a</i>) Frost splits sometimes occur at higher
+temperatures than 14&deg;F. (<i>b</i>) Most splits take place shortly
+before sunrise, <i>i.e.</i>, at the time of lowest air and soil
+temperature; they are never heard to take place at noon,
+afternoon, or evening. (<i>c</i>) They always occur between two roots
+or between the collars of two roots, (<i>d</i>) They are most
+frequent in old, stout-rooted, broad-crowned trees; in younger
+stands it is always the stoutest members that are found with
+frost splits, while in quite young stands they are altogether
+absent, (<i>e</i>) Trees on wet sites are most liable to splits, due
+to difference in wood structure, just as difference in wood
+structure makes different species vary in this regard. (<i>f</i>)
+Frost splits are most numerous less than three feet above the
+ground.</p>
+
+<p>When a tree is swayed by the wind the roots are counteracting
+forces, and the wood fibres are tested in tension and
+compression by the opposing forces; where the roots exercise
+tension stresses most effectively the effect of compression
+stresses is at a minimum; only where the pressure is in excess
+of the tension, <i>i.e.</i>, between the roots, can a separation of
+the fibre result. Hence, when by frost a tension on the entire
+periphery is established, and the <a name="pg064"></a> wind localizes additional
+strains, failure occurs. The stronger the compression and
+tension, the severer the strains and the oftener failures occur.
+The occurrence of reports of frost splits on wind-still days is
+believed by Busse to be due to the opening of old frost splits
+where the tension produced by the frost alone is sufficient.</p>
+
+<p>Frost splits may heal over temporarily, but usually open up
+again during the following winter. The presence of old splits is
+often indicated by a ridge of callous, the result of the
+cambium's effort to occlude the wound. Frost splits not only
+affect the value of lumber, but also afford an entrance into the
+living tree for disease and decay.</p>
+
+<a name="II_09"></a>
+<h3>SHAKES, GALLS, PITCH POCKETS</h3>
+
+<p><i>Heart shake</i> occurs in nearly all overmature timber, being more
+frequent in hardwoods (especially oak) than in conifers. In
+typical heart shake the centre of the hole shows indications of
+becoming hollow and radial clefts of varying size extend outward
+from the pith, being widest inward. It frequently affects only
+the butt log, but may extend to the entire hole and even the
+larger branches. It usually results from a shrinkage of the
+heartwood due probably to chemical changes in the wood.</p>
+
+<p>When it consists of a single cleft extending across the pith it
+is termed <i>simple heart shake</i>. Shake of this character in
+straight-grained trees affects only one or two central boards
+when cut into lumber, but in spiral-grained timber the damage is
+much greater. When shake consists of several radial clefts it is
+termed <i>star shake</i>. In some instances one or more of these
+clefts may extend nearly to the bark. In felled or converted
+timber clefts due to heart shake may be distinguished from
+seasoning cracks by the darker color of the exposed surfaces.
+Such clefts, however, tend to open up more and more as the
+timber seasons.</p>
+
+<p><i>Cup</i> or <i>ring shake</i> results from the pulling apart of two or
+more growth rings. It is one of the most serious defects to
+which sound timber is subject, as it seriously reduces the
+technical properties of wood. It is very common in sycamore and
+in western larch, particularly in the butt portion. Its
+occurrence is most frequent at the junction of two growth layers
+of very unequal <a name="pg065"></a> thickness. Consequently it is likely to occur in
+trees which have grown slowly for a time, then abruptly
+increased, due to improved conditions of light and food, as in
+thinning. Old timber is more subject to it than young trees. The
+damage is largely confined to the butt log. Cup shake is often
+associated with other forms of shake, and not infrequently shows
+traces of decay.</p>
+
+<p>The causes of cup shake are uncertain. The swaying action of the
+wind may result in shearing apart the growth layers, especially
+in trees growing in exposed places. Frost may in some instances
+be responsible for cup shake or at least a contributing factor,
+although trees growing in regions free from frost often have
+ring shake. Shrinkage of the heartwood may be concentric as well
+as radial in its action, thus producing cup shake instead of, or
+in connection with, heart shake.</p>
+
+<p>A local defect somewhat similar in effect to cup shake is known
+as <i>rind gall</i>. If the cambium layer is exposed by the removal
+of the entire bark or rind it will die. Subsequent growth over
+the damaged portion does not cohere with the wood previously
+formed by the old cambium. The defect resulting is termed rind
+gall. The most common causes of it are fire, gnawing, blazing,
+chipping, sun scald, lightning, and abrasions.</p>
+
+<p><i>Heart break</i> is a term applied to areas of compression failure
+along the grain found in occasional logs. Sometimes these breaks
+are invisible until the wood is manufactured into the finished
+article. The occurrence of this defect is mostly limited to the
+dense hardwoods, such as hickory and to heavy tropical species.
+It is the source of considerable loss in the fancy veneer
+industry, as the veneer from valuable logs so affected drops to
+pieces.</p>
+
+<p>The cause of heart break is not positively known. It is highly
+probable, however, that when the tree is felled the trunk
+strikes across a rock or another log, and the impact causes
+actual failure in the log as in a beam.</p>
+
+<p><i>Resin</i> or <i>pitch pockets</i> are of common occurrence in the wood
+of larch, spruce, fir, and especially of longleaf and other hard
+pines. They are due to accumulations of resin in openings
+between adjacent layers of growth. They are more frequent in
+trees growing alone than in those of dense stands. The pockets
+are usually a few inches in greatest dimension and affect only
+one or <a name="pg066"></a> two growth layers. They are hidden until exposed by the
+saw, rendering it impossible to cut lumber with reference to
+their position. Often several boards are damaged by a single
+pocket. In grading lumber, pitch pockets are classified as
+small, standard, and large, depending upon their width and
+length.</p>
+
+<a name="II_10"></a>
+<h3>INSECT INJURIES<a name="fn37r"></a><a href="#fn37"><sup><small>37</small></sup></a></h3>
+
+<p>The larv� of many insects are destructive to wood. Some attack
+the wood of living trees, others only that of felled or
+converted material. Every hole breaks the continuity of the
+fibres and impairs the strength, and if there are very many of
+them the material may be ruined for all purposes where strength
+is required.</p>
+
+<p>Some of the most common insects attacking the wood of living
+trees are the oak timber worm, the chestnut timber worm,
+carpenter worms, ambrosia beetles, the locust borer, turpentine
+beetles and turpentine borers, and the white pine weevil.</p>
+
+<p>The insect injuries to forest products may be classed according
+to the stage of manufacture of the material. Thus round timber
+with the bark on, such as poles, posts, mine props, and sawlogs,
+is subject to serious damage by the same class of insects as
+those mentioned above, particularly by the round-headed borers,
+timber worms, and ambrosia beetles. Manufactured unseasoned
+products are subject to damage from ambrosia beetles and other
+wood borers. Seasoned hardwood lumber of all kinds, rough
+handles, wagon stock, etc., made partially or entirely of
+sapwood, are often reduced in value from 10 to 90 per cent by a
+class of insects known as powder-post beetles. Finished hardwood
+products such as handles, wagon, carriage and machinery stock,
+especially if ash or hickory, are often destroyed by the
+powder-post beetles. Construction timbers in buildings, bridges
+and trestles, cross-ties, poles, mine props, fence posts, etc.,
+are sometimes seriously injured by wood-boring larv�, termites,
+black ants, carpenter bees, and powder-post beetles, and
+sometimes reduced <a name="pg067"></a> in value from 10 to 100 per cent. In tropical
+countries termites are a very serious pest in this respect.</p>
+
+<a name="II_11"></a>
+<h3>MARINE WOOD-BORER INJURIES</h3>
+
+<p>Vast amounts of timber used for piles in wharves and other
+marine structures are constantly being destroyed or seriously
+injured by marine borers. Almost invariably they are confined to
+salt water, and all the woods commonly used for piling are
+subject to their attacks. There are two genera of mollusks,
+<i>Xylotrya</i> and <i>Teredo</i>, and three of crustaceans, <i>Limnoria,
+Chelura</i>, and <i>Sphoeroma</i>, that do serious damage in many places
+along both the Atlantic and Pacific coasts.</p>
+
+<p>These mollusks, which are popularly known as "shipworms," are
+much alike in structure and mode of life. They attack the
+exposed surface of the wood and immediately begin to bore. The
+tunnels, often as large as a lead pencil, extend usually in a
+longitudinal direction and follow a very irregular, tangled
+course. Hard woods are apparently penetrated as readily as soft
+woods, though in the same timber the softer parts are preferred.
+The food consists of infusoria and is not obtained from the wood
+substance. The sole object of boring into the wood is to obtain
+shelter.</p>
+
+<p>Although shipworms can live in cold water they thrive best and
+are most destructive in warm water. The length of time required
+to destroy an average barked, unprotected pine pile on the
+Atlantic coast south from Chesapeake Bay and along the entire
+Pacific coast varies from but one to three years.</p>
+
+<p>Of the crustacean borers, <i>Limnoria</i>, or the "wood louse," is
+the only one of great importance, although <i>Sphoeroma</i> is
+reported destructive in places. <i>Limnoria</i> is about the size of
+a grain of rice and tunnels into the wood for both food and
+shelter. The galleries extend inward radially, side by side, in
+countless numbers, to the depth of about one-half inch. The thin
+wood partitions remaining are destroyed by wave action, so that
+a fresh surface is exposed to attack. Both hard and soft woods
+are damaged, but the rate is faster in the soft woods or softer
+portions of a wood.</p>
+
+<p>Timbers seriously attacked by marine borers are badly <a name="pg068"></a> weakened
+or completely destroyed. If the original strength of the
+material is to be preserved it is necessary to protect the wood
+from the borers. This is sometimes accomplished by proper
+injection of creosote oil, and more or less successfully by the
+use of various kinds of external coatings.<a name="fn38r"></a><a href="#fn38"><sup><small>38</small></sup></a> No treatment,
+however, has proved entirely satisfactory.</p>
+
+<a name="II_12"></a>
+<h3>FUNGOUS INJURIES<a name="fn39r"></a><a href="#fn39"><sup><small>39</small></sup></a></h3>
+
+<p>Fungi are responsible for almost all decay of wood. So far as
+known, all decay is produced by living organisms, either fungi
+or bacteria. Some species attack living trees, sometimes killing
+them, or making them hollow, or in the case of pecky cypress and
+incense cedar filling the wood with galleries like those of
+boring insects. A much larger variety work only in felled or
+dead wood, even after it is placed in buildings or manufactured
+articles. In any case the process of destruction is the same.
+The mycelial threads penetrate the walls of the cells in search
+of food, which they find either in the cell contents (starches,
+sugars, etc.), or in the cell wall itself. The breaking down of
+the cell walls through the chemical action of so-called
+"enzymes" secreted by the fungi follows, and the eventual
+product is a rotten, moist substance crumbling readily under the
+slightest pressure. Some species remove the ligneous matter and
+leave almost pure cellulose, which is white, like cotton; others
+dissolve the cellulose, leaving a brittle, dark brown mass of
+ligno-cellulose. Fungi (such as the bluing fungus) which merely
+stain wood usually do not affect its mechanical properties
+unless the attacks are excessive.</p>
+
+<p>It is evident, then, that the action of rot-causing fungi is to
+decrease the strength of wood, rendering it unsound, brittle,
+and <a name="pg069"></a> dangerous to use. The most dangerous kinds are the so-called
+"dry-rot" fungi which work in many kinds of lumber after it is
+placed in the buildings. They are particularly to be dreaded
+because unseen, working as they do within the walls or inside of
+casings. Several serious wrecks of large buildings have been
+attributed to this cause. It is stated<a name="fn40r"></a><a href="#fn40"><sup><small>40</small></sup></a> that in the three
+years (1911-1913) more than $100,000 was required to repair
+damage due to dry rot.</p>
+
+<p>Dry rot develops best at 75&deg;F. and is said to be killed by a
+temperature of 110&deg;F.<a name="fn41r"></a><a href="#fn41"><sup><small>41</small></sup></a> Fully 70 per cent humidity is
+necessary in the air in which a timber is surrounded for the
+growth of this fungus, and probably the wood must be quite near
+its fibre saturation condition. Nevertheless <i>Merulius
+lacrymans</i> (one of the most important species) has been found to
+live four years and eight months in a dry condition.<a name="fn42r"></a><a href="#fn42"><sup><small>42</small></sup></a>
+Thorough kiln-drying will kill this fungus, but will not prevent
+its redevelopment. Antiseptic treatment, such as creosoting, is
+the best prevention.</p>
+
+<p>All fungi require moisture and air<a name="fn43r"></a><a href="#fn43"><sup><small>43</small></sup></a> for their growth.
+Deprived of either of these the fungus dies or ceases to
+develop. Just what degree of moisture in wood is necessary for
+the "dry-rot" fungus has not been determined, but it is
+evidently considerably above that of thoroughly air-dry timber,
+probably more than 15 per cent moisture. Hence the importance of
+free circulation of air about all timbers in a building.</p>
+
+<p>Warmth is also conducive to the growth of fungi, the most
+favorable temperature being about 90&deg;F. They cannot grow in
+extreme cold, although no degree of cold such as occurs
+naturally will kill them. On the other hand, high temperature
+will kill them, but the spores may survive even the boiling
+temperature. Mould fungus has been observed to develop rapidly
+at 130&deg;F. in a dry kiln in moist air, a condition under which an
+animal cannot <a name="pg070"></a> live more than a few minutes. This fungus was
+killed, however, at about 140&deg; or 145&deg;F.<a name="fn44r"></a><a href="#fn44"><sup><small>44</small></sup></a></p>
+
+<p>The fungus (<i>Endothia parasitica</i> And.) which causes the
+chestnut blight kills the trees by girdling them and has no
+direct effect upon the wood save possibly the four or five
+growth rings of the sapwood.<a name="fn45r"></a><a href="#fn45"><sup><small>45</small></sup></a></p>
+
+<a name="II_13"></a>
+<h3>PARASITIC PLANT INJURIES.<a name="fn46r"></a><a href="#fn46"><sup><small>46</small></sup></a></h3>
+
+<p>The most common of the higher parasitic plants damaging timber
+trees are mistletoes. Many species of deciduous trees are
+attacked by the common mistletoe (<i>Phoradendron flavescens</i>). It
+is very prevalent in the South and Southwest and when present in
+sufficient quantity does considerable damage. There is also a
+considerable number of smaller mistletoes belonging to the genus
+<i>Razoumofskya (Arceuthobium)</i> which are widely distributed
+throughout the country, and several of them are common on
+coniferous trees in the Rocky Mountains and along the Pacific
+coast.</p>
+
+<p>One effect of the common mistletoe is the formation of large
+swellings or tumors. Often the entire tree may become stunted or
+distorted. The western mistletoe is most common on the branches,
+where it produces "witches' broom." It frequently attacks the
+trunk as well, and boards cut from such trees are filled with
+long, radial holes which seriously damage or destroy the value
+of the timber affected.</p>
+
+<a name="II_14"></a>
+v<h3>LOCALITY OF GROWTH</h3>
+
+<p>The data available regarding the effect of the locality of
+growth upon the properties of wood are not sufficient to warrant
+<a name="pg071"></a>
+definite conclusions. The subject has, however, been kept in
+mind in many of the U.S. Forest Service timber tests and the
+following quotations are assembled from various reports:</p>
+
+<p>"In both the Cuban and longleaf pine the locality where grown
+appears to have but little influence on weight or strength, and
+there is no reason to believe that the longleaf pine from one
+State is better than that from any other, since such variations
+as are claimed can be found on any 40-acre lot of timber in any
+State. But with loblolly and still more with shortleaf this
+seems not to be the case. Being widely distributed over many
+localities different in soil and climate, the growth of the
+shortleaf pine seems materially influenced by location. The wood
+from the southern coast and gulf region and even Arkansas is
+generally heavier than the wood from localities farther north.
+Very light and fine-grained wood is seldom met near the southern
+limit of the range, while it is almost the rule in Missouri,
+where forms resembling the Norway pine are by no means rare. The
+loblolly, occupying both wet and dry soils, varies accordingly."
+Cir. No. 12, p. 6.</p>
+
+<p>" ... It is clear that as all localities have their heavy and
+their light timber, so they all share in strong and weak, hard
+and soft material, and the difference in quality of material is
+evidently far more a matter of individual variation than of soil
+or climate." <i>Ibid.</i>, p.22</p>
+
+<p>"A representative committee of the Carriage Builders'
+Association had publicly declared that this important industry
+could not depend upon the supplies of southern timber, as the
+oak grown in the South lacked the necessary qualities demanded
+in carriage construction. Without experiment this statement
+could be little better than a guess, and was doubly unwarranted,
+since it condemned an enormous amount of material, and one
+produced under a great variety of conditions and by at least a
+dozen species of trees, involving, therefore, a complexity of
+problems difficult enough for the careful investigator, and
+entirely beyond the few unsystematic observations of the members
+of a committee on a flying trip through one of the greatest
+timber regions of the world.</p>
+
+<p>"A number of samples were at once collected (part of them
+supplied by the carriage builders' committee), and the fallacy
+of the broad statement mentioned was fully demonstrated by a
+short <a name="pg072"></a> series of tests and a more extensive study into structure
+and weight of these materials. From these tests it appears that
+pieces of white oak from Arkansas excelled well-selected pieces
+from Connecticut, both in stiffness and endwise compression (the
+two most important forms of resistance)." Report upon the
+forestry investigations of the U.S.D.A. 1877-1898, p. 331. See
+also Rep. of Div. of For., 1890, p. 209.</p>
+
+<p>"In some regions there are many small, stunted hickories, which
+most users will not touch. They have narrow sap, are likely to
+be birdpecked, and show very slow growth. Yet five of these
+trees from a steep, dry south slope in West Virginia had an
+average strength fully equal to that of the pignut from the
+better situation, and were superior in toughness, the work to
+maximum load being 36.8 as against 31.2 for pignut. The trees
+had about twice as many rings per inch as others from better
+situations.</p>
+
+<p>"This, however, is not very significant, as trees of the same
+species, age, and size, growing side by side under the same
+conditions of soil and situation, show great variation in their
+technical value. It is hard to account for this difference, but
+it seems that trees growing in wet or moist situations are
+rather inferior to those growing on fresher soil; also, it is
+claimed by many hickory users that the wood from limestone soils
+is superior to that from sandy soils.</p>
+
+<p>"One of the moot questions among hickory men is the relative
+value of northern and southern hickory. The impression prevails
+that southern hickory is more porous and brash than hickory from
+the north. The tests ... indicate that southern hickory is as
+tough and strong as northern hickory of the same age. But the
+southern hickories have a greater tendency to be shaky, and this
+results in much waste. In trees from southern river bottoms the
+loss through shakes and grub-holes in many cases amounts to as
+much as 50 per cent.</p>
+
+<p>"It is clear, therefore, that the difference in northern and
+southern hickory is not due to geographic location, but rather
+to the character of timber that is being cut. Nearly all of that
+from southern river bottoms and from the Cumberland Mountains is
+from large, old-growth trees; that from the north is from
+younger trees which are grown under more favorable conditions,
+and it is <a name="pg073"></a> due simply to the greater age of the southern trees
+that hickory from that region is lighter and more brash than
+that from the north." Bul. 80, pp. 52-55.</p>
+
+<a name="II_15"></a>
+<h3>SEASON OF CUTTING</h3>
+
+<p>It is generally believed that winter-felled timber has decided
+advantages over that cut at other seasons of the year, and to
+that cause alone are frequently ascribed much greater
+durability, less liability to check and split, better color, and
+even increased strength and toughness. The conclusion from the
+various experiments made on the subject is that while the time
+of felling may, and often does, affect the properties of wood,
+such result is due to the weather conditions rather than to the
+condition of the wood.</p>
+
+<p>There are two phases of this question. One is concerned with the
+physiological changes which might take place during the year in
+the wood of a living tree. The other deals with the purely
+physical results due to the weather, as differences in
+temperature, humidity, moisture, and other features to be
+mentioned later.</p>
+
+<p>Those who adhere to the first view maintain that wood cut in
+summer is quite different in composition from that cut in
+winter. One opinion is that in summer the "sap is up," while in
+winter it is "down," consequently winter-felled timber is drier.
+A variation of this belief is that in summer the sap contains
+certain chemicals which affect the properties of wood and does
+not contain them in winter. Again it is sometimes asserted that
+wood is actually denser in winter than in summer, as part of the
+wood substance is dissolved out in the spring and used for plant
+food, being restored in the fall.</p>
+
+<p>It is obvious that such views could apply only to sapwood, since
+it alone is in living condition at the time of cutting.
+Heartwood is dead wood and has almost no function in the
+existence of the tree other than the purely mechanical one of
+support. Heartwood does undergo changes, but they are gradual
+and almost entirely independent of the seasons.</p>
+
+<p>Sapwood might reasonably be expected to respond to seasonal
+changes, and to some extent it does. Just beneath the bark there
+is a thin layer of cells which during the growing season have
+not <a name="pg074"></a> attained their greatest density. With the exception of this
+one annual ring, or portion of one, the density of the wood
+substance of the sapwood is nearly the same the year round.
+Slight variations may occur due to impregnation with sugar and
+starch in the winter and its dissolution in the growing season.
+The time of cutting can have no material effect on the inherent
+strength and other mechanical properties of wood except in the
+outermost annual ring of growth.</p>
+
+<p>The popular belief that sap is up in the spring and summer and
+is down in the winter has not been substantiated by experiment.
+There are seasonal differences in the composition of sap, but so
+far as the amount of sap in a tree is concerned there is fully
+as much, if not more, during the winter than in summer.
+Winter-cut wood is not drier, to begin with, than
+summer-felled&mdash;in reality, it is likely to be wetter.<a name="fn47r"></a><a href="#fn47"><sup><small>47</small></sup></a></p>
+
+<p>The important consideration in regard to this question is the
+series of circumstances attending the handling of the timber
+after it is felled. Wood dries more rapidly in summer than in
+winter, not because there is less moisture at one time than
+another, but because of the higher temperature in summer. This
+greater heat is often accompanied by low humidity, and
+conditions are favorable for the rapid removal of moisture from
+the exposed portions of wood. Wood dries by evaporation, and
+other things being equal, this will proceed much faster in hot
+weather than in cold.</p>
+
+<p>It is a matter of common observation that when wood dries it
+shrinks, and if shrinkage is not uniform in all directions the
+material pulls apart, causing season checks. (<a href="#fig27">See Fig. 27</a>.) If
+evaporation proceeds more rapidly on the outside than inside,
+the greater shrinkage of the outer portions is bound to result
+in many checks, the number and size increasing with the degree
+of inequality of drying.</p>
+
+<p>In cold weather, drying proceeds slowly but uniformly, thus
+allowing the wood elements to adjust themselves with the least
+amount of rupturing. In summer, drying proceeds rapidly and
+<a name="pg075"></a>
+irregularly, so that material seasoned at that time is more
+likely to split and check.</p>
+
+<p>There is less danger of sap rot when trees are felled in winter
+because the fungus does not grow in the very cold weather, and
+the lumber has a chance to season to below the danger point
+before the fungus gets a chance to attack it. If the logs in
+each case could be cut into lumber immediately after felling and
+given exactly the same treatment, for example, kiln-dried, no
+difference due to the season of cutting would be noted.</p>
+
+<a name="II_16"></a>
+<h3>WATER CONTENT<a name="fn48r"></a><a href="#fn48"><sup><small>48</small></sup></a></h3>
+
+<p>Water occurs in living wood in three conditions, namely: (1) in
+the cell walls, (2) in the protoplasmic contents of the cells,
+and (3) as free water in the cell cavities and spaces. In
+heartwood it occurs only in the first and last forms. Wood that
+is thoroughly air-dried retains from 8 to 16 per cent of water
+in the cell walls, and none, or practically none, in the other
+forms. Even oven-dried wood retains a small percentage of
+moisture, but for all except chemical purposes, may be
+considered absolutely dry.</p>
+
+<p>The general effect of the water content upon the wood substance
+is to render it softer and more pliable. A similar effect of
+common observation is in the softening action of water on
+rawhide, paper, or cloth. Within certain limits the greater the
+water content the greater its softening effect.</p>
+
+<p>Drying produces a decided increase in the strength of wood,
+particularly in small specimens. An extreme example is the case
+of a completely dry spruce block two inches in section, which
+will sustain a permanent load four times as great as that which
+a green block of the same size will support.</p>
+
+<p>The greatest increase due to drying is in the ultimate crushing
+strength, and strength at elastic limit in endwise compression;
+these are followed by the modulus of rupture, and stress at
+elastic limit in cross-bending, while the modulus of elasticity
+is least affected. These ratios are shown in Table XV, but it is
+to be noted <a name="pg076"></a> that they apply only to wood in a much drier
+condition than is used in practice. For air-dry wood the ratios
+are considerably lower, particularly in the case of the ultimate
+strength and the elastic limit. Stiffness (within the elastic
+limit), while following a similar law, is less affected. In the
+case of shear parallel to the grain, the general effect of
+drying is to increase the strength, but this is often offset by
+small splits and checks caused by shrinkage.</p>
+
+<center>
+<table id="TABLE_15" summary="TABLE_XV" border="3" cellspacing="0" cellpadding="1" width="80%">
+<tbody>
+<tr><th colspan="7"> TABLE XV </th></tr>
+<tr><th colspan="7"> EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD,
+SHOWN IN RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM THE GREEN
+CONDITION TO KILN-DRY (3.5 PER CENT) </th></tr>
+<tr><td colspan="7"> (Forest Service Bul. 70, p. 89) </td></tr>
+<tr><th rowspan="2"> KIND OF STRENGTH </th>                                                   <th colspan="2"> Longleaf pine </th>  <th colspan="2"> Spruce </th>    <th colspan="2"> Chestnut </th></tr>
+<tr>                                                                                          <th> (1) </th> <th> (2) </th>         <th> (1) </th>  <th> (2) </th>   <th> (1) </th> <th> (2) </th></tr>
+<tr><td style="text-align: left;"> Crushing strength parallel to grain </td>                   <td> 2.89 </td><td> 2.60 </td>        <td> 3.71 </td> <td> 3.41 </td>  <td> 2.83 </td><td> 2.55 </td></tr>
+<tr><td style="text-align: left;"> Elastic limit in compression parallel to grain </td>        <td> 2.60 </td><td> 2.34 </td>        <td> 3.80 </td> <td> 3.49 </td>  <td> 2.40 </td><td> 2.26 </td></tr>
+<tr><td style="text-align: left;"> Modulus of rupture in bending </td>                         <td> 2.50 </td><td> 2.20 </td>        <td> 2.81 </td> <td> 2.50 </td>  <td> 2.09 </td><td> 1.82 </td></tr>
+<tr><td style="text-align: left;"> Stress at elastic limit in bending </td>                    <td> 2.90 </td><td> 2.55 </td>        <td> 2.90 </td> <td> 2.58 </td>  <td> 2.30 </td><td> 2.00 </td></tr>
+<tr><td style="text-align: left;"> Crushing strength at right angles to grain </td>            <td><br></td>  <td><br></td>          <td> 2.58 </td> <td> 2.48 </td>  <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Shearing strength parallel to grain </td>                   <td>2.01 </td> <td> 1.91 </td>        <td> 2.03 </td> <td> 1.95 </td>  <td> 1.55 </td><td> 1.47 </td></tr>
+<tr><td style="text-align: left;"> Modulus of elasticity in compression parallel to grain </td><td>1.63 </td> <td> 1.47 </td>        <td> 2.26 </td> <td> 2.08 </td>  <td> 1.43 </td><td> 1.29 </td></tr>
+<tr><td style="text-align: left;"> Modulus of elasticity in bending </td>                      <td>1.59 </td> <td> 1.35 </td>        <td> 1.43 </td> <td> 1.23 </td>  <td> 1.44 </td><td> 1.21 </td></tr>
+<tr><td style="text-align: left;" colspan="7"> NOTE.&mdash;The figures in the first
+column show the relative increase in strength between a green specimen
+and a kiln-dry specimen of equal size. The figures in the second column
+show the relative increase of strength of the same block after being
+dried from a green&nbsp;condition to 3.5 per cent moisture, correction
+having been made for shrinkage. That is, in the first column the
+strength values per actual&nbsp;unit of area are used; in the second
+the values per unit of area of green wood which shrinks to smaller size
+when dried. See also Cir. 108, Fig. 1, p. 8. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>The moisture content has a decided bearing also upon the manner
+in which wood fails. In compression tests on very dry <a name="pg077"></a> specimens
+the entire piece splits suddenly into pieces before any buckling
+takes place (<a href="#fig09">see Fig. 9</a>.), while with wet material the block
+gives way gradually, due to the buckling or bending of the walls
+of the fibres along one or more shearing planes. (<a href="#fig14">See Fig. 14</a>.)
+In bending tests on wet beams, first failure occurs by
+compression on top of the beam, gradually extending downward
+toward the neutral axis. Finally the beam ruptures at <a name="pg078"></a> the
+bottom. In the case of very dry beams the failure is usually by
+splitting or tension on the under side (<a href="#fig17">see Fig. 17</a>.), without
+compression on the upper, and is often sudden and without
+warning, and even while the load is still increasing. The effect
+varies somewhat with different species, chestnut, for example,
+becoming more brittle upon drying than do ash, hemlock, and
+longleaf pine. The tensile strength of wood is least affected by
+drying, as a rule.</p>
+
+<p>In drying wood no increase in strength results until the free
+water is evaporated and the cell walls begin to dry<a name="fn49r"></a><a href="#fn49"><sup><small>49</small></sup></a>. This
+critical point has been called the <i>fibre-saturation point</i>.
+(<a href="#fig24">See Fig. 24</a>.) Conversely, after the cell walls are saturated
+with water, any increase in the amount of water absorbed merely
+fills the cavities and intercellular spaces, and has no effect
+on the mechanical properties. Hence, soaking green wood does not
+lessen its strength unless the water is heated, whereupon a
+decided weakening results.</p>
+
+<p class="ctr"><img src="images/fig24.jpg" alt="Figure 24" id="fig24" /></p>
+<p class="imgtitle">Figure 24</p>
+<p class="imgtext"> Relation of the moisture content to the
+various strength values of spruce. FSP = fibre-saturation
+point.</p>
+
+<p>The strengthening effects of drying, while very marked in the
+case of small pieces, may be fully offset in structural timbers
+by inherent weakening effects due to the splitting apart of the
+wood elements as a result of irregular shrinkage, and in some
+cases also to the slitting of the cell walls (<a href="#fig25">see Fig. 25</a>).
+Consequently with large timbers in commercial use it is unsafe
+to count upon any greater strength, even after seasoning, than
+that of the green or fresh condition.</p>
+
+<p class="ctr"><img src="images/fig25.jpg" alt="Figure 25" id="fig25" /></p>
+<p class="imgtitle">Figure 25</p>
+<p class="imgtext"> Cross section of the wood of western
+larch showing fissures in the thick-walled cells of the late
+wood. Highly magnified. <i>Photo by U. S. Forest Service.</i></p>
+
+<p>In green wood the cells are all intimately joined together and
+are at their natural or normal size when saturated with water.
+The cell walls may be considered as made up of little particles
+with water between them. When wood is dried the films of water
+between the particles become thinner and thinner until almost
+entirely gone. As a result the cell walls grow thinner with loss
+of moisture,&mdash;in other words, the cell shrinks.</p>
+
+<p>It is at once evident that if drying does not take place
+uniformly throughout an entire piece of timber, the shrinkage as
+a <a name="pg080"></a> whole cannot be uniform. The process of drying is from the
+outside inward, and if the loss of moisture at the surface is
+met by a steady capillary current of water from the inside, the
+shrinkage, so far as the degree of moisture affected it, would
+be uniform. In the best type of dry kilns this condition is
+approximated by first heating the wood thoroughly in a moist
+atmosphere before allowing drying to begin.</p>
+
+<p>In air-seasoning and in ordinary dry kilns this condition too
+often is not attained, and the result is that a dry shell is
+formed which encloses a moist interior. (<a href="#fig26">See Fig. 26</a>.)
+Subsequent drying out of the inner portion is rendered more
+difficult by this "case-hardened" condition. As the outer part
+dries it is prevented from shrinking by the wet interior, which
+is still at its greatest volume. This outer portion must either
+check open or the fibres become strained in tension. If this
+outer shell dries while the fibres are thus strained they become
+"set" in this condition, and are no longer in tension. Later
+when the inner part dries, it tends to shrink away from the
+hardened outer shell, so that the inner fibres are now strained
+in tension and the outer fibres are in compression. If the
+stress exceeds the cohesion, numerous cracks open up, producing
+a "honey-combed" condition, or "hollow-horning," as it is
+called. If such a case-hardened stick of wood be resawed, the
+two halves will cup from the internal tension and external
+compression, with the concave surface inward.</p>
+
+<p class="ctr"><img src="images/fig26.jpg" alt="Figure 26" id="fig26" /></p>
+<p class="imgtitle">Figure 26</p>
+<p class="imgtext"> Progress of drying throughout the
+length of a chestnut beam, the black spots indicating the
+presence of free water in the wood. The first section at the
+left was cut one-fourth inch from the end, the next one-half
+inch, the next one inch, and all the others one inch apart. The
+illustration shows case-hardening very clearly. <i>Photo by U. S.
+Forest Service.</i></p>
+
+<p>For a given surface area the loss of water from wood is always
+greater from the ends than from the sides, due to the fact that
+the vessels and other water-carriers are cut across, allowing
+ready entrance of drying air and outlet for the water vapor.
+Water does not flow out of boards and timbers of its own accord,
+but must be evaporated, though it may be forced out of very
+sappy specimens by heat. In drying a log or pole with the bark
+on, most of the water must be evaporated through the ends, but
+in the case of peeled timbers and sawn boards the loss is
+greatest from the surface because the area exposed is so much
+greater.</p>
+
+<p>The more rapid drying of the ends causes local shrinkage, and
+were the material sufficiently plastic the ends would become
+bluntly tapering. The rigidity of the wood substance prevents
+this and the fibres are split apart. Later, as the remainder of
+the <a name="pg081"></a> stick dries many of the checks will come together, though
+some of the largest will remain and even increase in size as the
+drying proceeds. (<a href="#fig27">See Fig. 27</a>.)</p>
+
+<p class="ctr"><img src="images/fig27.jpg" alt="Figure 27" id="fig27" /></p>
+<p class="imgtitle">Figure 27</p>
+<p class="imgtext"> Excessive season checking. <i>Photo by U.S.
+Forest Service.</i></p>
+
+<p>A wood cell shrinks very little lengthwise. A dry wood cell is,
+therefore, practically of the same length as it was in a green
+or saturated condition, but is smaller in cross section, has
+thinner walls, and a larger cavity. It is at once evident that
+this fact makes shrinkage more irregular, for wherever cells
+cross each other at a decided angle they will tend to pull apart
+upon drying. This occurs wherever pith rays and wood fibres
+meet. A considerable portion of every wood is made up of these
+rays, which for the most part have their cells lying in a radial
+direction instead of longitudinally. (<a href="#fig00">See Frontispiece</a>.) In
+pine, over 15,000 of these occur on a square inch of a
+tangential section, and even in oak the very large rays which
+are readily visible to the eye as flakes <a name="pg082"></a> on quarter-sawed
+material represent scarcely one per cent of the number which the
+microscope reveals.</p>
+
+<p>A pith ray shrinks in height and width, that is, vertically and
+tangentially as applied to the position in a standing tree, but
+very little in length or radially. The other elements of the
+wood shrink radially and tangentially, but almost none
+lengthwise or vertically as applied to the tree. Here, then, we
+find the shrinkage of the rays tending to shorten a stick of
+wood, while the other cells resist it, and the tendency of a
+stick to get smaller in circumference is resisted by the endwise
+reaction or thrust of the rays. Only in a tangential direction,
+or around the stick in direction of the annual rings of growth,
+do the two forces coincide. Another factor to the same end is
+that the denser bands of late wood are continuous in a
+tangential direction, while radially they are separated by
+alternate zones of less dense early wood. Consequently the
+shrinkage along the rings (tangential) is fully twice as much as
+toward the centre (radial). (<a href="#TABLE_14">See Table XIV</a>.) This explains why
+some cracks open more and more as drying advances. (<a href="#fig27">See Fig.
+27</a>.)</p>
+
+<p>Although actual shrinkage in length is small, nevertheless the
+tendency of the rays to shorten a stick produces strains which
+are responsible for some of the splitting open of ties, posts,
+and sawed timbers with box heart. At the very centre of a tree
+the wood is light and weak, while farther out it becomes denser
+and stronger. Longitudinal shrinkage is accordingly least at the
+centre and greater toward the outside, tending to become
+greatest in the sapwood. When a round or a box-heart timber
+dries fast it splits radially, and as drying continues the cleft
+widens partly on account of the greater tangential shrinkage and
+also because the greater contraction of the outer fibres warps
+the sections apart. If a small hardwood stem is split while
+green for a short distance at the end and placed where it can
+dry out rapidly, the sections will become bow-shaped with the
+concave sides out. These various facts, taken together, explain
+why, for example, an oak tie, pole, or log may split open its
+entire length if drying proceeds rapidly and far enough. Initial
+stresses in the living trees produce a similar effect when the
+log is sawn into boards. This is especially so in <i>Eucalyptus
+globulus</i> and to a less extent with any rapidly grown wood.</p>
+
+<a name="pg083"></a>
+
+<p>The use of S-shaped thin steel clamps to prevent large checks
+and splits is now a common practice in this country with
+crossties and poles as it has been for a long time in European
+countries. These devices are driven into the butts of the
+timbers so as to cross incipient checks and prevent their
+widening. In place of the regular S-hook another of crimped iron
+has been devised. (<a href="#fig28">See Fig. 28</a>.) Thin straps of iron with one
+tapered edge are run between intermeshing cogs and crimped,
+after which they may be cut off any length desired. The time for
+driving S-irons of either form is when the cracks first appear.</p>
+
+<p class="ctr"><img src="images/fig28.jpg" alt="Figure 28" id="fig28" /></p>
+<p class="imgtitle">Figure 28</p>
+<p class="imgtext"> Control of season checking by the use
+of S-irons. <i>Photo by U. S. Forest Service.</i></p>
+
+<p>The tendency of logs to split emphasizes the importance of
+converting them into planks or timbers while in a green
+condition. Otherwise the presence of large checks may render
+much lumber worthless which might have been cut out in good
+condition. The loss would not be so great if logs were perfectly
+straight-grained, but this is seldom the case, most trees
+growing more or less spirally or irregularly. Large pieces crack
+more than smaller ones, quartered lumber less than that sawed
+through and through, thin pieces, especially veneers, less than
+thicker boards.</p>
+
+<p>In order to prevent cracks at the ends of boards, small straps
+of wood may be nailed on them or they may be painted. This
+<a name="pg084"></a>
+method is usually considered too expensive, except in the case
+of valuable material. Squares used for shuttles, furniture,
+gun-stocks, and tool handles should always be protected at the
+ends. One of the best means is to dip them into melted
+paraffine, which seals the ends and prevents loss of moisture
+there. Another method is to glue paper on the ends. In some
+cases abroad paper is glued on to all the surfaces of valuable
+exotic balks. Other substances sometimes employed for the
+purpose of sealing the wood are grease, carbolineum, wax, clay,
+petroleum, linseed oil, tar, and soluble glass. In place of
+solid beams, built-up material is often preferable, as the
+disastrous results of season checks are thereby largely overcome
+or minimized.</p>
+
+<a name="II_17"></a>
+<h3>TEMPERATURE</h3>
+
+<p>The effect of temperature on wood depends very largely upon the
+moisture content of the wood and the surrounding medium. If
+absolutely dry wood is heated in absolutely dry air the wood
+expands. The extent of this expansion is denoted by a
+coefficient corresponding to the increase in length or other
+dimensions for each degree rise in temperature divided by the
+original length or other dimension of the specimen. The
+coefficient of linear expansion of oak has been found to be
+.00000492; radial expansion, .0000544, or about eleven times the
+longitudinal. Spruce expands less than oak, the ratio of radial
+to longitudinal expansion being about six to one. Metals and
+glass expand equally in all directions, since they are
+homogeneous substances, while wood is a complicated structure.
+The coefficient of expansion of iron is .0000285, or nearly six
+times the coefficient of linear expansion of oak and seven times
+that of spruce<a name="fn50r"></a><a href="#fn50"><sup><small>50</small></sup></a>.</p>
+
+<p>Under ordinary conditions wood contains more or less moisture,
+so that the application of heat has a drying effect which is
+accompanied by shrinkage. This shrinkage completely obscures the
+expansion due to the heating.</p>
+
+<p>Experiments made at the Yale Forest School revealed the effect
+of temperature on the crushing strength of wet wood. In the case
+<a name="pg085"></a>
+of wet chestnut wood the strength decreases 0.42 per cent for
+each degree the water is heated above 60&deg; F.; in the case of
+spruce the decrease is 0.32 per cent.</p>
+
+<p>The effects of high temperature on wet wood are very marked.
+Boiling produces a condition of great pliability, especially in
+the case of hardwoods. If wood in this condition is bent and
+allowed to dry, it rigidly retains the shape of the bend, though
+its strength may be somewhat reduced. Except in the case of very
+dry wood <a name="pg086"></a> the effect of cold is to increase the strength and
+stiffness of wood. The freezing of any free water in the pores
+of the wood will augment these conditions.</p>
+
+<p>The effect of steaming upon the strength of cross-ties was
+investigated by the U.S. Forest Service in 1904. The conclusions
+were summarized as follows:</p>
+
+<p>"(1) The steam at pressure up to 40 pounds applied for 4 hours,
+or at a pressure of 20 pounds up to 20 hours, increases the
+weight of ties. At 40 pounds' pressure applied for 4 hours and
+at 20 pounds for 5 hours the wood began to be scorched.</p>
+
+<p>"(2) The steamed and saturated wood, when tested immediately
+after treatment, exhibited weaknesses in proportion to the
+pressure and duration of steaming. (<a href="#TABLE_16">See Table XVI</a>.) If allowed
+to air-dry subsequently the specimens regained the greater part
+of their strength, provided the pressure and duration had not
+exceeded those cited under (1). Subsequent immersion in water of
+the steamed wood and dried specimens showed that they were
+weaker than natural wood similarly dried and resoaked."<a name="fn51r"></a><a href="#fn51"><sup><small>51</small></sup></a></p>
+
+<center>
+<table id="TABLE_16" summary="TABLE_XVI" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th colspan="8"> TABLE XVI </th></tr>
+<tr><th colspan="8"> EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE </th></tr>
+<tr><td colspan="8"> (Forest Service, Cir. 39) </td></tr>
+<tr><th rowspan="5"> Treatment </th>                             <th colspan="3"> Cylinder conditions </th>  <th colspan="4"> Strength</th></tr>
+<tr>                                                             <th colspan="3"> Steaming </th><th colspan="2"> Static </th><th> Impact </th><th><br></th></tr><tr><th>Period </th><th> Pressure </th><th> Temperature </th><th> Bending modulus of rupture </th><th> Compression parallel to grain </th><th> Height of drop causing complete failure </th><th> Average of the three strengths </th></tr>
+<tr>                                                             <th rowspan="2"><i>Hrs.</i></th>
+                                                                              <th rowspan="2"><i>Lbs. per sq. inch</i></th>
+                                                                                            <th rowspan="2"> &deg;<i>F.</i></th>
+                                                                                                             <th> <i>Per cent</i> </th>
+                                                                                                                             <th> <i>Per cent</i> </th>
+                                                                                                                                            <th> <i>Per cent</i> </th>
+                                                                                                                                                            <th> <i>Per cent</i> </th></tr>
+<tr>                                                                                                                         <th colspan="4"> Untreated wood = 100% </th></tr>
+<tr><th rowspan="7"> Steam, at pressures various </th>           <td> 4 </td> <td><br></td> <td> 230[a] </td><td> 91.3 </td> <td> 79.1 </td><td> 96.4 </td> <td> 88.9 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 10 </td> <td> 238 </td>   <td> 78.2 </td> <td> 93.7 </td><td> 93.3 </td> <td> 88.4 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 20 </td> <td> 253 </td>   <td> 83.3 </td> <td> 84.2 </td><td> 91.4 </td> <td> 80.8 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 30 </td> <td> 269 </td>   <td> 80.4 </td> <td> 78.4 </td><td> 89.8 </td> <td> 82.9 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 40 </td> <td> 283 </td>   <td> 78.1 </td> <td> 74.4 </td><td> 74.0 </td> <td> 75.5 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 50 </td> <td> 292 </td>   <td> 75.8 </td> <td> 71.5 </td><td> 63.9 </td> <td> 70.4 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 100 </td><td> 337 </td>   <td> 41.4 </td> <td> 65.0 </td><td> 55.2 </td> <td> 53.9 </td></tr>
+<tr><th rowspan="8" colspan="1"> Steam, for various periods </th><td> 1 </td> <td> 20 </td> <td> 257 </td>   <td> 100.6 </td><td> 98.6 </td><td> 86.7 </td> <td> 95.3 </td></tr>
+<tr>                                                             <td> 2 </td> <td> 20 </td> <td> 267 </td>   <td> 88.4 </td> <td> 93.0 </td><td> 107.0 </td><td> 96.1 </td></tr>
+<tr>                                                             <td> 3 </td> <td> 20 </td> <td> 260 </td>   <td> 90.0 </td> <td> 93.6 </td><td> 84.1 </td> <td> 89.2 </td></tr>
+<tr>                                                             <td> 4 </td> <td> 20 </td> <td> 253 </td>   <td> 83.3 </td> <td> 84.2 </td><td> 91.4 </td> <td> 86.3 </td></tr>
+<tr>                                                             <td> 5 </td> <td> 20 </td> <td> 253 </td>   <td> 85.0 </td> <td> 78.1 </td><td> 84.2 </td> <td> 82.4 </td></tr>
+<tr>                                                             <td> 6 </td> <td> 20 </td> <td> 242 </td>   <td> 95.2 </td> <td> 89.8 </td><td> 76.0 </td> <td> 87.0 </td></tr>
+<tr>                                                             <td> 10 </td><td> 20 </td> <td> 255 </td>   <td> 73.7 </td> <td> 82.0 </td><td> 76.0 </td> <td> 77.2 </td></tr>
+<tr>                                                             <td> 20 </td><td> 20 </td> <td> 258 </td>   <td> 67.5 </td> <td> 65.0 </td><td> 99.0 </td> <td> 77.2 </td></tr>
+<tr><td style="text-align: left" rowspan="1" colspan="8"> [Footnote a: It will be noted that
+the temperature was 230&deg;. This is the maximum temperature by the
+maximum-temperature&nbsp;recording thermometer, and is due to the
+handling of the exhaust valve. The average temperature was that
+of&nbsp;exhaust
+steam.] </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>"(3) A high degree of steaming is injurious to wood in strength
+and spike-holding power. The degree of steaming at which
+pronounced harm results will depend upon the quality of the wood
+and its degree of seasoning, and upon the pressure (temperature)
+of steam and the duration of its application. For loblolly pine
+the limit of safety is certainly 30 pounds for 4 hours, or 20
+pounds for 6 hours."<a name="fn52r"></a><a href="#fn52"><sup><small>52</small></sup></a></p>
+
+<p>Experiments made at the Yale Forest School showed that steaming
+above 30 pounds' gauge pressure reduces the strength of wood
+permanently while wet from 25 to 75 per cent.</p>
+
+<a name="II_18"></a>
+<h3>PRESERVATIVES</h3>
+
+<p>The exact effects of chemical impregnation upon the mechanical
+properties of wood have not been fully determined, though they
+have been the subject of considerable investigation.<a name="fn53r"></a><a href="#fn53"><sup><small>53</small></sup></a> More
+<a name="pg087"></a>
+depends upon the method of treatment than upon the preservatives
+used. Thus preliminary steaming at too high pressure or for too
+long a period will materially weaken the wood, (<a href="#II_17">See Tempurature,
+<i>supra</i></a>.)</p>
+
+<p>The presence of zinc chloride does not weaken wood under static
+loading, although the indications are that the wood becomes
+brittle under impact. If the solution is too strong it will
+decompose the wood.</p>
+
+<p>Soaking in creosote oil causes wood to swell, and accordingly
+decreases the strength to some extent, but not nearly so much so
+as soaking in water.<a name="fn54r"></a><a href="#fn54"><sup><small>54</small></sup></a></p>
+
+<p>Soaking in kerosene seems to have no significant weakening
+effect.<a name="fn55r"></a><a href="#fn55"><sup><small>55</small></sup></a></p>
+
+<a name="pg088"></a>
+<a name="PART_III"></a>
+<h2>PART III<br>TIMBER TESTING<a name="fn56r"></a><a href="#fn56"><sup><small>56</small></sup></a></h2>
+
+<a name="III_01"></a>
+<h3>WORKING PLAN</h3>
+
+<p>Preliminary to making a series of timber tests it is very
+important that a working plan be prepared as a guide to the
+investigation. This should embrace: (1) the purpose of the
+tests; (2) kind, size, condition, and amount of material needed;
+(3) full description of the system of marking the pieces; (4)
+details of any special apparatus and methods employed; (5)
+proposed method of analyzing the data obtained and the nature of
+the final report. Great care should be taken in the preparation
+of this plan in order that all problems arising may be
+anticipated so far as possible and delays and unnecessary work
+avoided. A comprehensive study of previous investigations along
+the same or related lines should prove very helpful in outlining
+the work and preparing the report. (For sample working plan <a href="#APPENDIX">see
+Appendix</a>.)</p>
+
+<a name="III_02"></a>
+<h3>FORMS OF MATERIAL TESTED</h3>
+
+<p>In general, four forms of material are tested, namely: (1) large
+timbers, such as bridge stringers, car sills, large beams, and
+other pieces five feet or more in length, of actual sizes and
+grades in common use; (2) built-up structural forms and
+fastenings, such as built-up beams, trusses, and various kind of
+joints; (3) small clear pieces, such as are used in compression,
+shear, cleavage, and small cross-breaking tests; (4)
+manufactured articles, such as axles, spokes, shafts,
+wagon-tongues, cross-arms, insulator pins, barrels, and packing
+boxes.</p>
+
+<p>As the moisture content is of fundamental importance (<a href="#II_16">see Water Content</a>,
+pages 75-84.), all standard tests are usually <a name="pg089"></a> made in the
+green condition. Another series is also usually run in an
+air-dry condition of about 12 per cent moisture. In all cases
+the moisture is very carefully determined and stated with the
+results in the tables.</p>
+
+<a name="III_03"></a>
+<h3>SIZE OF TEST SPECIMENS</h3>
+
+<p>The size of the test specimen must be governed largely by the
+purpose for which the test is made. If the effect of a single
+factor, such as moisture, is the object of experiment, it is
+necessary to use small pieces of wood in order to eliminate so
+far as possible all disturbing factors. If the specimens are too
+large, it is impossible to secure enough perfect pieces from one
+tree to form a series for various tests. Moreover, the drying
+process with large timbers is very difficult and irregular, and
+requires a long period of time, besides causing checks and
+internal stresses which may obscure the results obtained.</p>
+
+<p>On the other hand, the smaller the dimensions of the test
+specimen the greater becomes the relative effect of the inherent
+factors affecting the mechanical properties. For example, the
+effect of a knot of given size is more serious in a small stick
+than in a large one. Moreover, the smaller the specimen the
+fewer growth rings it contains, hence there is greater
+opportunity for variation due to irregularities of grain.</p>
+
+<p>Tests on large timbers are considered necessary to furnish
+designers data on the probable strength of the different sizes
+and grades of timber on the market; their coefficients of
+elasticity under bending (since the stiffness rather than the
+strength often determines the size of a beam); and the manner of
+failure, whether in bending fibre stress or horizontal shear. It
+is believed that this information can only be obtained by direct
+tests on the different grades of car sills, stringers, and other
+material in common use.</p>
+
+<p>When small pieces are selected for test they very often are
+clear and straight-grained, and thus of so much better grade
+than the large sticks that tests upon them may not yield unit
+values applicable to the larger sizes. Extensive experiments
+show, however, (1) that the modulus of elasticity is
+approximately the <a name="pg090"></a> same for large timbers as for small clear
+specimens cut from them, and (2) that the fibre stress at
+elastic limit for large beams is, except in the weakest timbers,
+practically equal to the crushing strength of small clear pieces
+of the same material.<a name="fn57r"></a><a href="#fn57"><sup><small>57</small></sup></a></p>
+
+<a name="III_04"></a>
+<h3>MOISTURE DETERMINATION</h3>
+
+<p>In order for tests to be comparable, it is necessary to know the
+moisture content of the specimens at the zone of failure. This
+is determined from disks an inch thick cut from the timber
+immediately after testing.</p>
+
+<p>In cases, as in large beams, where it is desirable to know not
+only the average moisture content but also its distribution
+through the timber, the disks are cut up so as to obtain an
+outside, a middle, and an inner portion, of approximately equal
+areas. Thus in a section 10" &times; 12" the outer strip would be one
+inch wide, and the second one a little more than an inch and a
+quarter. Moisture determinations are made for each of the three
+portions separately.</p>
+
+<p>The procedure is as follows:</p>
+
+<p>(1) Immediately after sawing, loose splinters are removed and
+each section is weighed.</p>
+
+<p>(2) The material is put into a drying oven at 100&deg; C. (212&deg; F.)
+and dried until the variation in weight for a period of
+twenty-four hours is less than 0.5 per cent.</p>
+
+<p>(3) The disk is again carefully weighed.</p>
+
+<p>(4) The loss in weight expressed in per cent of the dry weight
+indicates the moisture content of the specimen from which the
+specimen was cut.</p>
+
+<a name="III_05"></a>
+<h3>MACHINE FOR STATIC TESTS</h3>
+
+<p>The standard screw machines used for metal tests are also used
+for wood, but in the case of wood tests the readings must be
+taken "on the fly," and the machine operated at a uniform speed
+without interruption from beginning to end of the test. This is
+on account of the time factor in the strength of wood. (See
+Speed of Testing Machine, <a href="#pg092">page 92</a>.)</p>
+
+<a name="pg091"></a>
+
+<p>The standard machines for static tests can be used for
+transverse bending, compression, tension, shear, and cleavage. A
+common form consists of three main parts, namely: (1) the
+straining mechanism, (2) the weighing apparatus, and (3) the
+machinery for communicating motion to the screws.</p>
+
+<p>The straining mechanism consists of two parts, one of which is a
+movable crosshead operated by four (sometimes two or three)
+upright steel straining screws which pass through openings in
+the platform and bear upward on the bed of the machine upon
+which the weighing platform rests as a fulcrum. At the lower
+ends of these screws are geared nuts all rotated simultaneously
+by a system of gears which cause the movable crosshead to rise
+and fall as desired.</p>
+
+<p>The stationary part of the straining mechanism, which is used
+only for tension and cleavage tests, consists of a steel cage
+above the movable crosshead and rests directly upon the weighing
+platform. The top of the cage contains a square hole into which
+one end of the test specimen may be clamped, the crosshead
+containing a similar clamp for the other end, in making tension
+tests.</p>
+
+<p>For testing long beams a special form of machine with an
+extended platform is used. (<a href="#fig29">See Fig. 29</a>.)</p>
+
+<p>The weighing platform rests upon knife edges carried by primary
+levers of the weighing apparatus, the fulcrum being on the bed
+of the machine, and any pressure upon it is directly transmitted
+through a series of levers to the weighing beam. This beam is
+adjusted by means of a poise running on a screw. In operation
+the beam is kept floating by means of another poise moved back
+and forth by a screw which is operated by a hand wheel or
+automatically. The larger units of stress are read from the
+graduations along the side of the beam, while the intermediate
+smaller weights are observed on the dial on the rear end of the
+beam.</p>
+
+<p>The machine is driven by power from a shaft or a motor and is so
+geared that various speeds are obtainable. One man can operate
+it.</p>
+
+<p>In making tests the operation of the straining screws is always
+downward so as to bring pressure to bear upon the weighing
+platform. For tests in tension and cleavage the specimen is
+placed <a name="pg092"></a> between the top of the stationary cage and the movable
+head and subjected to a pull. For tests in transverse bending,
+compression, and cleavage the specimen is placed between the
+movable head and the platform, and a direct compression force
+applied.</p>
+
+<p>Testing machines are usually calibrated to a portion of their
+capacity before leaving the factory. The delicacy of the
+weighing levers is verified by determining the number of pounds
+necessary to move the beam between the stops while a load of
+1,000 pounds rests on the platform. The usual requirement is
+that ten pounds should accomplish this movement.</p>
+
+<p>The size of machine suitable for compression tests on 2" &times; 2"
+sticks or for 2" &times; 2" beams with 26 to 36-inch span has a
+capacity of 30,000 pounds.</p>
+
+<a name="III_06"></a>
+<h3>SPEED OF TESTING MACHINE</h3>
+
+<p>In instructions for making static tests the rate of application
+of the stress, <i>i.e.</i>, the speed of the machine, is given
+because the strength of wood varies with the speed at which the
+fibres are strained. The speed of the crosshead of the testing
+machine is practically never constant, due to mechanical defects
+of the apparatus and variations in the speed of the motor, but
+so long as it does not exceed 25 per cent the results will not
+be appreciably affected. In fact, a change in speed of 50 per
+cent will not cause the strength of the wood to vary more than 2
+per cent.<a name="fn58r"></a><a href="#fn58"><sup><small>58</small></sup></a></p>
+
+<p>Following are the formul� used in determining the speed of the
+movable head of the machine in inches per minute (n):</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td>(1)</td> <td style="text-align: left;"> For endwise compression </td>        <td>n</td>   <td>=</td>   <td>Z l</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td><br></td><td><br></td><td>Z l<sup>2</sup></td></tr>
+<tr><td>(2)</td> <td style="text-align: left;"> For beams (centre loading) </td>     <td>n</td>   <td>=</td>   <td>------</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td><br></td><td><br></td><td>6h</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td><br></td><td><br></td><td> Z l<sup>2</sup></td></tr>
+<tr><td>(3)</td> <td style="text-align: left;"> For beams (third-point loading) </td><td>n</td>   <td>=</td>   <td>------</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td><br></td><td><br></td><td>5.4 h</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td><br></td><td><br></td><td><br></td></tr>
+<tr><td><br></td><td><br></td>                                                       <td>Z</td>   <td>=</td>   <td style="text-align: left;"> rate of fibre strain per inch of fibre length.</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td>l</td>   <td>=</td>   <td style="text-align: left;"> span of beam or length of compression specimen.</td></tr>
+<tr><td><br></td><td><br></td>                                                       <td>h</td>   <td>=</td>   <td style="text-align: left;"> height of beam.</td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg094"></a>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td colspan="4"> The values commonly used for Z are as follows:</td></tr>
+<tr><td style="text-align: left;"> Bending large beams </td>                     <td> Z </td><td> = </td><td> 0.0007 </td></tr>
+<tr><td style="text-align: left;"> Bending small beams </td>                     <td> Z </td><td> = </td><td> 0.0015 </td></tr>
+<tr><td style="text-align: left;"> Endwise compression-large specimens </td>     <td> Z </td><td> = </td><td> 0.0015 </td></tr>
+<tr><td style="text-align: left;"> Endwise compression-small specimens </td>     <td> Z </td><td> = </td><td> 0.003 </td></tr>
+<tr><td style="text-align: left;"> Right-angled compression-large specimens </td><td> Z </td><td> =</td> <td> 0.007 </td></tr>
+<tr><td style="text-align: left;"> Right-angled compression-small specimens </td><td> Z </td><td> = </td><td> 0.015 </td></tr>
+<tr><td style="text-align: left;"> Shearing parallel to the grain </td>          <td> Z </td><td> = </td><td> 0.015 </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>Example: At what speed should the crosshead move to give the
+required rate of fibre strain in testing a small beam 2" &times; 2" &times;
+30". (Span = 28".) Substituting these values in equation (2)
+above:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td><td><br></td><td> (0.0015 &times; 28<sup><small>2</small></sup>) </td><td><br></td><td><br></td></tr>
+<tr><td>n</td>   <td> = </td> <td> ----------------- </td>                             <td> = </td><td> 0.1 inch per minute. </td></tr>
+<tr><td><br></td><td><br></td><td> (6 &times; 2)</td>                                  <td><br></td><td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>In order that tests may be intelligently compared, it is
+important that account be taken of the speed at which the stress
+was applied. In determining the basis for a ratio between time
+and strength the rate of strain, which is controllable, and not
+the ratio of stress, which is circumstantial, should be used. In
+other words, the rate at which the movable head of the testing
+machine descends and not the rate of increase in the load is to
+be regulated. This ratio, to which the name <i>speed-strength
+modulus</i> has been given, may be expressed as a coefficient
+which, if multiplied into any proportional change in speed, will
+give the proportional change in strength. This ratio is derived
+from empirical curves. (<a href="#TABLE_17">See Table XVII</a>.)</p>
+
+<a name="pg093"></a>
+
+<center>
+<table id="TABLE_17" summary="TABLE_XVII" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th colspan="23"> TABLE XVII </th></tr>
+<tr><th colspan="23"> SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, <i>loc. cit.</i>) </th></tr>
+<tr><td rowspan="1" colspan="23"> (Values in parentheses are approximate) </td></tr>
+<tr><th colspan="2"> Rate of fibre strain.<br>Ten-thousandths inch per minute per inch </th>                    <th colspan="3"> 2/3 </th>                      <th colspan="3"> 2 </th>                        <th colspan="3"> 6 </th>                        <th colspan="3"> 18 </th>                       <th colspan="3"> 54 </th>                       <th colspan="3"> 162 </th>                      <th colspan="3"> 486 </th></tr>
+<tr><th rowspan="4" colspan="1"> COMPRESSION </th><th> Speed of crosshead.<br>Inches per minute</th><th colspan="3"> 0.000383 </th>                 <th colspan="3"> 0.00115 </th>                  <th colspan="3"> 0.00345 </th>                  <th colspan="3"> 0.0103</th>                    <th colspan="3"> 0.0310</th>                    <th colspan="3"> 0.0931</th>                    <th colspan="3"> .279</th></tr>
+<tr>                                              <th> Specimens </th>                                          <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th></tr>
+<tr>                                              <th> Relative&nbsp;crushing&nbsp;strength</th>                <td><br></td>   <td><br></td>   <td><br></td>   <td> 100.0 </td><td> 100.0 </td><td> 100.0 </td><td> 103.4 </td><td> 100.8 </td><td> 101.5 </td><td> 107.5 </td><td> 102.7 </td><td> 103.8 </td><td> 113.9 </td><td> 105.5 </td><td> 107.9 </td><td> 121.3 </td><td> 108.3 </td><td> 116.4 </td><td> 128.8 </td><td> 110.0 </td><td> 118.9 </td></tr>
+<tr>                                              <th> Speed-strength modulus, <i>T</i><br></th>                <td><br></td>   <td><br></td>   <td><br></td>   <td> 0.017 </td><td>(0.006)</td><td>(0.009)</td><td> 0.033 </td><td> 0.012 </td><td> 0.016 </td><td> 0.047 </td><td> 0.021 </td><td> 0.029 </td><td> 0.053 </td><td> 0.027 </td><td> 0.039 </td><td> 0.060 </td><td> 0.023 </td><td> 0.049 </td><td>(0.052)</td><td>(0.015)</td><td>(0.040)</td></tr>
+<tr><th rowspan="4"> BENDING </th>                <th> Speed of crosshead.<br>Inches per minute </th>              <th colspan="3"> 0.0072 </th>                   <th colspan="3"> 0.0216 </th>                   <th colspan="3"> 0.0648 </th>                   <th colspan="3"> 0.194 </th>                    <th colspan="3"> 0.583 </th>                    <th colspan="3"> 1.75 </th>                     <th colspan="3"> 5.25 </th></tr>
+<tr>                                              <th> Specimens </th>                                          <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th>  <th> Wet </th>  <th> Dry </th>  <th> All </th></tr>
+<tr>                                              <th> Relative crushing strength </th>                         <td> 97.4 </td> <td> 99.0 </td> <td> 98.2 </td> <td> 100.0 </td><td> 100.0 </td><td> 100.0 </td><td> 105.1 </td><td> 102.1 </td><td> 103.7 </td><td> 111.3 </td><td> 105.8 </td><td> 108.1 </td><td> 117.9 </td><td> 108.6 </td><td> 112.7 </td><td> 123.7 </td><td> 109.6 </td><td> 116.3 </td><td> 126.3 </td><td> 110.3 </td><td> 118.9 </td></tr>
+<tr>                                              <th> Speed-strength modulus, <i>T</i><br></th>                <td>(0.014)</td><td>(0.005)</td><td> 0.012 </td><td> 0.033 </td><td> 0.014 </td><td> 0.026 </td><td> 0.049 </td><td> 0.026 </td><td> 0.037 </td><td> 0.053 </td><td> 0.033 </td><td> 0.038 </td><td> 0.049 </td><td> 0.014 </td><td> 0.035 </td><td> 0.038 </td><td> 0.006 </td><td> 0.025 </td><td>(0.023)</td><td>(0.004)</td><td>(0.014)</td></tr>
+<tr align="left"><td rowspan="1" colspan="23"> NOTE.&mdash;The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_07"></a>
+<h3>BENDING LARGE BEAMS</h3>
+
+<p><i>Apparatus</i>: A static bending machine (described above), with a
+special crosshead for third-point loading and a long platform
+bearing knife-edge supports, is required. (<a href="#fig29">See Fig. 29</a>.)</p>
+
+<p class="ctr"><img src="images/fig29.jpg" alt="Figure 29" id="fig29" /></p>
+<p class="imgtitle">Figure 29</p>
+<p class="imgtext"> Static bending test on large beam. Note
+arrangement of wire and scale for measuring deflection; also
+method of applying load at "third-points."</p>
+
+<p><i>Preparing the material</i>: Standard sizes and grades of beams and
+timbers in common use are employed. The ends are roughly squared
+and the specimen weighed and measured, taking the
+cross-sectional dimensions midway of the length. Weights should
+be to the nearest pound, lengths to the nearest 0.1 inch, and
+cross-sectional dimensions to the nearest 0.01 inch.</p>
+
+<p><i>Marking and sketching</i>: The butt end of the beam is marked <a name="pg095"></a> <i>A</i>
+and the top end <i>B</i>. While facing <i>A</i>, the top side is marked
+<i>a</i>, the right hand <i>b</i>, the bottom <i>c</i>, the left hand <i>d</i>.
+Sketches are made of each side and end, showing (1) size,
+location, and condition of knots, checks, splits, and other
+defects; (2) irregularities of grain; (3) distribution of
+heartwood and sapwood; and on the ends: (4) the location of the
+pith and the arrangement of the growth rings, (5) number of
+rings per inch, and (6) the proportion of late wood.</p>
+
+<p>The number of rings per inch and the proportion of late wood
+should always be determined along a radius or a line normal to
+the rings. The average number of rings per inch is the total
+number of rings divided by the length of the line crossing them.
+The proportion of late wood is equal to the sum of the widths of
+the late wood crossed by the line, divided by the length of the
+line. Rings per inch should be to the nearest 0.1; late wood to
+the nearest 0.1 per cent.</p>
+
+<p>Since in large beams a great variation in rate of growth and
+<a name="pg096"></a>
+relative amount of late wood is likely in different parts of the
+section, it is advisable to consider the cross section in three
+volumes, namely, the upper and lower quarters and the middle
+half. The determination should be made upon each volume
+separately, and the average for the entire cross section
+obtained from these results.</p>
+
+<p>At the conclusion of the test the failure, as it appears on each
+surface, is traced on the sketches, with the failures numbered
+in the order of their occurrence. If the beam is subsequently
+cut up and used for other tests an additional sketch may be
+desirable to show the location of each piece.</p>
+
+<p><i>Adjusting specimen in machine</i>: The beam is placed in the
+machine with the side marked <i>a</i> on top, and with the ends
+projecting equally beyond the supports. In order to prevent
+crushing of the fibre at the points where the stress is applied
+it is necessary to use bearing blocks of maple or other hard
+wood with a convex surface in contact with the beam. Roller
+bearings should be placed between the bearing blocks and the
+knife edges of the crosshead to allow for the shortening due to
+flexure. (<a href="#fig29">See Fig. 29</a>.) Third-point loading is used, that is,
+the load is applied at two points one-third the span of the beam
+apart. (<a href="#fig30">See Fig. 30</a>.) This affords a uniform bending moment
+throughout the central third of the beam.</p>
+
+<p class="ctr"><img src="images/fig30.jpg" alt="Figure 30" id="fig30" /></p>
+<p class="imgtitle">Figure 30</p>
+<p class="imgtext"> Two methods of loading a beam, namely,
+third-point loading (upper), and centre loading (lower).</p>
+
+<p><i>Measuring the deflection</i>: The method of measuring the
+deflection should be such that any compression at the points of
+support or at the application of the load will not affect the
+reading. This may be accomplished by driving a small nail near
+each end of the beam, the exact location being on the neutral
+plane and vertically above each knife-edge support. Between
+these nails a fine wire is stretched free of the beam and kept
+taut by means of a rubber band or coiled spring on one end.
+Behind the wire at a point on the beam midway between the
+supports a steel scale graduated to hundredths of an inch is
+fastened vertically by means of thumb-tacks or small screws
+passing through holes in it. Attachment should be made on the
+neutral plane.</p>
+
+<p>The first reading is made when the scale beam is balanced at
+zero load, and afterward at regular increments of the load which
+is applied continuously and at a uniform speed. (See
+Speed of <a name="pg097"></a> Testing Machine, <a href="#pg092">page 92</a>.)
+If desired, however, the load may be
+read at regular increments of deflection. The deflection
+readings should be to the nearest 0.01 inch. To avoid error due
+to parallax, the readings may be taken by means of a reading
+telescope about ten feet distant and approximately on a level
+with the wire. A mirror fastened to the scale will increase the
+accuracy of the readings if the telescope is not used. As in all
+tests on timber, the strain must be continuous to rupture, not
+intermittent, and readings must be taken "on the fly." The
+weighing beam is kept balanced after the yield point is reached
+and the maximum load, and at least one point beyond it, noted.</p>
+
+<p><i>Log of the test</i>: The proper log sheet for this test consists
+of a piece of cross-section paper with space at the margin for
+notes. (<a href="#fig32">See Fig. 32</a>.) The load in some convenient unit (1,000 to
+10,000 pounds, depending upon the dimensions of the specimen) is
+entered on the ordinates, the deflection in tenths of an inch on
+the absciss�. The increments of load should be chosen so as to
+<a name="pg098"></a>
+furnish about ten points on the stress-strain diagram below the
+elastic limit.</p>
+
+<p>As the readings of the wire on the scale are made they are
+entered directly in their proper place on the cross-section
+paper. In many cases a test should be continued until complete
+failure results. The points where the various failures occur are
+indicated on the stress-strain diagram. A brief description of
+the failure is made on the margin of the log sheet, and the form
+traced on the sketches.</p>
+
+<p><i>Disposal of the specimen</i>: Two one-inch sections are cut from
+the region of failure to be used in determining the moisture
+content. (<a href="#III_04">See Moisture Determination, page 90</a>.) A two-inch section
+may be cut for subsequent reference and identification, and
+possible microscopic study. The remainder of the beam may be cut
+into small beams and compression pieces.</p>
+
+<p><i>Calculating the results</i>: The formul� used in calculating the
+results of tests on large rectangular simple beams loaded at
+third points of the span are as follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> 0.75 <i>P</i> </td></tr>
+<tr><td> (1) </td><td> <i>J</i> </td>                           <td> = </td><td> --------</td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>b h</i> </td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>l</i> (<i>P</i><sub><small>1</small></sub> + 0.75 <i>W</i>)</td></tr>
+<tr><td> (2) </td><td> <i>r</i> </td>                           <td> = </td><td> --------------------</td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>b h</i><sup><small>2</small></sup></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>l</i> (<i>P</i> + 0.75 <i>W</i>) </td></tr>
+<tr><td> (3) </td><td> <i>R</i> </td>                           <td> = </td><td> ---------------- </td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>b h</i><sup><small>2</small></sup></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> <i>P</i><sub><small>1</small></sub><i>l</i><small><sup>3</sup></small></td></tr>
+<tr><td> (4) </td><td> <i>E</i> </td>                           <td> = </td><td>---------------</td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> 4.7 <i>D b h</i> <small><sup>3</sup></small></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> 0.87 <i>P</i> <sub><small>1</small></sub> <i>D</i> </td></tr>
+<tr><td> (5) </td><td> <i>S</i> </td>                           <td> = </td><td>--------------</td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td> 2 <i>V</i> </td></tr>
+<tr><td><br></td> <td><br></td>                                 <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td> <i>b, h, l</i></td>                      <td> = </td><td style="text-align: left;"> breadth, height, and span of specimen, inches. </td></tr>
+<tr><td><br></td> <td> <i>D</i> </td>                           <td> = </td><td style="text-align: left;"> total deflection at elastic limit, inches. </td></tr>
+<tr><td><br></td> <td> <i>P</i> </td>                           <td> = </td><td style="text-align: left;"> maximum load, pounds. </td></tr>
+<tr><td><br></td> <td> <i>P</i><sub><small>1</small></sub> </td><td> = </td><td style="text-align: left;"> load at elastic limit, pounds. </td></tr>
+<tr><td><br></td> <td> <i>E</i> </td>                           <td> = </td><td style="text-align: left;"> modulus of elasticity, pounds per square inch. </td></tr>
+<tr><td><br></td> <td> <i>r</i> </td>                           <td> = </td><td style="text-align: left;"> fibre stress at elastic limit, pounds per sq. inch. </td></tr>
+<tr><td><br></td> <td> <i>R</i> </td>                           <td> = </td><td style="text-align: left;"> modulus of rupture, pounds per square inch. </td></tr>
+<tr><td><br></td> <td> <i>S</i> </td>                           <td> = </td><td style="text-align: left;"> elastic resilience or work to elastic limit, inch-pounds per cu. in. </td></tr>
+<tr><td><br></td> <td> <i>J</i> </td>                           <td> = </td><td style="text-align: left;"> greatest calculated longitudinal shear, pounds per square inch. </td></tr>
+<tr><td><br></td> <td> <i>V</i> </td>                           <td> = </td><td style="text-align: left;"> volume of beam, cubic inches. </td></tr>
+<tr><td><br></td> <td> <i>W</i> </td>                           <td> = </td><td style="text-align: left;"> weight of the beam. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg099"></a>
+
+<p>In large beams the weight should be taken into account in
+calculating the fibre stress. In (2) and (3) three-fourths of
+the weight of the beam is added to the load for this reason.</p>
+
+<a name="III_08"></a>
+<h3>BENDING SMALL BEAMS</h3>
+
+<p><i>Apparatus</i>: An ordinary static bending machine, a steel I-beam
+bearing two adjustable knife-edge supports to rest on the
+platform, and a special deflectometer, are required. (<a href="#fig31">See Fig.
+31</a>.)</p>
+
+<p class="ctr"><img src="images/fig31.jpg" alt="Figure 31" id="fig31" /></p>
+<p class="imgtitle">Figure 31</p>
+<p class="imgtext"> Static bending test on small beam. Note
+the use of the deflectometer with indicator and dial for
+measuring the deflection; also roller bearings between beam and
+supports.</p>
+
+<p><i>Preparing the material</i>: The specimens may be of any convenient
+size, though beams 2" &times; 2" &times; 30" tested over a 28-inch span, are
+considered best. The beams are surfaced on all four sides, care
+being taken that they are not damaged by the rollers of the
+surfacing machine. Material for these tests is sometimes cut
+from large beams after failure. The specimens are carefully
+weighed in grams, and all dimensions measured to the nearest
+0.01 inch. If to be tested in a green or fresh condition the
+<a name="pg100"></a>
+specimens should be kept in a damp box or covered with moist
+sawdust until needed. No defects should be allowed in these
+specimens.</p>
+
+<p><i>Marking and sketching</i>: Sketches are made of each end of the
+specimen to show the character of the growth, and after testing,
+the manner of failure is shown for all four sides. In obtaining
+data regarding the rate of growth and the proportion of late
+wood the same procedure is followed as with large beams.</p>
+
+<p><i>Adjusting specimen in machine</i>: The beam should be correctly
+centred in the machine and each end should have a plate with
+roller bearings between it and the support. Centre loading is
+used. Between the movable head of the machine and the specimen
+is placed a bearing block of maple or other hard wood, the lower
+surface of which is curved in a direction along the beam, the
+curvature of which should be slightly less than that of the beam
+at rupture, in order to prevent the edges from crushing into the
+fibres of the test piece.</p>
+
+<p><i>Measuring the deflection</i>: The method of measuring deflection
+of large beams can be used for small sizes, but because of the
+shortness of the span and consequent slight deformation in the
+latter, it is hardly accurate enough for good work. The special
+deflectometer shown in Fig. 31 allows closer reading, as it
+magnifies the deflection ten times. It rests on two small nails
+driven in the beam on the neutral plane and vertically above the
+supports. The fine wire on the wheel at the base of the
+indicator is attached to another small nail driven in the beam
+on the neutral plane midway between the end nails. All three
+nails should be in place before the beam is put into the
+machine. The indicator is adjustable by means of a thumb-screw
+at the base and is set at zero before the load is applied.
+Deflections are read to the nearest 0.001 inch. For rate of
+application of load see Speed of Testing Machine, <a href="#pg092">page 92</a>. The
+speed should be uniform from start to finish without stopping.
+Readings must be made "on the fly."</p>
+
+<p><i>Log of the test</i>: The log sheets used for small beams (see Fig.
+32) are the same as for large sizes and the procedure is
+practically identical. The stress-strain diagram is continued to
+or beyond the maximum load, and in a portion of the tests should
+be continued to six-inch deflection or until the specimen fails
+to support a load of 200 pounds. Deflection readings for equal
+increments of <a name="pg102"></a> load are taken until well beyond the elastic
+limit, after which the scale beam is kept balanced and the load
+read for each 0.1 inch deflection. The load and deflection at
+first failure, the maximum load, and any points of sudden change
+should be shown on the diagram, even though they do not occur at
+one of the regular points. A brief description of the failure
+and the nature of any defects is entered on the log sheet.</p>
+
+<a name="pg101"></a>
+
+<p class="ctr"><img src="images/fig32.jpg" alt="Figure 32" id="fig32" /></p>
+<p class="imgtitle">Figure 32</p>
+<p class="imgtext"> Sample log sheet, giving full details
+of a transverse bending test on a small pine beam.</p>
+
+<p><i>Calculating the results</i>: The formul� used in calculating the
+results of tests on small rectangular simple beams are as
+follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 0.75 <i>P</i> </td></tr>
+<tr><td> (1) </td><td> <i>J</i> </td><td> = </td> <td> -------- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>b h</i> </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 1.5 <i>P</i><sub><small>1</small></sub> <i>l</i> </td></tr>
+<tr><td> (2) </td><td> <i>r</i> </td><td> = </td> <td> ------------ </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>b h</i><sup><small>2</small></sup></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 1.5 <i>P l</i> </td></tr>
+<tr><td> (3) </td><td> <i>R</i> </td><td> = </td> <td> --------- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>b h</i><sup><small>2</small></sup></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P</i><sub><small>1</small></sub> <i>l</i> <sup><small>3</small></sup><br></td></tr>
+<tr><td> (4) </td><td> <i>E</i> </td><td> = </td> <td> ------------ </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 4 <i>D b h</i><sup><small>3</small></sup></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P</i><sub><small>1</small></sub> <i>D</i> </td></tr>
+<tr><td> (5) </td><td> <i>S</i> </td><td> = </td> <td> --------- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 2 <i>V</i> </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>The same legend is used as on <a href="#pg098">page 98</a>. The weight of
+the beam itself is disregarded.</p>
+
+<a name="III_09"></a>
+<h3>ENDWISE COMPRESSION</h3>
+
+<p><i>Apparatus</i>: An ordinary static testing machine and a
+compressometer are required. (<a href="#fig33">See Fig. 33</a>.)</p>
+
+<p class="ctr"><img src="images/fig33.jpg" alt="Figure 33" id="fig33" /></p>
+<p class="imgtitle">Figure 33</p>
+<p class="imgtext"> Endwise compression test, showing
+method of measuring the deformation by means of a
+compressometer.</p>
+
+<p><i>Preparing the material</i>: Two classes of specimens are commonly
+used, namely, (1) posts 24 inches in length, and (2) small clear
+blocks approximately 2" &times; 2" &times; 8". The specimens are surfaced on
+all four sides and both ends squared smoothly and evenly. They
+are carefully weighed, measured, rate of growth and proportion
+of late wood determined, as in bending tests. After the test a
+moisture section is cut and weighed. Ordinarily these specimens
+should be free from defects.</p>
+
+<p><i>Sketching</i>: Sketches are made of each end of the specimens to
+show the character of the growth. After testing, the manner of
+failure is shown for all four sides, and the various parts of
+the failure are numbered in the order of their occurrence.</p>
+
+<p><i>Adjusting specimen in machine</i>: The compressometer collars are
+adjusted, the distance between them being 20 inches for the
+posts and 6 inches for the blocks. If the two ends of the blocks
+are not exactly parallel a ball-and-socket block can be placed
+between <a name="pg103"></a> the upper end of the specimen and the movable head of
+the machine to overcome the irregularity. If the blocks are true
+they can simply be stood on end upon the platform and the
+movable head allowed to press directly upon the upper end.</p>
+
+<p><i>Measuring the deformation</i>: The deformation is measured by a
+compressometer. (<a href="#fig33">See Fig. 33</a>.) The latter registers to 0.001
+inch. In the case of posts the compression between the collars
+is communicated to the four points on the arms by means of brass
+rods; with short blocks, as in <a href="#fig33">Fig. 33</a>, the points of the arms
+are in direct contact with the collars. The operator lowers the
+fulcrum of the apparatus by moving the micrometer screws at such
+a rate that the set-screw in the rear end of the upper lever is
+kept barely touching the fixed arm below it, being guided by a
+bell operated by electric contact.</p>
+
+<p><i>Log of the test</i>: The load is applied continuously at a uniform
+rate of speed. (See Speed of Testing Maching, <a href="#pg092">page 92</a>.) <a name="pg104"></a> Readings
+are taken from the scale of the compressometer at regular
+increments of either load or compression. The stress-strain
+diagram is continued to at least one deformation point beyond
+the maximum load, and in event of sudden failure, the direction
+of the curve beyond the maximum point is indicated. A brief
+description of the failure is entered on the log sheet. (<a href="#fig34">See
+Fig. 34</a>.)</p>
+
+<a name="pg105"></a>
+
+<p class="ctr"><img src="images/fig34.jpg" alt="Figure 34" id="fig34" /></p>
+<p class="imgtitle">Figure 34</p>
+<p class="imgtext"> Sample log sheet of an endwise
+compression test on a short pine column.</p>
+
+<p>In short specimens the failure usually occurs in one or several
+planes diagonal to the axis of the specimen. If the ends are
+more moist than the middle a crushing may occur on the extreme
+ends in a horizontal plane. Such a test is not valid and should
+always be culled. If the grain is diagonal or the stress is
+unevenly applied a diagonal shear may occur from top to bottom
+of the test specimen. Such tests are also invalid and should be
+culled. When the plane (or several planes) of failure occurs
+through the body of the specimen the test is valid. It may
+sometimes be advantageous to allow the extreme ends to dry
+slightly before testing in order to bring the planes of failure
+within the body. This is a perfectly legitimate procedure
+provided no drying is allowed from the sides of the specimen,
+and the moisture disk is cut from the region of failure.</p>
+
+<p><i>Calculating the results:</i> The formul� used in calculating the
+results of tests on endwise compression are as follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P</i> </td></tr>
+<tr><td> (1) </td><td> <i>C</i> </td><td> = </td><td> ----- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>A</i> </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P</i><sub><small>1</small></sub></td></tr>
+<tr><td> (2) </td><td> <i>c</i> </td><td> = </td><td> ------- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>A</i> </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P</i><sub><small>1</small></sub> <i>l</i> </td></tr>
+<tr><td> (3) </td><td> <i>E</i> </td><td> = </td><td> --------- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>A D</i> </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> <i>P D</i> </td></tr>
+<tr><td> (4) </td><td> <i>S</i> </td><td> = </td><td> ----- </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td> 2 <i>V</i> </td></tr>
+<tr><td><br></td> <td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br></td> <td> <i>C</i> </td><td> = </td><td style="text-align: left;"> crushing strength, pounds per square inch. </td></tr>
+<tr><td><br></td> <td> <i>c</i> </td><td> = </td><td style="text-align: left;"> fibre strength at elastic limit, pounds per square inch. </td></tr>
+<tr><td><br></td> <td> <i>A</i> </td><td> = </td><td style="text-align: left;"> area of cross section, square inches. </td></tr>
+<tr><td><br></td> <td> <i>l</i> </td><td> = </td><td style="text-align: left;"> distance between centres of collars, inches. </td></tr>
+<tr><td><br></td> <td> <i>D</i> </td><td> = </td><td style="text-align: left;"> total shortening at elastic limit, inches. </td></tr>
+<tr><td><br></td> <td> <i>V</i> </td><td> = </td><td style="text-align: left;"> volume of specimen, cubic inches. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>Remainder of legend as on <a href="#pg098">page 98</a>.</p>
+
+<a name="III_10"></a>
+<h3>COMPRESSION ACROSS THE GRAIN</h3>
+
+<p><i>Apparatus</i>: An ordinary static testing machine, a bearing
+plate, and a deflectometer are required. (<a href="#fig35">See Fig. 35</a>.)</p>
+
+<p class="ctr"><img src="images/fig35.jpg" alt="Figure 35" id="fig35" /></p>
+<p class="imgtitle">Figure 35</p>
+<p class="imgtext"> Compression across the grain. Note
+method of measuring the deformation by means of a
+deflectomoter.</p>
+
+<a name="pg106"></a>
+
+<p><i>Preparing the material</i>: Two classes of specimens are used,
+namely, (1) sections of commercial sizes of ties, beams, and
+other timbers, and (2) small, clear specimens with the length
+several times the width. Sometimes small cubes are tested, but
+the results are hardly applicable to conditions in practice. In
+(2) the sides are surfaced and the ends squared. The specimens
+are then carefully measured and weighed, defects noted, rate of
+growth and proportion of late wood determined, as in bending
+tests. (<a href="#pg095">See page 95</a>.) After the test a
+moisture section is cut and weighed.</p>
+
+<p><i>Sketching</i>: Sketches are made as in endwise compression tests.
+(<a href="#pg102">See page 102</a>.)</p>
+
+<p><i>Adjusting specimen in machine</i>: The specimen is laid
+horizontally upon the platform of the machine and a steel
+bearing plate placed on its upper surface immediately beneath
+the centre of the movable head. For the larger specimens this
+plate is six inches wide; for the smaller sizes, two inches
+wide. The plate in all cases projects over the edges of the test
+piece, and in no case should the length of the latter be less
+than four times the width of the plate.</p>
+
+<a name="pg107"></a>
+
+<p><i>Measuring the deformation</i>: The compression is measured by
+means of a deflectometer (<a href="#fig35">see Fig. 35</a>), which, after the first
+increment of load is applied, is adjusted (by means of a small
+set screw) to read zero. The actual downward motion of the
+movable head (corresponding to the compression of the specimen)
+is multiplied ten times on the scale from which the readings are
+made.</p>
+
+<p><i>Log of the test</i>: The load is applied continuously and at
+uniform speed (see Speed of Testing Machine, <a href="#pg092">page 92</a>), until well
+beyond the elastic limit. The compression readings are taken at
+regular load increments and entered on the cross-section paper
+in the usual way. Usually there is no real maximum load in this
+case, as the strength continually increases as the fibres are
+crushed more compactly together.</p>
+
+<p><i>Calculating the results</i>: Ordinarily only the fibre stress at
+the elastic limit (<i>c</i>) is computed. It is equal to the load at
+elastic limit (P<sub><small>1</small></sub>) divided by the area under the plate (<i>B</i>).</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td>       <td><br></td><td> <i>P</i><sub><small>1</small></sub> </td><td rowspan="3"> <big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td> <i>c</i> </td> <td> = </td> <td> ------- </td></tr>
+<tr>                                                                  <td><br></td>       <td><br></td><td> <i>B</i> </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_11"></a>
+<h3>SHEAR ALONG THE GRAIN</h3>
+
+<p><i>Apparatus</i>: An ordinary static testing machine and a special
+tool designed for producing single shear are required. (<a href="#fig36">See
+Figs. 36</a> and <a href="#fig37">37</a>.) This shearing apparatus consists of a solid
+steel frame with set screws for clamping the block within it
+firmly in a vertical position. In the centre of the frame is a
+vertical slot in which a square-edged steel plate slides freely.
+When the testing block is in position, this plate impinges
+squarely along the upper surface of the tenon or lip, which, as
+vertical pressure is applied, shears off.</p>
+
+<p class="ctr"><img src="images/fig36.jpg" alt="Figure 36" id="fig36" /></p>
+<p class="imgtitle">Figure 36</p>
+<p class="imgtext"> Vertical section of shearing tool.</p>
+
+<p class="ctr"><img src="images/fig37.jpg" alt="Figure 37" id="fig37" /></p>
+<p class="imgtitle">Figure 37</p>
+<p class="imgtext"> Front view of shearing tool with test
+specimen and steel plate in position for testing.</p>
+
+<p><i>Preparing the material</i>: The specimens are usually in the form
+of small, clear, straight-grained blocks with a projecting tenon
+or lip to be sheared off. Two common forms and sizes are shown
+in Figure 38. Part of the blocks are cut so that the shearing
+surface is parallel to the growth rings, or tangential; others
+at <a name="pg109"></a> right angles to the growth rings, or radial. It is important
+that the upper surface of the tenon or lip be sawed exactly
+parallel to the base of the block. When the form with a tenon is
+used the under cut is extended a short distance horizontally
+into the block to prevent any compression from below.</p>
+
+<a name="pg108"></a>
+
+<p class="ctr"><img src="images/fig38.jpg" alt="Figure 38" id="fig38" /></p>
+<p class="imgtitle">Figure 38</p>
+<p class="imgtext"> Two forms of shear test specimens.</p>
+
+<p>In designing a shearing specimen it is necessary to take into
+consideration the proportions of the area of shear, since, if
+the length of the portion to be sheared off is too great in the
+direction of the shearing face, failure would occur by
+compression before the piece would shear. Inasmuch as the
+endwise compressive strength is sometimes not more than five
+times the shearing strength, the shearing surface should be less
+than five times the surface to which the load is applied. This
+condition is fulfilled in the specimens illustrated.</p>
+
+<p>Shearing specimens are frequently cut from beams after testing.
+In this case the specific gravity (dry), proportion of late
+wood, and rate of growth are assumed to be the same as already
+<a name="pg110"></a>
+recorded for the beams. In specimens not so taken, these
+quantities are determined in the usual way. The sheared-off
+portion is used for a moisture section.</p>
+
+<p><i>Adjusting specimen in machine</i>: The test specimen is placed in
+the shearing apparatus with the tenon or lip under the sliding
+plate, which is centred under the movable head of the machine.
+(<a href="#fig39">See Fig. 39</a>.) In order to reduce to a minimum the friction due
+to the lateral pressure of the plate against the bearings of the
+slot, the apparatus is sometimes placed upon several parallel
+steel rods to form a roller base. A slight initial load is
+applied to take up the lost motion of the machinery, and the
+beam balanced.</p>
+
+<p class="ctr"><img src="images/fig39.jpg" alt="Figure 39" id="fig39" /></p>
+<p class="imgtitle">Figure 39</p>
+<p class="imgtext"> Making a shearing test.</p>
+
+<p><i>Log of the test</i>: The load is applied continuously and at a
+uniform rate until failure, but no deformations are measured.
+The points noted are the maximum load and the length of time
+required to reach it. Sketches are made of the failure. If the
+failure is not pure shear the test is culled.</p>
+
+<p>The shearing strength per square inch is found by dividing the
+maximum load by the cross-sectional area.</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td rowspan="3"><big><big><big><big>(</big></big></big></big></td><td><br></td>      <td><br></td><td> <i>P</i> </td><td rowspan="3"> <big><big><big><big>)</big></big></big></big></td></tr>
+<tr>                                                                  <td> <i>Q</i> </td><td> = </td>   <td> --- </td></tr>
+<tr>                                                                  <td><br></td>      <td><br></td><td> <i>A</i> </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_12"></a>
+<h3>IMPACT TEST</h3>
+
+<p><i>Apparatus</i>: There are several types of impact testing
+machines.<a name="fn59r"></a><a href="#fn59"><sup><small>59</small></sup></a> One of the simplest and most efficient for use
+with wood is illustrated in <a href="#fig40">Figure 40</a>. The base of the machine
+is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500
+pounds. Two upright columns, each 8 feet long, act as guides for
+the striking head. At the top of the column is the hoisting
+mechanism for raising or lowering the striking weights. The
+power for operating the machine is furnished by a motor set on
+the top. The hoisting-mechanism is all controlled by a single
+operating lever, shown on the side of the column, whereby the
+striking weight may be raised, lowered, or stopped at the will
+of the operator. There is an automatic safety device for
+stopping the machine when the weight reaches the top.</p>
+
+<a name="pg111"></a>
+
+<p class="ctr"><img src="images/fig40.jpg" alt="Figure 40" id="fig40" /></p>
+<p class="imgtitle">Figure 40</p>
+<p class="imgtext"> Impact testing machine.</p>
+
+<p>The weight is lifted by a chain, one end of which passes over a
+<a name="pg112"></a>
+sprocket wheel in the hoisting mechanism. On the lower end of
+the chain is hung an electro-magnet of sufficient magnetic
+strength to support the heaviest striking weights. When it is
+desired to drop the striking weight the electric current is
+broken and reversed by means of an automatic switch and current
+breaker. The height of drop may be regulated by setting at the
+desired height on one of the columns a tripping pin which throws
+the switch on the magnet and so breaks and reverses the current.</p>
+
+<p>There are four striking weights, weighing respectively 50, 100,
+250, and 500 pounds, any one of which may be used, depending
+upon the desired energy of blow. When used for compression tests
+a flat steel head six inches in diameter is screwed into the
+lower end of the weight. For transverse tests, a well-rounded
+knife edge is screwed into the weight in place of the flat head.
+Knife edges for supporting the ends of the specimen to be
+tested, are securely bolted to the base of the machine.</p>
+
+<p>The record of the behavior of the specimen at time of impact is
+traced upon a revolving drum by a pencil fixed in the striking
+head. (<a href="#fig41">See Fig. 41</a>.) When a drop is made the pencil comes in
+contact with the drum and is held in place by a spring. The drum
+is revolved very slowly, either automatically or by hand. The
+speed of the drum can be recorded by a pencil in the end of a
+tuning fork which gives a known number of vibrations per second.</p>
+
+<p class="ctr"><img src="images/fig41.jpg" alt="Figure 41" id="fig41" /></p>
+<p class="imgtitle">Figure 41</p>
+<p class="imgtext"> Drum record of impact bending test.</p>
+
+<p>One size of this machine will handle specimens for transverse
+tests 9 inches wide and 6-foot span; the other, 12 inches wide
+and 8-foot span. For compression tests a free fall of about 6.5
+feet may be obtained. For transverse tests the fall is a little
+less, depending upon the size of the specimen.</p>
+
+<p>The machine is calibrated by dropping the hammer upon a copper
+cylinder. The axial compression of the plug is noted. The energy
+used in static tests to produce this axial compression under
+stress in a like piece of metal is determined. The external
+energy of the blow (<i>i.e.</i>, the weight of the hammer &times; the
+height of drop) is compared with the energy used in static tests
+at equal amounts of compression. For instance:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td style="text-align: left;"> Energy delivered, impact test</td><td style="width: 20px;"></td><td style="text-align: right;"> 35,000 inch-pounds</td></tr>
+<tr><td style="text-align: left;"> Energy computed from static test</td><td style="width: 20px;"></td><td style="text-align: right;"> 26,400 inch-pounds</td></tr>
+<tr><td style="text-align: left;"> Efficiency of blow of hammer</td><td style="width: 20px;"></td><td style="text-align: right;"> 75.3 per cent.</td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg113"></a>
+
+<p><i>Preparing the material</i>: The material used in making impact
+tests is of the same size and prepared in the same way as for
+static bending and compression tests. Bending in impact tests is
+more commonly used than compression, and small beams with
+28-inch span are usually employed.</p>
+
+<p><i>Method</i>: In making an impact bending test the hammer is allowed
+to rest upon the specimen and a zero or datum line is drawn. The
+hammer is then dropped from increasing heights and drum records
+taken until first failure. The first drop is one inch and the
+increase is by increments of one inch until a height of ten
+inches is reached, after which increments of two inches are used
+until complete failure occurs or 6-inch deflection is secured.</p>
+
+<a name="pg114"></a>
+
+<p>The 50-pound hammer is used when with drops up to 68 inches it
+is reasonably certain it will produce complete failure or 6-inch
+deflection in the case of all specimens of a species; for all
+other species a 100-pound hammer is used.</p>
+
+<p><i>Results</i>: The tracing on the drum (<a href="#fig41">see Fig. 41</a>) represents the
+actual deflection of the stick and the subsequent rebounds for
+each drop. The distance from the lowest point in each case to
+the datum line is measured and its square in tenths of a square
+inch entered as an abscissa on cross-section paper, with the
+height of drop in inches as the ordinate. The elastic limit is
+that point on the diagram where the square of the deflection
+begins to increase more rapidly than the height of drop. The
+difference between the datum line and the final resting point
+after each drop represents the set the material has received.</p>
+
+<p>The formul� used in calculating the results of impact tests in
+bending when the load is applied at the centre up to the elastic
+limit are as follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br></td><td><br></td><td><br></td><td> 3 <i>W H l</i> </td></tr>
+<tr><td> (1) </td><td> <i>r</i> </td><td> = </td><td> ----------- </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td> <i>D b h</i><sup><small>2</small></sup> </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td><br></td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td> <i>F S l</i><sup><small>2</small></sup> </td></tr>
+<tr><td> (2) </td><td> <i>E</i> </td><td> = </td><td> ----------- </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td> 6 <i>D h</i> </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td><br></td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td> <i>W H</i> </td></tr>
+<tr><td> (3) </td><td> <i>S</i> </td><td> = </td><td> ------- </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td> <i>l b h</i> </td></tr>
+<tr><td><br></td><td><br></td><td><br></td><td><br></td></tr>
+<tr><td><br></td><td> <i>H</i> </td><td> = </td><td style="text-align: left;"> height of drop of hammer, including deflection, inches. </td></tr>
+<tr><td><br></td><td> <i>S</i> </td><td> = </td><td style="text-align: left;"> modulus of elastic resilience, inch-pounds per cubic inch. </td></tr>
+<tr><td><br></td><td> <i>W</i> </td><td> = </td><td style="text-align: left;"> weight of hammer, pounds. </td></tr>
+</tbody>
+</table>
+</center>
+
+<p>Remainder of legend as on <a href="#pg098">page 98</a>.</p>
+
+<a name="III_13"></a>
+<h3>HARDNESS TEST: ABRASION AND INDENTATION</h3>
+
+<p><i>Abrasion</i>: The machine used by the U.S. Forest Service is a
+modified form of the Dorry abrasion machine. (<a href="#fig42">See Fig. 42</a>.) Upon
+the revolving horizontal disk is glued a commercial sandpaper,
+known as garnet paper, which is commonly employed in factories
+in finishing wood.</p>
+
+<a name="pg115"></a>
+
+<p class="ctr"><img src="images/fig42.jpg" alt="Figure 42" id="fig42" /></p>
+<p class="imgtitle">Figure 42</p>
+<p class="imgtext"> Abrasion machine for testing the
+wearing qualities of woods.</p>
+
+<p>A small block of the wood to be tested is fixed in one clamp and
+a similar block of some wood chosen as a standard, as sugar
+maple, at 10 per cent moisture, in the opposite, and held
+against the same zone of sandpaper by a weight of 26 pounds
+each. The size of the section under abrasion for each specimen
+is 2" &times; 2". <a name="pg116"></a> The conditions for wear are the same for both
+specimens. The speed of rotation is 68 revolutions a minute.</p>
+
+<p>The test is continued until the standard specimen is worn a
+specified amount, which varies with the kind of wood under test.
+A comparison of the wear of the two blocks affords a fair idea
+of their relative resistance to abrasion.</p>
+
+<p>Another method makes use of a sand blast to abrade the woods and
+is the one employed in New South Wales.<a name="fn60r"></a><a href="#fn60"><sup><small>60</small></sup></a> The apparatus
+consists essentially of a nozzle through which sand can be
+propelled at a high velocity against the test specimen by means
+of a steam jet.</p>
+
+<p>The wood to be tested is cut into blocks 3" &times; 3" &times; 1', and these
+are weighed to the nearest grain just before placing in the
+apparatus. Steam from the boiler at a pressure of about 43
+pounds per square inch is ejected from a nozzle in such a way
+that particles of fine quartz sand are caught up and thrown
+violently against the block which is being rotated. Only
+superheated steam strikes the block, thus leaving the wood dry.
+The test is continued for two minutes, after which the specimen
+is removed and immediately weighed.</p>
+
+<p>By comparison with the original weight the loss from <a name="pg117"></a> abrasion is
+determined, and by comparison with a certain wood chosen as a
+standard, a coefficient of wear-resistance can be obtained. The
+amount of wear will vary more or less according to the surface
+exposed, and in these tests quarter-sawed material was used with
+the edge grain to the blast.</p>
+
+<p><i>Indentation</i>: The tool used for this test consists of a punch
+with a hemispherical end or steel ball having a diameter of
+0.444 inch, giving a surface area of one-fourth square inch. It
+is fitted with a guard plate, which works loosely until the
+penetration has progressed to a depth of 0.222 inch, whereupon
+it tightens. (<a href="#fig43">See Fig. 43</a>.) The effect is that of sinking a ball
+half its diameter into the specimen. This apparatus is fitted
+into the movable head of the static testing machine.</p>
+
+<p class="ctr"><img src="images/fig43.jpg" alt="Figure 43" id="fig43" /></p>
+<p class="imgtitle">Figure 43</p>
+<p class="imgtext"> Design of tool for testing the hardness
+of woods by indentation.</p>
+
+<p>The wood to be tested is cut square with the grain into
+rectangular blocks measuring 2" &times; 2" &times; 6". A block is placed on
+the platform and the end of the punch forced into the wood at
+the rate of 0.25 inch per minute. The operator keeps moving the
+small handle of the guard plate back and forth until it
+tightens. At this instant the load is read and recorded.</p>
+
+<p>Two penetrations each are made on the tangential and radial
+surfaces, and one on each end of every specimen tested.</p>
+
+<p>In choosing the places on the block for the indentations, effort
+should be made to get a fair average of heartwood and sapwood,
+fine and coarse grain, early and late wood.</p>
+
+<p>Another method of testing by indentation involves the use <a name="pg118"></a> of a
+right-angled cone instead of a ball. For details of this test as
+used in New South Wales see <i>loc. cit.</i>, pp. 86-87.
+
+<a name="III_14"></a>
+<h3>CLEAVAGE TEST</h3>
+
+<p>A static testing machine and a special cleavage testing device
+are required. (<a href="#fig44">See Fig. 44</a>.) The latter consists essentially of
+two hooks, one of which is suspended from the centre of the top
+of the cage, the other extended above the movable head.</p>
+
+<p class="ctr"><img src="images/fig44.jpg" alt="Figure 44" id="fig44" /></p>
+<p class="imgtitle">Figure 44</p>
+<p class="imgtext"> Design of tool for cleavage test.</p>
+
+<p>The specimens are 2" &times; 2" &times; 3.75". At one end a one-inch hole is
+bored, with its centre equidistant from the two sides and 0.25
+inch from the end. (<a href="#fig45">See Fig. 45</a>.) This makes the cross section
+to be tested 2" &times; 3". Some of the blocks are cut radially and
+some tangentially, as indicated in the figure.</p>
+
+<p class="ctr"><img src="images/fig45.jpg" alt="Figure 45" id="fig45" /></p>
+<p class="imgtitle">Figure 45</p>
+<p class="imgtext"> Design of cleavage test specimen.</p>
+
+<p>The free ends of the hooks are fitted into the notch in the end
+of the specimen. The movable head of the machine is then made to
+descend at the rate of 0.25 inch per minute, pulling apart the
+hooks and splitting the block. The maximum load only is taken
+and the result expressed in pounds per square inch of width. A
+piece one-half inch thick is split off parallel to the failure
+and used for moisture determination.
+
+<a name="III_15"></a>
+<h3>TENSION TEST PARALLEL TO THE GRAIN</h3>
+
+<p>Since the tensile strength of wood parallel to the grain is
+greater than the compressive strength, and exceedingly greater
+than the shearing strength, it is very difficult to make
+satisfactory tension tests, as the head and shoulders of the
+test specimen (which is subjected <a name="pg119"></a> to both compression and shear)
+must be stronger than the portion subjected to a pure tensile
+stress.</p>
+
+<p>Various designs of test specimens have been made. The one first
+employed by the Division of Forestry<a name="fn61r"></a><a href="#fn61"><sup><small>61</small></sup></a> was prepared as
+follows: Sticks were cut measuring 1.5" &times; 2.5" &times; 16". The
+thickness at the centre was then reduced to three-eighths of an
+inch by cutting out circular segments with a band saw. This left
+a breaking section of 2.5" &times; 0.375". Care was taken to cut the
+specimen as nearly parallel to the grain as possible, so that
+its failure would occur in a condition of pure tension. The
+specimen was then placed between the plane wedge-shaped steel
+grips of the cage and the movable head of the static machine and
+pulled in two. Only the maximum load was recorded. (<a href="#fig46">See Fig. 46</a>,
+No. 1.)</p>
+
+<p class="ctr"><img src="images/fig46.jpg" alt="Figure 46" id="fig46" /></p>
+<p class="imgtitle">Figure 46</p>
+<p class="imgtext"> Designs of tension test specimens used
+in United States.</p>
+
+<a name="pg120"></a>
+
+<p>The difficulty of making such tests compared with the minor
+importance of the results is so great that they are at present
+omitted by the U.S. Forest Service. A form of specimen is
+suggested, however, and is as follows: "A rod of wood about one
+inch in diameter is bored by a hollow drill from the stick to be
+tested. The ends of this rod are inserted and glued in
+corresponding holes in permanent hardwood wedges. The specimen
+is then submitted to the ordinary tension test. The broken ends
+are punched from the wedges."<a name="fn62r"></a><a href="#fn62"><sup><small>62</small></sup></a> (See Fig. 46, No. 2.)</p>
+
+<p>The form used by the Department of Forestry of New South
+Wales<a name="fn63r"></a><a href="#fn63"><sup><small>63</small></sup></a> is as shown in Fig. 47. The specimen has a total
+length of 41 inches and is circular in cross section. On each
+end is a head 4 inches in diameter and 7 inches long. Below each
+head is a shoulder 8.5 inches long, which tapers from a diameter
+of 2.75 inches to 1.25 inches. In the middle is a cylindrical
+portion 1.25 inches in diameter and 10 inches long.</p>
+
+<p class="ctr"><img src="images/fig47.jpg" alt="Figure 47" id="fig47" /></p>
+<p class="imgtitle">Figure 47</p>
+<p class="imgtext"> Design of tension test specimen used in
+New South Wales.</p>
+
+<p>In making the test the specimen is fitted in the machine, and an
+extensometer attached to the middle portion and arranged to
+record the extension between the gauge points 8 inches apart.
+The area of the cross section then is 1.226 square inches, and
+the tensile strength is equal to the total breaking load applied
+divided by this area.</p>
+
+<a name="III_16"></a>
+<h3>TENSION TEST AT RIGHT ANGLES TO THE GRAIN</h3>
+
+<p>A static testing machine and a special testing device (<a href="#fig48">see Fig.
+48</a>) are required. The latter consists essentially of two double
+<a name="pg121"></a>
+hooks or clamps, one of which is suspended from the centre of
+the top of the cage, the other extended above the movable head.
+The specimens are 2" &times; 2" &times; 2.5". At each end a one-inch hole is
+bored with its centre equidistant from the two sides and 0.25
+inch from the ends. This makes the cross section to be tested 1"
+ &times; 2".</p>
+
+<p class="ctr"><img src="images/fig48.jpg" alt="Figure 48" id="fig48" /></p>
+<p class="imgtitle">Figure 48</p>
+<p class="imgtext"> Design of tool and specimen for testing
+tension at right angles to the grain.</p>
+
+<p>The free ends of the clamps are fitted into the notches in the
+<a name="pg122"></a>
+ends of the specimen. The movable head of the machine is then
+made to descend at the rate of 0.25 inch per minute, pulling the
+specimen in two at right angles to the grain. The maximum load
+only is taken and the result expressed in pounds per inch of
+width. A piece one-half inch thick is split off parallel to the
+failure and used for moisture determination.
+
+<a name="III_17"></a>
+<h3>TORSION TEST<a name="fn64r"></a><a href="#fn64"><sup><small>64</small></sup></a></h3>
+<p><i>Apparatus</i>: The torsion test is made in a Riehle-Miller
+torsional testing machine or its equivalent. (<a href="#fig49">See Fig. 49</a>.)</p>
+
+<p class="ctr"><img src="images/fig49.jpg" alt="Figure 49" id="fig49" /></p>
+<p class="imgtitle">Figure 49</p>
+<p class="imgtext"> Making a torsion test on hickory.</p>
+
+<p><i>Preparation of material</i>: The test pieces are cylindrical, 1.5
+inches in diameter and 18 inches gauge length, with squared ends
+4 inches long joined to the cylindrical portion with a fillet.
+The dimensions are carefully measured, and the usual data
+obtained in regard to the rate of growth, proportion of late
+wood, location <a name="pg123"></a> and kind of defects. The weight of the
+cylindrical portion of the specimen is obtained after the test.</p>
+
+<p><i>Making the test</i>: After the specimen is fitted in the machine
+the load is applied continuously at the rate of 22&deg; per minute.
+A troptometer is used in measuring the deformation. Readings are
+made until failure occurs, the points being entered on the
+cross-section paper. The character of the failure is described.
+Moisture determinations are made by the disk method.</p>
+
+<p><i>Results</i>: The conditions of ultimate rupture due to torsion
+appear not to be governed by definite mathematical laws; but
+where the material is not overstrained, laws may be assumed
+which are sufficiently exact for practical cases. The formul�
+commonly used for computations are as follows:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td><br> </td><td><br></td>      <td><br></td><td> 5.1 <i>M</i> </td></tr>
+<tr><td> (1) </td><td> <i>T</i> </td><td> = </td><td> ------- </td></tr>
+<tr><td><br> </td><td><br></td>      <td><br></td><td> <i>c</i><sup><small>3</small></sup> </td></tr>
+<tr><td><br> </td><td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br> </td><td><br></td>      <td><br></td><td> 114.6 <i>T f</i> </td></tr>
+<tr><td> (2) </td><td> <i>G</i> </td><td> = </td><td> ----------- </td></tr>
+<tr><td><br> </td><td><br></td>      <td><br></td><td> <i>a c</i> </td></tr>
+<tr><td><br> </td><td><br></td>      <td><br></td><td><br></td></tr>
+<tr><td><br> </td><td> <i>a</i> </td><td> = </td><td style="text-align: left;"> angle measured by troptometer at elastic limit, in degrees. </td></tr>
+<tr><td><br> </td><td> <i>c</i> </td><td> = </td><td style="text-align: left;"> diameter of specimen, inches. </td></tr>
+<tr><td><br> </td><td> <i>f</i> </td><td> = </td><td style="text-align: left;"> gauge length of specimen, inches. </td></tr>
+<tr><td><br> </td><td> <i>G</i> </td><td> = </td><td style="text-align: left;"> modulus of elasticity in shear across the grain, pounds per square inch. </td></tr>
+<tr><td><br> </td><td> <i>M</i> </td><td> = </td><td style="text-align: left;"> moment of torsion at elastic limit, inch-pounds. </td></tr>
+<tr><td><br> </td><td> <i>T</i> </td><td> = </td><td style="text-align: left;"> outer fibre torsional stress at elastic limit, pounds per square inch. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_18"></a>
+<h3>SPECIAL TESTS</h3>
+
+<a name="III_19"></a>
+<h4><i>Spike-pulling Test</i></h4>
+
+<p>Spike-pulling tests apply to problems of railroad maintenance,
+and the results are used to compare the spike-holding powers of
+various woods, both untreated and treated with different
+preservatives, and the efficiency of various forms of spikes.
+Special tests are also made in which the spike is subjected to a
+transverse load applied repetitively by a blow.</p>
+
+<p>For details of tests and results see:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td style="text-align: left;"> Cir. 38, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Instructions to engineers of timber tests, p. 26. </td></tr>
+<tr><td style="text-align: left;"> Cir. 46, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Holding force of railroad spikes in wooden ties. </td></tr>
+<tr><td style="text-align: left;"> Bul. 118, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Prolonging the life of cross-ties, pp. 37-40. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg124"></a>
+
+<a name="III_20"></a>
+<h4><i>Packing Boxes</i></h4>
+
+<p>Special tests on the strength of packing boxes of various woods
+have been made by the U.S. Forest Service to determine the
+merits of different kinds of woods as box material with the view
+of substituting new kinds for the more expensive ones now in
+use. The methods of tests consisted in applying a load along the
+diagonal of a box, an action similar to that which occurs when a
+box is dropped on one of its corners. The load was measured at
+each one-fourth inch in deflection, and notes were made of the
+primary and subsequent failures.</p>
+
+<p>For details of tests and results, see:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td style="text-align: left;"> Cir. 47, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Strength of packing boxes of various woods. </td></tr>
+<tr><td style="text-align: left;"> Cir. 214, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Tests of packing boxes of various forms. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_21"></a>
+<h4><i>Vehicle and Implement Woods</i></h4>
+
+<p>Tests were made by the U.S. Forest Service to obtain a better
+knowledge of the mechanical properties of the woods at present
+used in the manufacture of vehicles and implements and of those
+which might be substituted for them. Tests were made upon the
+following materials: hickory buggy spokes (<a href="#fig05">see Fig. 5</a>); hickory
+and red oak buggy shafts; wagon tongues; Douglas fir and
+southern pine cultivator poles.</p>
+
+<p>Details of the tests and results may be found in:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td style="text-align: left;"> Cir. 142, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Tests on vehicle and implement woods. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="III_22"></a>
+<h4><i>Cross-arms</i></h4>
+
+<p>In tests by the U.S. Forest Service on cross-arms a special
+apparatus was devised in which the load was distributed along
+the arm as in actual practice. The load was applied by rods
+passing through the pinholes in the arms. Nuts on these rods
+pulled down on the wooden bearing-blocks shaped to fit the upper
+side of the arm. The lower ends of these rods were attached to a
+system of equalizing levers, so arranged that the load at each
+pinhole would be the same. In all the tests the load was applied
+vertically by means of the static machine.</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td style="text-align: left;"> Cir. 204, </td><td style="width: 20px;"><br></td><td style="text-align: left;"> U.S.F.S.: </td><td style="width: 20px;"><br></td><td style="text-align: left;"> Strength tests of cross-arms. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg125"></a>
+
+<a name="III_23"></a>
+<h4><i>Other Tests</i></h4>
+
+<p>Many other kinds of tests are made as occasion demands. One kind
+consists of barrels and liquid containers, match-boxes, and
+explosive containers. These articles are subjected to shocks
+such as they would receive in transit and in handling, and also
+to hydraulic pressure.</p>
+
+<p>One of the most important tests from a practical standpoint is
+that of built-up structures such as compounded beams composed of
+small pieces bolted together, mortised joints, wooden trusses,
+etc. Tests of this kind can best be worked out according to the
+specific requirements in each case.</p>
+
+<a name="pg126"></a><br>
+<a name="pg127"></a>
+<a name="APPENDIX"></a><h2>APPENDIX</h2>
+
+<a name="APPENDIX_1"></a>
+<h3>SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE</h3>
+
+<hr>
+
+<h3>MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES</h3>
+<h4>Working Plan No. 124</h4>
+
+<hr>
+
+<h4>PURPOSE OF WORK</h4>
+
+<p>It is the general purpose of the work here outlined to provide:</p>
+
+<p>(<i>a</i>) Reliable data for comparing the mechanical properties of
+various species;</p>
+
+<p>(<i>b</i>) Data for the establishment of correct strength functions
+or working stresses;</p>
+
+<p>(<i>c</i>) Data upon which may be based analyses of the influence on
+the mechanical properties of such factors as:</p>
+
+<p>Locality;</p>
+
+<p>Distance of timber from the pith of the tree;</p>
+
+<p>Height of timber in the tree;</p>
+
+<p>Change from the green to the air-dried condition, etc.</p>
+
+<p>The mechanical properties which will be considered and the
+principal tests used to determine them are as follows:</p>
+
+<p>Strength and stiffness&mdash;</p>
+<p class="ind">Static bending;</p>
+<p class="ind">Compression parallel to grain;</p>
+<p class="ind">Compression perpendicular to grain;</p>
+<p class="ind">Shear.</p>
+
+<p>Toughness&mdash;</p>
+<p class="ind">Impact bending;</p>
+<p class="ind">Static bending;</p>
+<p class="ind">Work to maximum load and total work.</p>
+
+<a name="pg128"></a>
+
+<p>Cleavability&mdash;</p>
+<p class="ind">Cleavage test.</p>
+
+<p>Hardness&mdash;
+<p class="ind">Modification of Janka ball test for surface hardness.</p>
+
+<h4>MATERIAL</h4>
+
+<h5><i>Selection and Number of Trees</i></h5>
+
+<p>The material will be from trees selected in the forest by one
+qualified to determine the species. From each locality, three to
+five dominant trees of merchantable size and approximately
+average age will be so chosen as to be representative of the
+dominant trees of the species. Each species will eventually be
+represented by trees from five to ten localities. These
+localities will be so chosen as to be representative of the
+commercial range of the species. Trees from one to three
+localities will be used to represent each species until most of
+the important species have been tested.</p>
+
+<p>The 16-foot butt log will be taken from each tree selected and
+the entire merchantable hole of one average tree for each
+species.</p>
+
+<h5><i>Field Notes and Shipping Instructions</i></h5>
+
+<p>Field notes as outlined in Form&mdash;<i>a</i> Shipment Description,
+Manual of the Branch of Products, will be fully and carefully
+made by the collector. The age of each tree selected will be
+recorded and any other information likely to be of interest or
+importance will also be made a part of these field notes. Each
+log will have the bark left on. It will be plainly marked in
+accordance with directions given under Detailed Instructions.
+All material will be shipped to the laboratory immediately after
+being cut. No trees will be cut until the collector is notified
+that the laboratory is ready to receive the material.</p>
+
+<h4>DETAILED INSTRUCTIONS</h4>
+
+<h5><i>Part of Tree to be Tested</i></h5>
+
+<p>(<i>a</i>) For determining the value of tree and locality and the
+influence on the mechanical properties of distance from the
+pith, a 4-foot bolt will be cut from the top end of each 16-foot
+butt log.</p>
+
+<a name="pg129"></a>
+
+<p>(<i>b</i>) For investigating the variation of properties with the
+height of timber in the tree, all the logs from one average tree
+will be used.</p>
+
+<p>(<i>c</i>) For investigating the effect of drying the wood, the bolt
+next below that provided for in (<i>a</i>) will be used in the case
+of one tree from each locality.</p>
+
+<h5><i>Marking and Grouping of Material</i></h5>
+
+<p>The marking will be standard except as noted. Each log will be
+considered a "piece." The piece numbers will be plainly marked
+upon the butt end of each log by the collector. The north side
+of each log will also be marked.</p>
+
+<p>When only one bolt from a tree is used it will be designated by
+the number of the log from which it is cut. Whenever more than
+one bolt is taken from a tree, each 4-foot bolt or length of
+trunk will be given a letter (mark), <i>a, b, c,</i> etc., beginning
+at the stump.</p>
+
+<p>All bolts will be sawed into 2-1/2" &times; 2-1/2" sticks and the
+sticks marked according to the sketch, <a href="#fig50">Fig. 50</a>. The letters <i>N,
+E, S,</i> and <i>W</i> indicate the cardinal points when known; when
+these are unknown, <i>H, K, L,</i> and <i>M</i> will be used. Thus, <i>N5,
+K8, S7, M4</i> are stick numbers, the letter being a part of the
+stick number.</p>
+
+<p class="ctr"><img src="images/fig50.jpg" alt="Figure 50" id="fig50" /></p>
+<p class="imgtitle">Figure 50</p>
+<p class="imgtext"> Method of cutting and marking test
+specimens.</p>
+
+<p>Only straight-grained specimens, free from defects which will
+affect their strength, will be tested.</p>
+
+<h5><i>Care of Material</i></h5>
+
+<p>No material will be kept in the bolt or log long enough to be
+damaged or disfigured by checks, rot, or stains.</p>
+
+<a name="pg130"></a>
+
+<p><i>Green material</i>: The material to be tested green will be kept
+in a green state by being submerged in water until near the time
+of test. It will then be surfaced, sawed to length, and stored
+in damp sawdust at a temperature of 70&deg;F. (as nearly as
+practicable) until time of test. Care should be taken to avoid
+as much as possible the storage of green material in any form.</p>
+
+<p><i>Air-dry material</i>: The material to be air-dried will be cut
+into sticks 2-1/2" &times; 2-1/2" &times; 4'. The ends of these sticks will
+be paraffined to prevent checking. This material will be so
+piled as to leave an air space of at least one-half inch on each
+side of each stick, and in such a place that it will be
+protected from sunshine, rain, snow, and moisture from the
+ground. The sticks will be surfaced and cut to length just
+previous to test.</p>
+
+<h5><i>Order of Tests</i></h5>
+
+<p>The order of tests in all cases will be such as to eliminate so
+far as possible from the comparisons the effect of changes of
+condition of the specimens due to such factors as storage and
+weather conditions.</p>
+
+<p>The material used for determining the effect of height in tree
+will be tested in such order that the average time elapsing from
+time of cutting to time of test will be approximately the same
+for all bolts from any one tree.
+
+<h5><i>Tests on Green Material</i></h5>
+
+<p>The tests on all bolts, except those from which a comparison of
+green and dry timber is to be gotten, will be as follows:</p>
+
+<p><i>Static bending</i>: One stick from each pair. A pair consists of
+two adjacent sticks equidistant from the pith, as <i>N</i>7 and <i>N</i>8,
+or <i>H</i>5 and <i>H</i>6.</p>
+
+<p><i>Impact bending</i>: Four sticks; one to be taken from near the
+pith; one from near the periphery; and two representative of the
+cross section.</p>
+
+<p><i>Compression parallel to grain</i>: One specimen from each stick.
+These will be marked "1" in addition to the number of the stick
+from which they are taken.</p>
+
+<p><i>Compression perpendicular to grain</i>: One specimen from each of
+50 per cent of the static bending sticks. These will be marked
+<a name="pg131"></a>
+"2" in addition to the number of the stick from which they are
+cut.</p>
+
+<p><i>Hardness</i>: One specimen from each of the other 50 per cent of
+the static bending sticks. These specimens will be marked "4."</p>
+
+<p><i>Shear</i>: Six specimens from sticks not tested in bending or from
+the ends cut off in preparing the bending specimens. Two
+specimens will be taken from near the pith; two from near the
+periphery; and two that are representative of the average
+growth. One of each two will be tested in radial shear and the
+other in tangential shear. These specimens will have the mark
+"3."</p>
+
+<p><i>Cleavage</i>: Six specimens chosen and divided just as those for
+shearing. These specimens will have the mark "5." (For sketches
+showing radial and tangential cleavage, <a href="#fig45">see Fig. 45</a>.)</p>
+
+<p>When it is impossible to secure clear specimens for all of the
+above tests, tests will have precedence in the order in which
+they are named.</p>
+
+<h5><i>Tests to Determine the Effect of Air-drying</i></h5>
+
+<p>These tests will be made on material from the adjacent bolts
+mentioned in "<i>c</i>" under Part of Tree to be Tested. Both bolts
+will be cut as outlined above. One-half the sticks from each
+bolt will be tested green, the other half will be air-dried and
+tested. The division of green and air-dry will be according to
+the following scheme:</p>
+
+<center>
+<table summary="text">
+<tbody>
+<tr><td colspan="14"> STICK NUMBERS</td></tr>
+<tr><td> Lower bolt, </td><td> 1, </td><td><br></td><td><br></td><td> 4, </td><td> 5, </td><td><br></td><td><br></td><td> 8, </td><td> 9, </td><td><br></td><td rowspan="2" colspan="1"> etc. </td><td rowspan="2" colspan="1"><big><big><big><big>}</big></big></big></big></td><td rowspan="2" colspan="1"> Tested green </td></tr>
+<tr><td> Upper bolt, </td><td><br></td><td> 2, </td><td> 3, </td><td><br></td><td><br></td><td> 6, </td><td> 7, </td><td><br></td><td><br></td><td> 10, </td></tr>
+<tr><td> Lower bolt, </td><td><br></td><td> 2, </td><td> 3, </td><td><br></td><td><br></td><td> 6, </td><td> 7, </td><td><br></td><td><br></td><td> 10, </td><td rowspan="2" colspan="1"> etc. </td><td rowspan="2" colspan="1"> <big><big><big><big>}</big></big></big></big></td><td rowspan="2" colspan="1"> Air-dried and tested </td></tr>
+<tr><td> Upper bolt, </td><td> 1, </td><td><br></td><td><br></td><td> 4, </td><td> 5, </td><td><br></td><td><br></td><td> 8, </td><td> 9, </td><td><br></td></tr>
+</tbody>
+</table>
+</center>
+
+<p>All green sticks from these two bolts will be tested as if they
+were from the same bolt and according to the plan previously
+outlined for green material from single bolts. The tests on the
+air-dried material will be the same as on the green except for
+the difference of seasoning.</p>
+
+<a name="pg132"></a>
+
+<p>The material will be tested at as near 12 per cent moisture as
+is practicable. The approximate weight of the air-dried
+specimens at 12 per cent moisture will be determined by
+measuring while green 20 per cent of the sticks to be air-dried
+and assuming their dry gravity to be the same as that of the
+specimens tested green. This 20 per cent will be weighed as
+often as is necessary to determine the proper time of test.</p>
+
+<h5><i>Methods of Test</i></h5>
+
+<p>All tests will be made according to Circular 38 except in case
+of conflict with the instructions given below:</p>
+
+<p><i>Static bending</i>: The tests will be on specimens 2" &times; 2" &times; 30"
+on 28-inch span. Load will be applied at the centre.</p>
+
+<p>In all tests the load-deflection curve will be carried to or
+beyond the maximum load. In one-third of the tests the
+load-deflection curve will be continued to 6-inch deflection, or
+till the specimen fails to support a 200-pound load. Deflection
+readings for equal increments of load will be taken until well
+past the elastic limit, after which the scale beam will be kept
+balanced and the load read for each 0.1-inch deflection. The
+load and deflection at first failure, maximum load and points of
+sudden change, will be shown on the curve sheet even if they do
+not occur at one of the regular load or deflection increments.</p>
+
+<p><i>Impact bending</i>: The impact bending tests will be on specimens
+of the same size as those used in static bending. The span will
+be 28 inches.</p>
+
+<p>The tests will be by increment drop. The first drop will be 1
+inch and the increase will be by increments of 1 inch till a
+height of 10 inches is reached, after which increments of 2
+inches will be used until complete failure occurs or 6-inch
+deflection is secured.</p>
+
+<p>A 50-pound hammer will be used when with drops up to 68 inches
+it is practically certain that it will produce complete failure
+or 6-inch deflection in the case of all specimens of a species.
+For all other species, a 100-pound hammer will be used.</p>
+
+<p>In all cases drum records will be made until first failure. Also
+the height of drop causing complete failure or 6-inch deflection
+will be noted.</p>
+
+<p><i>Compression parallel to grain</i>: This test will be on specimens
+<a name="pg133"></a>
+2" &times; 2" &times; 8" in size. On 20 per cent of these tests
+load-compression curves for a 6-inch centrally located gauge
+length will be taken. Readings will be continued until the
+elastic limit is well passed. The other 80 per cent of the tests
+will be made for the purpose of obtaining the maximum load only.</p>
+
+<p><i>Compression perpendicular to grain</i>: This test will be on
+specimens 2" &times; 2" &times; 6" in size. The bearing plates will be 2
+inches wide. The rate of descent of the moving head will be
+0.024 inch per minute. The load-compression curve will be
+plotted to 0.1 inch compression and the test will then be
+discontinued.</p>
+
+<p><i>Hardness</i>: The tool shown in <a href="#fig43">Fig. 43</a> (an adaptation of the
+apparatus used by the German investigator, Janka) will be used.
+The rate of descent of the moving head will be 0.25 inch per
+minute. When the penetration has progressed to the point at
+which the plate "<i>a</i>" becomes tight, due to being pressed
+against the wood, the load will be read and recorded.</p>
+
+<p>Two penetrations will be made on a tangential surface, two on a
+radial, and one on each end of each specimen tested. The choice
+between the two radial and between the two tangential surfaces
+and the distribution of the penetrations over the surfaces will
+be so made as to get a fair average of heart and sap, slow and
+fast growth, and spring and summer wood. Specimens will be 2" &times;
+2" &times; 6".</p>
+
+<p><i>Shear</i>: The tests will be made with a tool slightly modified
+from that shown in Circular 38. The speed of descent of head
+will be 0.015 inch per minute. The only measurements to be made
+are those of the shearing area. The offset will be 1/8 inch.
+Specimens will be 2" &times; 2" &times; 2-1/2" in size. (For definition of
+offset and form of test specimen, <a href="#fig38">see Fig. 38</a>.)</p>
+
+<p><i>Cleavage</i>: The cleavage tests will be made on specimens of the
+form and size shown in <a href="#fig45">Fig. 45</a>. The apparatus will be as shown
+in Fig. 44. The maximum load only will be taken and the result
+expressed in pounds per inch of width. The speed of the moving
+head will be 0.25 inch per minute.</p>
+
+<h5><i>Moisture Determinations</i></h5>
+
+<p>Moisture determinations will be made on all specimens tested
+except those to be photographed or kept for exhibit. A 1-inch
+<a name="pg134"></a>
+disk will be cut from near the point of failure of bending and
+compression parallel specimens, from the portion under the plate
+in the case of the compression perpendicular specimens, and from
+the centre of the hardness test specimens. The beads from the
+shear specimens will be used as moisture disks. In the case of
+the cleavage specimens a piece 1/2 inch thick will be split off
+parallel to the failure and used as a moisture disk.</p>
+
+<h4>RECORDS</h4>
+
+<p>All records will be standard.</p>
+
+<h4>PHOTOGRAPHS</h4>
+
+<h5><i>Cross Sections</i></h5>
+
+<p>Just before cutting into sticks, the freshly cut end of at least
+one bolt from each tree will be photographed. A scale of inches
+will be shown in this photograph.</p>
+
+<h5><i>Specimens</i></h5>
+
+<p>Three photographs will be made of a group consisting of four 2"
+ &times; 2" &times; 30" specimens chosen from the material from each
+locality. Two of these specimens will be representative of
+average growth, one of fast and one of slow growth. These
+photographs will show radial, tangential, and end surfaces for
+each specimen.</p>
+
+<h5><i>Failures</i></h5>
+
+<p>Typical and abnormal failures of material from each site will be
+photographed.</p>
+
+<h5><i>Disposition of Material</i></h5>
+
+<p>The specimens photographed to show typical and abnormal failures
+will be saved for purposes of exhibit until deemed by the person
+in charge of the laboratory to be of no further value.</p>
+
+<a name="pg135"></a>
+
+<h3>SHRINKAGE AND SPECIFIC GRAVITY</h3>
+
+<h4>Appendix to Working Plan 124</h4>
+
+<hr>
+
+<h4>PURPOSE OF WORK</h4>
+
+<p>It is the purpose of this work to secure data on the shrinkage
+and specific gravity of woods tested under Project 124. The
+figures to be obtained are for use as average working values
+rather than as the basis for a detailed study of the principles
+involved.</p>
+
+<h4>MATERIAL</h4>
+
+<p>The material will be taken from that provided for mechanical
+tests.</p>
+
+<h4>RADIAL AND TANGENTIAL SHRINKAGE</h4>
+
+<h5><i>Specimens</i></h5>
+
+<p><i>Preparation</i>: Two specimens 1 inch thick, 4 inches wide, and 1
+inch long will be obtained from near the periphery of each "<i>d</i>"
+bolt. These will be cut from the sector-shaped sections left
+after securing the material for the mechanical tests or from
+disks cut from near the end of the bolt. They will be taken from
+adjoining pieces chosen so that the results will be comparable
+for use in determining radial and tangential shrinkage. (When a
+disk is used, care must be taken that it is green and has not
+been affected by the shrinkage and checking near the end of the
+bolt.)</p>
+
+<p>One of these specimens will be cut with its width in the radial
+direction and will be used for the determination of radial
+shrinkage. The other will have its width in the tangential
+direction and will be used for tangential shrinkage. These
+specimens will not be surfaced.</p>
+
+<p><i>Marking</i>: The shrinkage specimens will retain the shipment and
+piece numbers and marks of the bolts from which they are taken,
+and will have the additional mark <i>7</i>R or <i>7</i>T according as
+their widths are in the radial or tangential direction.</p>
+
+<a name="pg136"></a>
+
+<p><i>Shrinkage measurements</i>: The shrinkage specimens will be
+carefully weighed and measured soon after cutting. Rings per
+inch, per cent sap, and per cent summer wood will be measured.
+They will then be air-dried in the laboratory to constant
+weight, and afterward oven-dried at 100&deg;C. (212&deg;F.), when they
+will again be weighed and measured.</p>
+
+<h4>VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY</h4>
+
+<h5><i>Specimens</i></h5>
+
+<p><i>Selection and preparation</i>: Four 2" &times; 2" &times; 6" specimens will be
+cut from the mechanical test sticks of each "<i>d</i>" bolt; also
+from each of the composite bolts used in getting a comparison of
+green and air-dry. One of these specimens will be taken from
+near the pith and one from near the periphery; the other two
+will be representative of the average growth of the bolt. The
+sides of these specimens will be surfaced and the ends smooth
+sawn.</p>
+
+<p><i>Marking</i>: Each specimen will retain the shipment, piece, and
+stick numbers and mark of the stick from which it is cut, and
+will have the additional mark "<i>S</i>."</p>
+
+<p><i>Manipulation</i>: Soon after cutting, each specimen will be
+weighed and its volume will be determined by the method
+described below. The rings per inch and per cent summer wood,
+where possible, will be determined, and a carbon impression of
+the end of the specimen made. It will then be air-dried in the
+laboratory to a constant weight and afterward oven-dried at
+100&deg;C. When dry, the specimen will be taken from the oven,
+weighed, and a carbon impression of its end made. While still
+warm the specimen will be dipped in hot paraffine. The volume
+will then be determined by the following method:</p>
+
+<p>On one pan of a pair of balances is placed a container having in
+it water enough for the complete submersion of the test
+specimen. This container and water is balanced by weights placed
+on the other scale pan. The specimen is then held completely
+submerged and not touching the container while the scales are
+again balanced. The weight required to balance is the weight of
+water displaced by the specimen, and hence if in grams is
+numerically equal to the volume of the specimen in cubic
+centimetres. <a name="pg137"></a> A diagrammatic sketch of the arrangement of this
+apparatus is shown in <a href="#fig51">Fig. 51</a>.</p>
+
+<p class="ctr"><img src="images/fig51.jpg" alt="Figure 51" id="fig51" /></p>
+<p class="imgtitle">Figure 51</p>
+<p class="imgtext"> Diagram of specific gravity apparatus,
+showing a balance with container (<i>c</i>) filled with water in
+which the test block (<i>b</i>) is held submerged by a light rod
+(<i>a</i>) which is adjustable vertically and provided with a sharp
+point to be driven into the specimen.</p>
+
+<p>Air-dry specimens will be dipped in water and then wiped dry
+after the first weighing and just before being immersed for
+weighing their displacement. All displacement determinations
+will be made as quickly as possible in order to minimize the
+absorption of water by the specimen.</p>
+
+<a name="pg138"></a>
+<a name="APPENDIX_2"></a>
+<h3>STRENGTH VALUES FOR STRUCTURAL TIMBERS</h3>
+
+<h4>(From Cir. 189, U.S. Forest Service)</h4>
+
+<p>The following tables bring together in condensed form the
+average strength values resulting from a large number of tests
+made by the Forest Service on the principal structural timbers
+of the United States. These results are more completely
+discussed in other publications of the Service, a list of which
+is given on <a href="#pg157">pages 157</a>-159.</p>
+
+<p>The tests were made at the laboratories of the U.S. Forest
+Service, in cooperation with the following institutions: Yale
+Forest School, Purdue University, University of California,
+University of Oregon, University of Washington, University of
+Colorado, and University of Wisconsin.</p>
+
+<p>Tables <a href="#TABLE_18">XVIII</a> and <a href="#TABLE_19">XIX</a> give the average results obtained from
+tests on green material, while Tables <a href="#TABLE_20">XX</a> and <a href="#TABLE_21">XXI</a> give average
+results from tests on air-seasoned material. The small
+specimens, which were invariably 2" &times; 2" in cross section, were
+free from defects such as knots, checks, and cross grain; all
+other specimens were representative of material secured in the
+open market. The relation of stresses developed in different
+structural forms to those developed in the small clear specimens
+is shown for each factor in the column headed "Ratio to 2" &times;
+2"." Tests to determine the mechanical properties of different
+species are often confined to small, clear specimens. The ratios
+included in the tables may be applied to such results in order
+to approximate the strength of the species in structural sizes,
+and containing the defects usually encountered, when tests on
+such forms are not available.</p>
+
+<p>A comparison of the results of tests on seasoned material with
+those from tests on green material shows that, without
+exception, the strength of the 2" &times; 2" specimens is increased by
+lowering the moisture content, but that increase in strength of
+other sizes is much more erratic. Some specimens, in fact, show
+an apparent loss in strength due to seasoning. If structural
+timbers are <a name="pg139"></a> seasoned slowly, in order to avoid excessive
+checking, there should be an increase in their strength. In the
+light of these facts it is not safe to base working stresses on
+results secured from any but green material. For a discussion of
+factors of safety and safe working stresses for structural
+timbers see the Manual of the American Railway Engineering
+Association, Chicago, 1911. A table from that publication,
+giving working unit stresses for structural timber, is
+reproduced in this book, <a href="#TABLE_22">see Table XXII</a>.</p>
+
+<a name="pg140"></a>
+
+<center>
+<table id="TABLE_18" summary="TABLE_XVIII" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th rowspan="1" colspan="14"> TABLE XVIII </th></tr>
+<tr><th rowspan="1" colspan="14"> BENDING TESTS ON GREEN MATERIAL </th></tr>
+<tr><th rowspan="3" colspan="1"> Species </th>                              <th rowspan="1" colspan="2"> Sizes </th>
+                                                                                                        <th rowspan="2" colspan="1"> Number of tests </th>
+                                                                                                                           <th rowspan="2" colspan="1"> Per cent of moisture </th>
+                                                                                                                                          <th rowspan="2" colspan="1"> Rings per inch </th>
+                                                                                                                                                         <th rowspan="1" colspan="2"> F.S. at E.L. </th>
+                                                                                                                                                                                        <th rowspan="1" colspan="2"> M. of R. </th>
+                                                                                                                                                                                                                       <th rowspan="1" colspan="2"> M. of E. </th>
+                                                                                                                                                                                                                                                      <th rowspan="1" colspan="2"> Calculated shear </th></tr>
+<tr>                                                                        <th> Cross&nbsp;Section </th>
+                                                                                               <th> Span </th>
+                                                                                                                                                         <th> Average per sq. inch </th>
+                                                                                                                                                                         <th> Ratio to 2" by 2" </th>
+                                                                                                                                                                                        <th> Average per sq. inch</th>
+                                                                                                                                                                                                        <th>Ratio to 2" by 2" </th>
+                                                                                                                                                                                                                       <th> Average per sq. inch </th>
+                                                                                                                                                                                                                                       <th>Ratio to 2" by 2" </th>
+                                                                                                                                                                                                                                                      <th> Average per sq. inch </th>
+                                                                                                                                                                                                                                                                    <th>Ratio to 2" by 2" </th></tr>
+<tr>                                                                        <th><i>Inches</i></th>
+                                                                                               <th><i>Ins.</i></th>
+                                                                                                             <th><br></th> <th><br></th>  <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                         <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                                        <th><br></th>  <th>1,000 <i>lbs.</i></th>
+                                                                                                                                                                                                                                       <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                                                                                                    <th><br></th></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>                      <td> 12 by 12 </td><td> 138 </td><td> 4 </td>  <td> 28.6 </td><td> 9.7 </td> <td> 4,029 </td><td> 0.83 </td><td> 6,710 </td><td> 0.74 </td><td> 1,523 </td><td> 0.99 </td><td> 261 </td><td> 0.86 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 10 by 16 </td><td> 168 </td><td> 4 </td>  <td> 26.8 </td><td> 16.7 </td><td> 6,453 </td><td> .85 </td> <td> 6,453 </td><td> .71 </td> <td> 1,626 </td><td> 1.05 </td><td> 306 </td><td> 1.01 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 8 by 16 </td> <td> 156 </td><td> 7 </td>  <td> 28.4 </td><td> 14.6 </td><td> 3,147 </td><td> .64 </td> <td> 5,439 </td><td> .60 </td> <td> 1,368 </td><td> .89 </td ><td> 390 </td><td> 1.29 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 6 by 16 </td> <td> 132 </td><td> 1 </td>  <td> 40.3 </td><td> 21.8 </td><td> 4,120 </td><td> .83 </td> <td> 6,460 </td><td> .71 </td> <td> 1,190 </td><td> .77 </td> <td> 378 </td><td> 1.25 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 6 by 10 </td> <td> 180 </td><td> 1 </td>  <td> 31.0 </td><td> 6.2 </td> <td> 3,580 </td><td> .72 </td> <td> 6,500 </td><td> .72 </td> <td> 1,412 </td><td> .92 </td> <td> 175 </td><td> .58 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 6 by 8 </td>  <td> 180 </td><td> 2 </td>  <td> 27.0 </td><td> 8.2 </td> <td> 3,735 </td><td> .75 </td> <td> 5,745 </td><td> .63 </td> <td> 1,282 </td><td> .83 </td> <td> 121 </td><td> .40 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 15 </td> <td> 33.9 </td><td> 14.1 </td><td> 4,950 </td><td> 1.00 </td><td> 9,070 </td><td> 1.00 </td><td> 1,540 </td><td> 1:00 </td><td> 303 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>                        <td> 8 by 16 </td> <td> 180 </td><td> 191 </td><td> 31.5 </td><td> 11.0 </td><td> 3,968 </td><td> .76 </td> <td> 5,983 </td><td> .72 </td> <td> 1,517 </td><td> .95 </td> <td> 269 </td><td> .81 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 8 </td>  <td> 180 </td><td> 84 </td> <td> 30.1 </td><td> 10.8 </td><td> 3,693 </td><td> .71 </td> <td> 5,178 </td><td> .63 </td> <td> 1,533 </td><td> .96 </td> <td> 172 </td><td> .52 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 12 </td> <td> 180 </td><td> 27 </td> <td> 35.7 </td><td> 20.3 </td><td> 3,721 </td><td> .71 </td> <td> 5,276 </td><td> .64 </td> <td> 1,642 </td><td> 1.03 </td><td> 256 </td><td> .77 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 10 </td> <td> 180 </td><td> 26 </td> <td> 32.9 </td><td> 21.6 </td><td> 3,160 </td><td> .60 </td> <td> 4,699 </td><td> .57 </td> <td> 1,593 </td><td> 1.00 </td><td> 189 </td><td> .57 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 8 </td>  <td> 180 </td><td> 29 </td> <td> 33.6 </td><td> 17.6 </td><td> 3,593 </td><td> .69 </td> <td> 5,352 </td><td> .65 </td> <td> 1,607 </td><td> 1.01 </td><td> 171 </td><td> .51 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 24 </td> <td> 568 </td><td> 30.4 </td><td> 11.6 </td><td> 5,227 </td><td> 1.00 </td><td> 9,070 </td><td> 1.00 </td><td> 1,540 </td><td> 1.00 </td><td> 303 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Douglas&nbsp;fir&nbsp;(fire-killed) </td><td> 8 by 16 </td> <td> 180 </td><td> 30 </td> <td> 36.8 </td><td> 10.9 </td><td> 3,503 </td><td> .80 </td> <td> 4,994 </td><td> .64 </td> <td> 1,531 </td><td> .94 </td> <td> 330 </td><td> 1.19 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 12 </td> <td> 180 </td><td> 32 </td> <td> 34.2 </td><td> 17.7 </td><td> 3,489 </td><td> .80 </td> <td> 5,085 </td><td> .66 </td> <td> 1,624 </td><td> .99 </td> <td> 247 </td><td> .89 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 10 </td> <td> 180 </td><td> 32 </td> <td> 38.9 </td><td> 18.1 </td><td> 3,851 </td><td> .88 </td> <td> 5,359 </td><td> .69 </td> <td> 1,716 </td><td> 1.05 </td><td> 216 </td><td> .78 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 8 </td>  <td> 180 </td><td> 31 </td> <td> 37.0 </td><td> 15.7 </td><td> 3,403 </td><td> .78 </td> <td> 5,305 </td><td> .68 </td> <td> 1,676 </td><td> 1.02 </td><td> 169 </td><td> .61 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 290 </td><td> 33.2 </td><td> 17.2 </td><td> 4,360 </td><td> 1.00 </td><td> 7,752 </td><td> 1.00 </td><td> 1,636 </td><td> 1.00 </td><td> 277 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td>                     <td> 8 by 16 </td> <td> 180 </td><td> 12 </td> <td> 39.5 </td><td> 12.1 </td><td> 3,185 </td><td> .73 </td> <td> 5,407 </td><td> .70 </td> <td> 1,438 </td><td> 1.03 </td><td> 362 </td><td> 1.40 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 8 by 14 </td> <td> 180 </td><td> 12 </td> <td> 45.8 </td><td> 12.7 </td><td> 3,234 </td><td> .74 </td> <td> 5,781 </td><td> .75 </td> <td> 1,494 </td><td> 1.07 </td><td> 338 </td><td> 1.31 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 8 by 12 </td> <td> 180 </td><td> 24 </td> <td> 52.2 </td><td> 11.8 </td><td> 3,265 </td><td> .75 </td> <td> 5,503 </td><td> .71 </td> <td> 1,480 </td><td> 1.06 </td><td> 277 </td><td> 1.07 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 8 </td>  <td> 180 </td><td> 24 </td> <td> 47.8 </td><td> 11.5 </td><td> 3,519 </td><td> .81 </td> <td> 5,732 </td><td> .74 </td> <td> 1,485 </td><td> 1.06 </td><td> 185 </td><td> .72 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 254 </td><td> 51.7 </td><td> 13.6 </td><td> 4,350 </td><td> 1.00 </td><td> 7,710 </td><td> 1.00 </td><td> 1,395 </td><td> 1.00 </td><td> 258 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Western larch </td>                      <td> 8 by 16 </td> <td> 180 </td><td> 32 </td> <td> 51.0 </td><td> 25.3 </td><td> 3,276 </td><td> .77 </td> <td> 4,632 </td><td> .64 </td> <td> 1,272 </td><td> .97 </td> <td> 298 </td><td> 1.11 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 8 by 12 </td> <td> 180 </td><td> 30 </td> <td> 50.3 </td><td> 23.2 </td><td> 3,376 </td><td> .79 </td> <td> 5,286 </td><td> .73 </td> <td> 1,331 </td><td> 1.02 </td><td> 254 </td><td> .94 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 8 </td>  <td> 180 </td><td> 14 </td> <td> 56.0 </td><td> 25.6 </td><td> 3,528 </td><td> .83 </td> <td> 5,331 </td><td> .74 </td> <td> 1,432 </td><td> 1.09 </td><td> 169 </td><td> .63 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 28 </td> <td> 189 </td><td> 46.2 </td><td> 26.2 </td><td> 4,274 </td><td> 1.00 </td><td> 7,251 </td><td> 1.00 </td><td> 1,310 </td><td> 1.00 </td><td> 269 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Loblolly pine </td>                      <td> 8 by 16 </td> <td> 180 </td><td> 17 </td> <td> 15.8 </td><td> 6.1 </td> <td> 3,094 </td><td> .75 </td> <td> 5,394 </td><td> .69 </td> <td> 1,406 </td><td> .98 </td> <td> 383 </td><td> 1.44 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 12 </td> <td> 180 </td><td> 94 </td> <td> 60.9 </td><td> 5.9 </td> <td> 3,030 </td><td> .74 </td> <td> 5,028 </td><td> .64 </td> <td> 1,383 </td><td> .96 </td> <td> 221 </td><td> .83 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 44 </td> <td> 70.9 </td><td> 5.4 </td> <td> 4,100 </td><td> 1.00 </td><td> 7,870 </td><td> 1.00 </td><td> 1,440 </td><td> 1.00 </td><td> 265 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                           <td> 6 by 12 </td> <td> 162 </td><td> 15 </td> <td> 57.6 </td><td> 16.6 </td><td> 2,914 </td><td> .75 </td> <td> 4,500 </td><td> .66 </td> <td> 1,202 </td><td> 1.05 </td><td> 255 </td><td> 1.11 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 10 </td> <td> 162 </td><td> 15 </td> <td> 43.5 </td><td> 11.4 </td><td> 2,712 </td><td> .70 </td> <td> 4,611 </td><td> .68 </td> <td> 1,238 </td><td> 1.08 </td><td> 209 </td><td> .91 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 82 </td> <td> 38.8 </td><td> 14.0 </td><td> 3,875 </td><td> 1.00 </td><td> 6,820 </td><td> 1.00 </td><td> 1,141 </td><td> 1.00 </td><td> 229 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Western hemlock </td>                    <td> 8 by 16 </td> <td> 180 </td><td> 39 </td> <td> 42.5 </td><td> 15.6 </td><td> 3,516 </td><td> .80 </td> <td> 5,296 </td><td> .73 </td> <td> 1,445 </td><td> 1.01 </td><td> 261 </td><td> .92 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 28 </td> <td> 52 </td> <td> 51.8 </td><td> 12.1 </td><td> 4.406 </td><td> 1.00 </td><td> 7,294 </td><td> 1.00 </td><td> 1,428 </td><td> 1.00 </td><td> 284 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Redwood </td>                            <td> 8 by 16 </td> <td> 180 </td><td> 14 </td> <td> 86.5 </td><td> 19.9 </td><td> 3,734 </td><td> .79 </td> <td> 4,492 </td><td> .64 </td> <td> 1,016 </td><td> .96 </td> <td> 300 </td><td> 1.21 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 6 by 12 </td> <td> 180 </td><td> 14 </td> <td> 87.3 </td><td> 17.8 </td><td> 3,787 </td><td> .80 </td> <td> 4,451 </td><td> .64 </td> <td> 1,068 </td><td> 1.00 </td><td> 224 </td><td> .90 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 7 by 9 </td>  <td> 180 </td><td> 14 </td> <td> 79.8 </td><td> 16.7 </td><td> 4,412 </td><td> .93 </td> <td> 5,279 </td><td> .76 </td> <td> 1,324 </td><td> 1.25 </td><td> 199 </td><td> .80 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 3 by 14 </td> <td> 180 </td><td> 13 </td> <td> 86.1 </td><td> 23.7 </td><td> 3,506 </td><td> .74 </td> <td> 4,364 </td><td> .62 </td> <td> 947 </td>  <td> .89 </td> <td> 255 </td><td> 1.03 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 12 </td> <td> 180 </td><td> 12 </td> <td> 70.9 </td><td> 18.6 </td><td> 3,100 </td><td> .65 </td> <td> 3,753 </td><td> .54 </td> <td> 1,052 </td><td> .99 </td> <td> 187 </td><td> .75 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 10 </td> <td> 180 </td><td> 13 </td> <td> 55.8 </td><td> 20.0 </td><td> 3,285 </td><td> .69 </td> <td> 4,079 </td><td> .58 </td> <td> 1,107 </td><td> 1.04 </td><td> 169 </td><td> .68 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 8 </td>  <td> 180 </td><td> 13 </td> <td> 63.8 </td><td> 21.5 </td><td> 2,989 </td><td> .63 </td> <td> 4,063 </td><td> .58 </td> <td> 1,141 </td><td> 1.08 </td><td> 134 </td><td> .54 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 28 </td> <td> 157 </td><td> 75.5 </td><td> 19.1 </td><td> 4,750 </td><td> 1.00 </td><td> 6,980 </td><td> 1.00 </td><td> 1,061 </td><td> 1.00 </td><td> 248 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>                        <td> 6 by 12 </td> <td> 162 </td><td> 15 </td> <td> 50.3 </td><td> 12.5 </td><td> 2,305 </td><td> .82 </td> <td> 3,572 </td><td> .69 </td> <td> 987 </td>  <td> 1.03 </td><td> 201 </td><td> 1.17 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 12 </td> <td> 162 </td><td> 18 </td> <td> 47.9 </td><td> 14.7 </td><td> 2,648 </td><td> .94 </td> <td> 4,107 </td><td> .79 </td> <td> 1,255 </td><td> 1.31 </td><td> 238 </td><td> 1.38 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 10 </td> <td> 162 </td><td> 16 </td> <td> 45.7 </td><td> 13.3 </td><td> 2,674 </td><td> .95 </td> <td> 4,205 </td><td> .81 </td> <td> 1,306 </td><td> 1.36 </td><td> 198 </td><td> 1.15 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 30 </td> <td> 133 </td><td> 32.3 </td><td> 11.4 </td><td> 2,808 </td><td> 1.00 </td><td> 5,173 </td><td> 1.00 </td><td> 960 </td>  <td> 1.00 </td><td> 172 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Red spruce </td>                         <td> 2 by 10 </td> <td> 144 </td><td> 14 </td> <td> 32.5 </td><td> 21.9 </td><td> 2,394 </td><td> .66 </td> <td> 3,566 </td><td> .60 </td> <td> 1,180 </td><td> 1.02 </td><td> 181 </td><td> .80 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 26 </td> <td> 60 </td> <td> 37.3 </td><td> 21.3 </td><td> 3,627 </td><td> 1.00 </td><td> 5,900 </td><td> 1.00 </td><td> 1,157 </td><td> 1.00 </td><td> 227 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> White spruce </td>                       <td> 2 by 10 </td> <td> 144 </td><td> 16 </td> <td> 40.7 </td><td> 9.3 </td> <td> 2,239 </td><td> .72 </td> <td> 3,288 </td><td> .63 </td> <td> 1,081 </td><td> 1.08 </td><td> 166 </td><td> .83 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td>  <td> 26 </td> <td> 83 </td> <td> 58.3 </td><td> 10.2 </td><td> 3.090 </td><td> 1.00 </td><td> 5,185 </td><td> 1.00 </td><td> 998 </td>  <td> 1.00 </td><td> 199 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="14"><i>Note.&mdash;Following is an
+ explanation of the abbreviations used in the foregoing tables:</i><br>
+ F.S. at E.L. = Fiber stress at elastic limit.<br>
+ M. of E. = Modulus of elasticity.<br>
+ M. of R. = Modulus of rupture.<br>
+ Cr. str. at E.L. = Crushing strength at elastic limit.<br>
+ Cr. str. at max. ld. = Crushing strength at maximum load. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg141"></a>
+
+<center>
+<table id="TABLE_19" summary="TABLE_XIX" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th rowspan="1" colspan="15"> TABLE XIX </th></tr>
+<tr><th rowspan="1" colspan="15"> COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL </th></tr>
+<tr><th rowspan="3" colspan="1"> Species </th>                              <th rowspan="1" colspan="6"> Compression parallel to grain </th>
+                                                                                                                                                                          <th rowspan="1" colspan="5"> Compression perpendicular to grain </th>
+                                                                                                                                                                                                                                                                <th rowspan="1" colspan="3"> Shear </th></tr>
+<tr>                                                                        <th> Size of specimen </th>
+                                                                                             <th> No. of tests </th>
+                                                                                                           <th> Per cent of moisture </th>
+                                                                                                                          <th> Cr. str. at E. L. per square inch </th>
+                                                                                                                                          <th> M. of E. per square inch </th>
+                                                                                                                                                          <th> Cr. str. at max. ld., per square inch </th>
+                                                                                                                                                                          <th> Stress area </th>
+                                                                                                                                                                                           <th> Height </th>
+                                                                                                                                                                                                        <th> No. of tests </th>
+                                                                                                                                                                                                                      <th> Per cent of moisture </th>
+                                                                                                                                                                                                                                     <th>Cr. str. at max. ld., per square inch </th>
+                                                                                                                                                                                                                                                   <th> No. of tests </th>
+                                                                                                                                                                                                                                                                 <th> Per cent of moisture </th>
+                                                                                                                                                                                                                                                                                <th> Shear strength per square inch </th></tr>
+<tr>                                                                        <th><i>Inches</i></th>
+                                                                                             <th><br></th> <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                          <th>1,000 <i>lbs.</i></th>
+                                                                                                                                                          <th><i>Lbs.</i></th>
+                                                                                                                                                                          <th><i>Inches</i></th>
+                                                                                                                                                                                           <th><i>Inches</i></th>
+                                                                                                                                                                                                        <th><br></th> <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                                                                                   <th><br></th> <th><br></th>  <th><i>Lbs.</i></th></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>                      <td> 4 by 4 </td><td> 46 </td> <td> 26.3 </td><td> 3,480 </td><td><br></td>   <td> 4,800 </td><td> 4 by 4 </td><td> 4 </td> <td> 22 </td> <td> 25.3 </td><td> 568 </td><td> 44 </td> <td> 21.8 </td><td> 973 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 14 </td> <td> 34.7 </td><td><br></td>   <td><br></td>   <td> 4,400 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>                        <td> 6 by 6 </td><td> 515 </td><td> 30.7 </td><td> 2,780 </td><td> 1,181 </td><td> 3,500 </td><td> 4 by 8 </td><td> 16 </td><td> 259 </td><td> 30.3 </td><td> 570 </td><td> 531 </td><td> 29.7 </td><td> 765 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 6 </td><td> 170 </td><td> 30.9 </td><td> 2,720 </td><td> 2,123 </td><td> 3,490 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 902 </td><td> 29.8 </td><td> 3,500 </td><td> 1,925 </td><td> 4,030 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Douglas&nbsp;fir&nbsp;(fire-killed) </td><td> 6 by 6 </td><td> 108 </td><td> 34.8 </td><td> 2,620 </td><td> 1,801 </td><td> 3,290 </td><td> 6 by 8 </td><td> 16 </td><td> 24 </td> <td> 33.7 </td><td> 368 </td><td> 77 </td> <td> 35.8 </td><td> 631 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 204 </td><td> 37.9 </td><td><br></td>   <td><br></td>   <td> 3,430 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td>                     <td> 6 by 6 </td><td> 95 </td> <td> 41.2 </td><td> 2,514 </td><td> 1,565 </td><td> 3,436 </td><td> 5 by 8 </td><td> 16 </td><td> 12 </td> <td> 37.7 </td><td> 361 </td><td> 179 </td><td> 47.0 </td><td> 704 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 5 by 8 </td><td> 23 </td> <td> 43.5 </td><td> 2,241 </td><td> 1,529 </td><td> 3,423 </td><td> 5 by 8 </td><td> 14 </td><td> 12 </td> <td> 42.8 </td><td> 366 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 281 </td><td> 51.4 </td><td><br></td>   <td><br></td>   <td> 3,570 </td><td> 5 by 8 </td><td> 12 </td><td> 24 </td> <td> 53.0 </td><td> 325 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 5 by 5 </td><td> 8 </td> <td> 24 </td> <td> 47.0 </td><td> 344 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 2 </td><td> 2 </td> <td> 277 </td><td> 48.5 </td><td> 400 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Western larch </td>                      <td> 6 by 6 </td><td> 107 </td><td> 49.1 </td><td> 2,675 </td><td> 1,575 </td><td> 3,510 </td><td> 6 by 8 </td><td> 16 </td><td> 22 </td> <td> 43.6 </td><td> 417 </td><td> 179 </td><td> 40.7 </td><td> 700 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 491 </td><td> 50.6 </td><td> 3,026 </td><td> 1,545 </td><td> 3,696 </td><td> 6 by 8 </td><td> 12 </td><td> 20 </td> <td> 40.2 </td><td> 416 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 6 </td><td> 6 </td> <td> 53 </td> <td> 52.8 </td><td> 478 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 4 </td><td> 4 </td> <td> 30 </td> <td> 50.4 </td><td> 472 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Loblolly pine </td>                      <td> 8 by 8 </td><td> 14 </td> <td> 63.4 </td><td> 1,560 </td><td> 365 </td>  <td> 2,140 </td><td> 8 by 4 </td><td> 8 </td> <td> 16 </td> <td> 67.2 </td><td> 392 </td><td> 121 </td><td> 83.2 </td><td> 630 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 8 </td><td> 18 </td> <td> 60.0 </td><td> 2,430 </td><td> 691 </td>  <td> 3,560 </td><td> 4 by 4 </td><td> 8 </td> <td> 38 </td> <td> 44.6 </td><td> 546 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 53 </td> <td> 74.0 </td><td><br></td>   <td><br></td>   <td> 3,240 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>                           <td> 6 by 7 </td><td> 4 </td>  <td> 49.9 </td><td> 2,332 </td><td> 1,432 </td><td> 3,032 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td> 24 </td> <td> 39.2 </td><td> 668 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 7 </td><td> 6 </td>  <td> 27.7 </td><td> 2,444 </td><td> 1,334 </td><td> 3,360 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 165 </td><td> 36.8 </td><td><br></td>   <td><br></td>   <td> 3,190 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Western hemlock </td>                    <td> 6 by 6 </td><td> 82 </td> <td> 46.6 </td><td> 2,905 </td><td> 1,617 </td><td> 3,355 </td><td> 6 by 4 </td><td> 6 </td> <td> 30 </td> <td> 48.7 </td><td> 434 </td><td> 54 </td> <td> 65.7 </td><td> 630 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 131 </td><td> 55.6 </td><td> 2,938 </td><td> 1,737 </td><td> 3,392 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Redwood </td>                            <td> 6 by 6 </td><td> 34 </td> <td> 83.6 </td><td> 3,194 </td><td> 1,240 </td><td> 3,882 </td><td> 6 by 8 </td><td> 16 </td><td> 13 </td> <td> 86.7 </td><td> 473 </td><td> 148 </td><td> 84.2 </td><td> 742 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 143 </td><td> 36.8 </td><td> 3,490 </td><td> 1,222 </td><td> 3,980 </td><td> 6 by 6 </td><td> 12 </td><td> 14 </td> <td> 83.0 </td><td> 424 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 6 by 7 </td><td> 9 </td> <td> 13 </td> <td> 74.7 </td><td> 477 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 6 by 3 </td><td> 14 </td><td> 13 </td> <td> 75.6 </td><td> 411 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 6 by 2 </td><td> 12 </td><td> 12 </td> <td> 66.5 </td><td> 430 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 6 by 2 </td><td> 10 </td><td> 11 </td> <td> 55.0 </td><td> 423 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 6 by 2 </td><td> 8 </td> <td> 12 </td> <td> 56.7 </td><td> 396 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 2 </td><td> 2 </td> <td> 186 </td><td> 75.5 </td><td> 569 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>                        <td> 6 by 7 </td><td> 5 </td>  <td> 29.0 </td><td> 1,928 </td><td> 905 </td>  <td> 2,404 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td> 20 </td> <td> 26.7 </td><td> 589 </td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 4 by 7 </td><td> 8 </td>  <td> 28.4 </td><td> 2,154 </td><td> 1,063 </td><td> 2,652 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                                 <td> 2 by 2 </td><td> 178 </td><td> 26.8 </td><td><br></td>   <td><br></td>   <td> 2,504 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Red spruce </td>                         <td> 2 by 2 </td><td> 58 </td> <td> 35.4 </td><td><br></td>   <td><br></td>   <td> 2,750 </td><td> 2 by 2 </td><td> 2 </td> <td> 43 </td> <td> 31.8 </td><td> 310 </td><td> 30 </td> <td> 32.0 </td><td> 758 </td></tr>
+<tr><td style="text-align: left;"> White spruce </td>                       <td> 2 by 2 </td><td> 84 </td> <td> 61.0 </td><td><br></td>   <td><br></td>   <td> 2,370 </td><td> 2 by 2 </td><td> 2 </td> <td> 46 </td> <td> 50.4 </td><td> 270 </td><td> 40 </td> <td> 58.0 </td><td> 651 </td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="15"><i>Note.&mdash;Following is an
+ explanation of the abbreviations used in the foregoing tables:</i><br>
+ F.S. at E.L. = Fiber stress at elastic limit.<br>
+ M. of E. = Modulus of elasticity.<br>
+ M. of R. = Modulus of rupture.<br>
+ Cr. str. at E.L. = Crushing strength at elastic limit.<br>
+ Cr. str. at max. ld. = Crushing strength at maximum load. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg142"></a>
+
+<center>
+<table id="TABLE_20" summary="TABLE_XX" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th colspan="14"> TABLE XX </th></tr>
+<tr><th colspan="14"> BENDING TESTS ON AIR-SEASONED MATERIAL </th></tr>
+<tr><th rowspan="3"> Species </th>                           <th colspan="2"> Sizes </th>    <th rowspan="2"> Number of tests </th>
+                                                                                                           <th rowspan="2"> Per cent of moisture </th>
+                                                                                                                          <th rowspan="2"> Rings per inch </th>
+                                                                                                                                         <th colspan="2"> F.S. at E.L. </th>
+                                                                                                                                                                        <th colspan="2"> M. of R. </th> <th colspan="2"> M. of E. </th><th colspan="2"> Calculated shear </th></tr>
+<tr>                                                         <th> Cross&nbsp;Section </th>
+                                                                               <th> Span </th>
+                                                                                                                                         <th> Average per sq. inch </th>
+                                                                                                                                                         <th> Ratio to 2" by 2" </th>
+                                                                                                                                                                        <th> Average per sq. inch </th>
+                                                                                                                                                                                         <th> Ratio to 2" by 2" </th>
+                                                                                                                                                                                                        <th> Average per sq. inch </th>
+                                                                                                                                                                                                                        <th> Ratio to 2" by 2" </th>
+                                                                                                                                                                                                                                       <th> Average per sq. inch </th>
+                                                                                                                                                                                                                                                     <th> Ratio to 2" by 2" </th></tr>
+<tr>                                                         <th><i>Inches</i></th>
+                                                                               <th><i>Ins.</i></th>
+                                                                                             <th><br></th> <th><br></th>  <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                         <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                         <th><br></th>  <th> 1,000 <i>lbs.</i></th>
+                                                                                                                                                                                                                        <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                                                                                     <th><br></th></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>       <td> 8 by 16 </td><td> 180 </td><td> 5 </td>  <td> 22.2 </td><td> 16.0 </td><td> 3,390 </td><td> 0.50 </td><td> 4,274 </td> <td> 0.37 </td><td> 1,747 </td><td> 1.00 </td><td> 288 </td><td> 0.75 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 16 </td><td> 132 </td><td> 1 </td>  <td> 23.4 </td><td> 17.1 </td><td> 3,470 </td><td> .51 </td> <td> 6,610 </td> <td> .57 </td> <td> 1,501 </td><td> .86 </td> <td> 388 </td><td> 1.01 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 10 </td><td> 177 </td><td> 2 </td>  <td> 19.0 </td><td> 8.8 </td> <td> 4,560 </td><td> .68 </td> <td> 7,880 </td> <td> .68 </td> <td> 1,722 </td><td> .99 </td> <td> 214 </td><td> .56 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 11 </td><td> 180 </td><td> 1 </td>  <td> 18.4 </td><td> 23.9 </td><td> 3,078 </td><td> .46 </td> <td> 8,000 </td> <td> .69 </td> <td> 1,660 </td><td> .95 </td> <td> 251 </td><td> .66 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 8 </td> <td> 177 </td><td> 6 </td>  <td> 20.0 </td><td> 13.7 </td><td> 4,227 </td><td> .63 </td> <td> 8,196 </td> <td> .71 </td> <td> 1,634 </td><td> .94 </td> <td> 177 </td><td> .46 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 17 </td> <td> 15.9 </td><td> 13.9 </td><td> 6,750 </td><td> 1.00 </td><td> 11,520 </td><td> 1.00 </td><td> 1,740 </td><td> 1.00 </td><td> 383 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>         <td> 8 by 16 </td><td> 180 </td><td> 91 </td> <td> 20.8 </td><td> 13.1 </td><td> 4,563 </td><td> .68 </td> <td> 6,372 </td> <td> .61 </td> <td> 1,549 </td><td> .91 </td> <td> 269 </td><td> .64 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 5 by 8 </td> <td> 180 </td><td> 30 </td> <td> 14.9 </td><td> 12.2 </td><td> 5,065 </td><td> .76 </td> <td> 6,777 </td> <td> .65 </td> <td> 1,853 </td><td> 1.09 </td><td> 218 </td><td> .52 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 24 </td> <td> 211 </td><td> 19.0 </td><td> 16.4 </td><td> 6,686 </td><td> 1.00 </td><td> 10,378 </td><td> 1.00 </td><td> 1,695 </td><td> 1.00 </td><td> 419 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td>      <td> 8 by 16 </td><td> 180 </td><td> 3 </td>  <td> 17.0 </td><td> 12.3 </td><td> 4,220 </td><td> .54 </td> <td> 6,030 </td> <td> .50 </td> <td> 1,517 </td><td> .85 </td> <td> 398 </td><td> .98 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 8 by 14 </td><td> 180 </td><td> 3 </td>  <td> 16.0 </td><td> 12.3 </td><td> 4,253 </td><td> .55 </td> <td> 5,347 </td> <td> .44 </td> <td> 1,757 </td><td> .98 </td> <td> 307 </td><td> .76 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 8 by 12 </td><td> 180 </td><td> 7 </td>  <td> 16.0 </td><td> 12.4 </td><td> 5,051 </td><td> .65 </td> <td> 7,331 </td> <td> .60 </td> <td> 1,803 </td><td> 1.01 </td><td> 361 </td><td> .89 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 5 by 8 </td> <td> 180 </td><td> 6 </td>  <td> 12.2 </td><td> 22.5 </td><td> 7,123 </td><td> .92 </td> <td> 9,373 </td> <td> .77 </td> <td> 1,985 </td><td> 1.11 </td><td> 301 </td><td> .74 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 67 </td> <td> 14.2 </td><td> 13.7 </td><td> 7,780 </td><td> 1.00 </td><td> 12,120 </td><td> 1.00 </td><td> 1,792 </td><td> 1.00 </td><td> 404 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Western larch </td>       <td> 8 by 16 </td><td> 180 </td><td> 23 </td> <td> 18.3 </td><td> 21.9 </td><td> 3,343 </td><td> .57 </td> <td> 5,440 </td> <td> .53 </td> <td> 1,409 </td><td> .90 </td> <td> 349 </td><td> .96 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 8 by 12 </td><td> 180 </td><td> 29 </td> <td> 17.8 </td><td> 23.4 </td><td> 3,631 </td><td> .62 </td> <td> 6,186 </td> <td> .60 </td> <td> 1,549 </td><td> .99 </td> <td> 295 </td><td> .81 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 5 by 8 </td> <td> 180 </td><td> 10 </td> <td> 13.6 </td><td> 27.6 </td><td> 4,730 </td><td> .80 </td> <td> 7,258 </td> <td> .71 </td> <td> 1,620 </td><td> 1.04 </td><td> 221 </td><td> .61 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 240 </td><td> 16.1 </td><td> 26.8 </td><td> 5,880 </td><td> 1.00 </td><td> 10,254 </td><td> 1.00 </td><td> 1,564 </td><td> 1.00 </td><td> 364 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Loblolly pine </td>       <td> 8 by 16 </td><td> 180 </td><td> 14 </td> <td> 20.5 </td><td> 7.4 </td> <td> 4,195 </td><td> .81 </td> <td> 6,734 </td> <td> .72 </td> <td> 1,619 </td><td> 1.10 </td><td> 462 </td><td> 1.45 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 16 </td><td> 126 </td><td> 4 </td>  <td> 20.2 </td><td> 5.0 </td> <td> 2,432 </td><td> .47 </td> <td> 4,295 </td> <td> .46 </td> <td> 1,324 </td><td> .90 </td> <td> 266 </td><td> .84 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 10 </td><td> 174 </td><td> 3 </td>  <td> 21.3 </td><td> 4.7 </td> <td> 3,100 </td><td> .60 </td> <td> 6,167 </td> <td> .66 </td> <td> 1,449 </td><td> .99 </td> <td> 173 </td><td> .54 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 12 </td><td> 174 </td><td> 4 </td>  <td> 19.8 </td><td> 4.7 </td> <td> 2,713 </td><td> .52 </td> <td> 5,745 </td> <td> .61 </td> <td> 1,249 </td><td> .85 </td> <td> 185 </td><td> .58 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 8 by 8 </td> <td> 180 </td><td> 9 </td>  <td> 22.9 </td><td> 4.9 </td> <td> 2,903 </td><td> .56 </td> <td> 4,557 </td> <td> .48 </td> <td> 1,136 </td><td> .77 </td> <td> 93 </td> <td> .29 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 7 </td> <td> 144 </td><td> 2 </td>  <td> 21.1 </td><td> 5.0 </td> <td> 2,990 </td><td> .58 </td> <td> 4,968 </td> <td> .53 </td> <td> 1,286 </td><td> .88 </td> <td> 116 </td><td> .36 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 8 </td> <td> 132 </td><td> 8 </td>  <td> 19.5 </td><td> 9.1 </td> <td> 3,384 </td><td> .65 </td> <td> 6,194 </td> <td> .66 </td> <td> 1,200 </td><td> .82 </td> <td> 196 </td><td> .62 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 123 </td><td> 17.6 </td><td> 6.6 </td> <td> 5,170 </td><td> 1.00 </td><td> 9,400 </td> <td> 1.00 </td><td> 1,467 </td><td> 1.00 </td><td> 318 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>            <td> 6 by 12 </td><td> 162 </td><td> 5 </td>  <td> 23.0 </td><td> 15.1 </td><td> 3,434 </td><td> .45 </td> <td> 5,640 </td> <td> .43 </td> <td> 1,330 </td><td> .82 </td> <td> 318 </td><td> .75 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 10 </td><td> 162 </td><td> 4 </td>  <td> 14.4 </td><td> 9.7 </td> <td> 4,100 </td><td> .54 </td> <td> 5,320 </td> <td> .41 </td> <td> 1,386 </td><td> .84 </td> <td> 252 </td><td> .59 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 47 </td> <td> 11.3 </td><td> 16.2 </td><td> 7,630 </td><td> 1.00 </td><td> 13,080 </td><td> 1.00 </td><td> 1,620 </td><td> 1.00 </td><td> 425 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Western&nbsp;hemlock </td><td> 8 by 16 </td><td> 180 </td><td> 44 </td> <td> 17.7 </td><td> 17.8 </td><td> 4,398 </td><td> .69 </td> <td> 6,420 </td> <td> .62 </td> <td> 1,737 </td><td> 1.04 </td><td> 406 </td><td> 1.06 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 28 </td> <td> 311 </td><td> 17.9 </td><td> 19.4 </td><td> 6,333 </td><td> 1.00 </td><td> 10,369 </td><td> 1.00 </td><td> 1,666 </td><td> 1.00 </td><td> 382 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Redwood </td>             <td> 8 by 16 </td><td> 180 </td><td> 6 </td>  <td> 26.3 </td><td> 22.4 </td><td> 3,797 </td><td> .79 </td> <td> 4,428 </td> <td> .57 </td> <td> 1,107 </td><td> .96 </td> <td> 294 </td><td> 1.05 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 6 by 12 </td><td> 180 </td><td> 6 </td>  <td> 16.1 </td><td> 17.7 </td><td> 3,175 </td><td> .66 </td> <td> 3,353 </td> <td> .43 </td> <td> 728 </td>  <td> .64 </td> <td> 167 </td><td> .60 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 7 by 9 </td> <td> 180 </td><td> 6 </td>  <td> 15.9 </td><td> 15.2 </td><td> 3,280 </td><td> .69 </td> <td> 4,002 </td> <td> .51 </td> <td> 1,104 </td><td> .96 </td> <td> 147 </td><td> .53 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 3 by 14 </td><td> 180 </td><td> 6 </td>  <td> 13.1 </td><td> 24.4 </td><td><br></td>   <td><br></td>  <td> 5,033 </td> <td> .64 </td> <td><br></td>   <td><br></td>  <td> 291 </td><td> 1.04 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 12 </td><td> 180 </td><td> 5 </td>  <td> 13.8 </td><td> 14.4 </td><td> 3,928 </td><td> .82 </td> <td> 5,336 </td> <td> .68 </td> <td> 1,249 </td><td> 1.09 </td><td> 260 </td><td> .93 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 10 </td><td> 180 </td><td> 5 </td>  <td> 13.8 </td><td> 24.8 </td><td> 3,757 </td><td> .79 </td> <td> 4,606 </td> <td> .59 </td> <td> 1,198 </td><td> 1.05 </td><td> 186 </td><td> .67 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 8 </td> <td> 180 </td><td> 6 </td>  <td> 13.7 </td><td> 20.7 </td><td> 4,314 </td><td> .90 </td> <td> 5,050 </td> <td> .65 </td> <td> 1,313 </td><td> 1.15 </td><td> 166 </td><td> .60 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 28 </td> <td> 122 </td><td> 15.2 </td><td> 18.8 </td><td> 4,777 </td><td> 1.00 </td><td> 7,798 </td> <td> 1.00 </td><td> 1,146 </td><td> 1.00 </td><td> 279 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>         <td> 6 by 12 </td><td> 162 </td><td> 5 </td>  <td> 16.7 </td><td> 8.1 </td> <td> 2,968 </td><td> .56 </td> <td> 5,204 </td> <td> .61 </td> <td> 1,123 </td><td> .97 </td> <td> 286 </td><td> 1.02 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 10 </td><td> 162 </td><td> 5 </td>  <td> 13.7 </td><td> 12.0 </td><td> 5,170 </td><td> .98 </td> <td> 6,904 </td> <td> .82 </td> <td> 1,712 </td><td> 1.48 </td><td> 317 </td><td> 1.13 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td> <td> 30 </td> <td> 60 </td> <td> 14.9 </td><td> 11.2 </td><td> 5,280 </td><td> 1.00 </td><td> 8,470 </td> <td> 1.00 </td><td> 1,158 </td><td> 1.00 </td><td> 281 </td><td> 1.00 </td></tr>
+<tr><td style="text-align: left;" colspan="14"> <i>Note.&mdash;Following is an
+ explanation of the abbreviations used in the foregoing tables:</i><br>
+ F.S. at E.L. = Fiber stress at elastic limit.<br>
+ M. of E. = Modulus of elasticity.<br>
+ M. of R. = Modulus of rupture.<br>
+ Cr. str. at E.L. = Crushing strength at elastic limit.<br>
+ Cr. str. at max. ld. = Crushing strength at maximum load. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg143"></a>
+
+<center>
+<table id="TABLE_21" summary="TABLE_XXI" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th colspan="15"> TABLE XXI </th></tr>
+<tr><th colspan="15"> COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL </th></tr>
+<tr><th rowspan="3"> Species </th>                           <th colspan="6"> Compression parallel to grain </th>                                          <th colspan="5"> Compression perpendicular to grain </th>                <th colspan="3"> Shear </th></tr>
+<tr>                                                         <th> Size of specimen </th>
+                                                                              <th> No. of tests </th>
+                                                                                            <th> Per cent of moisture </th>
+                                                                                                           <th> Cr. str. at E. L. per square inch </th>
+                                                                                                                           <th> M. of E. per square inch </th>
+                                                                                                                                           <th> Cr. str. at max. ld., per square inch </th>
+                                                                                                                                                           <th> Stress area </th>
+                                                                                                                                                                            <th> Height </th>
+                                                                                                                                                                                         <th> No. of tests </th>
+                                                                                                                                                                                                       <th> Per cent of moisture </th>
+                                                                                                                                                                                                                      <th> Cr. str. at max. ld., per square inch </th>
+                                                                                                                                                                                                                                    <th> No. of tests </th>
+                                                                                                                                                                                                                                                  <th> Per cent of moisture </th>
+                                                                                                                                                                                                                                                                 <th> Shear strength </th></tr>
+<tr>                                                         <th><i>Inches</i></th>
+                                                                              <th><br></th> <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                           <th> 1,000 <i>lbs.</i></th>
+                                                                                                                                           <th><i>Lbs.</i></th>
+                                                                                                                                                           <th><i>Inches</i></th>
+                                                                                                                                                                            <th><i>Inches</i></th>
+                                                                                                                                                                                         <th><br></th> <th><br></th>  <th><i>Lbs.</i></th>
+                                                                                                                                                                                                                                    <th><br></th> <th><br></th>  <th><i>Lbs.</i></th></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>       <td> 4 by 5 </td><td> 46 </td> <td> 26.3 </td><td> 3,480 </td><td><br></td>   <td> 4,800 </td><td> 4 by 5 </td><td> 4 </td> <td> 22 </td> <td> 25.1 </td><td> 572 </td><td> 52 </td> <td> 20.2 </td><td> 984 </td></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>         <td> 6 by 6 </td><td> 259 </td><td> 20.3 </td><td> 3,271 </td><td> 1,038 </td><td> 4,258 </td><td> 4 by 8 </td><td> 16 </td><td> 44 </td> <td> 20.8 </td><td> 732 </td><td> 465 </td><td> 22.1 </td><td> 822 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 247 </td><td> 18.7 </td><td> 3,842 </td><td> 1,084 </td><td> 5,002 </td><td> 4 by 8 </td><td> 10 </td><td> 32 </td> <td> 18.1 </td><td> 584 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 4 </td><td> 8 </td> <td> 51 </td> <td> 20.2 </td><td> 638 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> <br></td>                 <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 4 </td><td> 6 </td> <td> 49 </td> <td> 24.0 </td><td> 613 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 4 </td><td> 4 </td> <td> 29 </td> <td> 24.8 </td><td> 603 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td>      <td> 6 by 6 </td><td> 29 </td> <td> 15.7 </td><td> 4,070 </td><td> 1,951 </td><td> 6,030 </td><td> 8 by 5 </td><td> 16 </td><td> 4 </td>  <td> 17.8 </td><td> 725 </td><td> 85 </td> <td><br></td>  <td> 1,135 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 57 </td> <td> 14.2 </td><td><br></td>   <td><br></td>   <td> 6,380 </td><td> 8 by 5 </td><td> 14 </td><td> 3 </td>  <td> 16.3 </td><td> 757 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 8 by 5 </td><td> 12 </td><td> 5 </td>  <td> 15.1 </td><td> 730 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 5 by 5 </td><td> 8 </td> <td> 6 </td>  <td> 13.0 </td><td> 918 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 2 </td><td> 2 </td> <td> 57 </td> <td> 13.9 </td><td> 926 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Western larch </td>       <td> 6 by 6 </td><td> 112 </td><td> 16.0 </td><td><br></td>   <td><br></td>   <td> 5,445 </td><td> 8 by 6 </td><td> 16 </td><td> 17 </td> <td> 18.8 </td><td> 491 </td><td> 193 </td><td> 15.0 </td><td> 905 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 4 </td><td> 81 </td> <td> 14.7 </td><td><br></td>   <td><br></td>   <td> 6,161 </td><td> 8 by 6 </td><td> 12 </td><td> 18 </td> <td> 17.6 </td><td> 526 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 270 </td><td> 14.8 </td><td><br></td>   <td><br></td>   <td> 5,934 </td><td> 5 by 4 </td><td> 8 </td> <td> 22 </td> <td> 13.3 </td><td> 735 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Loblolly pine </td>       <td> 6 by 6 </td><td> 23 </td> <td><br></td>  <td> 3,357 </td><td> 1,693 </td><td> 5,005 </td><td> 8 by 5 </td><td> 16 </td><td> 12 </td> <td> 19.8 </td><td> 602 </td><td> 156 </td><td> 11.3 </td><td> 1,115 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 5 by 5 </td><td> 10 </td> <td> 22.4 </td><td> 2,217 </td><td> 545 </td>  <td> 2,950 </td><td> 8 by 5 </td><td> 8 </td> <td> 7 </td>  <td> 22.9 </td><td> 679 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 8 </td><td> 8 </td>  <td> 19.4 </td><td> 3,010 </td><td> 633 </td>  <td> 3,920 </td><td> 4 by 5 </td><td> 8 </td> <td> 8 </td>  <td> 19.5 </td><td> 715 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 69 </td> <td><br></td>  <td><br></td>   <td><br></td>   <td> 5,547 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>            <td> 6 by 7 </td><td> 3 </td>  <td> 15.7 </td><td> 2,257 </td><td> 1,042 </td><td> 3,323 </td><td> 2 by 2 </td><td> 2 </td> <td> 57 </td> <td> 16.2 </td><td> 697 </td><td> 60 </td> <td> 14.0 </td><td> 879 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 7 </td><td> 3 </td>  <td> 13.6 </td><td> 3,780 </td><td> 1,301 </td><td> 4,823 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 4 </td><td> 57 </td> <td> 14.9 </td><td> 3,386 </td><td> 1,353 </td><td> 4,346 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 66 </td> <td> 14.6 </td><td><br></td>   <td><br></td>   <td> 4,790 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Western&nbsp;hemlock </td><td> 6 by 6 </td><td> 102 </td><td> 18.6 </td><td> 4,840 </td><td> 2,140 </td><td> 5,814 </td><td> 7 by 6 </td><td> 15 </td><td> 25 </td> <td> 18.2 </td><td> 514 </td><td> 131 </td><td> 17.7 </td><td> 924 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 463 </td><td> 17.0 </td><td> 4,560 </td><td> 1,923 </td><td> 5,403 </td><td> 6 by 6 </td><td> 6 </td> <td> 26 </td> <td> 16.8 </td><td> 431 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 4 by 4 </td><td> 4 </td> <td> 6 </td>  <td> 15.9 </td><td> 488 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Redwood </td>             <td> 6 by 6 </td><td> 18 </td> <td> 16.9 </td><td><br></td>   <td><br></td>   <td> 4,276 </td><td> 8 by 6 </td><td> 16 </td><td> 5 </td>  <td> 25.4 </td><td> 548 </td><td> 95 </td> <td> 12.4 </td><td> 671 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 115 </td><td> 14.6 </td><td><br></td>   <td><br></td>   <td> 5,119 </td><td> 6 by 6 </td><td> 12 </td><td> 6 </td>  <td> 14.7 </td><td> 610 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 7 by 6 </td><td> 9 </td> <td> 5 </td>  <td> 14.8 </td><td> 500 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 3 by 6 </td><td> 14 </td><td> 2 </td>  <td> 12.6 </td><td> 470 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 6 </td><td> 12 </td><td> 2 </td>  <td> 16.2 </td><td> 498 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 6 </td><td> 10 </td><td> 4 </td>  <td> 14.3 </td><td> 511 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 6 </td><td> 8 </td> <td> 2 </td>  <td> 13.2 </td><td> 429 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td><br></td>    <td><br></td> <td><br></td>  <td><br></td>   <td><br></td>   <td><br></td>   <td> 2 by 2 </td><td> 2 </td> <td> 145 </td><td> 13.8 </td><td> 564 </td><td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>         <td> 6 by 7 </td><td> 4 </td>  <td> 15.2 </td><td> 2,670 </td><td> 1,182 </td><td> 4,212 </td><td> 2 by 2 </td><td> 2 </td> <td> 36 </td> <td> 10.0 </td><td> 924 </td><td> 44 </td> <td> 11.9 </td><td> 1,145 </td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 7 </td><td> 2 </td>  <td> 22.2 </td><td> 3,275 </td><td> 1,724 </td><td> 4,575 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 4 by 4 </td><td> 55 </td> <td> 16.6 </td><td> 3,048 </td><td> 1,367 </td><td> 4,217 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;"><br></td>                  <td> 2 by 2 </td><td> 44 </td> <td> 11.2 </td><td><br></td>   <td><br></td>   <td> 7,550 </td><td><br></td>    <td><br></td><td><br></td> <td><br></td>  <td><br></td> <td><br></td> <td><br></td>  <td><br></td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="15"><i>Note.&mdash;Following is an explanation of the abbreviations used in the foregoing tables:</i><br> F.S. at E.L. = Fiber stress at elastic limit.<br> M. of E. = Modulus of elasticity.<br> M. of R. = Modulus of rupture.<br> Cr. str. at E.L. = Crushing strength at elastic limit.<br> Cr. str. at max. ld. = Crushing strength at maximum load. </td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg144"></a>
+
+<center>
+<table id="TABLE_22" summary="TABLE_XXII" border="3" cellspacing="0" cellpadding="1" width="100%">
+<tbody>
+<tr><th colspan="15"> TABLE XXII </th></tr>
+<tr><th colspan="15"> [b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMBER[c] </th></tr>
+<tr><th colspan="15"> EXPRESSED IN POUNDS PER SQUARE INCH </th></tr>
+<tr><td colspan="15"> (From Manual of the American Railway Engineering Assn., 1911, p. 153) </td></tr>
+<tr><td style="text-align: left;" colspan="15"> NOTE.&mdash;The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles the unit-stresses may be increased twenty-five (25) per cent. For buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit-stresses may be increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when the load is first applied, only fifty (50) per cent of the corresponding modulus of elasticity given in the table is to be employed. </td></tr>
+<tr><th rowspan="3" > KIND&nbsp;OF&nbsp;TIMBER </th>    <th colspan="3"> BENDING </th>                    <th colspan="4"> SHEARING </th>                               <th colspan="6"> COMPRESSION </th>                                                                                     <th rowspan="3"> Ratio of length of stringer to depth </th></tr>
+<tr>                                                    <th colspan="2"> Extreme fibre stress </th>
+                                                                                      <th> Modulus of elasticity </th>
+                                                                                                          <th colspan="2"> Parallel to the grain </th>
+                                                                                                                                         <th colspan="2"> Longitudinal shear in beams </th>
+                                                                                                                                                                        <th colspan="2"> Perpendicular to the grain </th>
+                                                                                                                                                                                                    <th colspan="2"> Parallel to the grain </th>
+                                                                                                                                                                                                                                     <th rowspan="2"> For columns under 15 diameters working stress </th>
+                                                                                                                                                                                                                                                   <th rowspan="2"> Formul&aelig;&nbsp;for&nbsp;working&nbsp;stress in long columns over 15 diameters </th></tr>
+<tr>                                                    <th> Average ultimate </th>
+                                                                       <th> Working stress </th>
+                                                                                      <th> Average </th>  <th> Average ultimate </th>
+                                                                                                                           <th> Working stress </th>
+                                                                                                                                         <th> Average ultimate </th>
+                                                                                                                                                          <th> Working stress </th>
+                                                                                                                                                                        <th> Elastic limit </th>
+                                                                                                                                                                                      <th> Working stress </th>
+                                                                                                                                                                                                    <th> Average ultimate </th>
+                                                                                                                                                                                                                      <th> Working stress </th></tr>
+<tr><td style="text-align: left;"> Douglas fir </td>    <td> 6100 </td><td> 1200 </td><td> 1,510,000 </td><td> 690 </td>   <td> 170 </td><td> 270 </td>   <td> 110 </td><td> 630 </td><td> 310 </td><td> 3600 </td>   <td> 1200 </td><td> 900 </td><td> 1200 (1 - <i>l </i>/ 60 <i>d</i>) </td><td> 10 </td></tr>
+<tr><td style="text-align: left;"> Longleaf pine </td>  <td> 6500 </td><td> 1300 </td><td> 1,610,000 </td><td> 720 </td>   <td> 180 </td><td> 300 </td>   <td> 120 </td><td> 520 </td><td> 260 </td><td> 3800 </td>   <td> 1300 </td><td> 980 </td><td> 1300 (1 - <i>l </i>/ 60 <i>d</i>) </td><td> 10 </td></tr>
+<tr><td style="text-align: left;"> Shortleaf pine </td> <td> 5600 </td><td> 1100 </td><td> 1,480,000 </td><td> 710 </td>   <td> 170 </td><td> 330 </td>   <td> 130 </td><td> 340 </td><td> 170 </td><td> 3400 </td>   <td> 1100 </td><td> 830 </td><td> 1100 (1 - <i>l </i>/ 60 <i>d</i>) </td><td> 10 </td></tr>
+<tr><td style="text-align: left;"> White pine </td>     <td> 4400 </td><td> 900 </td> <td> 1,130,000 </td><td> 400 </td>   <td> 100 </td><td> 180 </td>   <td> 70 </td> <td> 290 </td><td> 150 </td><td> 3000 </td>   <td> 1000 </td><td> 750 </td><td> 1000 (1 - <i>l </i>/ 60 <i>d</i>) </td><td> 10 </td></tr>
+<tr><td style="text-align: left;"> Spruce </td>         <td> 4800 </td><td> 1000 </td><td> 1,310,000 </td><td> 600 </td>   <td> 150 </td><td> 170 </td>   <td> 70 </td> <td> 370 </td><td> 180 </td><td> 3200 </td>   <td> 1100 </td><td> 830 </td><td> 1100 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Norway pine </td>    <td> 4200 </td><td> 800 </td> <td> 1,190,000 </td><td> 590[d] </td><td> 130 </td><td> 250 </td>   <td> 100 </td><td><br></td> <td> 150 </td><td> 2600[d] </td><td> 800 </td> <td> 600 </td><td> 800 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Tamarack </td>       <td> 4600 </td><td> 900 </td> <td> 1,220,000 </td><td> 670 </td>   <td> 170 </td><td> 260 </td>   <td> 100 </td><td><br></td> <td> 220 </td><td> 3200[d] </td><td> 1000 </td><td> 750 </td><td> 1000 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Western hemlock </td><td> 5800 </td><td> 1100 </td><td> 1,480,000 </td><td> 630 </td>   <td> 160 </td><td> 270[d] </td><td> 100 </td><td> 440 </td><td> 220 </td><td> 3500 </td>   <td> 1200 </td><td> 900 </td><td> 1200 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Redwood </td>        <td> 5000 </td><td> 900 </td> <td> 800,000 </td>  <td> 300 </td>   <td> 80 </td> <td><br></td>    <td><br></td> <td> 400 </td><td> 150 </td><td> 3300 </td>   <td> 900 </td> <td> 680 </td><td> 900 (1 - <i>l </i>/ 60 <i>d</i> )</td><td><br></td></tr>
+<tr><td style="text-align: left;"> Bald cypress </td>   <td> 4800 </td><td> 900 </td> <td> 1,150,000 </td><td> 500 </td>   <td> 120 </td><td><br></td>    <td><br></td> <td> 340 </td><td> 170 </td><td> 3900 </td>   <td> 1100 </td><td> 830 </td><td> 1100 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> Red cedar </td>      <td> 4200 </td><td> 800 </td> <td> 800,000 </td>  <td><br></td>    <td><br></td> <td><br></td>    <td><br></td> <td> 470 </td><td> 230 </td><td> 2800 </td>   <td> 900 </td> <td> 680 </td><td> 900 (1 - <i>l </i>/ 60 <i>d</i>) </td><td><br></td></tr>
+<tr><td style="text-align: left;"> White oak </td>      <td> 5700 </td><td> 1100 </td><td> 1,150,000 </td><td> 840 </td>   <td> 210 </td><td> 270 </td>   <td> 110 </td><td> 920 </td><td> 450 </td><td> 3500 </td>   <td> 1300 </td><td> 980 </td><td> 1300 (1 - <i>l </i>/ 60 <i>d</i>) </td><td> 12 </td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="13"> These unit-stresses are for a green condition of timber and are to be used without increasing the live load stresses for impact. </td><td rowspan="1" colspan="2"><i>l</i> = Length in inches.<br><i>d</i> = Least side in inches.</td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="15">[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.]<br></td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="15">[Footnote c: Green timber in exposed work.]<br></td></tr>
+<tr><td style="text-align: left;" rowspan="1" colspan="15">[Footnote d: Partially air-dry]<br></td></tr>
+</tbody>
+</table>
+</center>
+
+<a name="pg145"></a>
+<a name="BIBLIOGRAPHY"></a><h2>BIBLIOGRAPHY</h2>
+
+<hr>
+
+<h4><a href="#BIB_I">Part I:</a> Some general works on mechanics, materials of construction, and testing of materials.</h4>
+<h4><a href="#BIB_II">Part II:</a> Publications and articles on the mechanical properties of wood, and timber testing.</h4>
+<h4><a href="#BIB_III">Part III:</a> Publications of the U.S. Government on the mechanical properties of wood, and timber testing.</h4>
+
+<hr>
+
+<a name="pg146"></a>
+<a name="pg147"></a>
+<a name="BIB_I"></a>
+<h3>PART I.<br>SOME GENERAL WORKS ON MECHANICS, MATERIALS OF CONSTRUCTION, AND TESTING OF MATERIALS</h3>
+
+<p>ALLAN, WILLIAM: Strength of beams under transverse loads. New
+York, 1893.</p>
+
+<p>ANDERSON, SIR JOHN: The strength of materials and structures.
+London, 1902.</p>
+
+<p>BARLOW, PETER: Strength of materials, 1st ed. 1817; rev. 1867.</p>
+
+<p>BURR, WILLIAM H.: The elasticity and resistance of the materials
+of engineering. New York, 1911.</p>
+
+<p>CHURCH, IRVING P.: Mechanics of engineering. New York, 1911.</p>
+
+<p>HATFIELD, R.G.: Theory of transverse strain. 1877.</p>
+
+<p>HATT, W.K., and SCOFIELD, H.H.: Laboratory manual of testing
+materials. New York, 1913.</p>
+
+<p>JAMESON, J.M.: Exercises in mechanics. (Wiley technical series.)
+New York, 1913.</p>
+
+<p>JAMIESON, ANDREW: Strength of materials. (Applied mechanics and
+mechanical engineering, Vol. II.) London, 1911.</p>
+
+<p>JOHNSON, J.B.: The materials of construction. New York, 1910.</p>
+
+<p>KENT, WILLIAM: The strength of materials. New York, 1890.</p>
+
+<p>KOTTCAMP, J.P.: Exercises for the applied mechanics laboratory.
+(Wiley technical series.) New York, 1913.</p>
+
+<p>LANZA, GAETANO: Applied mechanics. New York, 1901.</p>
+
+<p>MERRIMAN, MANSFIELD: Mechanics of materials. New York, 1912.</p>
+
+<p>MURDOCK, H.E.: Strength of materials. New York, 1911.</p>
+
+<p>RANKINE, WILLIAM J.M.: A manual of applied mechanics. London,
+1901.</p>
+
+<p>THIL, A.: Conclusion de l'�tude pr�sent�e � la Commission des
+m�thodes d'essai des mat�riaux de construction. Paris, 1900.</p>
+
+<p>THURSTON, ROBERT H.: A treatise on non-metallic materials of
+engineering: stone, timber, fuel, lubricants, etc. (Materials of
+engineering, Part I.) New York, 1899.</p>
+
+<p>UNWIN, WILLIAM C.: The testing of materials of construction.
+London, 1899.</p>
+
+<p>WATERBURY, L.A.: Laboratory manual for testing materials of
+construction. New York, 1912.</p>
+
+<p>WOOD, DEVOLSON: A treatise on the  resistance of materials. New
+York, 1897.</p>
+
+<a name="pg148"></a>
+<a name="BIB_II"></a>
+<h3>PART II.<br>PUBLICATIONS AND ARTICLES ON THE MECHANICAL PROPERTIES OF WOOD, AND TIMBER TESTING</h3>
+
+<p>ABBOT, ARTHUR V.: Testing machines, their history, construction
+and use. Van Nostrand's Eng. Mag., Vol. XXX, 1884, pp. 204-214;
+325-344; 382-397; 477-490.</p>
+
+<p>ADAMS, E.E.: Tests to determine the strength of bolted timber
+joints. Cal. Jour, of Technology, Sept., 1904.</p>
+
+<p>ALVAREZ, ARTHUR C.: The strength of long seasoned Douglas fir
+and redwood. Univ. of Cal. Pub. in Eng., Vol. I, No. 2,
+Berkeley, 1913, pp. 11-20.</p>
+
+<p>BARLOW, PETER: An essay on the strength and stress of timber.
+London, 1817; 3d ed., 1826.</p>
+
+<p>&mdash;&mdash;: Experiments on the strength of different kinds of wood
+made in the carriage department, Royal Arsenal, Woolwich. Jour.
+Franklin Inst., Vol. X, 1832, pp. 49-52. Reprinted from
+Philosophical Mag. and Annals of Philos., No. 63, Mch., 1832.</p>
+
+<p>BATES, ONWARD: Pine stringers and floorbeams for bridges. Trans.
+Am. Soc. C.E., Vol. XXIII.</p>
+
+<p>BAUSCHINGER, JOHANN: Untersuchungen �ber die Elasticit�t und
+Festigkeit von Fichten- und Kiefernbauh�lzern. Mitt. a. d.
+mech.-tech. Laboratorium d. k. techn. Hochschule in M�nchen, 9.
+Hft., M�nchen, 1883.</p>
+
+<p>&mdash;&mdash;: Verhandlungen der M�nchener Conferenz und der von ihr
+gew�hlten st�ndigen Commission zur Vereinbarung einheitlicher
+Pr�fungsmethoden f�r Bau- und Constructions-material. <i>Ibid.</i>,
+14. Hft., 1886.</p>
+
+<p>&mdash;&mdash;: Untersuchungen �ber die Elasticit�t und Festigkeit
+verschiedener Nadelh�lzer. <i>Ibid.</i>, 16. Hft., 1887.</p>
+
+<p>BEARE, T. HUDSON: Timber: its strength and how to test it.
+Engineering, London, Dec. 9, 1904.</p>
+
+<p>BEAUVERIE, J.: Le bois. I. Paris, 1905, pp. 105-185.</p>
+
+<p>&mdash;&mdash;: Les bois industriels. Paris, 1910, pp. 55-77. Bending
+tests with wood, executed at the Danish State Testing
+Laboratory, Copenhagen. Proc. Int. Assn. Test. Mat., 1912,
+XXIII<sub><small>2</small></sub>, pp. 17. See also Eng. Record, Vol. LXVI, 1912, p.
+269.</p>
+
+<p>BERG, WALTER G.: Berg's complete timber test record. Chicago,
+1899. Reprint from Am. By. Bridges and Buildings. BOULGER, G.S.:
+Wood. London, 1908, pp. 112-121.</p>
+
+<p>BOUNICEAU,&mdash;: Note et exp�riences sur la torsion des bois.
+[N.p., n.d.]</p>
+
+<p>BOVEY, HENRY T.: Results of experiments at McGill University,
+<a name="pg149"></a>
+Montreal, on the strength of Canadian Douglas fir, red pine,
+white pine, and spruce. Trans. Can. Soc. C.E., Vol. IX, Part I,
+1895, pp. 69-236.</p>
+
+<p>BREUIL, M. PIERRE: Contribution to the discussion on the testing
+of wood. Proc. Int. Assn. Test. Mat., 1906, Disc, 1<i>e</i>, pp. 2.</p>
+
+<p>BROWN, T.S.: An Account of some experiments made by order of
+Col. Totten, at Fort Adams, Newport, R.I., to ascertain the
+relative stiffness and strength of the following kinds of
+timber, viz.: white pine (<i>Pinus strobus</i>), spruce (<i>Abies
+nigra</i>), and southern pine (<i>Pinus australis</i>), also called
+long-leaved pine. Jour. Franklin Inst., Vol. VII (n.s.), 1831,
+pp. 230-238.</p>
+
+<p>BUCHANAN, C.P.: Some tests of old timber. Eng. News, Vol. LXIV,
+No. 23, 1910, p. 67.</p>
+
+<p>BUSGEN, M.: Zur Bestimmung der Holzh�rten. Zeitschrift f. Forst-
+und Jagdwesen. Berlin, 1904, pp. 543-562.</p>
+
+<p>CHEVANDIER, E., et WERTHEIM, G.: M�moire sur les propri�t�s
+m�caniques du bois. Paris, 1846.</p>
+
+<p>CIESLAR, A.: Studien �ber die Qualit�t rasch erwachsenen
+Fichtenholzes. Centralblatt f. d. ges. Forstwesen, Wien, 1902,
+pp. 337-403.</p>
+
+<p>CLINE, McGARVEY: Forest Service investigations of American woods
+with special reference to investigations of mechanical
+properties. Proc. Int. Assn. Test. Mat., 1912, XXIII<sub><small>5</small></sub>, pp.
+17.</p>
+
+<p>&mdash;&mdash;: Forest Service tests to determine the influence of
+different methods and rates of loading on the strength and
+stiffness of timber. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908,
+pp. 535-540.</p>
+
+<p>&mdash;&mdash;: The Forest Products Laboratory: its purpose and work.
+Proc. Am. Soc. Test. Mat., Vol. X, 1910, pp. 477-489.</p>
+
+<p>&mdash;&mdash;: Specifications and grading rules for Douglas fir timber:
+an analysis of Forest Service tests on structural timbers. Proc.
+Am. Soc. Test. Mat., Vol. XI, 1911, pp. 744-766.</p>
+
+<p>Comparative strength and resistance of various tie timbers.
+Elec. Traction Weekly, Chicago, June 15, 1912.</p>
+
+<p>DAY, FRANK M.: Microscopic examination of timber with regard to
+its strength. 1883, pp. 6.</p>
+
+<p>DEWELL, H.D.: Tests of some joints used in heavy timber framing.
+Eng. News, Mch. 19, 1914, pp. 594-598; <i>et seq.</i></p>
+
+<p>D�RR, KARL: Die Festigkeit von Fichten- und Kiefernholz.
+Deutsche Bauzeitung, Berlin, Aug. 17, 1910. See also Zeitschrift
+d. ver. deutsch. Ing., Bd. 54, Nr. 36, 1910, p. 1503.</p>
+
+<p>DUPIN, CHARLES: Exp�riences sur la flexibilit�, la force, et
+l'�lasticit� des bois. Jour, de l'�cole Polytechnique, Vol. X,
+1815.</p>
+
+<p>DUPONT, ADOLPHE, et BOUQUET DE LA GRYE: Les bois indig�nes et
+�trangers. Paris, 1875, pp. 273-352.</p>
+
+<a name="pg150"></a>
+
+<p>ESTRADA, ESTEBAN DUQUE: On the strength and other properties of
+Cuban woods. Van Nostrand's Eng. Mag., Vol. XXIX, 1883, pp.
+417-426; 443-449.</p>
+
+<p>EVERETT, W.H.: Memorandum on mechanical tests of some Indian
+timbers. Govt. Bul. No. 6 (o.s.), Calcutta.</p>
+
+<p>EXNER, WILHELM FRANZ: Die mechanische Technologie des Holzes.
+Wien, 1871. (A translation and revision of Chevandier and
+Wertheim's M�moire sur les propri�t�s m�caniques du bois.)</p>
+
+<p>&mdash;&mdash;: Die technischen Eigenschaften der H�lzer. Lorey's Handbuch
+der Forstwissenschaft, II. Bd., 6. Kap., T�bingen, 1903.</p>
+
+<p>FERNOW, B.E.: Scientific timber testing. Digest of Physical
+Tests, Vol. I, No. 2, 1896, pp. 87-95.</p>
+
+<p>FOWKE, FRANCIS: Experiments on British colonial and other woods.
+1867.</p>
+
+<p>GARDNER, ROLAND: I. Mechanical tests, properties, and uses of
+thirty Philippine woods. II. Philippine sawmills, lumber market
+and prices. Bul. 4, Bu. For., P.I., 1906. (2d ed., 1907,
+contains tests of 34 woods.)</p>
+
+<p>GAYER, KARL: Forest utilization. (Vol. V, Schlich's Manual of
+Forestry. Translation of Die Forstbenutzung, Berlin, 1894.)
+London, 1908.</p>
+
+<p>GOLLNER, H.: Ueber die Festigkeit des Schwarzf�hrenholzes. Mitt.
+a. d. forstl. Versuchswesen Oesterreichs. II. Bd., 3. Hft.,
+Wien, 1881.</p>
+
+<p>GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der
+Baumaterialien. 3d ed., Berlin, 1880.</p>
+
+<p>GREEN, A.O.: Tasmanian timbers: their qualities and uses.
+Hobart, Tasmania, 1903, pp. 63.</p>
+
+<p>GREGORY, W.B.: Tests of creosoted timber. Trans. Am. Soc. C.E.,
+Vol. LXXVI, 1913, pp. 1192-1203. See also <i>ibid.</i>, Vol. LXX, p.
+37.</p>
+
+<p>GRISARD, JULES, et VANDENBERGHE, MAXIMILIEN: Les bois
+industriels, indig�nes et exotiques; synonymie et description
+des esp�ces, propri�t�s physiques des bois, qualit�s, d�fauts,
+usages et emplois. Paris, 189-. From Bul. de la Soci�t�
+nationale d'acclimatation de France, Vols. XXXVIII-XL.</p>
+
+<p>Hardwoods of Western Australia. Engineering, Vol. LXXXIII, Jan.
+11, 1907, pp. 35-37.</p>
+
+<p>HATT, WILLIAM KENDRICK: A Preliminary program for the timber
+test work to be undertaken by the Bureau of Forestry, United
+States Department of Agriculture. Proc. Am. Soc. Test. Mat.,
+Vol. III, 1903, pp. 308-343. Appendix I: Method of determining
+the effect of the rate of application of load on the strength of
+timber, pp. 325-327; App. II: A discussion on the <a name="pg151"></a> effect of
+moisture on strength and stiffness of timber, together with a
+plan of procedure for future tests, pp. 328-334.</p>
+
+<p>HATT, WILLIAM KENDRICK: Relation of timber tests to forest
+products. Proc. Int. Assn. Test. Mat., 1906, C 2 <i>e</i>, pp. 6.</p>
+
+<p>&mdash;&mdash;: Structural timber. Proc. Western Ry. Club, St. Louis, Mch.
+17, 1908.</p>
+
+<p>&mdash;&mdash;: Abstract of report on the present status of timber tests
+in the Forest Service, United States Department of Agriculture.
+Proc. Int. Assn. Test. Mat., 1909, XVL, pp. 10.</p>
+
+<p>&mdash;&mdash; and TURNER, W.P.: The Purdue University impact machine.
+Proc. Am. Soc. Test. Mat., Vol. VI, 1906, pp. 462-475.</p>
+
+<p>HAUPT, HERMAN: formul� for the strain upon timber. Center of
+gravity of an ungula and semi-cylinder. Jour. Franklin Inst.,
+Vol. XIX, 3d series, 1850, pp. 408-413.</p>
+
+<p>HEARDING, W.H.: Report upon experiments ... upon the compressive
+power of pine and hemlock timber. Washington, 1872, pp. 12.</p>
+
+<p>HOWE, MALVERD A.: Wood in compression; bearing values for
+inclined cuts. Eng. News, Vol. LXVIII, 1912, pp. 190-191.</p>
+
+<p>HOYER, EGBERT: Lehrbuch der vergleichenden mechanischen
+Technologie. 1878.</p>
+
+<p>IHLSENG, MANGUS C.: On the modulus of elasticity of some
+American woods as determined by vibration. Van Nostrand's Eng.
+Mag., Vol. XIX, 1878, pp. 8-9.</p>
+
+<p>&mdash;&mdash;: On a mode of measuring the velocity of sounds in woods.
+Am. Jour. Sci. and Arts, Vol. XVII, 1879.</p>
+
+<p>JACCARD, P.: �tude anatomique des bois comprim�s. Mitt. d. Schw.
+Centralanstalt f. d. forst. Versuchswesen. X. Bd., 1. Hft.,
+Zurich, 1910, pp. 53-101.</p>
+
+<p>JANKA, GABRIEL: Untersuchungen �ber die Elasticit�t und
+Festigkeit der �sterreichischen Bauh�lzer. I. Fichte S�dtirols;
+II. Fichte von Nordtirol vom Wienerwalde und Erzgebirge; III.
+Fichte aus den Karpaten, aus dem B�hmerwalde, Ternovanerwalde
+und den Zentralalpen. Technische Qualit�t des Fichtenholzes im
+allgemeinen; IV. L�rche aus dem Wienerwalde, aus Schlesien,
+Nord- und S�dtirol. Mitt. a. d. forst. Untersuchungswesen
+Oesterreichs, Wien, 1900-13.</p>
+
+<p>&mdash;&mdash;: Untersuchungen �ber Holzqualit�t. Centralblatt f. d. ges.
+Forstwesen. Wien, 1904, pp. 95-115.</p>
+
+<p>&mdash;&mdash;: Ueber neuere holztechnologische Untersuchungen. Oesterr.
+Vierteljahresschrift f�r Forstwesen, Wien, 1906, pp. 248-269.</p>
+
+<p>&mdash;&mdash;: Die H�rte des Holzes. Centralblatt f. d. ges. Forstwesen,
+Wien, 1906, pp. 193-202; 241-260.</p>
+
+<a name="pg152"></a>
+
+<p>JANKA, GABRIEL: Die Einwirkung von S�ss- und Salzw�ssern auf die
+gewerblichen Eigenschaften der Hauptholzarten. I. Teil.
+Untersuchungen u. Ergebnisse in mechanisch-technischer Hinsicht.
+Mitt. a. d. forst. Versuchswesen Oesterreichs, 33. Hft., Wien,
+1907.</p>
+
+<p>&mdash;&mdash;: Results of trials with timber carried out at the Austrian
+forestry testing-station at Mariabrunn. Proc. Int. Assn. Test.
+Mat., 1906, Disc. 2 <i>e</i>, pp. 7.</p>
+
+<p>&mdash;&mdash;: Ueber die an der k. k. forstlichen Versuchsanstalt
+Mariabrunnen gewonnenen Resultate der Holzfestigkeitspr�fungen.
+Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Wien, Aug. 9, 1907.</p>
+
+<p>&mdash;&mdash;: Ueber Holzh�rtepruf�ng. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 443-456.</p>
+
+<p>&mdash;&mdash;: Testing the hardness of wood by means of the ball test.
+Proc. Int. Assn. Test. Mat., 1912, XXIII<sub><small>3</small></sub>.</p>
+
+<p>JENNY, K.: Untersuchungen �ber die Festigkeit der H�lzer aus den
+L�ndern der ungarischen Krone. Budapest, 1873.</p>
+
+<p>JOHNSON, J.B.:  Time tests of timber in endwise compression.
+Paper before Section D, Am. Assn. for Adv. of Sci., Aug., 1898.</p>
+
+<p>JOHNSON, WALTER B.: Experiments on the adhesion of iron spikes
+of various forms when driven into different species of timbers.
+Jour. Franklin Inst., Vol. XIX (n.s.), 1837, pp. 281-292.</p>
+
+<p>JULIUS, G.A.: Western Australia timber tests, 1906. The physical
+characteristics of the hardwoods of Western Australia. Perth,
+1906, pp. 36.</p>
+
+<p>&mdash;&mdash;: Supplement to the Western Australia timber tests, 1906.
+The hardwoods of Australia. Perth, 1907, pp. 6.</p>
+
+<p>KARMARSH, CARL: Handbuch der mechanischen Technologie. I. Aufl.,
+1837; V. Aufl., 1875; verm. von H. Fisher, 1888.</p>
+
+<p>KIDDER, F.E.: Experiments on the transverse strength of southern
+and white pine. Van Nostrand's Eng. Mag., Vol. XXII, 1880, pp.
+166-168.</p>
+
+<p>&mdash;&mdash;: Experiments on the strength and stiffness of small spruce
+beams. <i>Ibid.</i>, Vol. XXIV, 1881, pp. 473-477.</p>
+
+<p>&mdash;&mdash;: Experiments on the fatigue of small spruce beams. Jour.
+Franklin Inst., Vol. CXIV, 1882, pp. 261-279.</p>
+
+<p>KIDWELL, EDGAR: The efficiency of built-up wooden beams. Trans.
+Am. Inst. Min. Eng., Feb., June, 1898.</p>
+
+<p>KIRKALDY, WM. G.: Illustrations of David Kirkaldy's system of
+mechanical testing. London, 1891.</p>
+
+<p>KUMMER, FREDERICK A.: The effects of preservative treatment on
+the strength of timber. Proc. Am. Soc. Test. Mat., Vol. IV,
+1904, pp. 434-438.</p>
+
+<a name="pg153"></a>
+
+<p>LABORD�RE, P., and ANSTETT, F.: Contribution to the study of
+means for improving the strength of wood for pavements. Proc.
+Int. Assn. Test. Mat., 1912, XXIII<sub><small>4</small></sub>, pp. 12.</p>
+
+<p>LANZA, GAETANO: An account of certain tests on the transverse
+strength and stiffness of large spruce beams. Trans. Am. Soc.
+Mech. Eng., Vol. IV, 1882, pp. 119-135. See also Jour. Franklin
+Inst., Vol. XCV, 1883, pp. 81-94.</p>
+
+<p>LASLETT, T.: Properties and characteristics of timber. Chatham,
+1867.</p>
+
+<p>&mdash;&mdash;: Timber and timber trees, native and foreign. (2d ed.
+revised and enlarged by H. Marshall Ward.) London and New York,
+1894.</p>
+
+<p>LEA, W.: Tables of strength and deflection of timber. London,
+1861.</p>
+
+<p>LEDEBUR, A.: Die Verarbeitung des Holzes auf mechanischem Wege.
+1881.</p>
+
+<p>LORENZ, N. VON: Analytische Untersuchung des Begriffes der
+Holzh�rte. Centralblatt f. d. ges. Forstwesen, Wien, 1909, pp.
+348-387.</p>
+
+<p>LUDWIG, PAUL: Die Regelprobe. Ein neues Verfahren zur
+H�rtebestimmung von Materialien. Berlin. 1908.</p>
+
+<p>MACFARLAND, H.B.: Tests of longleaf pine bridge timbers. Bul.
+149, Am. Ry. Eng. Assn., Sept., 1912. See also Eng. News, Dec.
+12, 1912, p. 1035.</p>
+
+<p>McKAY, DONALD: On the weight and strength of American
+ship-timber. Jour. Franklin Inst., Vol. XXXIX (3d series), 1860,
+p. 322.</p>
+
+<p>MALETTE, J.: Essais des bois de construction. Revue Technique,
+Apr. 25, 1905.</p>
+
+<p>MANN, JAMES: Australian timber: its strength, durability, and
+identification. Melbourne, 1900.</p>
+
+<p>MARTIN, CLARENCE A.: Tests on the relation between cross-bending
+and direct compressive strength in timber. Railroad Gazette,
+Mch. 13, 1903.</p>
+
+<p>Methods of testing metals and alloys ... Recommended by the
+Fourth Congress of the International Association for Testing
+Materials, held at Brussels, Sept. 3-6, 1906. London, 1907, pp.
+54. Methods of testing wood, pp. 39-49.</p>
+
+<p>MIKOLASCHEK, CARL: Untersuchungen �ber die Elasticit�t und
+Festigkeit der wichtigsten Bau- und Nutzh�lzer. Mitt. a. d.
+forstl. Versuchswesen Oesterreiches, II. Bd., 1. Hft., Wien,
+1879.</p>
+
+<p>MOELLER, JOSEPH: Die Rohstoffe des Tischler- und
+Drechslergewerbes. I. Theil: Das Holz. Kassel, 1883, pp. 68-122.</p>
+
+<p>MOLESWORTH, G.L.: Graphic diagrams of strength of teak beams.
+Roorke, 1881.</p>
+
+<a name="pg154"></a>
+
+<p>MORGAN, J.J.: Bending strength of yellow pine timber. Eng.
+Record, Vol. LXVII, 1913, pp. 608-609.</p>
+
+<p>MOROTO, K.: Untersuchungen �ber die Biegungselasticit�t und
+-Festigkeit der japanischen Bauh�lzer. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 346-355.</p>
+
+<p>NORDLINGER, H.: Die technischen Eigenschaften der H�lzer f�r
+Forst- und Baubeamte, Technologen und Gewerbetreibende.
+Stuttgart, 1860.</p>
+
+<p>&mdash;&mdash;: Druckfestigkeit des Holzes. 1882.</p>
+
+<p>&mdash;&mdash;: Die gewerblichen Eigenschaften der H�lzer. Stuttgart,
+1890.</p>
+
+<p>NORTH, A.T.: The grading of timber on the strength basis.
+Address before Western Society of Engineers. Lumber World
+Review, May 25, 1914, pp. 27-29.</p>
+
+<p>NORTON, W.A.: Results of experiments on the set of bars of wood,
+iron, and steel, after a transverse stress. Van Nostrand's Eng.
+Mag., Vol. XVII, 1877, pp. 531-535.</p>
+
+<p>PACCINOTTI E PERI: [Investigations into the elasticity of
+timbers.] Il Cimento, Vol. LVIII, 1845.</p>
+
+<p>PALACIO, E.: Tensile tests of timber. La Ingenieria, Buenos
+Aires, May 31, 1903, <i>et seq.</i></p>
+
+<p>PARENT,&mdash;: Exp�riences sur la r�sistance des bois de ch�ne et de
+sapin. M�moires de l'Acad�mie des Sciences, 1707-08.</p>
+
+<p>Propositions relatives � l'�tablissement d'un prec�d� uniforme
+pour l'essai des qualit�s techniques des bois. Proc. Int. Assn.
+Test. Mat., 1901, Annexe, pp. 13-28.</p>
+
+<p>ROGERS, CHARLES G.: A manual of forest engineering for India.
+Vol. I, Calcutta, 1900, pp. 50-91.</p>
+
+<p>RUDELOFF, M.: Der heutige Stand der Holzuntersuchungen. Mitt. a.
+d. k�niglichen tech. Versuchsanstalt, Berlin, IV, 1899.</p>
+
+<p>&mdash;&mdash;: Principles of a standard method of testing wood. Proc.
+Int. Assn. Test. Mat., 1906, 23 C, pp. 16.</p>
+
+<p>&mdash;&mdash;: Large <i>vs.</i> small test-pieces in testing wood. Proc. Int.
+Soc. Test. Mat., 1912, XXIII<sub><small>1</small></sub>, pp. 7.</p>
+
+<p>SARGENT, CHARLES SPRAGUE:  Woods of the United States, with an
+account of their structure, qualities, and uses. New York, 1885.</p>
+
+<p>SCHNEIDER, A.: Zusammengesetzte Tr�ger. Zeitschrift d. Oesterr.
+Ing. u. Arch. Ver., Nov. 24; Dec. 9, 1899.</p>
+
+<p>SCHWAPPACH, A.F.: Beitr�ge zur Kenntniss der Qualit�t des
+Rotbuchenholzes. Zeitschrift f. Forst- und Jagdwesen, Berlin,
+1894, pp. 513-539.</p>
+
+<p>&mdash;&mdash;: Untersuchungen �ber Raumgewicht und Druckfestigkeit des
+Holzes wichtiger Waldb�ume. Berlin, 1897-98.</p>
+
+<p>&mdash;&mdash;: Etablissement de m�thodes uniformes pour l'essai <a name="pg155"></a> � la
+compression des bois. Proc. Int. Assn. Test. Mat., 1901, Rapport
+23, pp. 28.</p>
+
+<p>SEBERT, H.: Notice sur les bois de la Nouvelle Cal�donie suivie
+de considerations g�nerates sur les propri�t�s m�caniques des
+bois et sur les prec�d�s employ�s pour les mesurer. Paris.</p>
+
+<p>SHERMAN, EDWARD C.: Crushing tests on water-soaked timbers. Eng.
+News, Vol. LXII, 1909, p. 22.</p>
+
+<p>SNOW, CHARLES H.: The principal species of wood: their
+characteristic properties. New York, 1908.</p>
+
+<p>STAUFFER, OTTMAR: Untersuchungen �ber specifisches
+Trockengewicht, sowie anatomisches Verhalten des Holzes der
+Birke. M�nchen, 1892.</p>
+
+<p>STENS, D.: Ueber die Eigenschaftenimpr�gnierter Grubenholzer,
+insbesondere �ber ihre Festigkeit. Gl�ckauf, Essen, Mch. 6,
+1907.</p>
+
+<p>Strength of wood for pavements. Can. Eng., Toronto, Sept. 12,
+1912.</p>
+
+<p>ST�BSCHEN-KISCHNER: Karmarsch-Heerins technisches Wr�terbuch. 3.
+Aufl., 1886.</p>
+
+<p>TALBOT, ARTHUR N.: Tests of timber beams. Bul. 41, Eng. Exp.
+Sta., Univ. of Ill., Urbana, 1910.</p>
+
+<p>Tests of wooden beams made at the Massachusetts Institute of
+Technology on spruce, white pine, yellow pine, and oak beams of
+commercial sizes. Technology Quarterly, Boston, Vol. VII, 1894.</p>
+
+<p>TETMAJER, L. v.: Zur Frage der Knickungsfestigkeit der
+Bauh�lzer. Schweizerische Bauzeitung, Bd. 11, Nr. 17.</p>
+
+<p>&mdash;&mdash;: Methoden und Resultate der Pr�fung der schweizerischen
+Bauh�lzer. Mitt. d. Anstalt z. Pr�fung v. Baumaterialien am
+eidgen�ssischen Polytechnicum in Z�rich. 2. Hft., 1884.</p>
+
+<p>&mdash;&mdash;: Methoden und Resultate der Pr�fung der schweizerischen
+Bauh�lzer. Mitt. d. Materialpr�fungs-Anstalt am Schweiz.
+Polytechnikum in Z�rich. Landesaustellungs-Ausgabe, 2. Hft.,
+Z�rich, 1896.</p>
+
+<p>THELEN, ROLF: The structural timbers  of the Pacific Coast.
+Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 558-567.</p>
+
+<p>THURSTON, R.H.: Torsional resistance of materials determined by
+a new apparatus with automatic registry. Jour. Franklin Inst.,
+Vol. LXV, 1873, pp. 254-260.</p>
+
+<p>&mdash;&mdash;: On the strength of American timber. <i>Ibid.</i>, Vol. LXXVIII,
+1879, pp. 217-235.</p>
+
+<p>&mdash;&mdash;: Experiments on the strength of yellow pine. <i>Ibid.</i>, Vol.
+LXXIX, 1880, pp. 157-163.</p>
+
+<p>&mdash;&mdash;: Influence of time on bending strength and elasticity.
+Proc. Am. Assn. for Adv. Sci., 1881. Also Proc. Inst. C.E., Vol.
+LXXI.</p>
+
+<p>&mdash;&mdash;: On the effect of prolonged stress upon the strength <a name="pg156"></a> and
+elasticity of pine timber. Jour. Franklin Inst., Vol. LXXX,
+1881, pp. 161-169.</p>
+
+<p>THURSTON, R.H.: On Flint's investigations of
+Nicaraguan woods. <i>Ibid.</i>, Vol. XCIV, 1887, pp. 289-315.</p>
+
+<p>TIEMANN, HARRY DONALD: The effect of moisture and other
+extrinsic factors upon the strength of wood. Proc. Am. Soc.
+Test. Mat., Vol. VII, 1907, pp. 582-594.</p>
+
+<p>&mdash;&mdash;: The effect of the speed of testing upon the strength of
+wood and the standardization of tests for speed. <i>Ibid.</i>, Vol.
+VIII, 1908, pp. 541-557.</p>
+
+<p>&mdash;&mdash;: The theory of impact and its application to testing
+materials. Jour. Franklin Inst., Vol. CLXVIII, 1909, pp.
+235-259; 336-364.</p>
+
+<p>&mdash;&mdash;: Some results of dead load bending tests of timber by means
+of a recording deflectometer. Proc. Am. Soc. Test. Mat., Vol.
+IX, 1909, pp. 534-548.</p>
+
+<p>TJADEN, M.E.H.: Het Indrukken van Paalkoppen in Kespen. De
+Ingenieur, Sept. 11, 1909.</p>
+
+<p>&mdash;&mdash;: Weerstand van Hout loodrecht op de Vezelrichting. <i>Ibid.</i>,
+May, 1911.</p>
+
+<p>&mdash;&mdash;: Buigvastheid van Hout. <i>Ibid.</i>, May 31, 1913.</p>
+
+<p>TRAUTWINE, JOHN C.: Shearing strength of some American woods.
+Jour. Franklin Inst., Vol. CIX, 1880, pp. 105-106.</p>
+
+<p>TREDGOLD, THOMAS: Elementary principles of carpentry. London,
+1870.</p>
+
+<p>TURNBULL, W.: A practical treatise on the strength and stiffness
+of timber. London, 1833.</p>
+
+<p>Untersuchungen �ber den Einfluss des Blauwerdens auf die
+Festigkeit von Kiefernholz. Mitt. a. d. k�nig. techn.
+Versuchsanstalten, I, 1897.</p>
+
+<p>Verfahren zur Pr�fung v. Metallen und Legierungen, von
+hydraulischen Bindemitteln, von Holz, von Ton-, Steinzeug- und
+Zementr�hren. Empfohlen v. dem in Br�ssel v. 3-6, IX, 1906,
+abgeh. IV. Kongress des internationalen Verbandes f. die
+Materialpr�fungen der Technik. Wien, 1907.</p>
+
+<p>WARREN, W.H.: Australian timbers. Sydney, 1892.</p>
+
+<p>&mdash;&mdash;: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Sydney, 1911.</p>
+
+<p>&mdash;&mdash;: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Proc. Int. Assn. Test. Mat., 1912,
+XXIII<sub><small>6</small></sub>, pp. 9.</p>
+
+<p>&mdash;&mdash;: The properties of New South Wales hardwood timbers.
+Builder, London, Nov. 1, 1912.</p>
+
+<p>&mdash;&mdash;: The hardwood timbers of New South Wales, Australia. Jour.
+Soc. of Arts, London, Dec. 6, 1912.</p>
+
+<a name="pg157"></a>
+
+<p>WELLINGTON, A.M.: Experiments on impregnated timber. Railroad
+Gazette, 1880.</p>
+
+<p>WIJKANDER, &mdash;&mdash;: Untersuchung der Festigkeitseigenschaften
+schwedischer Holzarten in der Materialpr�fungsanstalt des
+Chalmers'schen Institutes ausgef�hrt. 1897.</p>
+
+<p>WING, CHARLES B.: Transverse strength of the Douglas fir. Eng.
+News, Vol. XXXIII, Mch. 14, 1895.</p>
+
+<a name="BIB_III"></a>
+<h3>PART III.<br>PUBLICATIONS OF THE U.S. GOVERNMENT ON THE MECHANICAL PROPERTIES OF WOOD, AND TIMBER TESTING</h3>
+
+<h4>MISCELLANEOUS</h4>
+
+<p>House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol.
+IX, Tenth Census report.) Report on the forests of North America
+(exclusive of Mexico). Part II, The Woods of the United States.</p>
+
+<p>House Report No. 1442, 53d Cong., 2d sess. Investigations and
+tests of American timber. 1894, pp. 4.</p>
+
+<p>War Dept. Doc. 1. Resolutions of the conventions held at Munich,
+Dresden, Berlin, and Vienna, for the purpose of adopting uniform
+methods for testing construction materials with regard to their
+mechanical properties. By J. Bauschinger. Translated by O.M.
+Carter and E.A. Gieseler. 1896, pp. 44.</p>
+
+<p>War Dept. Doc. 11. On tests of construction materials.
+Translations from the French and from the German. By O.M. Carter
+and E.A. Gieseler. 1896, pp. 84.</p>
+
+<p>House Doc. No. 181, 55th Cong., 3d sess. Report upon the
+forestry investigations of the U.S. Department of Agriculture,
+1877-1898. By B.E. Fernow, 1899, pp. 401. Contains chapter on
+The work in timber physics in the Division of Forestry, by
+Filibert Roth, pp. 330-395.
+<h4>FOREST SERVICE</h4>
+<p>Cir. 7&mdash;The Government timber tests [189-], pp. 4.</p>
+
+<p>Cir. 8&mdash;Strength of "boxed" or "turpentine" timber. 1892, pp. 4.</p>
+
+<p>Bul. 6&mdash;Timber Physics. Pt. I. Preliminary report. 1. Need of
+the investigation. 2. Scope and historical development of the
+science of "timber physics." 3. Organization and methods of
+timber examinations in the Division of Forestry. By B.E. Fernow,
+1892, pp. 57.</p>
+
+<p>Unnumbered Cir.&mdash;Instructions for the collection of test pieces
+of pines for timber investigations [1893], pp. 4.</p>
+
+<a name="pg158"></a>
+
+<p>Cir. 9&mdash;Effect of turpentine gathering on the timber of longleaf
+pine. By B.E. Fernow [1893], p. 1.</p>
+
+<p>Bul. 8&mdash;Timber physics. Pt. II. Progress report. Results of
+investigations on longleaf pine. 1893, pp. 92.</p>
+
+<p>Bul. 10&mdash;Timber: an elementary discussion of the characteristics
+and properties of wood. By Filibert Roth. 1895, pp. 88.</p>
+
+<p>Bul. 12&mdash;Economical designing of timber trestle bridges. By A.L.
+Johnson, 1896, pp. 57.</p>
+
+<p>Cir. 12&mdash;Southern pine, mechanical and physical properties.
+1896, pp. 12.</p>
+
+<p>Cir. 15&mdash;Summary of mechanical tests on thirty-two species of
+American woods. 1897, pp. 12.</p>
+
+<p>Cir. 18&mdash;Progress in timber physics. 1898, pp. 20.</p>
+
+<p>Cir. 19&mdash;Progress in timber physics: Bald cypress (<i>Taxodium
+distichum</i>). By Filibert Roth, 1898, pp. 24.</p>
+
+<p>Y.B. Extr. 288&mdash;Tests on the physical properties of woods. By
+F.E. Olmstead, 1902, pp. 533-538.</p>
+
+<p>Unnumbered Cir.&mdash;Timber tests. [1903], pp. 15.</p>
+
+<p>Unnumbered Cir.&mdash;Timber preservation and timber testing at the
+Louisiana Purchase Exposition. 1904, pp. 6.</p>
+
+<p>Cir. 32&mdash;Progress report on the strength of structural timber.
+By W.K. Hatt, 1904, pp. 28.</p>
+
+<p>Bul. 58&mdash;The red gum. By Alfred Chittenden. Includes a
+discussion of The mechanical properties of red gum wood, by W.K.
+Hatt. 1905, pp. 56.</p>
+
+<p>Cir. 38&mdash;Instructions to engineers of timber tests. By W.K.
+Hatt, 1906, pp. 55. Revised edition, 1909, pp. 56.</p>
+
+<p>Cir. 39&mdash;Experiments on the strength of treated timber. By W.K.
+Hatt, 1906, pp. 31. Revised edition, 1908.</p>
+
+<p>Bul. 70&mdash;Effect of moisture upon the strength and stiffness of
+wood. By H.D. Tiemann, 1906, pp. 144.</p>
+
+<p>Cir. 46&mdash;Holding force of railroad spikes in wooden ties. By
+W.K. Hatt, 1906, pp. 7.</p>
+
+<p>Cir. 47&mdash;Strength of packing boxes of various woods. By W.K.
+Hatt, 1906, pp. 7.</p>
+
+<p>Cir. 108&mdash;The strength of wood as influenced by moisture. By
+H.D. Tiemann, 1907, pp. 42.</p>
+
+<p>Cir. 115&mdash;Second progress report on the strength of structural
+timber. By W.K. Hatt, 1907, pp. 39.</p>
+
+<p>Cir. 142&mdash;Tests of vehicle and implement woods. By H.B. Holroyd
+and H.S. Betts, 1908, pp. 29.</p>
+
+<p>Cir. 146&mdash;Experiments with railway cross-ties. By H.B. Eastman,
+1908, pp. 32.</p>
+
+<p>Cir. 179&mdash;Utilization of California eucalypts. By H.S. Betts and
+C. Stowell Smith, 1910, pp. 30.</p>
+
+<a name="pg159"></a>
+
+<p>Bul. 75&mdash;California tanbark oak. Part II, Utilization of the
+wood of tanbark oak, by H.S. Betts, 1911, pp. 24-32.</p>
+
+<p>Bul. 88&mdash;Properties and uses of Douglas fir. By McGarvey Cline
+and J.B. Knapp, 1911, pp. 75.</p>
+
+<p>Cir. 189&mdash;Strength values for structural timbers. By McGarvey
+Cline, 1912, pp. 8.</p>
+
+<p>Cir. 193&mdash;Mechanical properties of redwood. By A.L. Heim, 1912,
+pp. 32.</p>
+
+<p>Bul. 108&mdash;Tests of structural timbers. By McGarvey Cline and
+A.L. Heim, 1912, pp. 1231.</p>
+
+<p>Bul. 112&mdash;Fire-killed Douglas fir: a study of its rate of
+deterioration, usability, and strength. By J.B. Knapp, 1912, pp.
+18.</p>
+
+<p>Bul. 115&mdash;Mechanical properties of western hemlock. By O.P.M.
+Goss, 1913, pp. 45.</p>
+
+<p>Bul. 122&mdash;Mechanical properties of western larch. By O.P.M.
+Goss, 1913, pp. 45.</p>
+
+<p>Cir. 213&mdash;Mechanical properties of woods grown in the United
+States. 1913, pp. 4.</p>
+
+<p>Cir. 214&mdash;Tests of packing boxes of various forms. By John A.
+Newlin, 1913, pp. 23.</p>
+
+<p>Review Forest Service Investigations. 1913. [Outline of
+investigations.] Vol. I, pp. 17-21. A microscopic study of the
+mechanical failure of wood, by Warren D. Brush. Vol. II, pp.
+33-38.</p>
+
+<p>Bul. 67, U.S.D.A.&mdash;Tests of Rocky Mountain woods for telephone
+poles. By Norman deW. Betts and A.L. Heim, 1914, pp. 28.</p>
+
+<p>Bul. 77, U.S.D.A.&mdash;Rocky Mountain mine timbers. By Norman deW.
+Betts, 1914, pp. 34.</p>
+
+<p>Bul. 86, U.S.D.A.&mdash;Tests of wooden barrels. By J.A. Newlin,
+1914, pp. 12.
+<h4>REPORTS OF TESTS ON THE STRENGTH OF STRUCTURAL MATERIAL, MADE AT THE WATERTOWN ARSENAL, MASS.</h4>
+<p>House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of
+wood grown on the Pacific slope, pp. 19-93.</p>
+
+<p>Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of
+white and yellow pines to forces of compression in the direction
+of the fibers, as used for columns, or posts, pp. 239-395.</p>
+
+<p>Senate Ex. Doc. No. 5, 48th Cong., 1st sess., 1884. Tests of
+California laurel wood by compression, indentation, shearing,
+transverse tension, pp. 223-236. Tests of North American woods
+(under supervision of Prof. C.S. Sargent in charge of the
+forestry division of the Tenth Census), with 16 photographs of
+fractures of American woods, pp. 237-347.</p>
+
+<a name="pg160"></a>
+
+<p>Senate Ex. Doc. No. 35, 49th Cong., 1st sess., 1885. Adhesion of
+nails, spikes, and screws in various woods. Experiments on the
+resistance of cut nails, wire nails (steel), wood screws, lag
+screws in white pine, yellow pine, chestnut, white oak, and
+laurel, pp. 448-471.</p>
+
+<p>House Ex. Doc. No. 14, 51st Cong., 1st sess., 1890. Adhesion of
+spikes and bolts in railroad ties, pp. 595-617.</p>
+
+<p>House Ex. Doc. No. 161, 52d Cong., 1st sess., 1892. Adhesion of
+nails in wood, pp. 744-745.</p>
+
+<p>House Ex. Doc. No. 92, 53d Cong., 3d sess., 1895.
+Woods&mdash;compression tests (endwise compression), pp. 471-476.</p>
+
+<p>House Doc. No. 54, 54th Cong., 1st sess., 1896. Compression
+tests on Douglas fir wood, pp. 536-563. Expansion and
+contraction of oak and pine wood, pp. 567-574.</p>
+
+<p>House Doc. No. 164, 55th Cong., 2d sess., 1898. Compression
+tests of timber posts, pp. 405-411. New posts of yellow pine and
+spruce, pp. 413-450; Old yellow pine posts from Boston Fire
+Brick Co. building, No. 394 Federal St., Boston, Mass., pp.
+451-473.</p>
+
+<p>House Doc. No. 143, 55th Cong., 3d sess., 1899. Fire-proofed
+wood (endwise and transverse tests), pp. 676-681.</p>
+
+<p>House Doc. No. 190, 56th Cong., 2d sess., 1901. Cypress wood for
+United States Engineer Corps; compression and transverse tests,
+pp. 1121-1126. Old white pine and red oak from roof trusses of
+Old South Church, Boston, Mass., pp. 1127-1130. Compression of
+rubber, balata, and wood buffers, pp. 1149-1158.</p>
+
+<p>House Doc. No. 335, 57th Cong., 2d sess., 1903. Douglas fir and
+white oak woods. Transverse and shearing tests; also
+observations on heat conductivity of sticks over wood fires and
+a stick exposed to low temperature. Expansion crosswise the
+grain of wood after submersion, pp. 519-561. Adhesion of lag
+screws and bolts in wood, pp. 563-578.</p>
+
+<a name="pg161"></a>
+<a name="INDEX"></a><h2>INDEX</h2>
+
+<a name="Abrasion"></a>
+<p>Abrasion,
+          <a href="#pg039">39</a>,
+          <a href="#pg114">114</a>-117
+</p>
+
+<p>Annual rings,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Apparatus, testing,
+          <a href="#pg094">94</a>,
+          <a href="#pg099">99</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg107">107</a>,
+          <a href="#pg110">110</a>-111,
+          <a href="#pg114">114</a>,
+          <a href="#pg117">117</a>,
+          <a href="#pg118">118</a>,
+          <a href="#pg121">121</a>,
+          <a href="#pg122">122</a>,
+          <a href="#pg132">132</a>,
+          <a href="#pg133">133</a>,
+          <a href="#pg136">136</a>
+</p>
+
+<p>Arborvit�,
+          <a href="#pg006">6</a>,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Ash,
+          <a href="#pg015">15</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg066">66</a>,
+          <a href="#pg078">78</a>
+</p>
+<p class="ind">black,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">white,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg045">45</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Aspen, largetooth,
+          <a href="#pg013">13</a>
+</p>
+
+<p>Axis, neutral,
+          <a href="#pg023">23</a>
+</p>
+
+<p>Basswood,
+          <a href="#pg009">6</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Beams,
+          <a href="#pg015">15</a>,
+          <a href="#pg024">24</a>-37,
+          <a href="#pg092">92</a>,
+          <a href="#pg094">94</a>
+</p>
+<p class="ind">cantilever,
+          <a href="#pg024">24</a>
+</p>
+<p class="ind">continuous,
+          <a href="#pg024">24</a>
+</p>
+<p class="ind">simple,
+          <a href="#pg024">24</a>
+</p>
+
+<p>Beech,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Bending large beams,
+          <a href="#pg094">94</a>-99
+</p>
+<p class="ind">small beams,
+          <a href="#pg099">99</a>-102,
+          <a href="#pg132">132</a>
+</p>
+<p class="ind">strength,
+          <a href="#pg002">2</a>,
+          <a href="#pg022">22</a>-37,
+          <a href="#pg026">26</a>,
+          <a href="#pg075">75</a>
+</p>
+
+<p>Bibliography,
+          <a href="#pg145">145</a>-160
+</p>
+
+<p>Birch,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">yellow,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Bird-peck,
+          <a href="#pg059">59</a>,
+          <a href="#pg072">72</a>
+</p>
+
+<p>Black check,
+          <a href="#pg059">59</a>
+</p>
+
+<p>Boiling, effect of,
+          <a href="#pg006">6</a>,
+          <a href="#pg085">85</a>
+</p>
+
+<p>Bow, flexure of a,
+          <a href="#pg004">4</a>
+</p>
+
+<p>Boxheart,
+          <a href="#pg054">54</a>,
+          <a href="#pg082">82</a>
+</p>
+
+<p>Brash,
+          <a href="#pg006">6</a>
+</p>
+
+<p>Breaking strength of beams,
+          <a href="#pg015">15</a>
+</p>
+
+<p>Brittleness,
+          <a href="#pg006">6</a>,
+          <a href="#pg034">34</a>,
+          <a href="#pg037">37</a>,
+          <a href="#pg038">38</a>
+</p>
+
+<p>Buckeye,
+          <a href="#pg013">13</a>,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Buckling of fibres,
+          <a href="#pg015">15</a>,
+          <a href="#pg077">77</a>
+</p>
+
+<p>Butternut,
+          <a href="#pg013">13</a>
+</p>
+
+<p>Cantilever,
+          <a href="#pg024">24</a>
+</p>
+
+<p>Calibration of testing machines,
+          <a href="#pg092">92</a>,
+          <a href="#pg112">112</a>
+</p>
+
+<p>Case-hardening,
+          <a href="#pg080">80</a>
+</p>
+
+<p>Catalpa,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Cedar, Central American,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">incense,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">red,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">white,
+          <a href="#pg022">22</a>
+</p>
+
+<p>Checking,
+          <a href="#pg054">54</a>,
+          <a href="#pg061">61</a>,
+          <a href="#pg074">74</a>,
+          <a href="#pg075">75</a>-84
+</p>
+
+<p><i>Chelura</i>,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Cherry, black,
+          <a href="#pg013">13</a>,
+          <a href="#pg022">22</a>
+</p>
+
+<p>Chestnut,
+          <a href="#pg015">15</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg049">49</a>,
+          <a href="#pg050">50</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg066">66</a>,
+          <a href="#pg078">78</a>
+</p>
+
+<p>Cleavability,
+          <a href="#pg002">2</a>,
+          <a href="#pg041">41</a>
+</p>
+
+<p>Cleavage,
+          <a href="#pg041">41</a>,
+          <a href="#pg118">118</a>,
+          <a href="#pg133">133</a>
+</p>
+
+<p>Coefficient of elasticity (see <a href="#Modulus_of_elasticity">Modulus of elasticity</a>)
+</p>
+<p class="ind">expansion,
+          <a href="#pg084">84</a>
+</p>
+
+<p>Cold, effect of,
+          <a href="#pg086">86</a>
+</p>
+
+<p>Color,
+          <a href="#pg050">50</a>,
+          <a href="#pg058">58</a>-59
+</p>
+
+<p>Column, long,
+          <a href="#pg012">12</a>,
+          <a href="#pg014">14</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">short,
+          <a href="#pg015">15</a>,
+          <a href="#pg102">102</a>-104
+</p>
+
+<p>Compression across the grain,
+          <a href="#pg094">94</a>,
+          <a href="#pg104">104</a>-107,
+          <a href="#pg133">133</a>
+</p>
+<a name="C_endwise"></a>
+<p class="ind">endwise,
+          <a href="#pg012">12</a>,
+          <a href="#pg092">92</a>,
+          <a href="#pg094">94</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg132">132</a>
+</p>
+<p class="ind">failure,
+          <a href="#pg034">34</a>,
+          <a href="#pg104">104</a>
+</p>
+<p class="ind">parallel to grain (<a href="#C_endwise">see C. endwise</a>)
+</p>
+<p class="ind">perpendicular to grain (<a href="#C_right_angles_to_grain">see C. right angles to grain</a>)</p>
+<a name=C_right_angles_to_grain></a>
+<p class="ind">right angles to grain,
+          <a href="#pg094">94</a>,
+          <a href="#pg133">133</a>
+</p>
+
+<p>Compressive strength,
+          <a href="#pg001">1</a>,
+          <a href="#pg009">9</a>,
+          <a href="#pg023">23</a>
+</p>
+
+<p>Compressometer,
+          <a href="#pg103">103</a>
+</p>
+
+<p>Coniferous wood,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Cottonwood,
+          <a href="#pg044">44</a>,
+          <a href="#pg045">45</a>
+</p>
+
+<p>Creosote, effect of,
+          <a href="#pg087">87</a>
+</p>
+
+<p>Cross-arms, testing,
+          <a href="#pg124">124</a>
+</p>
+
+<p>Cross grain,
+          <a href="#pg008">8</a>,
+          <a href="#pg059">59</a>-61
+</p>
+
+<p>Cross-grained tension failure,
+          <a href="#pg034">34</a>
+</p>
+
+<p>Crushing strength,
+          <a href="#pg002">2</a>,
+          <a href="#pg009">9</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg075">75</a>
+</p>
+<p class="ind">formula for,
+          <a href="#pg104">104</a>
+</p>
+
+<p>Cucumber tree,
+          <a href="#pg013">13</a>
+</p>
+
+<p>Cup shake,
+          <a href="#pg064">64</a>
+</p>
+
+<p>Cypress, bald,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg068">68</a>,
+          <a href="#pg144">144</a>
+</p>
+
+<p>
+Dead Load (<a href="#Load">see Load</a>)
+</p>
+
+<p>Definitions,
+          <a href="#pg002">2</a>-7
+</p>
+
+<p>Deflection,
+          <a href="#pg025">25</a>,
+          <a href="#pg030">30</a>
+</p>
+<p class="ind">measuring,
+          <a href="#pg096">96</a>-97,
+          <a href="#pg100">100</a>
+</p>
+
+<p>Deflectometer,
+          <a href="#pg099">99</a>,
+          <a href="#pg106">106</a>-107
+</p>
+
+<p>Deformation, measuring,
+          <a href="#pg103">103</a>,
+          <a href="#pg107">107</a>
+</p>
+
+<p>Density,
+          <a href="#pg054">54</a>-58
+</p>
+
+<p>Diffuse-porous,
+          <a href="#pg044">44</a>,
+          <a href="#pg050">50</a>
+</p>
+
+<p>Dogwood,
+          <a href="#pg022">22</a>
+</p>
+
+<p>Drying,
+          <a href="#pg075">75</a>-84
+</p>
+<p class="ind">effect of,
+          <a href="#pg075">75</a>-78,
+          <a href="#pg138">138</a>-139
+</p>
+
+<p>Dry rot,
+          <a href="#pg069">69</a>
+</p>
+
+<p>Durability,
+          <a href="#pg053">53</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg075">75</a>
+</p>
+
+<a name="Early_wood"></a>
+<p>Early Wood,
+          <a href="#pg044">44</a>,
+          <a href="#pg082">82</a>
+</p>
+
+<p>Ease of working, factors affecting,
+          <a href="#pg048">48</a>,
+          <a href="#pg050">50</a>
+</p>
+
+<p>Ebony,
+          <a href="#pg022">22</a>
+</p>
+
+<p>Elasticity, modulus of,
+          <a href="#pg006">6</a>,
+          <a href="#pg025">25</a>,
+          <a href="#pg089">89</a>
+</p>
+<p class="ind">formul� for,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg114">114</a>,
+          <a href="#pg123">123</a>
+</p>
+
+<p>Elastic limit,
+          <a href="#pg002">2</a>,
+          <a href="#pg005">5</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg062">62</a>
+</p>
+<p class="ind">resilience,
+          <a href="#pg006">6</a>
+</p>
+<p class="ind2">formul� for,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>
+</p>
+
+<p>Elm,
+          <a href="#pg008">8</a>,
+          <a href="#pg038">38</a>,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">rock,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">slippery,
+          <a href="#pg013">13</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">white,
+          <a href="#pg013">13</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Elongation,
+          <a href="#pg003">3</a>,
+          <a href="#pg007">7</a>,
+          <a href="#pg033">33</a>
+</p>
+
+<p><i>Eucalyptus globulus</i>,
+          <a href="#pg054">54</a>,
+          <a href="#pg078">78</a>,
+          <a href="#pg082">82</a>
+</p>
+<p class="ind"><i>viminalis</i>,
+          <a href="#pg054">54</a>
+</p>
+
+<p>Factor of Safety,
+          <a href="#pg029">29</a>,
+          <a href="#pg139">139</a>
+</p>
+
+<p>Failures, bending,
+          <a href="#pg033">33</a>-37,
+          <a href="#pg077">77</a>,
+          <a href="#pg078">78</a>
+</p>
+<p class="ind">compression, endwise,
+          <a href="#pg012">12</a>,
+          <a href="#pg015">15</a>-19,
+          <a href="#pg104">104</a>
+</p>
+<p class="ind">cross-grained,
+          <a href="#pg010">10</a>,
+          <a href="#pg076">76</a>,
+          <a href="#pg077">77</a>,
+          <a href="#pg107">107</a>
+</p>
+<p class="ind">shearing,
+          <a href="#pg019">19</a>
+</p>
+<p class="ind">tension,
+          <a href="#pg008">8</a>
+</p>
+<p class="ind">torsion,
+          <a href="#pg038">38</a>,
+          <a href="#pg039">39</a>
+</p>
+
+<p>Fibre-saturation point,
+          <a href="#pg078">78</a>
+</p>
+
+<p>Fibre strain, rate of,
+          <a href="#pg092">92</a>-93
+</p>
+<p class="ind">stress,
+          <a href="#pg015">15</a>
+</p>
+<p class="ind2">at elastic limit,
+          <a href="#pg054">54</a>,
+          <a href="#pg062">62</a>,
+          <a href="#pg075">75</a>,
+          <a href="#pg123">123</a>
+</p>
+<p class="ind3">formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg107">107</a>,
+          <a href="#pg114">114</a>
+</p>
+
+<p>Fir, Alpine,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">amabilis,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">Douglas,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg055">55</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg124">124</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">white,
+          <a href="#pg009">9</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Flexibility,
+          <a href="#pg005">5</a>,
+          <a href="#pg037">37</a>
+</p>
+
+<p>Flexure,
+          <a href="#pg004">4</a>,
+          <a href="#pg012">12</a>,
+          <a href="#pg060">60</a>
+</p>
+
+<p>Formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg107">107</a>,
+          <a href="#pg110">110</a>,
+          <a href="#pg114">114</a>,
+          <a href="#pg123">123</a>
+</p>
+
+<p>Frost splits,
+          <a href="#pg062">62</a>-64
+</p>
+
+<p>Fungi 59,
+          <a href="#pg068">68</a>-70,
+          <a href="#pg075">75</a>
+</p>
+
+<p>Grain, cross,
+          <a href="#pg008">8</a>,
+          <a href="#pg059">59</a>-61
+</p>
+<p class="ind">diagonal,
+          <a href="#pg060">60</a>
+</p>
+<p class="ind">spiral,
+          <a href="#pg060">60</a>
+</p>
+
+<p>Growth, in diameter,
+          <a href="#pg043">43</a>,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">locality of, effect,
+          <a href="#pg070">70</a>-73
+</p>
+<p class="ind">rate of, effect,
+          <a href="#pg043">43</a>,
+          <a href="#pg050">50</a>,
+          <a href="#pg072">72</a>
+</p>
+<p class="ind">rings,
+          <a href="#pg044">44</a>,
+          <a href="#pg052">52</a>
+</p>
+<p class="ind2">measuring,
+          <a href="#pg095">95</a>-96
+</p>
+
+<p>Gum 22,
+          <a href="#pg044">44</a>,
+          <a href="#pg051">51</a>
+</p>
+<p class="ind">red,
+          <a href="#pg027">27</a>,
+          <a href="#pg045">45</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Hackberry,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Hardness,
+          <a href="#pg002">2</a>,
+          <a href="#pg039">39</a>-41,
+          <a href="#pg054">54</a>,
+          <a href="#pg114">114</a>-118,
+          <a href="#pg133">133</a>
+</p>
+
+<p>Heart break,
+          <a href="#pg065">65</a>
+</p>
+<p class="ind">shake,
+          <a href="#pg064">64</a>
+</p>
+
+<p>Heartwood,
+          <a href="#pg050">50</a>-54,
+          <a href="#pg058">58</a>,
+          <a href="#pg073">73</a>,
+          <a href="#pg075">75</a>
+</p>
+
+<p>Heat, effect of,
+          <a href="#pg084">84</a>-86
+</p>
+
+<p>Hemlock,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg015">15</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg045">45</a>,
+          <a href="#pg056">56</a>,
+          <a href="#pg078">78</a>
+</p>
+<p class="ind">western,
+          <a href="#pg048">48</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+
+<p>Hickory,
+          <a href="#pg008">8</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg038">38</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg043">43</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg049">49</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg053">53</a>,
+          <a href="#pg055">55</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg065">65</a>,
+          <a href="#pg066">66</a>,
+          <a href="#pg072">72</a>,
+          <a href="#pg124">124</a>
+</p>
+<p class="ind">big shellbark,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">bitternut,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">mockernut,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">nutmeg,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">pignut,
+          <a href="#pg006">6</a>,
+          <a href="#pg007">7</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">shagbark,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">water,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Hollow-horning,
+          <a href="#pg080">80</a>
+</p>
+
+<p>Honey-combing,
+          <a href="#pg054">54</a>,
+          <a href="#pg080">80</a>
+</p>
+
+<p>Impact,
+          <a href="#pg030">30</a>-33,
+          <a href="#pg110">110</a>-114,
+          <a href="#pg132">132</a>
+</p>
+
+<p>Implement woods, testing,
+          <a href="#pg124">124</a>
+</p>
+
+<p>Indentation,
+          <a href="#pg039">39</a>,
+          <a href="#pg117">117</a>-118
+</p>
+
+<p>Injuries, fungous,
+          <a href="#pg052">52</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg068">68</a>-70
+</p>
+<p class="ind">insect,
+          <a href="#pg052">52</a>,
+          <a href="#pg066">66</a>-67,
+          <a href="#pg072">72</a>
+</p>
+<p class="ind">marine wood-borer,
+          <a href="#pg067">67</a>-68
+</p>
+<p class="ind">parasitic plant,
+          <a href="#pg070">70</a>
+</p>
+
+<p>Kerosene, effect of,
+          <a href="#pg087">87</a>
+</p>
+
+<p>Knots,
+          <a href="#pg051">51</a>,
+          <a href="#pg052">52</a>,
+          <a href="#pg061">61</a>-62,
+          <a href="#pg089">89</a>
+</p>
+
+<p>Larch,
+          <a href="#pg008">8</a>
+</p>
+<p class="ind">western,
+          <a href="#pg036">36</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg064">64</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>
+</p>
+
+<a name="Late_wood"></a>
+<p>Late wood,
+          <a href="#pg044">44</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg082">82</a>
+</p>
+<p class="ind">measuring,
+          <a href="#pg096">96</a>
+</p>
+<p class="ind">relation to strength,
+          <a href="#pg047">47</a>-49
+</p>
+
+<p>Limit of elasticity,
+          <a href="#pg005">5</a>
+</p>
+
+<p><i>Limnoria</i>,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Live load,
+          <a href="#pg028">28</a>
+</p>
+<a name="Load"></a>
+<p>Load, application of,
+          <a href="#pg029">29</a>
+</p>
+<p class="ind">concentrated,
+          <a href="#pg028">28</a>
+</p>
+<p class="ind">dead,
+          <a href="#pg028">28</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">immediate,
+          <a href="#pg028">28</a>
+</p>
+<p class="ind">kinds of,
+          <a href="#pg026">26</a>
+</p>
+<p class="ind">live,
+          <a href="#pg028">28</a>
+</p>
+<p class="ind">maximum,
+          <a href="#pg029">29</a>
+</p>
+<p class="ind">permanent,
+          <a href="#pg028">28</a>
+</p>
+<p class="ind">safe,
+          <a href="#pg029">29</a>
+</p>
+<p class="ind">uniform,
+          <a href="#pg026">26</a>
+</p>
+
+<p>Loading, centre,
+          <a href="#pg097">97</a>,
+          <a href="#pg100">100</a>
+</p>
+<p class="ind">static,
+          <a href="#pg029">29</a>
+</p>
+<p class="ind">sudden,
+          <a href="#pg029">29</a>
+</p>
+<p class="ind">third-point,
+          <a href="#pg097">97</a>
+</p>
+<p class="ind">vibratory,
+          <a href="#pg029">29</a>
+</p>
+
+<p>Locust,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">black,
+          <a href="#pg013">13</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg051">51</a>
+</p>
+<p class="ind">honey,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Log of tests,
+          <a href="#pg097">97</a>-98,
+          <a href="#pg100">100</a>-102,
+          <a href="#pg103">103</a>-104,
+          <a href="#pg107">107</a>,
+          <a href="#pg110">110</a>,
+          <a href="#pg114">114</a>
+</p>
+
+<p>Machine for static tests,
+          <a href="#pg090">90</a>-92
+Maple,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg051">51</a>
+</p>
+<p class="ind">red,
+          <a href="#pg013">13</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">silver,
+          <a href="#pg013">13</a>
+</p>
+<p class="ind">sugar,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Marking test specimens,
+          <a href="#pg094">94</a>-95,
+          <a href="#pg100">100</a>,
+          <a href="#pg129">129</a>-131
+</p>
+
+<p>Material for tests,
+          <a href="#pg088">88</a>-90,
+          <a href="#pg094">94</a>,
+          <a href="#pg099">99</a>-100,
+          <a href="#pg102">102</a>,
+          <a href="#pg106">106</a>,
+          <a href="#pg107">107</a>-110,
+          <a href="#pg113">113</a>,
+          <a href="#pg115">115</a>-116,
+          <a href="#pg117">117</a>,
+          <a href="#pg118">118</a>,
+          <a href="#pg119">119</a>-120,
+          <a href="#pg121">121</a>,
+          <a href="#pg122">122</a>,
+          <a href="#pg123">123</a>-125,
+          <a href="#pg128">128</a>-134,
+          <a href="#pg135">135</a>,
+          <a href="#pg136">136</a>
+</p>
+
+<p>Mechanical properties, definition of,
+          <a href="#pg001">1</a>
+</p>
+<p class="ind">factors affecting,
+          <a href="#pg043">43</a>-87
+</p>
+
+<p>Medullary rays (<a href="#Rays">see Rays</a>)
+</p>
+
+<p>Mistletoe,
+          <a href="#pg070">70</a>
+</p>
+
+<a name="Modulus_of_elasticity"></a>
+<p>Modulus of elasticity,
+          <a href="#pg006">6</a>,
+          <a href="#pg025">25</a>,
+          <a href="#pg089">89</a>
+</p>
+<p class="ind2">formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg114">114</a>,
+          <a href="#pg123">123</a>
+</p>
+<p class="ind">of rupture,
+          <a href="#pg026">26</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg062">62</a>,
+          <a href="#pg075">75</a>
+</p>
+<p class="ind2">formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>
+</p>
+<p class="ind">speed-strength,
+          <a href="#pg094">94</a>
+</p>
+
+<p>Moisture, determination,
+          <a href="#pg090">90</a>,
+          <a href="#pg133">133</a>-134
+</p>
+<p class="ind">effect of,
+          <a href="#pg006">6</a>,
+          <a href="#pg008">8</a>,
+          <a href="#pg017">17</a>,
+          <a href="#pg033">33</a>,
+          <a href="#pg075">75</a>-84,
+          <a href="#pg138">138</a>-139
+</p>
+
+<p>Mulberry,
+          <a href="#pg044">44</a>,
+          <a href="#pg051">51</a>
+</p>
+
+<p>Natural shape and size,
+          <a href="#pg002">2</a>
+</p>
+
+<p>Neutral axis,
+          <a href="#pg023">23</a>
+</p>
+<p class="ind">plane,
+          <a href="#pg023">23</a>,
+          <a href="#pg033">33</a>
+</p>
+
+<p>Oak,
+          <a href="#pg015">15</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg043">43</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg049">49</a>,
+          <a href="#pg055">55</a>,
+          <a href="#pg060">60</a>,
+          <a href="#pg066">66</a>,
+          <a href="#pg071">71</a>,
+          <a href="#pg084">84</a>
+</p>
+<p class="ind">black,
+          <a href="#pg040">40</a>
+</p>
+<p class="ind">bur,
+          <a href="#pg013">13</a>
+</p>
+<p class="ind">live,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">post,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">red,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>,
+          <a href="#pg124">124</a>
+</p>
+<p class="ind">southern,
+          <a href="#pg052">52</a>,
+          <a href="#pg071">71</a>
+</p>
+<p class="ind">swamp white,
+          <a href="#pg009">9</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">tanbark,
+          <a href="#pg056">56</a>
+</p>
+<p class="ind">white,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg056">56</a>,
+          <a href="#pg072">72</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">yellow,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Osage orange,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg056">56</a>
+</p>
+
+<p>Oven-dry,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Packing Boxes, testing,
+          <a href="#pg124">124</a>
+</p>
+
+<p>Permanent set,
+          <a href="#pg005">5</a>
+</p>
+
+<p>Permeability,
+          <a href="#pg054">54</a>
+</p>
+
+<p>Pitch pockets,
+          <a href="#pg065">65</a>
+</p>
+
+<p>Pith rays (<a href="#Rays">see Rays</a>)
+</p>
+
+<p>Pine,
+          <a href="#pg044">44</a>,
+          <a href="#pg045">45</a>,
+          <a href="#pg055">55</a>
+</p>
+<p class="ind">Cuban,
+          <a href="#pg071">71</a>
+</p>
+<p class="ind">loblolly,
+          <a href="#pg036">36</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg071">71</a>,
+          <a href="#pg085">85</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>
+</p>
+<p class="ind">lodgepole,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">longleaf,
+          <a href="#pg006">6</a>,
+          <a href="#pg007">7</a>,
+          <a href="#pg008">8</a>,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg014">14</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg046">46</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg065">65</a>,
+          <a href="#pg071">71</a>,
+          <a href="#pg078">78</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">northern yellow,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">Norway (<a href="#Red_pine">see Red pine</a>)
+</p>
+<a name="Red_pine"></a>
+<p class="ind">red,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">shortleaf,
+          <a href="#pg013">13</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg071">71</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+<p class="ind">southern yellow,
+          <a href="#pg022">22</a>,
+          <a href="#pg124">124</a>
+</p>
+<p class="ind">sugar,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">western yellow,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">white,
+          <a href="#pg013">13</a>,
+          <a href="#pg015">15</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg051">51</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg144">144</a>
+</p>
+
+<p>Plane, neutral,
+          <a href="#pg023">23</a>,
+          <a href="#pg033">33</a>
+</p>
+
+<p>Plasticity,
+          <a href="#pg006">6</a>
+</p>
+
+<p>Pliability,
+          <a href="#pg005">5</a>,
+          <a href="#pg038">38</a>,
+          <a href="#pg085">85</a>
+</p>
+
+<p>Poplar,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">yellow,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Pores,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Preservatives, effect of,
+          <a href="#pg086">86</a>
+</p>
+
+<a name="Rays"></a>
+<p>Rays,
+          <a href="#pg060">60</a>
+</p>
+<p class="ind">effect on compression failure,
+          <a href="#pg017">17</a>,
+          <a href="#pg018">18</a>
+</p>
+<p class="ind">shrinkage,
+          <a href="#pg081">81</a>-82
+</p>
+
+<p>Redwood,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+
+<a name="Resilience"></a>
+<p>Resilience,
+          <a href="#pg002">2</a>,
+          <a href="#pg005">5</a>,
+          <a href="#pg049">49</a>
+</p>
+<p class="ind">elastic,
+          <a href="#pg006">6</a>
+</p>
+<p class="ind2">formul� for,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg104">104</a>,
+          <a href="#pg114">114</a>
+</p>
+
+<p>Resin, effect of,
+          <a href="#pg059">59</a>
+</p>
+<p class="ind">pockets,
+          <a href="#pg065">65</a>
+</p>
+
+<p>Rind-gall,
+          <a href="#pg065">65</a>
+</p>
+
+<p>Ring, annual,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">growth,
+          <a href="#pg044">44</a>,
+          <a href="#pg052">52</a>
+</p>
+<p class="ind">-shake,
+          <a href="#pg064">64</a>-65
+</p>
+<p class="ind">-porous,
+          <a href="#pg044">44</a>,
+          <a href="#pg059">59</a>
+</p>
+
+<p>Rot,
+          <a href="#pg068">68</a>,
+          <a href="#pg069">69</a>
+</p>
+
+<p>Rupture, modulus of,
+          <a href="#pg026">26</a>,
+          <a href="#pg054">54</a>,
+          <a href="#pg062">62</a>,
+          <a href="#pg075">75</a>
+</p>
+<p class="ind">formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>
+</p>
+
+<p>Safe Load,
+          <a href="#pg029">29</a>
+</p>
+
+<p>Sap,
+          <a href="#pg073">73</a>,
+          <a href="#pg074">74</a>
+</p>
+<p class="ind">-stain,
+          <a href="#pg059">59</a>
+</p>
+<p class="ind">-wood,
+          <a href="#pg050">50</a>-54,
+          <a href="#pg073">73</a>,
+          <a href="#pg074">74</a>
+</p>
+
+<p>Sassafras,
+          <a href="#pg051">51</a>
+</p>
+
+<p>Season checks,
+          <a href="#pg061">61</a>,
+          <a href="#pg078">78</a>-84
+</p>
+<p class="ind">of cutting, effect of,
+          <a href="#pg073">73</a>-75
+</p>
+
+<p>Seasoning,
+          <a href="#pg055">55</a>,
+          <a href="#pg074">74</a>,
+          <a href="#pg075">75</a>-84
+</p>
+
+<p>Second-growth,
+          <a href="#pg049">49</a>,
+          <a href="#pg050">50</a>
+</p>
+
+<p>Set,
+          <a href="#pg005">5</a>
+</p>
+
+<p>Shake,
+          <a href="#pg064">64</a>-66,
+          <a href="#pg072">72</a>
+</p>
+<p class="ind">cup,
+          <a href="#pg064">64</a>
+</p>
+<p class="ind">heart,
+          <a href="#pg064">64</a>
+</p>
+<p class="ind">ring,
+          <a href="#pg064">64</a>
+</p>
+<p class="ind">star,
+          <a href="#pg064">64</a>
+</p>
+
+<p>Shear 3,
+          <a href="#pg019">19</a>-22,
+          <a href="#pg133">133</a>
+</p>
+<p class="ind">across the grain,
+          <a href="#pg019">19</a>,
+          <a href="#pg021">21</a>,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">along the grain,
+          <a href="#pg019">19</a>,
+          <a href="#pg021">21</a>,
+          <a href="#pg076">76</a>,
+          <a href="#pg094">94</a>,
+          <a href="#pg107">107</a>-110
+</p>
+<p class="ind2">formul�,
+          <a href="#pg098">98</a>,
+          <a href="#pg102">102</a>
+</p>
+<p class="ind">horizontal,
+          <a href="#pg024">24</a>
+</p>
+<p class="ind">failure,
+          <a href="#pg035">35</a>-37
+</p>
+<p class="ind">longitudinal,
+          <a href="#pg021">21</a>,
+          <a href="#pg024">24</a>
+</p>
+<p class="ind">oblique,
+          <a href="#pg021">21</a>,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">transverse,
+          <a href="#pg023">23</a>
+</p>
+<p class="ind">vertical,
+          <a href="#pg023">23</a>
+</p>
+
+<p>Shearing strength,
+          <a href="#pg002">2</a>,
+          <a href="#pg019">19</a>
+</p>
+
+<p>Shipping dry,
+          <a href="#pg057">57</a>
+</p>
+
+<p>Shipworms,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Shock,
+          <a href="#pg030">30</a>,
+          <a href="#pg049">49</a>
+</p>
+
+<p>Shortening,
+          <a href="#pg003">3</a>,
+          <a href="#pg033">33</a>
+</p>
+
+<p>Shrinkage,
+          <a href="#pg054">54</a>,
+          <a href="#pg058">58</a>,
+          <a href="#pg074">74</a>,
+          <a href="#pg076">76</a>,
+          <a href="#pg078">78</a>-82,
+          <a href="#pg135">135</a>-137
+</p>
+
+<p><i>S</i>-irons,
+          <a href="#pg083">83</a>
+</p>
+
+<p>Site, effect on wood,
+          <a href="#pg048">48</a>,
+          <a href="#pg049">49</a>,
+          <a href="#pg070">70</a>-73
+</p>
+
+<p>Size of test specimens, effect of,
+          <a href="#pg089">89</a>-90,
+          <a href="#pg138">138</a>-139
+</p>
+
+<p>Sketching test specimens,
+          <a href="#pg094">94</a>-95,
+          <a href="#pg100">100</a>,
+          <a href="#pg102">102</a>,
+          <a href="#pg106">106</a>
+</p>
+
+<p>Softwood,
+          <a href="#pg044">44</a>,
+          <a href="#pg060">60</a>
+</p>
+
+<p>Span,
+          <a href="#pg025">25</a>
+</p>
+
+<p>Specific gravity,
+          <a href="#pg055">55</a>,
+          <a href="#pg135">135</a>-137
+</p>
+
+<p>Speed of testing machine,
+          <a href="#pg093">93</a>-94
+</p>
+<p class="ind">-strength modulus,
+          <a href="#pg094">94</a>
+</p>
+
+<p><i>Sph�eroma</i>,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Spike-pulling test,
+          <a href="#pg123">123</a>
+</p>
+
+<p>Spiral grain,
+          <a href="#pg060">60</a>
+</p>
+
+<p>Splintering tension failure,
+          <a href="#pg034">34</a>
+</p>
+
+<p>Splitting,
+          <a href="#pg041">41</a>,
+          <a href="#pg060">60</a>
+</p>
+
+<p>Spring wood (<a href="#Early_wood">see Early wood</a>)
+</p>
+
+<p>Spruce,
+          <a href="#pg014">14</a>,
+          <a href="#pg015">15</a>,
+          <a href="#pg022">22</a>,
+          <a href="#pg044">44</a>,
+          <a href="#pg059">59</a>,
+          <a href="#pg075">75</a>,
+          <a href="#pg084">84</a>,
+          <a href="#pg144">144</a>
+</p><p class="ind">Engelmann,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>
+</p>
+<p class="ind">red,
+          <a href="#pg007">7</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>
+</p>
+<p class="ind">white,
+          <a href="#pg013">13</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">140</a>
+</p>
+
+<p>Static tests, machine for,
+          <a href="#pg090">90</a>-92
+</p>
+
+<p>Steaming, effect of,
+          <a href="#pg085">85</a>,
+          <a href="#pg087">87</a>
+</p>
+
+<p>Stiffness,
+          <a href="#pg001">1</a>,
+          <a href="#pg004">4</a>,
+          <a href="#pg005">5</a>,
+          <a href="#pg006">6</a>,
+          <a href="#pg025">25</a>,
+          <a href="#pg026">26</a>,
+          <a href="#pg062">62</a>,
+          <a href="#pg076">76</a>
+</p>
+
+<p>Strain, definition of,
+          <a href="#pg002">2</a>
+</p>
+<p class="ind">unit,
+          <a href="#pg003">3</a>
+</p>
+
+<p>Stress, compressive,
+          <a href="#pg003">3</a>
+</p>
+<p class="ind">definition of,
+          <a href="#pg002">2</a>
+</p>
+<p class="ind">due to impact,
+          <a href="#pg031">31</a>,
+          <a href="#pg032">32</a>
+</p>
+<p class="ind">external,
+          <a href="#pg002">2</a>,
+          <a href="#pg033">33</a>
+</p>
+<p class="ind">internal,
+          <a href="#pg002">2</a>
+</p>
+<p class="ind">shearing,
+          <a href="#pg003">3</a>,
+          <a href="#pg021">21</a>
+</p>
+<p class="ind">tensile,
+          <a href="#pg003">3</a>,
+          <a href="#pg062">62</a>
+</p>
+<p class="ind">torsional,
+          <a href="#pg038">38</a>
+</p>
+
+<p>Stress, unit,
+          <a href="#pg003">3</a>
+</p>
+<p class="ind">-strain diagram,
+          <a href="#pg003">3</a>,
+          <a href="#pg097">97</a>-98,
+          <a href="#pg100">100</a>
+</p>
+
+<p>Structural timbers, strength of,
+          <a href="#pg138">138</a>-144
+</p>
+
+<p>Summer wood, (<a href="#Late_wood">see Late wood</a>)
+</p>
+
+<p>Sycamore,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg064">64</a>
+</p>
+
+<p>Tamarack,
+          <a href="#pg009">9</a>,
+          <a href="#pg013">13</a>,
+          <a href="#pg016">16</a>,
+          <a href="#pg020">20</a>,
+          <a href="#pg027">27</a>,
+          <a href="#pg032">32</a>,
+          <a href="#pg036">36</a>,
+          <a href="#pg040">40</a>,
+          <a href="#pg042">42</a>,
+          <a href="#pg048">48</a>,
+          <a href="#pg057">57</a>,
+          <a href="#pg140">140</a>,
+          <a href="#pg141">141</a>,
+          <a href="#pg142">142</a>,
+          <a href="#pg143">143</a>,
+          <a href="#pg144">144</a>
+</p>
+
+<p>Temperature, effect of,
+          <a href="#pg084">84</a>-86
+</p>
+
+<p>Tensile strength,
+          <a href="#pg001">1</a>,
+          <a href="#pg007">7</a>,
+          <a href="#pg023">23</a>,
+          <a href="#pg078">78</a>
+</p>
+<p class="ind">parallel to grain,
+          <a href="#pg007">7</a>,
+          <a href="#pg008">8</a>
+</p>
+<p class="ind">right angles to grain,
+          <a href="#pg008">8</a>
+</p>
+
+<p>Tension,
+          <a href="#pg007">7</a>
+</p>
+<p class="ind">failures,
+          <a href="#pg034">34</a>
+</p>
+<p class="ind">tests,
+          <a href="#pg118">118</a>-122
+</p>
+
+<p>Teredo,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Tests, impact,
+          <a href="#pg031">31</a>-33
+</p>
+<p class="ind">timber,
+          <a href="#pg088">88</a>-136
+</p>
+
+<p>Test specimens, size of,
+          <a href="#pg089">89</a>-90
+</p>
+
+<p>Timber testing,
+          <a href="#pg088">88</a>-136
+</p>
+
+<p>Vehicle woods, testing,
+          <a href="#pg124">124</a>
+</p>
+
+<p>Variability of wood,
+          <a href="#pg001">1</a>,
+          <a href="#pg002">2</a>,
+          <a href="#pg043">43</a>
+</p>
+
+<p>Walnut, black,
+          <a href="#pg022">22</a>
+</p>
+<p class="ind">common,
+          <a href="#pg022">22</a>
+</p>
+
+<p>Water content,
+          <a href="#pg055">55</a>,
+          <a href="#pg073">73</a>,
+          <a href="#pg074">74</a>
+</p>
+<p class="ind">effect of,
+          <a href="#pg006">06</a>,
+          <a href="#pg008">08</a>,
+          <a href="#pg017">17</a>,
+          <a href="#pg033">33</a>,
+          <a href="#pg075">75</a>-84
+</p>
+
+<p>Wear, resistance to (<a href="#Abrasion">see Abrasion</a>)
+</p>
+
+<p>Weight, relation to mechanical properties,
+          <a href="#pg054">54</a>-55
+</p>
+
+<p>Willow,
+          <a href="#pg044">44</a>
+</p>
+<p class="ind">black,
+          <a href="#pg013">13</a>
+</p>
+
+<p>Work (<a href="#Resilience">see Resilience</a>),
+          <a href="#pg030">30</a>,
+          <a href="#pg054">54</a>
+</p>
+
+<p>Working plan,
+          <a href="#pg088">88</a>,
+          <a href="#pg127">127</a>-137
+</p>
+
+<p>Xylotrya,
+          <a href="#pg067">67</a>
+</p>
+
+<p>Yellow Poplar,
+          <a href="#pg044">44</a>
+</p>
+
+<p>Zinc Chloride, effect of,
+          <a href="#pg087">87</a>
+</p>
+
+<a name="FOOTNOTES"></a><h2>FOOTNOTES</h2>
+
+<a name="fn01"></a><a href="#fn01r">return</a>
+<p>[Footnote 1: This is in accordance with the discovery made in
+1678 by Robert Hooke, and is known as <i>Hooke's law</i>.]</p>
+
+<a name="fn02"></a><a href="#fn02r">return</a>
+<p>[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. (<a href="#fig32">See Fig. 32</a>.)]</p>
+
+<a name="fn03"></a><a href="#fn03r">return</a>
+<p>[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.]</p>
+
+<a name="fn04"></a><a href="#fn04r">return</a>
+<p>[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.]</p>
+
+<a name="fn05"></a><a href="#fn05r">return</a>
+<p>[Footnote 5: See Bulletin 70, <i>op. cit.</i>, p. 129.]</p>
+
+<a name="fn06"></a><a href="#fn06r">return</a>
+<p>[Footnote 6: Jaccard, P.: �tude anatomique des bois comprim�s.
+Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band,
+1. Heft. Zurich, 1910, p. 66.]</p>
+
+<a name="fn07"></a><a href="#fn07r">return</a>
+<p>[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&mdash;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, <i>op. cit.</i>, pp. 57 <i>et seq.</i>]</p>
+
+<a name="fn08"></a><a href="#fn08r">return</a>
+<p>[Footnote 8: Shear should not be confused with ordinary cutting
+or incision.]</p>
+
+<a name="fn09"></a><a href="#fn09r">return</a>
+<p>[Footnote 9: While in reality this relationship does not exactly
+hold, the formul� for beams are based on its assumption.]</p>
+
+<a name="fn10"></a><a href="#fn10r">return</a>
+<p>[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 formul� are required. See any book on mechanics.]</p>
+
+<a name="fn11"></a><a href="#fn11r">return</a>
+<p>[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.]</p>
+
+<a name="fn12"></a><a href="#fn12r">return</a>
+<p>[Footnote 12: A fourth might be added, namely, <b>vibratory</b>, or
+<b>harmonic repetition</b>, which is frequently serious in the case
+of bridges.]</p>
+
+<a name="fn13"></a><a href="#fn13r">return</a>
+<p>[Footnote 13: Johnson, J.B.: The materials of construction, pp.
+81-82.]</p>
+
+<a name="fn14"></a><a href="#fn14r">return</a>
+<p>[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.]</p>
+
+<a name="fn15"></a><a href="#fn15r">return</a>
+<p>[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912,
+XXIII<sub><small>2</small></sub>, pp. 12-13.]</p>
+
+<a name="fn16"></a><a href="#fn16r">return</a>
+<p>[Footnote 16: See articles by Gabriel Janka listed in
+bibliography, <a href="#pg151">pages 151-152</a>.]</p>
+
+<a name="fn17"></a><a href="#fn17r">return</a>
+<p>[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 &amp; Sons, 1912.]</p>
+
+<a name="fn18"></a><a href="#fn18r">return</a>
+<p>[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.
+29.]</p>
+
+<a name="fn19"></a><a href="#fn19r">return</a>
+<p>[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, p. 37.]</p>
+
+<a name="fn20"></a><a href="#fn20r">return</a>
+<p>[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]</p>
+
+<a name="fn21"></a><a href="#fn21r">return</a>
+<p>[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.
+20-21.]</p>
+
+<a name="fn22"></a><a href="#fn22r">return</a>
+<p>[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]</p>
+
+<a name="fn23"></a><a href="#fn23r">return</a>
+<p>[Footnote 23: Bul. 80: The commercial hickories, p. 50.]</p>
+
+<a name="fn24"></a><a href="#fn24r">return</a>
+<p>[Footnote 24: <i>Loc. cit.</i>]</p>
+
+<a name="fn25"></a><a href="#fn25r">return</a>
+<p>[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 and <i>Eucalyptus globulus</i> and <i>viminalis</i>. 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.&mdash;H.D. Tiemann.]</p>
+
+<a name="fn26"></a><a href="#fn26r">return</a>
+<p>[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]</p>
+
+<a name="fn27"></a><a href="#fn27r">return</a>
+<p>[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.]</p>
+
+<a name="fn28"></a><a href="#fn28r">return</a>
+<p>[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126,
+appendix.]</p>
+
+<a name="fn29"></a><a href="#fn29r">return</a>
+<p>[Footnote 29: See Burke, H.E.: Black check in western hemlock.
+Cir. No. 61, U.S. Bu. Entomology, 1905.]</p>
+
+<a name="fn30"></a><a href="#fn30r">return</a>
+<p>[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees
+and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]</p>
+
+<a name="fn31"></a><a href="#fn31r">return</a>
+<p>[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.</p>
+
+<p>Weiss, Howard, and Barnum, Charles T.: The prevention of
+sapstain in lumber. Cir. 192, U.S. Forest Service, Washington,
+1911, pp. 16-17.]</p>
+
+<a name="fn32"></a><a href="#fn32r">return</a>
+<p>[Footnote 32: See Standard classification of structural timber.
+Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
+Contains three plates showing standard defects.]</p>
+
+<a name="fn33"></a><a href="#fn33r">return</a>
+<p>[Footnote 33: Bul. 108, pp. 52 <i>et seq.</i>]</p>
+
+<a name="fn34"></a><a href="#fn34r">return</a>
+<p>[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
+properties of western hemlock, p. 20.]</p>
+
+<a name="fn35"></a><a href="#fn35r">return</a>
+<p>[Footnote 35: Hartig, R.: The diseases of trees (trans. by
+Somerville and Ward), London and New York, 1894, pp. 282-294.]</p>
+
+<a name="fn36"></a><a href="#fn36r">return</a>
+<p>[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
+Centralb., XXXII, 2, 1910, pp. 74-81.]</p>
+
+<a name="fn37"></a><a href="#fn37r">return</a>
+<p>[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.]</p>
+
+<a name="fn38"></a><a href="#fn38r">return</a>
+<p>[Footnote 38: See Smith, C. Stowell: Preservation of piling
+against marine wood borers. Cir. 128, U.S. Forest Service, 1908,
+pp. 15.]</p>
+
+<a name="fn39"></a><a href="#fn39r">return</a>
+<p>[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.</p>
+
+<p>Meineoke, E.P.: Forest tree diseases common in California and
+Nevada, U.S. Forest Service, Washington, D.C., 1914.</p>
+
+<p>Hartig, R.: The diseases of trees. London and New York, 1894.]</p>
+
+<a name="fn40"></a><a href="#fn40r">return</a>
+<p>[Footnote 40: Dry rot in factory timbers, by Inspection Dept.
+Associated Factory Mutual Fire Insurance Cos., 31 Milk Street,
+Boston, 1913.]</p>
+
+<a name="fn41"></a><a href="#fn41r">return</a>
+<p>[Footnote 41: Falck, Richard: Die Meruliusfa�le des Bauholzes,
+Hausschwammforschungen, 6. Heft., Jena, 1912.]</p>
+
+<a name="fn42"></a><a href="#fn42r">return</a>
+<p>[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]</p>
+
+<a name="fn43"></a><a href="#fn43r">return</a>
+<p>[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.]</p>
+
+<a name="fn44"></a><a href="#fn44r">return</a>
+<p>[Footnote 44: Experiments in kiln-drying <i>Eucalyptus</i> in
+Berkeley, U.S. Forest Service.]</p>
+
+<a name="fn45"></a><a href="#fn45r">return</a>
+<p>[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.]</p>
+
+<a name="fn46"></a><a href="#fn46r">return</a>
+<p>[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.</p>
+
+<p>Bray, Wm. L.: The mistletoe pest in the Southwest. Bul. 166,
+U.S. Bu. Plant Ind., Washington, 1910.</p>
+
+<p>Meinecke, E.P.: Forest tree diseases common in California and
+Nevada. U.S. Forest Service, Washington, 1914, pp. 54-58.]</p>
+
+<a name="fn47"></a><a href="#fn47r">return</a>
+<p>[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.</p>
+
+<p>Kempfer, Wm. H.: The air-seasoning of timber. In Bul. 161, Am.
+Ry. Eng. Assn., 1913, p. 214.]</p>
+
+<a name="fn48"></a><a href="#fn48r">return</a>
+<p>[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.]</p>
+
+<a name="fn49"></a><a href="#fn49r">return</a>
+<p>[Footnote 49: The wood of <i>Eucalyptus globulus</i> (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.]</p>
+
+<a name="fn50"></a><a href="#fn50r">return</a>
+<p>[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev.
+ed.), p. 75.]</p>
+
+<a name="fn51"></a><a href="#fn51r">return</a>
+<p>[Footnote 51: Cir. 39. Experiments on the strength of treated
+timber, p. 18.]</p>
+
+<a name="fn52"></a><a href="#fn52r">return</a>
+<p>[Footnote 52: <i>Ibid.</i>, p. 21. See also Cir. 108, p. 19, table
+5.]</p>
+
+<a name="fn53"></a><a href="#fn53r">return</a>
+<p>[Footnote 53: Hatt, W. K.: Experiments on the strength of
+treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]</p>
+
+<a name="fn54"></a><a href="#fn54r">return</a>
+<p>[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by
+the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p.
+7.]</p>
+
+<a name="fn55"></a><a href="#fn55r">return</a>
+<p>[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.]</p>
+
+<a name="fn56"></a><a href="#fn56r">return</a>
+<p>[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.]</p>
+
+<a name="fn57"></a><a href="#fn57r">return</a>
+<p>[Footnote 57: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, pp. 53-54.]</p>
+
+<a name="fn58"></a><a href="#fn58r">return</a>
+<p>[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.]</p>
+
+<a name="fn59"></a><a href="#fn59r">return</a>
+<p>[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.]</p>
+
+<a name="fn60"></a><a href="#fn60r">return</a>
+<p>[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.]</p>
+
+<a name="fn61"></a><a href="#fn61r">return</a>
+<p>[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]</p>
+
+<a name="fn62"></a><a href="#fn62r">return</a>
+<p>[Footnote 62: Cir. 38: Instructions to engineers of timber
+tests, 1906, p. 24.]</p>
+
+<a name="fn63"></a><a href="#fn63r">return</a>
+<p>[Footnote 63: Warren, W.H.: The strength, elasticity, and other
+properties of New South Wales hardwood timbers, 1911, pp.
+58-62.]</p>
+
+<a name="fn64"></a><a href="#fn64r">return</a>
+<p>[Footnote 64: Wood is so seldom subjected to a pure stress of
+this kind that the torsion test is usually omitted.]</p>
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of The Mechanical Properties of Wood
+by Samuel J. Record
+
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+Project Gutenberg's The Mechanical Properties of Wood, by Samuel J. Record
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
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+
+Title: The Mechanical Properties of Wood
+       Including a Discussion of the Factors Affecting the Mechanical
+       Properties, and Methods of Timber Testing
+
+
+Author: Samuel J. Record
+
+Release Date: May 8, 2004 [EBook #12299]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK THE MECHANICAL PROPERTIES OF WOOD ***
+
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+Produced by Curtis Weyant, GF Untermeyer and PG Distributed
+Proofreaders.  Scans provided by Case Western Reserve University's
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+
+
+THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+
+[Illustration: Frontispiece. _Photo by the author_.
+
+Photomicrograph of a small block of western hemlock. At the top
+is the cross section showing to the right the late wood of one
+season's growth, to the left the early wood of the next season.
+The other two sections are longitudinal and show the fibrous
+character of the wood. To the left is the radial section with
+three rays crossing it. To the right is the tangential section
+upon which the rays appear as vertical rows of beads. X 35.]
+
+
+
+
+THE MECHANICAL PROPERTIES OF WOOD
+
+_Including a Discussion of the Factors Affecting the Mechanical
+Properties, and Methods of Timber Testing_
+
+
+BY SAMUEL J. RECORD, M.A., M.F. ASSISTANT PROFESSOR OF FOREST
+PRODUCTS, YALE UNIVERSITY
+
+
+FIRST EDITION FIRST THOUSAND
+
+1914
+
+
+BY THE SAME AUTHOR
+
+Identification of the Economic Woods of the United States.
+8vo, vi + 117 pages, 15 figures. Cloth, $1.25 net.
+
+
+TO THE STAFF OF THE FOREST PRODUCTS LABORATORY, AT MADISON,
+WISCONSIN IN APPRECIATION OF THE MANY OPPORTUNITIES AFFORDED AND
+COURTESIES EXTENDED THE AUTHOR
+
+
+
+
+PREFACE
+
+
+
+This book was written primarily for students of forestry to whom
+a knowledge of the technical properties of wood is essential.
+The mechanics involved is reduced to the simplest terms and
+without reference to higher mathematics, with which the students
+rarely are familiar. The intention throughout has been to avoid
+all unnecessarily technical language and descriptions, thereby
+making the subject-matter readily available to every one
+interested in wood.
+
+Part I is devoted to a discussion of the mechanical properties
+of wood--the relation of wood material to stresses and strains.
+Much of the subject-matter is merely elementary mechanics of
+materials in general, though written with reference to wood in
+particular. Numerous tables are included, showing the various
+strength values of many of the more important American woods.
+
+Part II deals with the factors affecting the mechanical
+properties of wood. This is a subject of interest to all who are
+concerned in the rational use of wood, and to the forester it
+also, by retrospection, suggests ways and means of regulating
+his forest product through control of the conditions of
+production. Attempt has been made, in the light of all data at
+hand, to answer many moot questions, such as the effect on the
+quality of wood of rate of growth, season of cutting, heartwood
+and sapwood, locality of growth, weight, water content,
+steaming, and defects.
+
+Part III describes methods of timber testing. They are for the
+most part those followed by the U.S. Forest Service. In schools
+equipped with the necessary machinery the instructions will
+serve to direct the tests; in others a study of the text with
+reference to the illustrations should give an adequate
+conception of the methods employed in this most important line
+of research.
+
+The appendix contains a copy of the working plan followed by the
+U.S. Forest Service in the extensive investigations covering the
+mechanical properties of the woods grown in the United States.
+It contains many valuable suggestions for the independent
+investigator. In addition four tables of strength values for
+structural timbers, both green and air-seasoned, are included.
+The relation of the stresses developed in different structural
+forms to those developed in the small clear specimens is given.
+
+In the bibliography attempt was made to list all of the
+important publications and articles on the mechanical properties
+of wood, and timber testing. While admittedly incomplete, it
+should prove of assistance to the student who desires a fuller
+knowledge of the subject than is presented here.
+
+The writer is indebted to the U.S. Forest Service for nearly all
+of his tables and photographs as well as many of the data upon
+which the book is based, since only the Government is able to
+conduct the extensive investigations essential to a thorough
+understanding of the subject. More than eighty thousand tests
+have been made at the Madison laboratory alone, and the work is
+far from completion.
+
+The writer also acknowledges his indebtedness to Mr. Emanuel
+Fritz, M.E., M.F., for many helpful suggestions in the
+preparation of Part I; and especially to Mr. Harry Donald
+Tiemann, M.E., M.F., engineer in charge of Timber Physics at the
+Government Forest Products Laboratory, Madison, Wisconsin, for
+careful revision of the entire manuscript.
+
+SAMUEL J. RECORD.
+YALE FOREST SCHOOL, _July 1, 1914_.
+
+
+
+
+CONTENTS
+
+
+
+      PREFACE
+
+       PART I THE MECHANICAL PROPERTIES OF WOOD
+
+              Introduction
+              Fundamental considerations and definitions
+              Tensile strength
+              Compressive or crushing strength
+              Shearing strength
+              Transverse or bending strength: Beams
+              Toughness: Torsion
+              Hardness
+              Cleavability
+
+      PART II FACTORS AFFECTING THE MECHANICAL PROPERTIES OF
+                WOOD
+
+              Introduction
+              Rate of growth
+              Heartwood and sapwood
+              Weight, density, and specific gravity
+              Color
+              Cross grain
+              Knots
+              Frost splits
+              Shakes, galls, pitch pockets
+              Insect injuries
+              Marine wood-borer injuries
+              Fungous injuries
+              Parasitic plant injuries
+              Locality of growth
+              Season of cutting
+              Water content
+              Temperature
+              Preservatives
+
+     PART III TIMBER TESTING
+
+              Working plan
+              Forms of material tested
+              Size of test specimens
+              Moisture determination
+              Machine for static tests
+              Speed of testing machine
+              Bending large beams
+              Bending small beams
+              Endwise compression
+              Compression across the grain
+              Shear along the grain
+              Impact test
+              Hardness test: Abrasion and indentation
+              Cleavage test
+              Tension test parallel to the grain
+              Tension test at right angles to the grain
+              Torsion test
+              Special tests
+              Spike pulling test
+              Packing boxes
+              Vehicle and implement woods
+              Cross-arms
+              Other tests
+
+     APPENDIX
+
+              Sample working plan of United States Forest
+                Service
+              Strength values for structural timbers
+
+ BIBLIOGRAPHY
+
+      Part I: Some general works on mechanics, materials of
+                construction, and testing of materials
+     Part II: Publications and articles on the mechanical
+                properties of wood, and timber testing
+    Part III: Publications of the United States Government on
+                the mechanical properties of wood, and timber
+                testing
+
+ILLUSTRATIONS
+
+ Frontispiece Photomicrograph of a small block of western
+                hemlock
+           1. Stress-strain diagrams of two longleaf pine beams
+           2. Compression across the grain
+           3. Side view of failures in compression across the
+                grain
+           4. End view of failures in compression across the
+                grain
+           5. Testing a buggy-spoke in endwise compression
+           6. Unequal distribution of stress in a long column due
+                to lateral bending
+           7. Endwise compression of a short column
+           8. Failures of a short column of green spruce
+           9. Failures of short columns of dry chestnut
+          10. Example of shear along the grain
+          11. Failures of test specimens in shear along the
+                grain
+          12. Horizontal shear in a beam
+          13. Oblique shear in a short column
+          14. Failure of a short column by oblique shear
+          15. Diagram of a simple beam
+          16. Three common forms of beams--(1) simple,
+                (2) cantilever, (3) continuous
+          17. Characteristic failures of simple beams
+          18. Failure of a large beam by horizontal shear
+          19. Torsion of a shaft
+          20. Effect of torsion on different grades of hickory
+          21. Cleavage of highly elastic wood
+          22. Cross-sections of white ash, red gum, and eastern
+                hemlock
+          23. Cross-section of longleaf pine
+          24. Relation of the moisture content to the various
+                strength values of spruce
+          25. Cross-section of the wood of western larch showing
+                fissures in the thick-walled cells of the late
+                wood
+          26. Progress of drying throughout the length of a
+                chestnut beam
+          27. Excessive season checking
+          28. Control of season checking by the use of S-irons
+          29. Static bending test on a large beam
+          30. Two methods of loading a beam
+          31. Static bending test on a small beam
+          32. Sample log sheet, giving full details of a
+                transverse bending test on a small pine beam
+          33. Endwise compression test
+          34. Sample log sheet of an endwise compression test on
+                a short pine column
+          35. Compression across the grain
+          36. Vertical section of shearing tool
+          37. Front view of shearing tool
+          38. Two forms of shear test specimens
+          39. Making a shearing test
+          40. Impact testing machine
+          41. Drum record of impact bending test
+          42. Abrasion machine for testing the wearing qualities
+                of woods
+          43. Design of tool for testing the hardness of woods
+                by indentation
+          44. Design of tool for cleavage test
+          45. Design of cleavage test specimen
+          46. Designs of tension test specimens used in United
+                States
+          47. Design of tension test specimen used in New South
+                Wales
+          48. Design of tool and specimen for testing tension at
+                right angles to the grain
+          49. Making a torsion test on hickory
+          50. Method of cutting and marking test specimens
+          51. Diagram of specific gravity apparatus
+
+       TABLES
+
+           I. Comparative strength of iron, steel, and wood
+          II. Ratio of strength of wood in tension and in
+                compression
+         III. Right-angled tensile strength of small clear
+                pieces of 25 woods in green condition
+          IV. Results of compression tests across the grain on
+                51 woods in green condition, and comparison with
+                white oak
+           V. Relation of fibre stress at elastic limit in
+                bending to the crushing strength of blocks cut
+                therefrom in pounds per square inch
+          VI. Results of endwise compression tests on small
+                clear pieces of 40 woods in green condition
+         VII. Shearing strength along the grain of small clear
+                pieces of 41 woods in green condition
+        VIII. Shearing strength across the grain of various
+                American woods
+          IX. Results of static bending tests on small clear
+                beams of 49 woods in green condition
+           X. Results of impact bending tests on small clear
+                beams of 34 woods in green condition
+          XI. Manner of first failure of large beams
+         XII. Hardness of 32 woods in green condition, as
+                indicated by the load required to imbed a
+                0.444-inch steel ball to one-half its diameter
+        XIII. Cleavage strength of small clear pieces of 32
+                woods in green condition
+         XIV. Specific gravity, and shrinkage of 51 American
+                woods
+          XV. Effect of drying on the mechanical properties of
+                wood, shown in ratio of increase due to reducing
+                moisture content from the green condition to
+                kiln-dry
+         XVI. Effect of steaming on the strength of green
+                loblolly pine
+        XVII. Speed-strength moduli, and relative increase in
+                strength at rates of fibre strain increasing in
+                geometric ratio
+       XVIII. Results of bending tests on green structural
+                timbers
+         XIX. Results of compression and shear tests on green
+                structural timbers
+          XX. Results of bending tests on air-seasoned
+                structural timbers
+         XXI. Results of compression and shear tests on
+                air-seasoned structural timbers
+        XXII. Working unit stresses for structural timber
+                expressed in pounds per square inch
+
+
+
+
+PART I THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+INTRODUCTION
+
+
+The mechanical properties of wood are its fitness and ability to
+resist applied or external forces. By external force is meant
+any force outside of a given piece of material which tends to
+deform it in any manner. It is largely such properties that
+determine the use of wood for structural and building purposes
+and innumerable other uses of which furniture, vehicles,
+implements, and tool handles are a few common examples.
+
+Knowledge of these properties is obtained through
+experimentation either in the employment of the wood in practice
+or by means of special testing apparatus in the laboratory.
+Owing to the wide range of variation in wood it is necessary
+that a great number of tests be made and that so far as possible
+all disturbing factors be eliminated. For comparison of
+different kinds or sizes a standard method of testing is
+necessary and the values must be expressed in some defined
+units. For these reasons laboratory experiments if properly
+conducted have many advantages over any other method.
+
+One object of such investigation is to find unit values for
+strength and stiffness, etc. These, because of the complex
+structure of wood, cannot have a constant value which will be
+exactly repeated in each test, even though no error be made. The
+most that can be accomplished is to find average values, the
+amount of variation above and below, and the laws which govern
+the variation. On account of the great variability in strength
+of different specimens of wood even from the same stick and
+appearing to be alike, it is important to eliminate as far as
+possible all extraneous factors liable to influence the results
+of the tests.
+
+The mechanical properties of wood considered in this book are:
+(1) stiffness and elasticity, (2) tensile strength, (3)
+compressive or crushing strength, (4) shearing strength, (5)
+transverse or bending strength, (6) toughness, (7) hardness, (8)
+cleavability, (9) resilience. In connection with these,
+associated properties of importance are briefly treated.
+
+In making use of figures indicating the strength or other
+mechanical properties of wood for the purpose of comparing the
+relative merits of different species, the fact should be borne
+in mind that there is a considerable range in variability of
+each individual material and that small differences, such as a
+few hundred pounds in values of 10,000 pounds, cannot be
+considered as a criterion of the quality of the timber. In
+testing material of the same kind and grade, differences of 25
+per cent between individual specimens may be expected in
+conifers and 50 per cent or even more in hardwoods. The figures
+given in the tables should be taken as indications rather than
+fixed values, and as applicable to a large number collectively
+and not to individual pieces.
+
+
+
+FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS
+
+
+Study of the mechanical properties of a material is concerned
+mostly with its behavior in relation to stresses and strains,
+and the factors affecting this behavior. A ~stress~ is a
+distributed force and may be defined as the mutual action (1) of
+one body upon another, or (2) of one part of a body upon another
+part. In the first case the stress is _external_; in the other
+_internal_. The same stress may be internal from one point of
+view and external from another. An external force is always
+balanced by the internal stresses when the body is in
+equilibrium.
+
+If no external forces act upon a body its particles assume
+certain relative positions, and it has what is called its
+_natural shape and size_. If sufficient external force is
+applied the natural shape and size will be changed. This
+distortion or deformation of the material is known as the
+~strain~. Every stress produces a corresponding strain, and
+within a certain limit (see _elastic limit_, in FUNDAMENTAL
+CONSIDERATIONS AND DEFINITIONS, above) the strain is directly
+proportional to the stress producing it.[1] The same intensity
+of stress, however, does not produce the same strain in
+different materials or in different qualities of the same
+material. No strain would be produced in a perfectly rigid body,
+but such is not known to exist.
+
+[Footnote 1: This is in accordance with the discovery made in
+1678 by Robert Hooke, and is known as _Hooke's law_.]
+
+Stress is measured in pounds (or other unit of weight or force).
+A ~unit stress~ is the stress on a unit of the sectional
+      {                P  }
+area. { Unit stress = --- } For instance, if a load (P) of one
+      {                A  }
+hundred pounds is uniformly supported by a vertical post with a
+cross-sectional area (A) of ten square inches, the unit
+compressive stress is ten pounds per square inch.
+
+Strain is measured in inches (or other linear unit). A ~unit
+strain~ is the strain per unit of length. Thus if a post 10
+inches long before compression is 9.9 inches long under the
+compressive stress, the total strain is 0.1 inch, and the unit
+           l     0.1
+strain is --- = ----- = 0.01 inch per inch of length.
+           L     10
+
+As the stress increases there is a corresponding increase in the
+strain. This ratio may be graphically shown by means of a
+diagram or curve plotted with the increments of load or stress
+as ordinates and the increments of strain as abscissae. This is
+known as the ~stress-strain diagram~. Within the limit mentioned
+above the diagram is a straight line. (See Fig. 1.) If the
+results of similar experiments on different specimens are
+plotted to the same scales, the diagrams furnish a ready means
+for comparison. The greater the resistance a material offers to
+deformation the steeper or nearer the vertical axis will be the
+line.
+
+[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf
+pine beams. E.L. = elastic limit. The areas of the triangles
+0(EL)A and 0(EL)B represent the elastic resilience of the dry
+and green beams, respectively.]
+
+There are three kinds of internal stresses, namely, (1)
+~tensile~, (2) ~compressive~, and (3) ~shearing~. When external
+forces act upon a bar in a direction away from its ends or a
+direct pull, the stress is a tensile stress; when toward the
+ends or a direct push, compressive stress. In the first instance
+the strain is an _elongation_; in the second a _shortening_.
+Whenever the forces tend to cause one portion of the material to
+slide upon another adjacent to it the action is called a
+_shear_. The action is that of an ordinary pair of shears. When
+riveted plates slide on each other the rivets are sheared off.
+
+These three simple stresses may act together, producing compound
+stresses, as in flexure. When a bow is bent there is a
+compression of the fibres on the inner or concave side and an
+elongation of the fibres on the outer or convex side. There is
+also a tendency of the various fibres to slide past one another
+in a longitudinal direction. If the bow were made of two or more
+separate pieces of equal length it would be noted on bending
+that slipping occurred along the surfaces of contact, and that
+the ends would no longer be even. If these pieces were securely
+glued together they would no longer slip, but the tendency to do
+so would exist just the same. Moreover, it would be found in the
+latter case that the bow would be much harder to bend than where
+the pieces were not glued together--in other words, the
+_stiffness_ of the bow would be materially increased.
+
+~Stiffness~ is the property by means of which a body acted upon
+by external forces tends to retain its natural size and shape,
+or resists deformation. Thus a material that is difficult to
+bend or otherwise deform is stiff; one that is easily bent or
+otherwise deformed is _flexible_. Flexibility is not the exact
+counterpart of stiffness, as it also involves toughness and
+pliability.
+
+If successively larger loads are applied to a body and then
+removed it will be found that at first the body completely
+regains its original form upon release from the stress--in other
+words, the body is ~elastic~. No substance known is perfectly
+elastic, though many are practically so under small loads.
+Eventually a point will be reached where the recovery of the
+specimen is incomplete. This point is known as the ~elastic
+limit~, which may be defined as the limit beyond which it is
+impossible to carry the distortion of a body without producing a
+permanent alteration in shape. After this limit has been
+exceeded, the size and shape of the specimen after removal of
+the load will not be the same as before, and the difference or
+amount of change is known as the ~permanent set~.
+
+Elastic limit as measured in tests and used in design may be
+defined as that unit stress at which the deformation begins to
+increase in a faster ratio than the applied load. In practice
+the elastic limit of a material under test is determined from
+the stress-strain diagram. It is that point in the line where
+the diagram begins perceptibly to curve.[2] (See Fig. 1.)
+
+[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.)]
+
+~Resilience~ is the amount of work done upon a body in deforming
+it. Within the elastic limit it is also a measure of the
+potential energy stored in the material and represents the
+amount of work the material would do upon being released from a
+state of stress. This may be graphically represented by a
+diagram in which the abscissae represent the amount of deflection
+and the ordinates the force acting. The area included between
+the stress-strain curve and the initial line (which is zero)
+represents the work done. (See Fig. 1.) If the unit of space is
+in inches and the unit of force is in pounds the result is
+inch-pounds. If the elastic limit is taken as the apex of the
+triangle the area of the triangle will represent the ~elastic
+resilience~ of the specimen. This amount of work can be applied
+repeatedly and is perhaps the best measure of the toughness of
+the wood as a working quality, though it is not synonymous with
+toughness.
+
+Permanent set is due to the ~plasticity~ of the material. A
+perfectly plastic substance would have no elasticity and the
+smallest forces would cause a set. Lead and moist clay are
+nearly plastic and wood possesses this property to a greater or
+less extent. The plasticity of wood is increased by wetting,
+heating, and especially by steaming and boiling. Were it not for
+this property it would be impossible to dry wood without
+destroying completely its cohesion, due to the irregularity of
+shrinkage.
+
+A substance that can undergo little change in shape without
+breaking or rupturing is ~brittle~. Chalk and glass are common
+examples of brittle materials. Sometimes the word _brash_ is
+used to describe this condition in wood. A brittle wood breaks
+suddenly with a clean instead of a splintery fracture and
+without warning. Such woods are unfitted to resist shock or
+sudden application of load.
+
+The measure of the stiffness of wood is termed the ~modulus of
+elasticity~ (or _coefficient of elasticity_). It is the ratio of
+stress per unit of area to the deformation per unit of
+        {      unit stress  }
+length. { E = ------------- } It is a number indicative of
+        {      unit strain  }
+stiffness, not of strength, and only applies to conditions
+within the elastic limit. It is nearly the same whether derived
+from compression tests or from tension tests.
+
+A large modulus indicates a stiff material. Thus in green wood
+tested in static bending it varies from 643,000 pounds per
+square inch for arborvitae to 1,662,000 pounds for longleaf pine,
+and 1,769,000 pounds for pignut hickory. (See Table IX.) The
+values derived from tests of small beams of dry material are
+much greater, approaching 3,000,000 for some of our woods. These
+values are small when compared with steel which has a modulus of
+elasticity of about 30,000,000 pounds per square inch. (See
+Table I.)
+
+|------------------------------------------------------------------------------|
+|                                   TABLE I                                    |
+|------------------------------------------------------------------------------|
+|                COMPARATIVE STRENGTH OF IRON, STEEL, AND WOOD                 |
+|------------------------------------------------------------------------------|
+|                         | Sp.  | Modulus of | Tensile  | Crushing | Modulus  |
+|        MATERIAL         | gr., | elasticity | strength | strength |   of     |
+|                         | dry  | in bending |          |          | rupture  |
+|-------------------------+----- +------------+----------+----------+----------|
+|                         |      | Lbs. per   | Lbs. per | Lbs. per | Lbs. per |
+|                         |      |  sq. in.   | sq. in.  | sq. in.  | sq. in.  |
+|                         |      |            |          |          |          |
+| Cast iron, cold blast   |      |            |          |          |          |
+|   (Hodgkinson)          | 7.1  | 17,270,000 |  16,700  |  106,000 |  38,500  |
+| Bessenger steel,        |      |            |          |          |          |
+|   high grade (Fairbain) | 7.8  | 29,215,000 |  88,400  |  225,600 |          |
+| Longleaf pine,          |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .63 |  2,800,000 |          |   13,000 |  21,000  |
+| Redspruce,              |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .41 |  1,800,000 |          |    8,800 |  14,500  |
+| Pignut hickory,         |      |            |          |          |          |
+|   3.5% moisture (U.S.)  |  .86 |  2,370,000 |          |   11,130 |  24,000  |
+|------------------------------------------------------------------------------|
+| NOTE.--Great variation may be found in different samples of metals as well   |
+| as of wood. The examples given represent reasonable values.                  |
+|------------------------------------------------------------------------------|
+
+
+
+TENSILE STRENGTH
+
+
+~Tension~ results when a pulling force is applied to opposite
+ends of a body. This external pull is communicated to the
+interior, so that any portion of the material exerts a pull or
+tensile force upon the remainder, the ability to do so depending
+upon the property of cohesion. The result is an elongation or
+stretching of the material in the direction of the applied
+force. The action is the opposite of compression.
+
+Wood exhibits its greatest strength in tension parallel to the
+grain, and it is very uncommon in practice for a specimen to be
+pulled in two lengthwise. This is due to the difficulty of
+making the end fastenings secure enough for the full tensile
+strength to be brought into play before the fastenings shear off
+longitudinally. This is not the case with metals, and as a
+result they are used in almost all places where tensile strength
+is particularly needed, even though the remainder of the
+structure, such as sills, beams, joists, posts, and flooring,
+may be of wood. Thus in a wooden truss bridge the tension
+members are steel rods.
+
+The tensile strength of wood parallel to the grain depends upon
+the strength of the fibres and is affected not only by the
+nature and dimensions of the wood elements but also by their
+arrangement. It is greatest in straight-grained specimens with
+thick-walled fibres. Cross grain of any kind materially reduces
+the tensile strength of wood, since the tensile strength at
+right angles to the grain is only a small fraction of that
+parallel to the grain.
+
+|--------------------------------------------------------------|
+|                           TABLE II                           |
+|--------------------------------------------------------------|
+|   RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION    |
+|           (Bul. 10, U. S. Div. of Forestry, p. 44)           |
+|--------------------------------------------------------------|
+|               |       Ratio:    |  A stick 1 square inch in  |
+|               |                 |      cross section.        |
+|               |      Tensile    |                            |
+| KIND OF WOOD  |      strength   |   Weight required to--     |
+|               | R = ----------- +----------------------------|
+|               |     compressive | Pull apart | Crush endwise |
+|               |      strength   |            |               |
+|---------------+-----------------+------------+---------------|
+| Hickory       |        3.7      |     32,000 |     8,500     |
+| Elm           |        3.8      |     29,000 |     7,500     |
+| Larch         |        2.3      |     19,400 |     8,600     |
+| Longleaf Pine |        2.2      |     17,300 |     7,400     |
+|--------------------------------------------------------------|
+| NOTE.--Moisture condition not given.                         |
+|--------------------------------------------------------------|
+
+Failure of wood in tension parallel to the grain occurs
+sometimes in flexure, especially with dry material. The tension
+portion of the fracture is nearly the same as though the piece
+were pulled in two lengthwise. The fibre walls are torn across
+obliquely and usually in a spiral direction. There is
+practically no pulling apart of the fibres, that is, no
+separation of the fibres along their walls, regardless of their
+thickness. The nature of tension failure is apparently not
+affected by the moisture condition of the specimen, at least not
+so much so as the other strength values.[3]
+
+[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.]
+
+Tension at right angles to the grain is closely related to
+cleavability. When wood fails in this manner the thin fibre
+walls are torn in two lengthwise while the thick-walled fibres
+are usually pulled apart along the primary wall.
+
+|--------------------------------------------|
+|                 TABLE III                  |
+|--------------------------------------------|
+|  TENSILE STRENGTH AT RIGHT ANGLES TO THE   |
+| GRAIN OF SMALL CLEAR PIECES OF 25 WOODS IN |
+|              GREEN CONDITION               |
+|         (Forest Service Cir. 213)          |
+|--------------------------------------------|
+|                  |    When    |    When    |
+|   COMMON NAME    | surface of | surface of |
+|   OF SPECIES     | failure is | failure is |
+|                  |   radial   | tangential |
+|------------------+------------+------------|
+|                  |  Lbs. per  |  Lbs. per  |
+|                  |  sq. inch  |  sq. inch  |
+|                  |            |            |
+|    Hardwoods     |            |            |
+|                  |            |            |
+| Ash, white       |      645   |      671   |
+| Basswood         |      226   |      303   |
+| Beech            |      633   |      969   |
+| Birch, yellow    |      446   |      526   |
+| Elm, slippery    |      765   |      832   |
+| Hackberry        |      661   |      786   |
+| Locust, honey    |    1,133   |    1,445   |
+| Maple, sugar     |      610   |      864   |
+| Oak, post        |      714   |      924   |
+|   red            |      639   |      874   |
+|   swamp white    |      757   |      909   |
+|   white          |      622   |      749   |
+|   yellow         |      728   |      929   |
+| Sycamore         |      540   |      781   |
+| Tupelo           |      472   |      796   |
+|                  |            |            |
+|    Conifers      |            |            |
+|                  |            |            |
+| Arborvitae        |      241   |      235   |
+| Cypress, bald    |      242   |      251   |
+| Fir, white       |      213   |      304   |
+| Hemlock          |      271   |      323   |
+| Pine, longleaf   |      240   |      298   |
+|   red            |      179   |      205   |
+|   sugar          |      239   |      304   |
+|   western yellow |      230   |      252   |
+|   white          |      225   |      285   |
+| Tamarack         |      236   |      274   |
+|--------------------------------------------|
+
+
+
+COMPRESSIVE OR CRUSHING STRENGTH
+
+
+~Compression across the grain~ is very closely related to
+hardness and transverse shear. There are two ways in which wood
+is subjected to stress of this kind, namely, (1) with the load
+acting over the entire area of the specimen, and (2) with a load
+concentrated over a portion of the area. (See Fig. 2.) The
+latter is the condition more commonly met with in practice, as,
+for example, where a post rests on a horizontal sill, or a rail
+rests on a cross-tie. The former condition, however, gives the
+true resistance of the grain to simple crushing.
+
+[Illustration: FIG. 2.--Compression across the grain.]
+
+The first effect of compression across the grain is to compact
+the fibres, the load gradually but irregularly increasing as the
+density of the material is increased. If the specimen lies on a
+flat surface and the load is applied to only a portion of the
+upper area, the bearing plate indents the wood, crushing the
+upper fibres without affecting the lower part. (See Fig. 3.) As
+the load increases the projecting ends sometimes split
+horizontally. (See Fig. 4.) The irregularities in the load are
+due to the fact that the fibres collapse a few at a time,
+beginning with those with the thinnest walls. The projection of
+the ends increases the strength of the material directly beneath
+the compressing weight by introducing a beam action which helps
+support the load. This influence is exerted for a short distance
+only.
+
+[Illustration: FIG. 3.--Side view of failures in compression
+across the grain, showing crushing of blocks under bearing
+plate. Specimen at right shows splitting at ends.]
+
+[Illustration: FIG. 4.--End view of failures in compression
+across the grain, showing splitting of the ends of the test
+specimens.]
+
+When wood is used for columns, props, posts, and spokes, the
+weight of the load tends to shorten the material endwise. This
+is ~endwise compression~, or compression parallel to the grain.
+In the case of long columns, that is, pieces in which the length
+is very great compared with their diameter, the failure is by
+sidewise bending or flexure, instead of by crushing or
+splitting. (See Fig. 5.) A familiar instance of this action is
+afforded by a flexible walking-stick. If downward pressure is
+exerted with the hand on the upper end of the stick placed
+vertically on the floor, it will be noted that a definite amount
+of force must be applied in each instance before decided flexure
+takes place. After this point is reached a very slight increase
+of pressure very largely increases the deflection, thus
+obtaining so great a leverage about the middle section as to
+cause rupture.
+
+[Illustration: FIG. 5.--Testing a buggy spoke in endwise
+compression, illustrating the failure by sidewise bending of a
+long column fixed only at the lower end. _Photo by U. S. Forest
+Service_]
+
+The lateral bending of a column produces a combination of
+bending with compressive stress over the section, the
+compressive stress being maximum at the section of greatest
+deflection on the concave side. The convex surface is under
+tension, as in an ordinary beam test. (See Fig. 6.) If the same
+stick is braced in such a way that flexure is prevented, its
+supporting strength is increased enormously, since the
+compressive stress acts uniformly over the section, and failure
+is by crushing or splitting, as in small blocks. In all columns
+free to bend in any direction the deflection will be seen in the
+direction in which the column is least stiff. This sidewise
+bending can be overcome by making pillars and columns thicker in
+the middle than at the ends, and by bracing studding, props, and
+compression members of trusses. The strength of a column also
+depends to a considerable extent upon whether the ends are free
+to turn or are fixed.
+
+[Illustration: FIG. 6.--Unequal distribution of stress in a long
+column due to lateral bending.]
+
+|-------------------------------------------------------|
+|                       TABLE IV                        |
+|-------------------------------------------------------|
+|   RESULTS OF COMPRESSION TESTS ACROSS THE GRAIN ON    |
+|   51 WOODS IN GREEN CONDITION, AND COMPARISON WITH    |
+|                       WHITE OAK                       |
+|                 (U. S. Forest Service)                |
+|-------------------------------------------------------|
+|                       | Fibre stress  | Fiber stress  |
+|      COMMON NAME      |  at elastic   | in per cent   |
+|      OF SPECIES       |     limit     | of white oak, |
+|                       | perpendicular | or 853 pounds |
+|                       |   to grain    | per sq. in.   |
+|-----------------------+---------------+---------------|
+|                       |   Lbs. per    |               |
+|                       |   sq. inch    |    Per cent   |
+|                       |               |               |
+| Osage orange          |     2,260     |      265.0    |
+| Honey locust          |     1,684     |      197.5    |
+| Black locust          |     1,426     |      167.2    |
+| Post oak              |     1,148     |      134.6    |
+| Pignut hickory        |     1,142     |      133.9    |
+| Water hickory         |     1,088     |      127.5    |
+| Shagbark hickory      |     1,070     |      125.5    |
+| Mockernut hickory     |     1,012     |      118.6    |
+| Big shellbark hickory |       997     |      116.9    |
+| Bitternut hickory     |       986     |      115.7    |
+| Nutmeg hickory        |       938     |      110.0    |
+| Yellow oak            |       857     |      100.5    |
+| White oak             |       853     |      100.0    |
+| Bur oak               |       836     |       98.0    |
+| White ash             |       828     |       97.1    |
+| Red oak               |       778     |       91.2    |
+| Sugar maple           |       742     |       87.0    |
+| Rock elm              |       696     |       81.6    |
+| Beech                 |       607     |       71.2    |
+| Slippery elm          |       599     |       70.2    |
+| Redwood               |       578     |       67.8    |
+| Bald cypress          |       548     |       64.3    |
+| Red maple             |       531     |       62.3    |
+| Hackberry             |       525     |       61.6    |
+| Incense cedar         |       518     |       60.8    |
+| Hemlock               |       497     |       58.3    |
+| Longleaf pine         |       491     |       57.6    |
+| Tamarack              |       480     |       56.3    |
+| Silver maple          |       456     |       53.5    |
+| Yellow birch          |       454     |       53.2    |
+| Tupelo                |       451     |       52.9    |
+| Black cherry          |       444     |       52.1    |
+| Sycamore              |       433     |       50.8    |
+| Douglas fir           |       427     |       50.1    |
+| Cucumber tree         |       408     |       47.8    |
+| Shortleaf pine        |       400     |       46.9    |
+| Red pine              |       358     |       42.0    |
+| Sugar pine            |       353     |       41.1    |
+| White elm             |       351     |       41.2    |
+| Western yellow pine   |       348     |       40.8    |
+| Lodgepole pine        |       348     |       40.8    |
+| Red spruce            |       345     |       40.5    |
+| White pine            |       314     |       36.8    |
+| Engelman spruce       |       290     |       34.0    |
+| Arborvitae             |       288     |       33.8    |
+| Largetooth aspen      |       269     |       31.5    |
+| White spruce          |       262     |       30.7    |
+| Butternut             |       258     |       30.3    |
+| Buckeye (yellow)      |       210     |       24.6    |
+| Basswood              |       209     |       24.5    |
+| Black willow          |       193     |       22.6    |
+|-------------------------------------------------------|
+
+The complexity of the computations depends upon the way in which
+the stress is applied and the manner in which the stick bends.
+Ordinarily where the length of the test specimen is not greater
+than four diameters and the ends are squarely faced (see Fig.
+7), the force acts uniformly over each square inch of area and
+the crushing strength is equal to the maximum load (P) divided
+                                      {      P  }
+by the area of the cross-section (A). { C = --- }
+                                      {      A  }
+
+[Illustration: FIG. 7.--Endwise compression of a short column.]
+
+It has been demonstrated[4] that the ultimate strength in
+compression parallel to the grain is very nearly the same as the
+extreme fibre stress at the elastic limit in bending. (See Table
+V.) In other words, the transverse strength of beams at elastic
+limit is practically equal to the compressive strength of the
+same material in short columns. It is accordingly possible to
+calculate the approximate breaking strength of beams from the
+compressive strength of short columns except when the wood is
+brittle. Since tests on endwise compression are simpler, easier
+to make, and less expensive than transverse bending tests, the
+importance of this relation is obvious, though it does not do
+away with the necessity of making beam tests.
+
+[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.]
+
+|-------------------------------------------------------------------------------|
+|                                    TABLE V                                    |
+|-------------------------------------------------------------------------------|
+|   RELATION OF FIBRE STRESS AT ELASTIC LIMIT (r) IN BENDING TO THE CRUSHING    |
+|        STRENGTH (C) OF BLOCKS CUT THEREFROM,  IN POUNDS PER SQUARE INCH       |
+|                        (Forest Service Bul. 70, p. 90)                        |
+|-------------------------------------------------------------------------------|
+|                                 LONGLEAF PINE                                 |
+|-------------------------------------------------------------------------------|
+|                          | Soaked | Green  | 14    | 11.5  | 9.5   | Kiln-dry |
+|    MOISTURE CONDITION    | 50 per | 23 per | per   | per   | per   | 6.2 per  |
+|                          | cent   | cent   | cent  | cent  | cent  | cent     |
+| -------------------------+--------+--------+-------+-------+-------+----------|
+| Number of tests averaged |      5 |      5 |     5 |     5 |     4 |        5 |
+| _r_ in bending           |  4,920 |  5,944 | 6,924 | 7,852 | 9,280 |   11,550 |
+| _C_ in compression       |  4,668 |  5,100 | 6,466 | 7,466 | 8,985 |   10,910 |
+| Per cent _r_ is in       |        |        |       |       |       |          |
+|   excess of _C_          |    5.5 |   16.5 |   7.1 |   5.2 |   3.3 |      5.9 |
+|-------------------------------------------------------------------------------|
+|                                    SPRUCE                                     |
+|-------------------------------------------------------------------------------|
+|                                  | Soaked | Green  | 10    | 8.1   | Kiln-dry |
+|        MOISTURE CONDITION        | 30 per | 30 per | per   | per   | 3.9 per  |
+|                                  | cent   | cent   | cent  | cent  | cent     |
+|----------------------------------+--------+--------+-------+-------+----------|
+| Number of tests averaged         |      5 |      4 |     5 |     3 |        4 |
+| _r_ in bending                   |  3,002 |  3,362 | 6,458 | 8,400 |   10,170 |
+| _C_ in compression               |  2,680 |  3,025 | 6,120 | 7,610 |    9,335 |
+| Per cent _r_                     |        |        |       |       |          |
+|   is in excess of _C_            |   12.0 |   11.1 |   5.5 |  10.4 |      9.0 |
+|-------------------------------------------------------------------------------|
+
+When a short column is compressed until it breaks, the manner of
+failure depends partly upon the anatomical structure and partly
+upon the degree of humidity of the wood. The fibres (tracheids
+in conifers) act as hollow tubes bound closely together, and in
+giving way they either (1) buckle, or (2) bend.[5]
+
+[Footnote 5: See Bulletin 70, _op. cit._, p. 129.]
+
+The first is typical of any dry thin-walled cells, as is usually
+the case in seasoned white pine and spruce, and in the early
+wood of hard pines, hemlock, and other species with decided
+contrast between the two portions of the growth ring. As a rule
+buckling of a tracheid begins at the bordered pits which form
+places of least resistance in the walls. In hardwoods such as
+oak, chestnut, ash, etc., buckling occurs only in the
+thinnest-walled elements, such as the vessels, and not in the
+true fibres.
+
+According to Jaccard[6] the folding of the cells is accompanied
+by characteristic alterations of their walls which seem to split
+them into extremely thin layers. When greatly magnified, these
+layers appear in longitudinal sections as delicate threads
+without any definite arrangements, while on cross section they
+appear as numerous concentric strata. This may be explained on
+the ground that the growth of a fibre is by successive layers
+which, under the influence of compression, are sheared apart.
+This is particularly the case with thick-walled cells such as
+are found in late wood.
+
+[Footnote 6: Jaccard, P.: Etude anatomique des bois comprimes.
+Mit. d. Schw. Centralanstalt f.d. forst. Versuchswesen. X. Band,
+1. Heft. Zurich, 1910, p. 66.]
+
+|-------------------------------------------------------|
+|                       TABLE VI                        |
+|-------------------------------------------------------|
+|  RESULTS OF ENDWISE COMPRESSION TESTS ON SMALL CLEAR  |
+|         PIECES OF 40 WOODS IN GREEN CONDITION         |
+|               (Forest Service Cir. 213)               |
+|-------------------------------------------------------|
+|                   |   Fibre   |          |  Modulus   |
+|    COMMON NAME    | stress at | Crushing |     of     |
+|    OF SPECIES     |  elastic  | strength | elasticity |
+|                   |   limit   |          |            |
+|-------------------+-----------+----------+------------|
+|                   | Lbs. per  | Lbs. per |  Lbs. per  |
+|                   | sq. inch  | sq. inch |  sq. inch  |
+|                   |           |          |            |
+|    Hardwoods      |           |          |            |
+|                   |           |          |            |
+| Ash, white        |   3,510   |   4,220  |  1,531,000 |
+| Basswood          |     780   |   1,820  |  1,016,000 |
+| Beech             |   2,770   |   3,480  |  1,412,000 |
+| Birch, yellow     |   2,570   |   3,400  |  1,915,000 |
+| Elm, slippery     |   3,410   |   3,990  |  1,453,000 |
+| Hackberry         |   2,730   |   3,310  |  1,068,000 |
+| Hickory,          |           |          |            |
+|   big shellbark   |   3,570   |   4,520  |  1,658,000 |
+|   bitternut       |   4,330   |   4,570  |  1,616,000 |
+|   mockernut       |   3,990   |   4,320  |  1,359,000 |
+|   nutmeg          |   3,620   |   3,980  |  1,411,000 |
+|   pignut          |   3,520   |   4,820  |  1,980,000 |
+|   shagbark        |   3,730   |   4,600  |  1,943,000 |
+|   water           |   3,240   |   4,660  |  1,926,000 |
+| Locust, honey     |   4,300   |   4,970  |  1,536,000 |
+| Maple, sugar      |   3,040   |   3,670  |  1,463,000 |
+| Oak, post         |   2,780   |   3,330  |  1,062,000 |
+|   red             |   2,290   |   3,210  |  1,295,000 |
+|   swamp white     |   3,470   |   4,360  |  1,489,000 |
+|   white           |   2,400   |   3,520  |    946,000 |
+|   yellow          |   2,870   |   3,700  |  1,465,000 |
+| Osage orange      |   3,980   |   5,810  |  1,331,000 |
+| Sycamore          |   2,320   |   2,790  |  1,073,000 |
+| Tupelo            |   2,280   |   3,550  |  1,280,000 |
+|                   |           |          |            |
+|    Conifers       |           |          |            |
+|                   |           |          |            |
+| Arborvitae         |   1,420   |   1,990  |    754,000 |
+| Cedar, incense    |   2,710   |   3,030  |    868,000 |
+| Cypress, bald     |   3,560   |   3,960  |  1,738,000 |
+| Fir, alpine       |   1,660   |   2,060  |    882,000 |
+|   amabilis        |   2,763   |   3,040  |  1,579,000 |
+|   Douglas         |   2,390   |   2,920  |  1,440,000 |
+|   white           |   2,610   |   2,800  |  1,332,000 |
+| Hemlock           |   2,110   |   2,750  |  1,054,000 |
+| Pine, lodgepole   |   2,290   |   2,530  |  1,219,000 |
+|   longleaf        |   3,420   |   4,280  |  1,890,000 |
+|   red             |   2,470   |   3,080  |  1,646,000 |
+|   sugar           |   2,340   |   2,600  |  1,029,000 |
+|   western yellow  |   2,100   |   2,420  |  1,271,000 |
+|   white           |   2,370   |   2,720  |  1,318,000 |
+| Redwood           |   3,420   |   3,820  |  1,175,000 |
+| Spruce, Engelmann |   1,880   |   2,170  |  1,021,000 |
+| Tamarack          |   3,010   |   3,480  |  1,596,000 |
+|-------------------------------------------------------|
+
+The second case, where the fibres bend with more or less regular
+curves instead of buckling, is characteristic of any green or
+wet wood, and in dry woods where the fibres are thick-walled. In
+woods in which the fibre walls show all gradations of
+thickness--in other words, where the transition from the
+thin-walled cells of the early wood to the thick-walled cells of
+the late wood is gradual--the two kinds of failure, namely,
+buckling and bending, grade into each other. In woods with very
+decided contrast between early and late wood the two forms are
+usually distinct. Except in the case of complete failure the
+cavity of the deformed cells remains open, and in hardwoods this
+is true not only of the wood fibres but also of the tube-like
+vessels. In many cases longitudinal splits occur which isolate
+bundles of elements by greater or less intervals. The splitting
+occurs by a tearing of the fibres or rays and not by the
+separation of the rays from the adjacent elements.
+
+[Illustration: FIG. 8.--Failures of short columns of green
+spruce.]
+
+[Illustration: FIG. 9.--Failures of short columns of dry
+chestnut.]
+
+Moisture in wood decreases the stiffness of the fibre walls and
+enlarges the region of failure. The curve which the fibre walls
+make in the region of failure is more gradual and also more
+irregular than in dry wood, and the fibres are more likely to be
+separated.
+
+In examining the lines of rupture in compression parallel to the
+grain it appears that there does not exist any specific type,
+that is, one that is characteristic of all woods. Test blocks
+taken from different parts of the same log may show very decided
+differences in the manner of failure, while blocks that are much
+alike in the size, number, and distribution of the elements of
+unequal resistance may behave very similarly. The direction of
+rupture is, according to Jaccard, not influenced by the
+distribution of the medullary rays.[7] These are curved with the
+bundles of fibres to which they are attached. In any case the
+failure starts at the weakest points and follows the lines of
+least resistance. The plane of failure, as visible on radial
+surfaces, is horizontal, and on the tangential surface it is
+diagonal.
+
+[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. 57 _et seq._]
+
+
+
+SHEARING STRENGTH
+
+
+Whenever forces act upon a body in such a way that one portion
+tends to slide upon another adjacent to it the action is called
+a ~shear~.[8] In wood this shearing action may be (1) ~along the
+grain~, or (2) ~across the grain~. A tenon breaking out its
+mortise is a familiar example of shear along the grain, while
+the shoving off of the tenon itself would be shear across the
+grain. The use of wood for pins or tree-nails involves
+resistance to shear across the grain. Another common instance of
+the latter is where the steel edge of the eye of an axe or
+hammer tends to cut off the handle. In Fig. 10 the action of the
+wooden strut tends to shear off along the grain the portion _AB_
+of the wooden tie rod, and it is essential that the length of
+this portion be great enough to guard against it. Fig. 11 shows
+characteristic failures in shear along the grain.
+
+[Footnote 8: Shear should not be confused with ordinary cutting
+or incision.]
+
+[Illustration: FIG. 10.--Example of shear along the grain.]
+
+[Illustration: FIG. 11.--Failures of test specimens in shear
+along the grain. In the block at the left the surface of failure
+is radial; in the one at the right, tangential]
+
+|---------------------------------------------|
+|                  TABLE VII                  |
+|---------------------------------------------|
+| SHEARING STRENGTH ALONG THE GRAIN OF SMALL  |
+| CLEAR PIECES OF 41 WOODS IN GREEN CONDITION |
+|          (Forest Service Cir. 213)          |
+|---------------------------------------------|
+|                   |    When    |    When    |
+|   COMMON NAME     | surface of | surface of |
+|   OF SPECIES      | failure is | failure is |
+|                   |   radial   | tangential |
+|-------------------+------------+------------|
+|                   |  Lbs. per  |  Lbs. per  |
+|                   |  sq. inch  |  sq. inch  |
+|                   |            |            |
+|    Hardwoods      |            |            |
+|                   |            |            |
+| Ash, black        |      876   |      832   |
+|   white           |    1,360   |    1,312   |
+| Basswood          |      560   |      617   |
+| Beech             |    1,154   |    1,375   |
+| Birch, yellow     |    1,103   |    1,188   |
+| Elm, slippery     |    1,197   |    1,174   |
+|   white           |      778   |      872   |
+| Hackberry         |    1,095   |    1,161   |
+| Hickory,          |            |            |
+|   big shellbark   |    1,134   |    1,191   |
+|   bitternut       |    1,134   |    1,348   |
+|   mockernut       |    1,251   |    1,313   |
+|   nutmeg          |    1,010   |    1,053   |
+|   pignut          |    1,334   |    1,457   |
+|   shagbark        |    1,230   |    1,297   |
+|   water           |    1,390   |    1,490   |
+| Locust, honey     |    1,885   |    2,096   |
+| Maple, red        |    1,130   |    1,330   |
+|   sugar           |    1,193   |    1,455   |
+| Oak, post         |    1,196   |    1,402   |
+|   red             |    1,132   |    1,195   |
+|   swamp white     |    1,198   |    1,394   |
+|   white           |    1,096   |    1,292   |
+|   yellow          |    1,162   |    1,196   |
+| Sycamore          |      900   |    1,102   |
+| Tupelo            |      978   |    1,084   |
+|                   |            |            |
+|    Conifers       |            |            |
+|                   |            |            |
+| Arborvitae         |      617   |      614   |
+| Cedar, incense    |      613   |      662   |
+| Cypress, bald     |      836   |      800   |
+| Fir, alpine       |      573   |      654   |
+|   amabilis        |      517   |      639   |
+|   Douglas         |      853   |      858   |
+|   white           |      742   |      723   |
+| Hemlock           |      790   |      813   |
+| Pine, lodgepole   |      672   |      747   |
+|   longleaf        |    1,060   |      953   |
+|   red             |      812   |      741   |
+|   sugar           |      702   |      714   |
+|   western yellow  |      686   |      706   |
+|   white           |      649   |      639   |
+| Spruce, Engelmann |      607   |      624   |
+| Tamarack          |      883   |      843   |
+|---------------------------------------------|
+
+Both shearing stresses may act at the same time. Thus the weight
+carried by a beam tends to shear it off at right angles to the
+axis; this stress is equal to the resultant force acting
+perpendicularly at any point, and in a beam uniformly loaded and
+supported at either end is maximum at the points of support and
+zero at the centre. In addition there is a shearing force
+tending to move the fibres of the beam past each other in a
+longitudinal direction. (See Fig. 12.) This longitudinal shear
+is maximum at the neutral plane and decreases toward the upper
+and lower surfaces.
+
+[Illustration: FIG. 12.--Horizontal shear in a beam.]
+
+Shearing across the grain is so closely related to compression
+at right angles to the grain and to hardness that there is
+little to be gained by making separate tests upon it. Knowledge
+of shear parallel to the grain is important, since wood
+frequently fails in that way. The value of shearing stress
+parallel to the grain is found by dividing the maximum load in
+pounds (P) by the area of the cross section in inches (A).
+
+     {          P  }
+     { Shear = --- }
+     {          A  }
+
+Oblique shearing stresses are developed in a bar when it is
+subjected to direct tension or compression. The maximum shearing
+stress occurs along a plane when it makes an angle of 45 degrees
+                                                       P
+with the axis of the specimen. In this case, shear = -----. When
+                                                      2 A
+the value of the angle [Greek: theta] is less than 45 degrees,
+                             P
+the shear along the plane = --- sin [Greek: theta] cos [Greek:
+                             A
+theta]. (See Fig. 13.) The effect of oblique shear is often
+visible in the failures of short columns. (See Fig. 14.)
+
+[Illustration: FIG. 13.--Oblique shear in a short column.]
+
+[Illustration: FIG. 14.--Failure of short column by oblique
+shear.]
+
+|---------------------------------------------------------------------------|
+|                                TABLE VIII                                 |
+|---------------------------------------------------------------------------|
+|       SHEARING STRENGTH ACROSS THE GRAIN OF VARIOUS AMERICAN WOODS        |
+|  (J.C. Trautwine. Jour. Franklin Institute. Vol. 109, 1880, pp. 105-106)  |
+|---------------------------------------------------------------------------|
+|     KIND OF WOOD      | Lbs. per |        KIND OF WOOD         | Lbs. per |
+|                       | sq. inch |                             | sq. inch |
+|-----------------------+----------+-----------------------------+----------|
+| Ash                   |   6,280  | Hickory                     |   7,285  |
+| Beech                 |   5,223  | Locust                      |   7,176  |
+| Birch                 |   5,595  | Maple                       |   6,355  |
+| Cedar (white)         |   1,372  | Oak                         |   4,425  |
+| Cedar (white)         |   1,519  | Oak (live)                  |   8,480  |
+| Cedar (Central Amer.) |   3,410  | Pine (white)                |   2,480  |
+| Cherry                |   2,945  | Pine (northern yellow)      |   4,340  |
+| Chestnut              |   1,536  | Pine (southernyellow)       |   5,735  |
+| Dogwood               |   6,510  | Pine (very resinous yellow) |   5,053  |
+| Ebony                 |   7,750  | Poplar                      |   4,418  |
+| Gum                   |   5,890  | Spruce                      |   3,255  |
+| Hemlock               |   2,750  | Walnut (black)              |   4,728  |
+| Hickory               |   6,045  | Walnut (common)             |   2,830  |
+|---------------------------------------------------------------------------|
+| NOTE.--Two specimens of each were tested. All were fairly seasoned and    |
+| without defects. The piece sheared off was 5/8 in. The single circular    |
+| area of each pin was 0.322 sq. in.                                        |
+|---------------------------------------------------------------------------|
+
+
+
+TRANSVERSE OR BENDING STRENGTH: BEAMS
+
+
+When external forces acting in the same plane are applied at
+right angles to the axis of a bar so as to cause it to bend,
+they occasion a shortening of the longitudinal fibres on the
+concave side and an elongation of those on the convex side.
+Within the elastic limit the relative stretching and contraction
+of the fibres is directly[9] proportional to their distances
+from a plane intermediate between them--the ~neutral plane~.
+(N_{1} P in Fig. 15.) Thus the fibres half-way between the
+neutral plane and the outer surface experience only half as much
+shortening or elongation as the outermost or extreme fibres.
+Similarly for other distances. The elements along the neutral
+plane experience no tension or compression in an axial
+direction. The line of intersection of this plane and the plane
+of section is known as the ~neutral axis~ (N A in Fig. 15) of
+the section.
+
+[Footnote 9: While in reality this relationship does not exactly
+hold, the formulae for beams are based on its assumption.]
+
+[Illustration: FIG. 15.--Diagram of a simple beam. N_{1} P =
+neutral plane, N A = neutral axis of section R S.]
+
+If the bar is symmetrical and homogeneous the neutral plane is
+located half-way between the upper and lower surfaces, so long
+as the deflection does not exceed the elastic limit of the
+material. Owing to the fact that the tensile strength of wood is
+from two to nearly four times the compressive strength, it
+follows that at rupture the neutral plane is much nearer the
+convex than the concave side of the bar or beam, since the sum
+of all the compressive stresses on the concave portion must
+always equal the sum of the tensile stresses on the convex
+portion. The neutral plane begins to change from its central
+position as soon as the elastic limit has been passed. Its
+location at any time is very uncertain.
+
+The external forces acting to bend the bar also tend to rupture
+it at right angles to the neutral plane by causing one
+transverse section to slip past another. This stress at any
+point is equal to the resultant perpendicular to the axis of the
+forces acting at this point, and is termed the ~transverse
+shear~ (or in the case of beams, ~vertical shear~).
+
+In addition to this there is a shearing stress, tending to move
+the fibres past one another in an axial direction, which is
+called ~longitudinal shear~ (or in the case of beams,
+~horizontal shear~). This stress must be taken into
+consideration in the design of timber structures. It is maximum
+at the neutral plane and decreases to zero at the outer elements
+of the section. The shorter the span of a beam in proportion to
+its height, the greater is the liability of failure in
+horizontal shear before the ultimate strength of the beam is
+reached.
+
+
+_Beams_
+
+There are three common forms of beams, as follows:
+
+(1) ~Simple beam~--a bar resting upon two supports, one near
+each end. (See Fig. 16, No. 1.)
+
+(2) ~Cantilever beam~--a bar resting upon one support or
+fulcrum, or that portion of any beam projecting out of a wall or
+beyond a support. (See Fig. 16, No. 2.)
+
+(3) ~Continuous beam~--a bar resting upon more than two
+supports. (See Fig. 16, No. 3.)
+
+[Illustration: FIG. 16.--Three common forms of beams. 1. Simple.
+2. Cantilever. 3. Continuous.]
+
+
+_Stiffness of Beams_
+
+The two main requirements of a beam are stiffness and strength.
+The formulae for the _modulus of elasticity (E)_ or measure of
+stiffness of a rectangular prismatic simple beam loaded at the
+centre and resting freely on supports at either end is:[10]
+
+[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 formulae are required. See any book on mechanics.]
+
+           P' l^{3}
+     E = -------------
+          4 D b h^{3}
+
+   b = breadth or width of beam, inches.
+   h = height or depth of beam, inches.
+   l = span (length between points of supports) of beam, inches.
+   D = deflection produced by load P', inches.
+  P' = load at or below elastic limit, pounds.
+
+From this formulae it is evident that for rectangular beams of
+the same material, mode of support, and loading, the deflection
+is affected as follows:
+
+(1) It is inversely proportional to the width for beams of the
+same length and depth. If the width is tripled the deflection is
+one-third as great.
+
+(2) It is inversely proportional to the cube of the depth for
+beams of the same length and breadth. If the depth is tripled
+the deflection is one twenty-seventh as great.
+
+(3) It is directly proportional to the cube of the span for
+beams of the same breadth and depth. Tripling the span gives
+twenty-seven times the deflection.
+
+The number of pounds which concentrated at the centre will
+deflect a rectangular prismatic simple beam one inch may be
+found from the preceding formulae by substituting D = 1" and
+solving for P'. The formulae then becomes:
+
+                              4 E b h^{3}
+     Necessary weight (P') = -------------
+                                 l^{3}
+
+In this case the values for E are read from tables prepared from
+data obtained by experimentation on the given material.
+
+
+_Strength of Beams_
+
+The measure of the breaking strength of a beam is expressed in
+terms of unit stress by a _modulus of rupture_, which is a
+purely hypothetical expression for points beyond the elastic
+limit. The formulae used in computing this modulus is as follows:
+
+          1.5 P l
+     R = ---------
+          b h{^2}
+
+  b, h, l = breadth, height, and span, respectively, as in
+    preceding formulae.
+  R = modulus of rupture, pounds per square inch.
+  P = maximum load, pounds.
+
+In calculating the fibre stress at the elastic limit the same
+formulae is used except that the load at elastic limit (P_{1}) is
+substituted for the maximum load (P).
+
+From this formulae it is evident that for rectangular prismatic
+beams of the same material, mode of support, and loading, the
+load which a given beam can support varies as follows:
+
+(1) It is directly proportional to the breadth for beams of the
+same length and depth, as is the case with stiffness.
+
+(2) It is directly proportional to the square of the height for
+beams of the same length and breadth, instead of as the cube of
+this dimension as in stiffness.
+
+(3) It is inversely proportional to the span for beams of the
+same breadth and depth and not to the cube of this dimension as
+in stiffness.
+
+The fact that the strength varies as the _square_ of the height
+and the stiffness as the _cube_ explains the relationship of
+bending to thickness. Were the law the same for strength and
+stiffness a thin piece of material such as a sheet of paper
+could not be bent any further without breaking than a thick
+piece, say an inch board.
+|-------------------------------------------------------------------------------------|
+|                                      TABLE IX                                       |
+|-------------------------------------------------------------------------------------|
+| RESULTS OF STATIC BENDING TESTS ON SMALL CLEAR BEAMS OF 49 WOODS IN GREEN CONDITION |
+|                             (Forest Service Cir. 213)                               |
+|-------------------------------------------------------------------------------------|
+|                 |   Fibre   |          |            |        Work in Bending        |
+|   COMMON NAME   | stress at | Modulus  |  Modulus   |-------------------------------|
+|   OF SPECIES    |  elastic  |    of    |     of     |    To    |    To    |         |
+|                 |   limit   | rupture  | elasticity | elastic  | maximum  |  Total  |
+|                 |           |          |            |  limit   |   load   |         |
+|-----------------+-----------+----------+------------+----------+----------+---------|
+|                 |           |          |            | In.-lbs. | In.-lbs. | In.-lb. |
+|                 | Lbs. per  | Lbs. per |  Lbs. per  | per cu.  | per cu.  | per     |
+|                 | sq. in.   | sq. in.  |  sq. in.   | inch     | inch     | inch    |
+|                 |           |          |            |          |          |         |
+|   Hardwoods     |           |          |            |          |          |         |
+|                 |           |          |            |          |          |         |
+| Ash, black      |    2,580  |   6,000  |    960,000 |    0.41  |    13.1  |   38.9  |
+|   white         |    5,180  |   9,920  |  1,416,000 |    1.10  |    20.0  |   43.7  |
+| Basswood        |    2,480  |   4,450  |   842,000  |     .45  |     5.8  |    8.9  |
+| Beech           |    4,490  |   8,610  |  1,353,000 |     .96  |    14.1  |   31.4  |
+| Birch, yellow   |    4,190  |   8,390  |  1,597,000 |     .62  |    14.2  |   31.5  |
+| Elm, rock       |    4,290  |   9,430  |  1,222,000 |     .90  |    19.4  |   47.4  |
+|   slippery      |    5,560  |   9,510  |  1,314,000 |    1.32  |    11.7  |   44.2  |
+|   white         |    2,850  |   6,940  |  1,052,000 |     .44  |    11.8  |   27.4  |
+| Gum, red        |    3,460  |   6,450  |  1,138,000 |          |          |         |
+| Hackberry       |    3,320  |   7,800  |  1,170,000 |     .56  |    19.6  |   52.9  |
+| Hickory,        |           |          |            |          |          |         |
+|   big shellbark |    6,370  |  11,110  |  1,562,000 |    1.47  |    24.3  |   78.0  |
+|   bitternut     |    5,470  |  10,280  |  1,399,000 |    1.22  |    20.0  |   75.5  |
+|   mockernut     |    6,550  |  11,110  |  1,508,000 |    1.50  |    31.7  |   84.4  |
+|   nutmeg        |    4,860  |   9,060  |  1,289,000 |    1.06  |    22.8  |   58.2  |
+|   pignut        |    5,860  |  11,810  |  1,769,000 |    1.12  |    30.6  |   86.7  |
+|   shagbark      |    6,120  |  11,000  |  1,752,000 |    1.22  |    18.3  |   72.3  |
+|   water         |    5,980  |  10,740  |  1,563,000 |    1.29  |    18.8  |   52.9  |
+| Locust, honey   |    6,020  |  12,360  |  1,732,000 |    1.28  |    17.3  |   64.4  |
+| Maple, red      |    4,450  |   8,310  |  1,445,000 |     .78  |     9.8  |   17.1  |
+|   sugar         |    4,630  |   8,860  |  1,462,000 |     .88  |    12.7  |   32.0  |
+| Oak, post       |    4,720  |   7,380  |    913,000 |    1.39  |     9.1  |   17.4  |
+|   red           |    3,490  |   7,780  |  1,268,000 |     .60  |    11.4  |   26.0  |
+|   swamp white   |    5,380  |   9,860  |  1,593,000 |    1.05  |    14.5  |   37.6  |
+|   tanbark       |    6,580  |  10,710  |  1,678,000 |    1.49  |          |         |
+|   white         |    4,320  |   8,090  |  1,137,000 |     .95  |    12.1  |   36.7  |
+|   yellow        |    5,060  |   8,570  |  1,219,000 |    1.20  |    11.7  |   30.7  |
+| Osage orange    |    7,760  |  13,660  |  1,329,000 |    2.53  |    37.9  |  101.7  |
+| Sycamore        |    2,820  |   6,300  |    961,000 |     .51  |     7.1  |   13.6  |
+| Tupelo          |    4,300  |   7,380  |  1,045,000 |    1.00  |     7.8  |   20.9  |
+|                 |           |          |            |          |          |         |
+|   Conifers      |           |          |            |          |          |         |
+|                 |           |          |            |          |          |         |
+| Arborvitae       |    2,600  |   4,250  |    643,000 |     .60  |     5.7  |    9.5  |
+| Cedar, incense  |    3,950  |   6,040  |    754,000 |          |          |         |
+| Cypress, bald   |    4,430  |   7,110  |  1,378,000 |     .96  |     5.1  |   15.4  |
+| Fir, alpine     |    2,366  |   4,450  |    861,000 |     .66  |     4.4  |    7.4  |
+|   amabilis      |    4,060  |   6,570  |  1,323,000 |          |          |         |
+|   Douglas       |    3,570  |   6,340  |  1,242,000 |     .59  |     6.6  |   13.6  |
+|   white         |    3,880  |   5,970  |  1,131,000 |     .77  |     5.2  |   14.9  |
+| Hemlock         |    3,410  |   5,770  |    917,000 |     .73  |     6.6  |   12.9  |
+| Pine, lodgepole |    3,080  |   5,130  |  1,015,000 |     .54  |     5.1  |    7.4  |
+|   longleaf      |    5,090  |   8,630  |  1,662,000 |     .88  |     8.1  |   34.8  |
+|   red           |    3,740  |   6,430  |  1,384,000 |     .59  |     5.8  |   28.0  |
+|   shortleaf     |    4,360  |   7,710  |  1,395,000 |          |          |         |
+|   sugar         |    3,330  |   5,270  |    966,000 |     .66  |     5.0  |   11.6  |
+|   west, yellow  |    3,180  |   5,180  |  1,111,000 |     .52  |     4.3  |   15.6  |
+| White           |    3,410  |   5,310  |  1,073,000 |     .62  |     5.9  |   13.3  |
+| Redwood         |    4,530  |   6,560  |  1,024,000 |          |          |         |
+| Spruce,         |           |          |            |          |          |         |
+|   Engelmann     |    2,740  |   4,550  |    866,000 |     .50  |     4.8  |    6.1  |
+|   red           |    3,440  |   5,820  |  1,143,000 |     .62  |     6.0  |         |
+|   white         |    3,160  |   5,200  |    968,000 |     .58  |     6.6  |         |
+| Tamarack        |    4,200  |   7,170  |  1,236,000 |     .84  |     7.2  |   30.0  |
+|-------------------------------------------------------------------------------------|
+
+
+_Kinds of Loads_
+
+There are various ways in which beams are loaded, of which the
+following are the most important:
+
+(1) ~Uniform load~ occurs where the load is spread evenly over
+the beam.
+
+(2) ~Concentrated load~ occurs where the load is applied at
+single point or points.
+
+(3) ~Live~ or ~immediate load~ is one of momentary or short
+duration at any one point, such as occurs in crossing a bridge.
+
+(4) ~Dead~ or ~permanent load~ is one of constant and
+indeterminate duration, as books on a shelf. In the case of a
+bridge the weight of the structure itself is the dead load. All
+large beams support a uniform dead load consisting of their own
+weight.
+
+The effect of dead load on a wooden beam may be two or more
+times that produced by an immediate load of the same weight.
+Loads greater than the elastic limit are unsafe and will
+generally result in rupture if continued long enough. A beam may
+be considered safe under permanent load when the deflections
+diminish during equal successive periods of time. A continual
+increase in deflection indicates an unsafe load which is almost
+certain to rupture the beam eventually.
+
+Variations in the humidity of the surrounding air influence the
+deflection of dry wood under dead load, and increased
+deflections during damp weather are cumulative and not recovered
+by subsequent drying. In the case of longleaf pine, dry beams
+may with safety be loaded permanently to within three-fourths of
+their elastic limit as determined from ordinary static tests.
+Increased moisture content, due to greater humidity of the air,
+lowers the elastic limit of wood so that what was a safe load
+for the dry material may become unsafe.
+
+When a dead load not great enough to rupture a beam has been
+removed, the beam tends gradually to recover its former shape,
+but the recovery is not always complete. If specimens from such
+a beam are tested in the ordinary testing machine it will be
+found that the application of the dead load did not affect the
+stiffness, ultimate strength, or elastic limit of the material.
+In other words, the deflections and recoveries produced by live
+loads are the same as would have been produced had not the beam
+previously been subjected to a dead load.[11]
+
+[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.]
+
+~Maximum load~ is the greatest load a material will support and
+is usually greater than the load at rupture.
+
+~Safe load~ is the load considered safe for a material to
+support in actual practice. It is always less than the load at
+elastic limit and is usually taken as a certain proportion of
+the ultimate or breaking load.
+
+The ratio of the breaking to the safe load is called the factor
+of safety. (Factor of safety = ultimate strength / safe load) In
+order to make due allowance for the natural variations and
+imperfections in wood and in the aggregate structure, as well as
+for variations in the load, the factor of safety is usually as
+high as 6 or 10, especially if the safety of human life depends
+upon the structure. This means that only from one-sixth to
+one-tenth of the computed strength values is considered safe to
+use. If the depth of timbers exceeds four times their thickness
+there is a great tendency for the material to twist when loaded.
+It is to overcome this tendency that floor joists are braced at
+frequent intervals. Short deep pieces shear out or split before
+their strength in bending can fully come into play.
+
+
+_Application of Loads_
+
+There are three[12] general methods in which loads may be
+applied to beams, namely:
+
+[Footnote 12: A fourth might be added, namely, ~vibratory~, or
+~harmonic repetition~, which is frequently serious in the case
+of bridges.]
+
+(1) ~Static loading~ or the gradual imposition of load so that
+the moving parts acquire no appreciable momentum. Loads are so
+applied in the ordinary testing machine.
+
+(2) ~Sudden imposition of load without initial velocity.~ "Thus
+in the case of placing a load on a beam, if the load be brought
+into contact with the beam, but its weight sustained by external
+means, as by a cord, and then this external support be
+_suddenly_ (instantaneously) removed, as by quickly cutting the
+cord, then, although the load is already touching the beam (and
+hence there is no real impact), yet the beam is at first
+offering no resistance, as it has yet suffered no deformation.
+Furthermore, as the beam deflects the resistance increases, but
+does not come to be equal to the load until it has attained its
+normal deflection. In the meantime there has been an unbalanced
+force of gravity acting, of a constantly diminishing amount,
+equal at first to the entire load, at the normal deflection. But
+at this instant the load and the beam are in motion, the
+hitherto unbalanced force having produced an accelerated
+velocity, and this velocity of the weight and beam gives to them
+an energy, or _vis viva_, which must now spend itself in
+overcoming an _excess_ of resistance over and above the imposed
+load, and the whole mass will not stop until the deflection (as
+well as the resistance) has come to be equal to _twice_ that
+corresponding to the static load imposed. Hence we say the
+effect of a suddenly imposed load is to produce twice the
+deflection and stress of the same load statically applied. It
+must be evident, however, that this case has nothing in common
+with either the ordinary 'static' tests of structural materials
+in testing-machines, or with impact tests."[13]
+
+[Footnote 13: Johnson, J.B.: The materials of construction, pp.
+81-82.]
+
+(3) ~Impact, shock,~ or ~blow.~[14] There are various common
+uses of wood where the material is subjected to sudden shocks
+and jars or impact. Such is the action on the felloes and spokes
+of a wagon wheel passing over a rough road; on a hammer handle
+when a blow is struck; on a maul when it strikes a wedge.
+
+[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.]
+
+Resistance to impact is resistance to energy which is measured
+by the product of the force into the space through which it
+moves, or by the product of one-half the moving mass which
+causes the shock into the square of its velocity. The work done
+upon the piece at the instant the velocity is entirely removed
+from the striking body is equal to the total energy of that
+body. It is impossible, however, to get all of the energy of the
+striking body stored in the specimen, though the greater the
+mass and the shorter the space through which it moves, or, in
+other words, the greater the proportion of weight and the
+smaller the proportion of velocity making up the energy of the
+striking body, the more energy the specimen will absorb. The
+rest is lost in friction, vibrations, heat, and motion of the
+anvil.
+
+In impact the stresses produced become very complex and
+difficult to measure, especially if the velocity is high, or the
+mass of the beam itself is large compared to that of the weight.
+
+The difficulties attending the measurement of the stresses
+beyond the elastic limit are so great that commonly they are not
+reckoned. Within the elastic limit the formulae for calculating
+the stresses are based on the assumption that the deflection is
+proportional to the stress in this case as in static tests.
+
+A common method of making tests upon the resistance of wood to
+shock is to support a small beam at the ends and drop a heavy
+weight upon it in the middle. (See Fig. 40.) The height of the
+weight is increased after each drop and records of the
+deflection taken until failure. The total work done upon the
+specimen is equal to the area of the stress-strain diagram plus
+the effect of local inertia of the molecules at point of
+contact.
+
+The stresses involved in impact are complicated by the fact that
+there are various ways in which the energy of the striking body
+may be spent:
+
+(_a_) It produces a local deformation of both bodies at the
+surface of contact, within or beyond the elastic limit. In
+testing wood the compression of the substance of the steel
+striking-weight may be neglected, since the steel is very hard
+in comparison with the wood. In addition to the compression of
+the fibres at the surface of contact resistance is also offered
+by the inertia of the particles there, the combined effect of
+which is a stress at the surface of contact often entirely out
+of proportion to the compression which would result from the
+action of a static force of the same magnitude. It frequently
+exceeds the crushing strength at the extreme surface of contact,
+as in the case of the swaging action of a hammer on the head of
+an iron spike, or of a locomotive wheel on the steel rail. This
+is also the case when a bullet is shot through a board or a pane
+of glass without breaking it as a whole.
+
+(_b_) It may move the struck body as a whole with an accelerated
+velocity, the resistance consisting of the inertia of the body.
+This effect is seen when a croquet ball is struck with a mallet.
+
+(_c_) It may deform a fixed body against its external supports
+and resistances. In making impact tests in the laboratory the
+test specimen is in reality in the nature of a cushion between
+two impacting bodies, namely, the striking weight and the base
+of the machine. It is important that the mass of this base be
+sufficiently great that its relative velocity to that of the
+common centre of gravity of itself and the striking weight may
+be disregarded.
+
+(_d_) It may deform the struck body as a whole against the
+resisting stresses developed by its own inertia, as, for
+example, when a baseball bat is broken by striking the ball.
+
+|-------------------------------------------------------|
+|                        TABLE X                        |
+|-------------------------------------------------------|
+| RESULTS OF IMPACT BENDING TESTS ON SMALL CLEAR BEAMS  |
+|            OF 34 WOODS IN GREEN CONDITION             |
+|               (Forest Service Cir. 213)               |
+|-------------------------------------------------------|
+|                   |   Fibre   |            | Work in  |
+|    COMMON NAME    | stress at | Modulus of | bending  |
+|    OF SPECIES     |  elastic  | elasticity |    to    |
+|                   |   limit   |            | elastic  |
+|                   |           |            |  limit   |
+|-------------------+-----------+------------+----------|
+|                   |           |            | In.-lbs. |
+|                   | Lbs. per  |  Lbs. per  | per cu.  |
+|                   | sq. in.   |  sq. in.   | inch     |
+|                   |           |            |          |
+|    Hardwoods      |           |            |          |
+|                   |           |            |          |
+| Ash, black        |    7,840  |    955,000 |    3.69  |
+|   white           |   11,710  |  1,564,000 |    4.93  |
+| Basswood          |    5,480  |    917,000 |    1.84  |
+| Beech             |   11,760  |  1,501,000 |    5.10  |
+| Birch, yellow     |   11,080  |  1,812,000 |    3.79  |
+| Elm, rock         |   12,090  |  1,367,000 |    6.52  |
+|   slippery        |   11,700  |  1,569,000 |    4.86  |
+|   white           |    9,910  |  1,138,000 |    4.82  |
+| Hackberry         |   10,420  |  1,398,000 |    4.48  |
+| Locust, honey     |   13,460  |  2,114,000 |    4.76  |
+| Maple, red        |   11,670  |  1,411,000 |    5.45  |
+|   sugar           |   11,680  |  1,680,000 |    4.55  |
+| Oak, post         |   11,260  |  1,596,000 |    4.41  |
+|   red             |   10,580  |  1,506,000 |    4.16  |
+|   swamp white     |   13,280  |  2,048,000 |    4.79  |
+|   white           |    9,860  |  1,414,000 |    3.84  |
+|   yellow          |   10,840  |  1,479,000 |    4.44  |
+| Osage orange      |   15,520  |  1,498,000 |    8.92  |
+| Sycamore          |    8,180  |  1,165,000 |    3.22  |
+| Tupelo            |    7,650  |  1,310,000 |    2.49  |
+|                   |           |            |          |
+|    Conifers       |           |            |          |
+|                   |           |            |          |
+| Arborvitae         |    5,290  |    778,000 |    2.04  |
+| Cypress, bald     |    8,290  |  1,431,000 |    2.71  |
+| Fir, alpine       |    5,280  |    980,000 |    1.59  |
+|   Douglas         |    8,870  |  1,579,000 |    2.79  |
+|   white           |    7,230  |  1,326,000 |    2.21  |
+| Hemlock           |    6,330  |  1,025,000 |    2.19  |
+| Pine, lodgepole   |    6,870  |  1,142,000 |    2.31  |
+|   longleaf        |    9,680  |  1,739,000 |    3.02  |
+|   red             |    7,480  |  1,438,000 |    2.18  |
+|   sugar           |    6,740  |  1,083,000 |    2.34  |
+|   western yellow  |    7,070  |  1,115,000 |    2.51  |
+|   white           |    6,490  |  1,156,000 |    2.06  |
+| Spruce, Engelmann |    6,300  |  1,076,000 |    2.09  |
+| Tamarack          |    7,750  |  1,263,000 |    2.67  |
+|-------------------------------------------------------|
+
+Impact testing is difficult to conduct satisfactorily and the
+data obtained are of chief value in a relative sense, that is,
+for comparing the shock-resisting ability of woods of which like
+specimens have been subjected to exactly identical treatment.
+Yet this test is one of the most important made on wood, as it
+brings out properties not evident from other tests. Defects and
+brittleness are revealed by impact better than by any other kind
+of test. In common practice nearly all external stresses are of
+the nature of impact. In fact, no two moving bodies can come
+together without impact stress. Impact is therefore the
+commonest form of applied stress, although the most difficult to
+measure.
+
+
+_Failures in Timber Beams_
+
+If a beam is loaded too heavily it will break or fail in some
+characteristic manner. These failures may be classified
+according to the way in which they develop, as tension,
+compression, and horizontal shear; and according to the
+appearance of the broken surface, as brash, and fibrous. A
+number of forms may develop if the beam is completely ruptured.
+
+Since the tensile strength of wood is on the average about three
+times as great as the compressive strength, a beam should,
+therefore, be expected to fail by the formation in the first
+place of a fold on the compression side due to the crushing
+action, followed by failure on the tension side. This is usually
+the case in green or moist wood. In dry material the first
+visible failure is not infrequently on the lower or tension
+side, and various attempts have been made to explain why such is
+the case.[15]
+
+[Footnote 15: See Proc. Int. Assn. for Testing Materials, 1912,
+XXIII_{2}, pp. 12-13.]
+
+Within the elastic limit the elongations and shortenings are
+equal, and the neutral plane lies in the middle of the beam.
+(See TRANSVERSE OR BENDING STRENGTH: BEAMS, above.) Later the
+top layer of fibres on the upper or compression side fail, and
+on the load increasing, the next layer of fibres fail, and so
+on, even though this failure may not be visible. As a result the
+shortenings on the upper side of the beam become considerably
+greater than the elongations on the lower side. The neutral
+plane must be presumed to sink gradually toward the tension
+side, and when the stresses on the outer fibres at the bottom
+have become sufficiently great, the fibres are pulled in two,
+the tension area being much smaller than the compression area.
+The rupture is often irregular, as in direct tension tests.
+Failure may occur partially in single bundles of fibres some
+time before the final failure takes place. One reason why the
+failure of a dry beam is different from one that is moist, is
+that drying increases the stiffness of the fibres so that they
+offer more resistance to crushing, while it has much less effect
+upon the tensile strength.
+
+There is considerable variation in tension failures depending
+upon the toughness or the brittleness of the wood, the
+arrangement of the grain, defects, etc., making further
+classification desirable. The four most common forms are:
+
+(1)~Simple tension,~ in which there is a direct pulling in two
+of the wood on the under side of the beam due to a tensile
+stress parallel to the grain, (See Fig. 17, No. 1.) This is
+common in straight-grained beams, particularly when the wood is
+seasoned.
+
+[Illustration: FIG. 17.--Characteristic failures of simple
+beams.]
+
+(2)~Cross-grained tension,~ in which the fracture is caused by a
+tensile force acting oblique to the grain. (See Fig. 17, No. 2.)
+This is a common form of failure where the beam has diagonal,
+spiral or other form of cross grain on its lower side. Since the
+tensile strength of wood across the grain is only a small
+fraction of that with the grain it is easy to see why a
+cross-grained timber would fail in this manner.
+
+(3)~Splintering tension,~ in which the failure consists of a
+considerable number of slight tension failures, producing a
+ragged or splintery break on the under surface of the beam. (See
+Fig. 17, No. 3.) This is common in tough woods. In this case the
+surface of fracture is fibrous.
+
+(4)~Brittle tension,~ in which the beam fails by a clean break
+extending entirely through it. (See Fig. 17, No. 4.) It is
+characteristic of a brittle wood which gives way suddenly
+without warning, like a piece of chalk. In this case the surface
+of fracture is described as brash.
+
+~Compression failure~ (see Fig. 17, No. 5) has few variations
+except that it appears at various distances from the neutral
+plane of the beam. It is very common in green timbers. The
+compressive stress parallel to the fibres causes them to buckle
+or bend as in an endwise compressive test. This action usually
+begins on the top side shortly after the elastic limit is
+reached and extends downward, sometimes almost reaching the
+neutral plane before complete failure occurs. Frequently two or
+more failures develop at about the same time.
+
+~Horizontal shear failure,~ in which the upper and lower
+portions of the beam slide along each other for a portion of
+their length either at one or at both ends (see Fig. 17, No. 6),
+is fairly common in air-dry material and in green material when
+the ratio of the height of the beam to the span is relatively
+large. It is not common in small clear specimens. It is often
+due to shake or season checks, common in large timbers, which
+reduce the actual area resisting the shearing action
+considerably below the calculated area used in the formulae for
+horizontal shear. (See page 98 for this formulae.) For this
+reason it is unsafe, in designing large timber beams, to use
+shearing stresses higher than those calculated for beams that
+failed in horizontal shear. The effect of a failure in
+horizontal shear is to divide the beam into two or more beams
+the combined strength of which is much less than that of the
+original beam. Fig. 18 shows a large beam in which two failures
+in horizontal shear occurred at the same end. That the parts
+behave independently is shown by the compression failure below
+the original location of the neutral plane.
+
+[Illustration: FIG. 18.--Failure of a large beam by horizontal
+shear. _Photo by U. S, Forest Service._]
+
+Table XI gives an analysis of the causes of first failure in 840
+large timber beams of nine different species of conifers. Of the
+total number tested 165 were air-seasoned, the remainder green.
+The failure occurring first signifies the point of greatest
+weakness in the specimen under the particular conditions of
+loading employed (in this case, third-point static loading).
+
+|-----------------------------------------------------------|
+|                         TABLE XI                          |
+|-----------------------------------------------------------|
+|          MANNER OF FIRST FAILURE OF LARGE BEAMS           |
+|             (Forest Service Bul. 108, p. 56)              |
+|-----------------------------------------------------------|
+|                  | Total  | Per cent of total failing by  |
+|   COMMON NAME    | number |---------+-------------+-------|
+|   OF SPECIES     | of     | Tension | Compression | Shear |
+|                  | tests  |         |             |       |
+|------------------+--------+---------+-------------+-------|
+| Longleaf pine:   |        |         |             |       |
+|   green          |    17  |    18   |      24     |   58  |
+|   dry            |     9  |    22   |      22     |   56  |
+| Douglas fir:     |        |         |             |       |
+|   green          |   191  |    27   |      72     |    1  |
+|   dry            |    91  |    19   |      76     |    5  |
+| Shortleaf pine:  |        |         |             |       |
+|   green          |    48  |    27   |      56     |   17  |
+|   dry            |    13  |    54   |             |   46  |
+| Western larch:   |        |         |             |       |
+|   green          |    62  |    23   |      71     |    6  |
+|   dry            |    52  |    54   |      19     |   27  |
+| Loblolly pine:   |        |         |             |       |
+|   green          |   111  |    40   |      53     |    7  |
+|   dry            |    25  |    60   |      12     |   28  |
+| Tamarack:        |        |         |             |       |
+|   green          |    30  |    37   |      53     |   10  |
+|   dry            |     9  |    45   |      22     |   33  |
+| Western hemlock: |        |         |             |       |
+|   green          |    39  |    21   |      74     |    5  |
+|   dry            |    44  |    11   |      66     |   23  |
+| Redwood:         |        |         |             |       |
+|   green          |    28  |    43   |      50     |    7  |
+|   dry            |    12  |    83   |      17     |       |
+| Norway pine:     |        |         |             |       |
+|   green          |    49  |    18   |      76     |    6  |
+|   dry            |    10  |    30   |      60     |   10  |
+|-----------------------------------------------------------|
+| NOTE.--These tests were made on timbers ranging in cross  |
+| section from 4" x 10" to 8" x 16", and with a span of 15  |
+| feet.                                                     |
+|-----------------------------------------------------------|
+
+
+
+TOUGHNESS: TORSION
+
+
+Toughness is a term applied to more than one property of wood.
+Thus wood that is difficult to split is said to be tough. Again,
+a tough wood is one that will not rupture until it has deformed
+considerably under loads at or near its maximum strength, or one
+which still hangs together after it has been ruptured and may be
+bent back and forth without breaking apart. Toughness includes
+flexibility and is the reverse of brittleness, in that tough
+woods break gradually and give warning of failure. Tough woods
+offer great resistance to impact and will permit rougher
+treatment in manipulations attending manufacture and use.
+Toughness is dependent upon the strength, cohesion, quality,
+length, and arrangement of fibre, and the pliability of the
+wood. Coniferous woods as a rule are not as tough as hardwoods,
+of which hickory and elm are the best examples.
+
+The torsion or twisting test is useful in determining the
+toughness of wood. If the ends of a shaft are turned in opposite
+directions, or one end is turned and the other is fixed, all of
+the fibres except those at the axis tend to assume the form of
+helices. (See Fig. 19.) The strain produced by torsion or
+twisting is essentially shear transverse and parallel to the
+fibres, combined with longitudinal tension and transverse
+compression. Within the elastic limit the strains increase
+directly as the distance from the axis of the specimen. The
+outer elements are subjected to tensile stresses, and as they
+become twisted tend to compress those near the axis. The
+elongated elements also contract laterally. Cross sections which
+were originally plane become warped. With increasing strain the
+lateral adhesion of the outer fibres is destroyed, allowing them
+to slide past each other, and reducing greatly their power of
+resistance. In this way the strains on the fibres nearer the
+axis are progressively increased until finally all of the
+elements are sheared apart. It is only in the toughest materials
+that the full effect of this action can be observed. (See Fig.
+20.) Brittle woods snap off suddenly with only a small amount of
+torsion, and their fracture is irregular and oblique to the axis
+of the piece instead of frayed out and more nearly perpendicular
+to the axis as is the case with tough woods.
+
+[Illustration: FIG. 19.--Torsion of a shaft.]
+
+[Illustration: FIG. 20.--Effect of torsion on different grades
+of hickory. _Photo by U. S. Forest Service._]
+
+
+
+HARDNESS
+
+
+The term _hardness_ is used in two senses, namely: (1)
+resistance to indentation, and (2) resistance to abrasion or
+scratching. In the latter sense hardness combined with toughness
+is a measure of the wearing ability of wood and is an important
+consideration in the use of wood for floors, paving blocks,
+bearings, and rollers. While resistance to indentation is
+dependent mostly upon the density of the wood, the wearing
+qualities may be governed by other factors such as toughness,
+and the size, cohesion, and arrangement of the fibres. In use
+for floors, some woods tend to compact and wear smooth, while
+others become splintery and rough. This feature is affected to
+some extent by the manner in which the wood is sawed; thus
+edge-grain pine flooring is much better than flat-sawn for
+uniformity of wear.
+
+|-------------------------------------------------------------------|
+|                             TABLE XII                             |
+|-------------------------------------------------------------------|
+|             HARDNESS OF 32 WOODS IN GREEN CONDITION,              |
+|            AS INDICATED BY THE LOAD REQUIRED TO IMBED             |
+|         A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER          |
+|                     (Forest Service Cir. 213)                     |
+|-------------------------------------------------------------------|
+| COMMON NAME OF SPECIES | Average |   End   | Radial  | Tangential |
+|                        |         | surface | surface |  surface   |
+|------------------------+---------+---------+---------+------------|
+|                        |  Pounds |  Pounds |  Pounds |   Pounds   |
+|                        |         |         |         |            |
+|    Hardwoods           |         |         |         |            |
+|                        |         |         |         |            |
+|  1 Osage orange        |   1,971 |   1,838 |   2,312 |    1,762   |
+|  2 Honey locust        |   1,851 |   1,862 |   1,860 |    1,832   |
+|  3 Swamp white oak     |   1,174 |   1,205 |   1,217 |    1,099   |
+|  4 White oak           |   1,164 |   1,183 |   1,163 |    1,147   |
+|  5 Post oak            |   1,099 |   1,139 |   1,068 |    1,081   |
+|  6 Black oak           |   1,069 |   1,093 |   1,083 |    1,031   |
+|  7 Red oak             |   1,043 |   1,107 |   1,020 |    1,002   |
+|  8 White ash           |   1,046 |   1,121 |   1,000 |    1,017   |
+|  9 Beech               |     942 |   1,012 |     897 |      918   |
+| 10 Sugar maple         |     937 |     992 |     918 |      901   |
+| 11 Rock elm            |     910 |     954 |     883 |      893   |
+| 12 Hackberry           |     799 |     829 |     795 |      773   |
+| 13 Slippery elm        |     788 |     919 |     757 |      687   |
+| 14 Yellow birch        |     778 |     827 |     768 |      739   |
+| 15 Tupelo              |     738 |     814 |     666 |      733   |
+| 16 Red maple           |     671 |     766 |     621 |      626   |
+| 17 Sycamore            |     608 |     664 |     560 |      599   |
+| 18 Black ash           |     551 |     565 |     542 |      546   |
+| 19 White elm           |     496 |     536 |     456 |      497   |
+| 20 Basswood            |     239 |     273 |     226 |      217   |
+|                        |         |         |         |            |
+|    Conifers            |         |         |         |            |
+|                        |         |         |         |            |
+|  1 Longleaf pine       |    532  |     574 |     502 |      521   |
+|  2 Douglas fir         |    410  |     415 |     399 |      416   |
+|  3 Bald cypress        |    390  |     460 |     355 |      354   |
+|  4 Hemlock             |    384  |     463 |     354 |      334   |
+|  5 Tamarack            |    384  |     401 |     380 |      370   |
+|  6 Red pine            |    347  |     355 |     345 |      340   |
+|  7 White fir           |    346  |     381 |     322 |      334   |
+|  8 Western yellow pine |    328  |     334 |     307 |      342   |
+|  9 Lodgepole pine      |    318  |     316 |     318 |      319   |
+| 10 White pine          |    299  |     304 |     294 |      299   |
+| 11 Engelmann pine      |    266  |     272 |     253 |      274   |
+| 12 Alpine fir          |    241  |     284 |     203 |      235   |
+|-------------------------------------------------------------------|
+| NOTE.--Black locust and hickory are not included in this table,   |
+| but their position would be near the head of the list.            |
+|-------------------------------------------------------------------|
+
+Tests for either form of hardness are of comparative value only.
+Tests for indentation are commonly made by penetrations of the
+material with a steel punch or ball.[16] Tests for abrasion are
+made by wearing down wood with sandpaper or by means of a sand
+blast.
+
+[Footnote 16: See articles by Gabriel Janka listed in
+bibliography, pages 151-152.]
+
+
+
+CLEAVABILITY
+
+
+_Cleavability_ is the term used to denote the facility with
+which wood is split. A splitting stress is one in which the
+forces act normally like a wedge. (See Fig. 21.) The plane of
+cleavage is parallel to the grain, either radially or
+tangentially.
+
+[Illustration: FIG. 21.--Cleavage of highly elastic wood. The
+cleft runs far ahead of the wedge.]
+
+This property of wood is very important in certain uses such as
+firewood, fence rails, billets, and squares. Resistance to
+splitting or low cleavability is desirable where wood must hold
+nails or screws, as in box-making. Wood usually splits more
+readily along the radius than parallel to the growth rings
+though exceptions occur, as in the case of cross grain.
+
+Splitting involves transverse tension, but only a portion of the
+fibres are under stress at a time. A wood of little stiffness
+and strong cohesion across the grain is difficult to split,
+while one with great stiffness, such as longleaf pine, is easily
+split. The form of the grain and the presence of knots greatly
+affect this quality.
+
+|---------------------------------------------|
+|                 TABLE XIII                  |
+|---------------------------------------------|
+| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF  |
+|         32 WOODS IN GREEN CONDITION         |
+|          (Forest Service Cir. 213)          |
+|---------------------------------------------|
+|                   |    When    |    When    |
+|   COMMON NAME     | surface of | surface of |
+|   OF SPECIES      | failure is | failure is |
+|                   |   radial   | tangential |
+|-------------------+------------+------------|
+|                   |  Lbs. per  |  Lbs. per  |
+|                   |  sq. inch  |  sq. inch  |
+|                   |            |            |
+|    Hardwoods      |            |            |
+|                   |            |            |
+| Ash, black        |     275    |     260    |
+|   white           |     333    |     346    |
+| Bashwood          |     130    |     168    |
+| Beech             |     339    |     527    |
+| Birch, yellow     |     294    |     287    |
+| Elm, slippery     |     401    |     424    |
+|   white           |     210    |     270    |
+| Hackberry         |     422    |     436    |
+| Locust, honey     |     552    |     610    |
+| Maple, red        |     297    |     330    |
+|   sugar           |     376    |     513    |
+| Oak, post         |     354    |     487    |
+|   red             |     380    |     470    |
+|   swamp white     |     428    |     536    |
+|   white           |     382    |     457    |
+|   yellow          |     379    |     470    |
+| Sycamore          |     265    |     425    |
+| Tupelo            |     277    |     380    |
+|                   |            |            |
+|    Conifers       |            |            |
+|                   |            |            |
+| Arborvitae         |     148    |     139    |
+| Cypress, bald     |     167    |     154    |
+| Fir, alpine       |     130    |     133    |
+|   Douglas         |     139    |     127    |
+|   white           |     145    |     187    |
+| Hemlock           |     168    |     151    |
+| Pine, lodgepole   |     142    |     140    |
+|   longleaf        |     187    |     180    |
+|   red             |     161    |     154    |
+|   sugar           |     168    |     189    |
+|   western yellow  |     162    |     187    |
+|   white           |     144    |     160    |
+| Spruce, Engelmann |     110    |     135    |
+| Tamarack          |     167    |     159    |
+|---------------------------------------------|
+
+
+
+
+PART II FACTORS  AFFECTING THE MECHANICAL PROPERTIES OF WOOD
+
+
+
+INTRODUCTION
+
+
+Wood is an organic product--a structure of infinite variation of
+detail and design.[17] It is on this account that no two woods
+are alike--in reality no two specimens from the same log are
+identical. There are certain properties that characterize each
+species, but they are subject to considerable variation. Oak,
+for example, is considered hard, heavy, and strong, but some
+pieces, even of the same species of oak, are much harder,
+heavier, and stronger than others. With hickory are associated
+the properties of great strength, toughness, and resilience, but
+some pieces are comparatively weak and brash and ill-suited for
+the exacting demands for which good hickory is peculiarly
+adapted.
+
+[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.]
+
+It follows that no definite value can be assigned to the
+properties of any wood and that tables giving average results of
+tests may not be directly applicable to any individual stick.
+With sufficient knowledge of the intrinsic factors affecting the
+results it becomes possible to infer from the appearance of
+material its probable variation from the average. As yet too
+little is known of the relation of structure and chemical
+composition to the mechanical and physical properties to permit
+more than general conclusions.
+
+
+
+RATE OF GROWTH
+
+
+To understand the effect of variations in the rate of growth it
+is first necessary to know how wood is formed. A tree increases
+in diameter by the formation, between the old wood and the inner
+bark, of new woody layers which envelop the entire stem, living
+branches, and roots. Under ordinary conditions one layer is
+formed each year and in cross section as on the end of a log
+they appear as rings--often spoken of as _annual rings_. These
+growth layers are made up of wood cells of various kinds, but
+for the most part fibrous. In timbers like pine, spruce,
+hemlock, and other coniferous or softwood species the wood cells
+are mostly of one kind, and as a result the material is much
+more uniform in structure than that of most hardwoods. (See
+Frontispiece.) There are no vessels or pores in coniferous wood
+such as one sees so prominently in oak and ash, for example.
+(See Fig. 22.)
+
+[Illustration: FIG. 22.--Cross sections of a ring-porous
+hardwood (white ash), a diffuse-porous hardwood (red gum), and a
+non-porous or coniferous wood (eastern hemlock). X 30.
+_Photomicrographs by the author._]
+
+The structure of the hardwoods is more complex. They are more or
+less filled with vessels, in some cases (oak, chestnut, ash)
+quite large and distinct, in others (buckeye, poplar, gum) too
+small to be seen plainly without a small hand lens. In
+discussing such woods it is customary to divide them into two
+large classes--_ring-porous_ and _diffuse-porous_. (See Fig.
+22.) In ring-porous species, such as oak, chestnut, ash, black
+locust, catalpa, mulberry, hickory, and elm, the larger vessels
+or pores (as cross sections of vessels are called) become
+localized in one part of the growth ring, thus forming a region
+of more or less open and porous tissue. The rest of the ring is
+made up of smaller vessels and a much greater proportion of wood
+fibres. These fibres are the elements which give strength and
+toughness to wood, while the vessels are a source of weakness.
+
+In diffuse-porous woods the pores are scattered throughout the
+growth ring instead of being collected in a band or row.
+Examples of this kind of wood are gum, yellow poplar, birch,
+maple, cottonwood, basswood, buckeye, and willow. Some species,
+such as walnut and cherry, are on the border between the two
+classes, forming a sort of intermediate group.
+
+If one examines the smoothly cut end of a stick of almost any
+kind of wood, he will note that each growth ring is made up of
+two more or less well-defined parts. That originally nearest the
+centre of the tree is more open textured and almost invariably
+lighter in color than that near the outer portion of the ring.
+The inner portion was formed early in the season, when growth
+was comparatively rapid and is known as _early wood_ (also
+spring wood); the outer portion is the _late wood_, being
+produced in the summer or early fall. In soft pines there is not
+much contrast in the different parts of the ring, and as a
+result the wood is very uniform in texture and is easy to work.
+In hard pine, on the other hand, the late wood is very dense and
+is deep-colored, presenting a very decided contrast to the soft,
+straw-colored early wood. (See Fig. 23.) In ring-porous woods
+each season's growth is always well defined, because the large
+pores of the spring abut on the denser tissue of the fall
+before. In the diffuse-porous, the demarcation between rings is
+not always so clear and in not a few cases is almost, if not
+entirely, invisible to the unaided eye. (See Fig. 22.)
+
+[Illustration: FIG. 23.--Cross section of longleaf pine showing
+several growth rings with variations in the width of the
+dark-colored late wood. Seven resin ducts are visible. X 33.
+_Photomicrograph by U.S. Forest Service_]
+
+If one compares a heavy piece of pine with a light specimen it
+will be seen at once that the heavier one contains a larger
+proportion of late wood than the other, and is therefore
+considerably darker. The late wood of all species is denser than
+that formed early in the season, hence the greater the
+proportion of late wood the greater the density and strength.
+When examined under a microscope the cells of the late wood are
+seen to be very thick-walled and with very small cavities, while
+those formed first in the season have thin walls and large
+cavities. The strength is in the walls, not the cavities. In
+choosing a piece of pine where strength or stiffness is the
+important consideration, the principal thing to observe is the
+comparative amounts of early and late wood. The width of ring,
+that is, the number per inch, is not nearly so important as the
+proportion of the late wood in the ring.
+
+It is not only the proportion of late wood, but also its
+quality, that counts. In specimens that show a very large
+proportion of late wood it may be noticeably more porous and
+weigh considerably less than the late wood in pieces that
+contain but little. One can judge comparative density, and
+therefore to some extent weight and strength, by visual
+inspection.
+
+The conclusions of the U.S. Forest Service regarding the effect
+of rate of growth on the properties of Douglas fir are
+summarized as follows:
+
+"1. In general, rapidly grown wood (less than eight rings per
+inch) is relatively weak. A study of the individual tests upon
+which the average points are based shows, however, that when it
+is not associated with light weight and a small proportion of
+summer wood, rapid growth is not indicative of weak wood.
+
+"2. An average rate of growth, indicated by from 12 to 16 rings
+per inch, seems to produce the best material.
+
+"3. In rates of growths lower than 16 rings per inch, the
+average strength of the material decreases, apparently
+approaching a uniform condition above 24 rings per inch. In such
+slow rates of growth the texture of the wood is very uniform,
+and naturally there is little variation in weight or strength.
+
+"An analysis of tests on large beams was made to ascertain if
+average rate of growth has any relation to the mechanical
+properties of the beams. The analysis indicated conclusively
+that there was no such relation. Average rate of growth [without
+consideration also of density], therefore, has little
+significance in grading structural timber."[18] This is because
+of the wide variation in the percentage of late wood in
+different parts of the cross section.
+
+[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.
+29.]
+
+Experiments seem to indicate that for most species there is a
+rate of growth which, in general, is associated with the
+greatest strength, especially in small specimens. For eight
+conifers it is as follows:[19]
+
+[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, p. 37.]
+
+                  Rings per inch
+Douglas fir             24
+Shortleaf pine          12
+Loblolly pine            6
+Western larch           18
+Western hemlock         14
+Tamarack                20
+Norway pine             18
+Redwood                 30
+
+No satisfactory explanation can as yet be given for the real
+causes underlying the formation of early and late wood. Several
+factors may be involved. In conifers, at least, rate of growth
+alone does not determine the proportion of the two portions of
+the ring, for in some cases the wood of slow growth is very hard
+and heavy, while in others the opposite is true. The quality of
+the site where the tree grows undoubtedly affects the character
+of the wood formed, though it is not possible to formulate a
+rule governing it. In general, however, it may be said that
+where strength or ease of working is essential, woods of
+moderate to slow growth should be chosen. But in choosing a
+particular specimen it is not the width of ring, but the
+proportion and character of the late wood which should govern.
+
+In the case of the ring-porous hardwoods there seems to exist a
+pretty definite relation between the rate of growth of timber
+and its properties. This may be briefly summed up in the general
+statement that the more rapid the growth or the wider the rings
+of growth, the heavier, harder, stronger, and stiffer the wood.
+This, it must be remembered, applies only to ring-porous woods
+such as oak, ash, hickory, and others of the same group, and is,
+of course, subject to some exceptions and limitations.
+
+In ring-porous woods of good growth it is usually the middle
+portion of the ring in which the thick-walled, strength-giving
+fibres are most abundant. As the breadth of ring diminishes,
+this middle portion is reduced so that very slow growth produces
+comparatively light, porous wood composed of thin-walled vessels
+and wood parenchyma. In good oak these large vessels of the
+early wood occupy from 6 to 10 per cent of the volume of the
+log, while in inferior material they may make up 25 per cent or
+more. The late wood of good oak, except for radial grayish
+patches of small pores, is dark colored and firm, and consists
+of thick-walled fibres which form one-half or more of the wood.
+In inferior oak, such fibre areas are much reduced both in
+quantity and quality. Such variation is very largely the result
+of rate of growth.
+
+Wide-ringed wood is often called "second-growth," because the
+growth of the young timber in open stands after the old trees
+have been removed is more rapid than in trees in the forest, and
+in the manufacture of articles where strength is an important
+consideration such "second-growth" hardwood material is
+preferred. This is particularly the case in the choice of
+hickory for handles and spokes. Here not only strength, but
+toughness and resilience are important. The results of a series
+of tests on hickory by the U.S. Forest Service show that "the
+work or shock-resisting ability is greatest in wide-ringed wood
+that has from 5 to 14 rings per inch, is fairly constant from 14
+to 38 rings, and decreases rapidly from 38 to 47 rings. The
+strength at maximum load is not so great with the most
+rapid-growing wood; it is maximum with from 14 to 20 rings per
+inch, and again becomes less as the wood becomes more closely
+ringed. The natural deduction is that wood of first-class
+mechanical value shows from 5 to 20 rings per inch and that
+slower growth yields poorer stock. Thus the inspector or buyer
+of hickory should discriminate against timber that has more than
+20 rings per inch. Exceptions exist, however, in the case of
+normal growth upon dry situations, in which the slow-growing
+material may be strong and tough."[20]
+
+[Footnote 20: Bul. 80: The commercial hickories, pp. 48-50.]
+
+The effect of rate of growth on the qualities of chestnut wood
+is summarized by the same authority as follows: "When the rings
+are wide, the transition from spring wood to summer wood is
+gradual, while in the narrow rings the spring wood passes into
+summer wood abruptly. The width of the spring wood changes but
+little with the width of the annual ring, so that the narrowing
+or broadening of the annual ring is always at the expense of the
+summer wood. The narrow vessels of the summer wood make it
+richer in wood substance than the spring wood composed of wide
+vessels. Therefore, rapid-growing specimens with wide rings have
+more wood substance than slow-growing trees with narrow rings.
+Since the more the wood substance the greater the weight, and
+the greater the weight the stronger the wood, chestnuts with
+wide rings must have stronger wood than chestnuts with narrow
+rings. This agrees with the accepted view that sprouts (which
+always have wide rings) yield better and stronger wood than
+seedling chestnuts, which grow more slowly in diameter."[21]
+
+[Footnote 21: Bul. 53: Chestnut in southern Maryland, pp.
+20-21.]
+
+In diffuse-porous woods, as has been stated, the vessels or
+pores are scattered throughout the ring instead of collected in
+the early wood. The effect of rate of growth is, therefore, not
+the same as in the ring-porous woods, approaching more nearly
+the conditions in the conifers. In general it may be stated that
+such woods of medium growth afford stronger material than when
+very rapidly or very slowly grown. In many uses of wood,
+strength is not the main consideration. If ease of working is
+prized, wood should be chosen with regard to its uniformity of
+texture and straightness of grain, which will in most cases
+occur when there is little contrast between the late wood of one
+season's growth and the early wood of the next.
+
+
+
+HEARTWOOD AND SAPWOOD
+
+
+Examination of the end of a log of many species reveals a
+darker-colored inner portion--the _heartwood_, surrounded by a
+lighter-colored zone--the _sapwood_. In some instances this
+distinction in color is very marked; in others, the contrast is
+slight, so that it is not always easy to tell where one leaves
+off and the other begins. The color of fresh sapwood is always
+light, sometimes pure white, but more often with a decided tinge
+of green or brown.
+
+Sapwood is comparatively new wood. There is a time in the early
+history of every tree when its wood is all sapwood. Its
+principal functions are to conduct water from the roots to the
+leaves and to store up and give back according to the season the
+food prepared in the leaves. The more leaves a tree bears and
+the more thrifty its growth, the larger the volume of sapwood
+required, hence trees making rapid growth in the open have
+thicker sapwood for their size than trees of the same species
+growing in dense forests. Sometimes trees grown in the open may
+become of considerable size, a foot or more in diameter, before
+any heartwood begins to form, for example, in second-growth
+hickory, or field-grown white and loblolly pines.
+
+As a tree increases in age and diameter an inner portion of the
+sapwood becomes inactive and finally ceases to function. This
+inert or dead portion is called heartwood, deriving its name
+solely from its position and not from any vital importance to
+the tree, as is shown by the fact that a tree can thrive with
+its heart completely decayed. Some, species begin to form
+heartwood very early in life, while in others the change comes
+slowly. Thin sapwood is characteristic of such trees as
+chestnut, black locust, mulberry, Osage orange, and sassafras,
+while in maple, ash, gum, hickory, hackberry, beech, and
+loblolly pine, thick sapwood is the rule.
+
+There is no definite relation between the annual rings of growth
+and the amount of sapwood. Within the same species the
+cross-sectional area of the sapwood is roughly proportional to
+the size of the crown of the tree. If the rings are narrow, more
+of them are required than where they are wide. As the tree gets
+larger, the sapwood must necessarily become thinner or increase
+materially in volume. Sapwood is thicker in the upper portion of
+the trunk of a tree than near the base, because the age and the
+diameter of the upper sections are less.
+
+When a tree is very young it is covered with limbs almost, if
+not entirely, to the ground, but as it grows older some or all
+of them will eventually die and be broken off. Subsequent growth
+of wood may completely conceal the stubs which, however, will
+remain as knots. No matter how smooth and clear a log is on the
+outside, it is more or less knotty near the middle. Consequently
+the sapwood of an old tree, and particularly of a forest-grown
+tree, will be freer from knots than the heartwood. Since in most
+uses of wood, knots are defects that weaken the timber and
+interfere with its ease of working and other properties, it
+follows that sapwood, because of its position in the tree, may
+have certain advantages over heartwood.
+
+It is really remarkable that the inner heartwood of old trees
+remains as sound as it usually does, since in many cases it is
+hundreds of years, and in a few instances thousands of years,
+old. Every broken limb or root, or deep wound from fire,
+insects, or falling timber, may afford an entrance for decay,
+which, once started, may penetrate to all parts of the trunk.
+The larvae of many insects bore into the trees and their tunnels
+remain indefinitely as sources of weakness. Whatever advantages,
+however, that sapwood may have in this connection are due solely
+to its relative age and position.
+
+If a tree grows all its life in the open and the conditions of
+soil and site remain unchanged, it will make its most rapid
+growth in youth, and gradually decline. The annual rings of
+growth are for many years quite wide, but later they become
+narrower and narrower. Since each succeeding ring is laid down
+on the outside of the wood previously formed, it follows that
+unless a tree materially increases its production of wood from
+year to year, the rings must necessarily become thinner. As a
+tree reaches maturity its crown becomes more open and the annual
+wood production is lessened, thereby reducing still more the
+width of the growth rings. In the case of forest-grown trees so
+much depends upon the competition of the trees in their struggle
+for light and nourishment that periods of rapid and slow growth
+may alternate. Some trees, such as southern oaks, maintain the
+same width of ring for hundreds of years. Upon the whole,
+however, as a tree gets larger in diameter the width of the
+growth rings decreases.
+
+It is evident that there may be decided differences in the grain
+of heartwood and sapwood cut from a large tree, particularly one
+that is overmature. The relationship between width of growth
+rings and the mechanical properties of wood is discussed under
+Rate of Growth. In this connection, however, it may be stated
+that as a general rule the wood laid on late in the life of a
+tree is softer, lighter, weaker, and more even-textured than
+that produced earlier. It follows that in a large log the
+sapwood, because of the time in the life of the tree when it was
+grown, may be inferior in hardness, strength, and toughness to
+equally sound heartwood from the same log.
+
+After exhaustive tests on a number of different woods the U.S.
+Forest Service concludes as follows: "Sapwood, except that from
+old, overmature trees, is as strong as heartwood, other things
+being equal, and so far as the mechanical properties go should
+not be regarded as a defect."[22] Careful inspection of the
+individual tests made in the investigation fails to reveal any
+relation between the proportion of sapwood and the breaking
+strength of timber.
+
+[Footnote 22: Bul. 108: Tests of structural timber, p. 35.]
+
+In the study of the hickories the conclusion was: "There is an
+unfounded prejudice against the heartwood. Specifications place
+white hickory, or sapwood, in a higher grade than red hickory,
+or heartwood, though there is no inherent difference in
+strength. In fact, in the case of large and old hickory trees,
+the sapwood nearest the bark is comparatively weak, and the best
+wood is in the heart, though in young trees of thrifty growth
+the best wood is in the sap."[23] The results of tests from
+selected pieces lying side by side in the same tree, and also
+the average values for heartwood and sapwood in shipments of the
+commercial hickories without selection, show conclusively that
+"the transformation of sapwood into heartwood does not affect
+either the strength or toughness of the wood.... It is true,
+however, that sapwood is usually more free from latent defects
+than heartwood."[24]
+
+[Footnote 23: Bul. 80: The commercial hickories, p. 50.]
+
+[Footnote 24: _Loc. cit._]
+
+Specifications for paving blocks often require that longleaf
+pine be 90 per cent heart. This is on the belief that sapwood is
+not only more subject to decay, but is also weaker than
+heartwood. In reality there is no sound basis for discrimination
+against sapwood on account of strength, provided other
+conditions are equal. It is true that sapwood will not resist
+decay as long as heartwood, if both are untreated with
+preservatives. It is especially so of woods with deep-colored
+heartwood, and is due to infiltrations of tannins, oils, and
+resins, which make the wood more or less obnoxious to
+decay-producing fungi. If, however, the timbers are to be
+treated, sapwood is not a defect; in fact, because of the
+relative ease with which it can be impregnated with
+preservatives it may be made more desirable than heartwood.[25]
+
+[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 and _Eucalyptus globulus_ and _viminalis_. 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.]
+
+In specifications for structural timbers reference is sometimes
+made to "boxheart," meaning the inclusion of the pith or centre
+of the tree within a cross section of the timber. From numerous
+experiments it appears that the position of the pith does not
+bear any relation to the strength of the material. Since most
+season checks, however, are radial, the position of the pith may
+influence the resistance of a seasoned beam to horizontal shear,
+being greatest when the pith is located in the middle half of
+the section.[26]
+
+[Footnote 26: Bul. 108, U.S. Forest Service, p. 36.]
+
+
+
+WEIGHT, DENSITY, AND SPECIFIC GRAVITY
+
+
+From data obtained from a large number of tests on the strength
+of different woods it appears that, other things being equal,
+the crushing strength parallel to the grain, fibre stress at
+elastic limit in bending, and shearing strength along the grain
+of wood vary in direct proportion to the weight of dry wood per
+unit of volume when green. Other strength values follow
+different laws. The hardness varies in a slightly greater ratio
+than the square of the density. The work to the breaking point
+increases even more rapidly than the cube of density. The
+modulus of rupture in bending lies between the first power and
+the square of the density. This, of course, is true only in case
+the greater weight is due to increase in the amount of wood
+substance. A wood heavy with resin or other infiltrated
+substance is not necessarily stronger than a similar specimen
+free from such materials. If differences in weight are due to
+degree of seasoning, in other words, to the relative amounts of
+water contained, the rules given above will of course not hold,
+since strength increases with dryness. But of given specimens of
+pine or of oak, for example, in the green condition, the
+comparative strength may be inferred from the weight. It is not
+permissible, however, to compare such widely different woods as
+oak and pine on a basis of their weights.[27]
+
+[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.]
+
+The weight of wood substance, that is, the material which
+composes the walls of the fibres and other cells, is practically
+the same in all species, whether pine, hickory, or cottonwood,
+being a little greater than half again as heavy as water. It
+varies slightly from beech sapwood, 1.50, to Douglas fir
+heartwood, 1.57, averaging about 1.55 at 30 deg. to 35 deg. C., in terms
+of water at its greatest density 4 deg. C. The reason any wood
+floats is that the air imprisoned in its cavities buoys it up.
+When this is displaced by water the wood becomes water-logged
+and sinks. Leaving out of consideration infiltrated substances,
+the reason a cubic foot of one kind of dry wood is heavier than
+that of another is because it contains a greater amount of wood
+substance.  ~Density~ is merely the weight of a unit of volume,
+as 35 pounds per cubic foot, or 0.56 grams per cubic centimetre.
+~Specific gravity~ or relative density is the ratio of the
+density of any material to the density of distilled water at 4 deg.
+C. (39.2 deg. F.). A cubic foot of distilled water at 4 deg. C. weighs
+62.43 pounds. Hence the specific gravity of a piece of wood with
+a density of 35 pounds is 35 / 62.43 = 0.561. To find the weight
+per cubic foot when the specific gravity is given, simply
+multiply by 62.43. Thus, 0.561 X 62.43 = 35. In the metric
+system, since the weight of a cubic centimetre of pure water is
+one gram, the density in grams per cubic centimetre has the same
+numerical value as the specific gravity.
+
+Since the amount of water in wood is extremely variable it
+usually is not satisfactory to refer to the density of green
+wood. For scientific purposes the density of "oven-dry" wood is
+used; that is, the wood is dried in an oven at a temperature of
+100 deg.C. (212 deg.F.) until a constant weight is attained. For
+commercial purposes the weight or density of air-dry or
+"shipping-dry" wood is used. This is usually expressed in pounds
+per thousand board feet, a board foot being considered as
+one-twelfth of a cubic foot.
+
+Wood shrinks greatly in drying from the green to the oven-dry
+condition. (See Table XIV.) Consequently a block of wood
+measuring a cubic foot when green will measure considerably less
+when oven-dry. It follows that the density of oven-dry wood does
+not represent the weight of the dry wood substance in a cubic
+foot of green wood. In other words, it is not the weight of a
+cubic foot of green wood minus the weight of the water which it
+contains. Since the latter is often a more convenient figure to
+use and much easier to obtain than the weight of oven-dry wood,
+it is commonly expressed in tables of "specific gravity or
+density of dry wood."
+
+|----------------------------------------------------------------------------|
+|                                 TABLE XIV                                  |
+|----------------------------------------------------------------------------|
+|            SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS            |
+|                         (Forest Service Cir. 213)                          |
+|----------------------------------------------------------------------------|
+|                 |         |  Specific gravity  | Shrinkage from green to   |
+|                 | Mois-   | oven-dry, based on | oven-dry condition        |
+|   COMMON NAME   |  ture   |--------------------+---------------------------|
+|   OF SPECIES    | content | Volume  |  Volume  | In     |        | Tangen- |
+|                 |         |   when  |   when   | volume | Radial | tial    |
+|                 |         |  green  | oven-dry |        |        |         |
+|-----------------+---------+---------+----------+--------+--------+---------|
+|                 |   Per   |         |          |  Per   |  Per   |   Per   |
+|                 |   cent  |         |          |  cent  |  cent  |   cent  |
+|                 |         |         |          |        |        |         |
+|    Hardwoods    |         |         |          |        |        |         |
+|                 |         |         |          |        |        |         |
+| Ash, black      |     77  |  0.466  |          |        |        |         |
+|   white         |     38  |   .550  |   0.640  |  12.6  |   4.3  |    6.4  |
+|     "           |     47  |   .516  |    .590  |  11.7  |        |         |
+| Basswood        |    110  |   .315  |    .374  |  14.5  |   6.2  |    8.4  |
+| Beech           |     61  |   .556  |    .669  |  16.5  |   4.6  |   10.5  |
+| Birch, yellow   |     72  |   .545  |    .661  |  17.0  |   7.9  |    9.0  |
+| Elm, rock       |     46  |   .578  |          |        |        |         |
+|   slippery      |     57  |   .541  |    .639  |  15.5  |   5.1  |    9.9  |
+|   white         |     66  |   .430  |          |        |        |         |
+| Gum, red        |     71  |   .434  |          |        |        |         |
+| Hackberry       |     50  |   .504  |    .576  |  14.0  |   4.2  |    8.9  |
+| Hickory,        |         |         |          |        |        |         |
+|   big shellbark |     64  |   .601  |          |  17.6  |   7.4  |   11.2  |
+|    "      "     |     55  |   .666  |          |  20.9  |   7.9  |   14.2  |
+|   bitternut     |     65  |   .624  |          |        |        |         |
+|   mockernut     |     64  |   .606  |          |  16.5  |   6.9  |   10.4  |
+|       "         |     57  |   .662  |          |  18.9  |   8.4  |   11.4  |
+|       "         |     48  |   .666  |          |        |        |         |
+|   nutmeg        |     76  |   .558  |          |        |        |         |
+|   pignut        |     59  |   .627  |          |  15.0  |   5.6  |    9.8  |
+|     "           |     54  |   .667  |          |  15.3  |   6.3  |    9.5  |
+|     "           |     55  |   .667  |          |  16.9  |   6.8  |   10.9  |
+|     "           |     52  |   .667  |          |  21.2  |   8.5  |   13.8  |
+|   shagbark      |     65  |   .608  |          |  16.0  |   6.5  |   10.2  |
+|      "          |     58  |   .646  |          |  18.4  |   7.9  |   11.4  |
+|      "          |     64  |   .617  |          |        |        |         |
+|      "          |     60  |   .653  |          |  15.5  |   6.5  |    9.7  |
+|   water         |     74  |   .630  |          |        |        |         |
+| Locust, honey   |     53  |   .695  |    .759  |   8.6  |        |         |
+| Maple, red      |     69  |   .512  |          |        |        |         |
+|   sugar         |     57  |   .546  |    .643  |  14.3  |   4.9  |    9.1  |
+|     "           |     56  |   .577  |          |        |        |         |
+| Oak, post       |     64  |   .590  |    .732  |  16.0  |   5.7  |   10.6  |
+|   red           |     80  |   .568  |    .660  |  13.1  |   3.7  |    8.3  |
+|   swamp white   |     74  |   .637  |    .792  |  17.7  |   5.5  |   10.6  |
+|   tanbark       |     88  |   .585  |          |        |        |         |
+|   white         |     58  |   .594  |    .704  |  15.8  |   6.2  |    8.3  |
+|     "           |     62  |   .603  |    .696  |  14.3  |   4.9  |    9.0  |
+|     "           |     78  |   .600  |    .708  |  16.0  |   4.8  |    9.2  |
+|   yellow        |     77  |   .573  |    .669  |  14.2  |   4.5  |    9.7  |
+|      "          |     80  |   .550  |          |        |        |         |
+| Osage orange    |     31  |   .761  |    .838  |   8.9  |        |         |
+| Sycamore        |     81  |   .454  |    .526  |  13.5  |   5.0  |    7.3  |
+| Tupelo          |    121  |   .475  |    .545  |  12.4  |   4.4  |    7.9  |
+|----------------------------------------------------------------------------|
+
+|----------------------------------------------------------------------------|
+|                             TABLE XIV (CONT.)                              |
+|----------------------------------------------------------------------------|
+|            SPECIFIC GRAVITY, AND SHRINKAGE OF 51 AMERICAN WOODS            |
+|                         (Forest Service Cir. 213)                          |
+|----------------------------------------------------------------------------|
+|                 |         |  Specific gravity  | Shrinkage from green to   |
+|                 |         | oven-dry, based on |    oven-dry condition     |
+|   COMMON NAME   |         |--------------------+---------------------------|
+|   OF SPECIES    |  Mois-  | Volume  |  Volume  |   In   |        | Tangen- |
+|                 |   ture  |   when  |   when   | volume | Radial |  tial   |
+|                 | content |  green  | oven-dry |        |        |         |
+|-----------------+---------+---------+----------+--------+--------+---------|
+|                 |   Per   |         |          |  Per   |  Per   |   Per   |
+|                 |   cent  |         |          |  cent  |  cent  |   cent  |
+|                 |         |         |          |        |        |         |
+|    Conifers     |         |         |          |        |        |         |
+|                 |         |         |          |        |        |         |
+| Arborvitae       |     55  |   .293  |    .315  |   7.0  |   2.1  |    4.9  |
+| Cedar, incense  |     80  |   .363  |          |        |        |         |
+| Cypress, bald   |     79  |   .452  |    .513  |  11.5  |   3.8  |    6.0  |
+| Fir, alpine     |     47  |   .306  |    .321  |   9.0  |   2.5  |    7.1  |
+|   amabilis      |    117  |   .383  |          |        |        |         |
+|   Douglas       |     32  |   .418  |    .458  |  10.9  |   3.7  |    6.6  |
+|   white         |    156  |   .350  |    .437  |  10.2  |   3.4  |    7.0  |
+| Hemlock (east.) |    129  |   .340  |    .394  |   9.2  |   2.3  |    5.0  |
+| Pine, lodgepole |     44  |   .370  |    .415  |  11.3  |   4.2  |    7.1  |
+|           "     |     58  |   .371  |    .407  |  10.1  |   3.6  |    5.9  |
+|   longleaf      |     63  |   .528  |    .599  |  12.8  |   6.0  |    7.6  |
+|   red or Nor    |     54  |   .440  |    .507  |  11.5  |   4.5  |    7.2  |
+|   shortleaf     |     52  |   .447  |          |        |        |         |
+|   sugar         |    123  |   .360  |    .386  |   8.4  |   2.9  |    5.6  |
+|   west yellow   |     98  |   .353  |    .395  |   9.2  |   4.1  |    6.4  |
+|     "    "      |    125  |   .377  |    .433  |  11.5  |   4.3  |    7.3  |
+|     "    "      |     93  |   .391  |    .435  |   9.9  |   3.8  |    5.8  |
+|   white         |     74  |   .363  |    .391  |   7.8  |   2.2  |    5.9  |
+| Redwood         |     81  |   .334  |          |        |        |         |
+|    "            |     69  |   .366  |          |        |        |         |
+| Spruce,         |         |         |          |        |        |         |
+|   Engelmann     |     45  |   .325  |    .359  |  10.5  |   3.7  |    6.9  |
+|       "         |    156  |   .299  |    .335  |  10.3  |   3.0  |    6.2  |
+|   red           |     31  |   .396  |          |        |        |         |
+|   white         |     41  |   .318  |          |        |        |         |
+| Tamarack        |     52  |   .491  |    .558  |  13.6  |   3.7  |    7.4  |
+|----------------------------------------------------------------------------|
+
+This weight divided by 62.43 gives the specific gravity per
+green volume. It is purely a fictitious quantity. To convert
+this figure into actual density or specific gravity of the dry
+wood, it is necessary to know the amount of shrinkage in volume.
+If S is the percentage of shrinkage from the green to the
+oven-dry condition, based on the green volume; D, the density of
+the dry wood per cubic foot while green; and d the actual
+                                   D
+density of oven-dry wood, then ---------- = d.
+                                1 - .0 S
+
+This relation becomes clearer from the following analysis:
+Taking V and W as the volume and weight, respectively, when
+green, and v and w as the corresponding volume and weight when
+                     w         W         V - v
+oven-dry, then, d = --- ; D = --- ; S = ------- X 100, and
+                     v         V           V
+     V - v
+s = ------- X 100, in which S is the percentage of shrinkage
+       v
+from the green to the oven-dry condition, based on the green
+volume, and s the same based on the oven-dry volume.
+
+In tables of specific gravity or density of wood it should
+always be stated whether the dry weight per unit of volume when
+green or the dry weight per unit of volume when dry is intended,
+since the shrinkage in volume may vary from 6 to 50 per cent,
+though in conifers it is usually about 10 per cent, and in
+hardwoods nearer 15 per cent. (See Table XIV.)
+
+
+
+COLOR
+
+
+In species which show a distinct difference between heartwood
+and sapwood the natural color of heartwood is invariably darker
+than that of the sapwood, and very frequently the contrast is
+conspicuous. This is produced by deposits in the heartwood of
+various materials resulting from the process of growth,
+increased possibly by oxidation and other chemical changes,
+which usually have little or no appreciable effect on the
+mechanical properties of the wood. (See HEARTWOOD AND SAPWOOD,
+above.) Some experiments[28] on very resinous longleaf pine
+specimens, however, indicate an increase in strength. This is
+due to the resin which increases the strength when dry. Spruce
+impregnated with crude resin and dried is greatly increased in
+strength thereby.
+
+[Footnote 28: Bul. 70, U.S. Forest Service, p. 92; also p. 126,
+appendix.]
+
+Since the late wood of a growth ring is usually darker in color
+than the early wood, this fact may be used in judging the
+density, and therefore the hardness and strength of the
+material. This is particularly the case with coniferous woods.
+In ring-porous woods the vessels of the early wood not
+infrequently appear on a finished surface as darker than the
+denser late wood, though on cross sections of heartwood the
+reverse is commonly true. Except in the manner just stated the
+color of wood is no indication of strength.
+
+Abnormal discoloration of wood often denotes a diseased
+condition, indicating unsoundness. The black check in western
+hemlock is the result of insect attacks.[29] The reddish-brown
+streaks so common in hickory and certain other woods are mostly
+the result of injury by birds.[30] The discoloration is merely
+an indication of an injury, and in all probability does not of
+itself affect the properties of the wood. Certain rot-producing
+fungi impart to wood characteristic colors which thus become
+criterions of weakness. Ordinary sap-staining is due to fungous
+growth, but does not necessarily produce a weakening effect.[31]
+
+[Footnote 29: See Burke, H.E.: Black check in western hemlock.
+Cir. No. 61, U.S. Bu. Entomology, 1905.]
+
+[Footnote 30: See McAtee, W.L.: Woodpeckers in relation to trees
+and wood products. Bul. No. 39, U.S. Biol. Survey, 1911.]
+
+[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.]
+
+
+
+CROSS GRAIN
+
+
+_Cross grain_ is a very common defect in timber. One form of it
+is produced in lumber by the method of sawing and has no
+reference to the natural arrangement of the wood elements. Thus
+if the plane of the saw is not approximately parallel to the
+axis of the log the grain of the lumber cut is not parallel to
+the edges and is termed diagonal. This is likely to occur where
+the logs have considerable taper, and in this case may be
+produced if sawed parallel to the axis of growth instead of
+parallel to the growth rings.
+
+Lumber and timber with diagonal grain is always weaker than
+straight-grained material, the extent of the defect varying with
+the degree of the angle the fibres make with the axis of the
+stick. In the vicinity of large knots the grain is likely to be
+cross. The defect is most serious where wood is subjected to
+flexure, as in beams.
+
+_Spiral grain_ is a very common defect in a tree, and when
+excessive renders the timber valueless for use except in the
+round. It is produced by the arrangement of the wood fibres in a
+spiral direction about the axis instead of exactly vertical.
+Timber with spiral grain is also known as "torse wood." Spiral
+grain usually cannot be detected by casual inspection of a
+stick, since it does not show in the so-called visible grain of
+the wood, by which is commonly meant a sectional view of the
+annual rings of growth cut longitudinally. It is accordingly
+very easy to allow spiral-grained material to pass inspection,
+thereby introducing an element of weakness in a structure.
+
+There are methods for readily detecting spiral grain. The
+simplest is that of splitting a small piece radially. It is
+necessary, of course, that the split be radial, that is, in a
+plane passing through the axis of the log, and not tangentially.
+In the latter case it is quite probable that the wood would
+split straight, the line of cleavage being between the growth
+rings.
+
+In inspection, the elements to examine are the rays. In the case
+of oak and certain other hardwoods these rays are so large that
+they are readily seen not only on a radial surface, but on the
+tangential as well. On the former they appear as flakes, on the
+latter as short lines. Since these rays are between the fibres
+it naturally follows that they will be vertical or inclined
+according as the tree is straight-grained or spiral-grained.
+While they are not conspicuous in the softwoods, they can be
+seen upon close scrutiny, and particularly so if a small hand
+magnifier is used.
+
+When wood has begun to dry and check it is very easy to see
+whether or not it is straight- or spiral-grained, since the
+checks will for the most part follow along the rays. If one
+examines a row of telephone poles, for example, he will probably
+find that most of them have checks running spirally around them.
+If boards were sawed from such a pole after it was badly checked
+they would fall to pieces of their own weight. The only way to
+get straight material would be to split it out.
+
+It is for this reason that split billets and squares are
+stronger than most sawed material. The presence of the spiral
+grain has little, if any, effect on the timber when it is used
+in the round, but in sawed material the greater the pitch of the
+spiral the greater is the defect.
+
+
+
+KNOTS
+
+
+_Knots_ are portions of branches included in the wood of the
+stem or larger branch. Branches originate as a rule from the
+central axis of a stem, and while living increase in size by the
+addition of annual woody layers which are a continuation of
+those of the stem. The included portion is irregularly conical
+in shape with the tip at the pith. The direction of the fibre is
+at right angles or oblique to the grain of the stem, thus
+producing local cross grain.
+
+During the development of a tree most of the limbs, especially
+the lower ones, die, but persist for a time--often for years.
+Subsequent layers of growth of the stem are no longer intimately
+joined with the dead limb, but are laid around it. Hence dead
+branches produce knots which are nothing more than pegs in a
+hole, and likely to drop out after the tree has been sawed into
+lumber. In grading lumber and structural timber, knots are
+classified according to their form, size, soundness, and the
+firmness with which they are held in place.[32]
+
+[Footnote 32: See Standard classification of structural timber.
+Yearbook Am. Soc. for Testing Materials, 1913, pp. 300-303.
+Contains three plates showing standard defects.]
+
+Knots materially affect checking and warping, ease in working,
+and cleavability of timber. They are defects which weaken timber
+and depreciate its value for structural purposes where strength
+is an important consideration. The weakening effect is much more
+serious where timber is subjected to bending and tension than
+where under compression. The extent to which knots affect the
+strength of a beam depends upon their position, size, number,
+direction of fibre, and condition. A knot on the upper side is
+compressed, while one on the lower side is subjected to tension.
+The knot, especially (as is often the case) if there is a season
+check in it, offers little resistance to this tensile stress.
+Small, knots, however, may be so located in a beam along the
+neutral plane as actually to increase the strength by tending to
+prevent longitudinal shearing. Knots in a board or plank are
+least injurious when they extend through it at right angles to
+its broadest surface. Knots which occur near the ends of a beam
+do not weaken it. Sound knots which occur in the central portion
+one-fourth the height of the beam from either edge are not
+serious defects.
+
+Extensive experiments by the U.S. Forest Service[33] indicate
+the following effects of knots on structural timbers:
+
+[Footnote 33: Bul. 108, pp. 52 _et seq._]
+
+(1) Knots do not materially influence the stiffness of
+structural timber.
+
+(2) Only defects of the most serious character affect the
+elastic limit of beams. Stiffness and elastic strength are more
+dependent upon the quality of the wood fibre than upon defects
+in the beam.
+
+(3) The effect of knots is to reduce the difference between the
+fibre stress at elastic limit and the modulus of rupture of
+beams. The breaking strength is very susceptible to defects.
+
+(4) Sound knots do not weaken wood when subject to compression
+parallel to the grain.[34]
+
+[Footnote 34: Bul. 115, U.S. Forest Service: Mechanical
+properties of western hemlock, p. 20.]
+
+
+
+FROST SPLITS
+
+
+A common defect in standing timber results from radial splits
+which extend inward from the periphery of the tree, and almost,
+if not always, near the base. It is most common in trees which
+split readily, and those with large rays and thin bark. The
+primary cause of the splitting is frost, and various theories
+have been advanced to explain the action.
+
+R. Hartig[35] believes that freezing forces out a part of the
+imbibition water of the cell walls, thereby causing the wood to
+shrink, and if the interior layers have not yet been cooled,
+tangential strains arise which finally produce radial clefts.
+
+[Footnote 35: Hartig, R.: The diseases of trees (trans. by
+Somerville and Ward), London and New York, 1894, pp. 282-294.]
+
+Another theory holds that the water is not driven out of the
+cell walls, but that difference in temperature conditions of
+inner and outer layers is itself sufficient to set up the
+strains, resulting in splitting. An air temperature of 14 deg.F. or
+less is considered necessary to produce frost splits.
+
+A still more recent theory is that of Busse[36] who considers
+the mechanical action of the wind a very important factor. He
+observed: (_a_) Frost splits sometimes occur at higher
+temperatures than 14 deg.F. (_b_) Most splits take place shortly
+before sunrise, _i.e._, at the time of lowest air and soil
+temperature; they are never heard to take place at noon,
+afternoon, or evening. (_c_) They always occur between two roots
+or between the collars of two roots, (_d_) They are most
+frequent in old, stout-rooted, broad-crowned trees; in younger
+stands it is always the stoutest members that are found with
+frost splits, while in quite young stands they are altogether
+absent, (_e_) Trees on wet sites are most liable to splits, due
+to difference in wood structure, just as difference in wood
+structure makes different species vary in this regard. (_f_)
+Frost splits are most numerous less than three feet above the
+ground.
+
+[Footnote 36: Busse, W.: Frost-, Ring- und Kernrisse. Forstwiss.
+Centralb., XXXII, 2, 1910, pp. 74-81.]
+
+When a tree is swayed by the wind the roots are counteracting
+forces, and the wood fibres are tested in tension and
+compression by the opposing forces; where the roots exercise
+tension stresses most effectively the effect of compression
+stresses is at a minimum; only where the pressure is in excess
+of the tension, _i.e._, between the roots, can a separation of
+the fibre result. Hence, when by frost a tension on the entire
+periphery is established, and the wind localizes additional
+strains, failure occurs. The stronger the compression and
+tension, the severer the strains and the oftener failures occur.
+The occurrence of reports of frost splits on wind-still days is
+believed by Busse to be due to the opening of old frost splits
+where the tension produced by the frost alone is sufficient.
+
+Frost splits may heal over temporarily, but usually open up
+again during the following winter. The presence of old splits is
+often indicated by a ridge of callous, the result of the
+cambium's effort to occlude the wound. Frost splits not only
+affect the value of lumber, but also afford an entrance into the
+living tree for disease and decay.
+
+
+
+SHAKES, GALLS, PITCH POCKETS
+
+
+_Heart shake_ occurs in nearly all overmature timber, being more
+frequent in hardwoods (especially oak) than in conifers. In
+typical heart shake the centre of the hole shows indications of
+becoming hollow and radial clefts of varying size extend outward
+from the pith, being widest inward. It frequently affects only
+the butt log, but may extend to the entire hole and even the
+larger branches. It usually results from a shrinkage of the
+heartwood due probably to chemical changes in the wood.
+
+When it consists of a single cleft extending across the pith it
+is termed _simple heart shake_. Shake of this character in
+straight-grained trees affects only one or two central boards
+when cut into lumber, but in spiral-grained timber the damage is
+much greater. When shake consists of several radial clefts it is
+termed _star shake_. In some instances one or more of these
+clefts may extend nearly to the bark. In felled or converted
+timber clefts due to heart shake may be distinguished from
+seasoning cracks by the darker color of the exposed surfaces.
+Such clefts, however, tend to open up more and more as the
+timber seasons.
+
+_Cup_ or _ring shake_ results from the pulling apart of two or
+more growth rings. It is one of the most serious defects to
+which sound timber is subject, as it seriously reduces the
+technical properties of wood. It is very common in sycamore and
+in western larch, particularly in the butt portion. Its
+occurrence is most frequent at the junction of two growth layers
+of very unequal thickness. Consequently it is likely to occur in
+trees which have grown slowly for a time, then abruptly
+increased, due to improved conditions of light and food, as in
+thinning. Old timber is more subject to it than young trees. The
+damage is largely confined to the butt log. Cup shake is often
+associated with other forms of shake, and not infrequently shows
+traces of decay.
+
+The causes of cup shake are uncertain. The swaying action of the
+wind may result in shearing apart the growth layers, especially
+in trees growing in exposed places. Frost may in some instances
+be responsible for cup shake or at least a contributing factor,
+although trees growing in regions free from frost often have
+ring shake. Shrinkage of the heartwood may be concentric as well
+as radial in its action, thus producing cup shake instead of, or
+in connection with, heart shake.
+
+A local defect somewhat similar in effect to cup shake is known
+as _rind gall_. If the cambium layer is exposed by the removal
+of the entire bark or rind it will die. Subsequent growth over
+the damaged portion does not cohere with the wood previously
+formed by the old cambium. The defect resulting is termed rind
+gall. The most common causes of it are fire, gnawing, blazing,
+chipping, sun scald, lightning, and abrasions.
+
+_Heart break_ is a term applied to areas of compression failure
+along the grain found in occasional logs. Sometimes these breaks
+are invisible until the wood is manufactured into the finished
+article. The occurrence of this defect is mostly limited to the
+dense hardwoods, such as hickory and to heavy tropical species.
+It is the source of considerable loss in the fancy veneer
+industry, as the veneer from valuable logs so affected drops to
+pieces.
+
+The cause of heart break is not positively known. It is highly
+probable, however, that when the tree is felled the trunk
+strikes across a rock or another log, and the impact causes
+actual failure in the log as in a beam.
+
+_Resin_ or _pitch pockets_ are of common occurrence in the wood
+of larch, spruce, fir, and especially of longleaf and other hard
+pines. They are due to accumulations of resin in openings
+between adjacent layers of growth. They are more frequent in
+trees growing alone than in those of dense stands. The pockets
+are usually a few inches in greatest dimension and affect only
+one or two growth layers. They are hidden until exposed by the
+saw, rendering it impossible to cut lumber with reference to
+their position. Often several boards are damaged by a single
+pocket. In grading lumber, pitch pockets are classified as
+small, standard, and large, depending upon their width and
+length.
+
+
+
+INSECT INJURIES[37]
+
+
+[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.]
+
+The larvae of many insects are destructive to wood. Some attack
+the wood of living trees, others only that of felled or
+converted material. Every hole breaks the continuity of the
+fibres and impairs the strength, and if there are very many of
+them the material may be ruined for all purposes where strength
+is required.
+
+Some of the most common insects attacking the wood of living
+trees are the oak timber worm, the chestnut timber worm,
+carpenter worms, ambrosia beetles, the locust borer, turpentine
+beetles and turpentine borers, and the white pine weevil.
+
+The insect injuries to forest products may be classed according
+to the stage of manufacture of the material. Thus round timber
+with the bark on, such as poles, posts, mine props, and sawlogs,
+is subject to serious damage by the same class of insects as
+those mentioned above, particularly by the round-headed borers,
+timber worms, and ambrosia beetles. Manufactured unseasoned
+products are subject to damage from ambrosia beetles and other
+wood borers. Seasoned hardwood lumber of all kinds, rough
+handles, wagon stock, etc., made partially or entirely of
+sapwood, are often reduced in value from 10 to 90 per cent by a
+class of insects known as powder-post beetles. Finished hardwood
+products such as handles, wagon, carriage and machinery stock,
+especially if ash or hickory, are often destroyed by the
+powder-post beetles. Construction timbers in buildings, bridges
+and trestles, cross-ties, poles, mine props, fence posts, etc.,
+are sometimes seriously injured by wood-boring larvae, termites,
+black ants, carpenter bees, and powder-post beetles, and
+sometimes reduced in value from 10 to 100 per cent. In tropical
+countries termites are a very serious pest in this respect.
+
+
+
+MARINE WOOD-BORER INJURIES
+
+
+Vast amounts of timber used for piles in wharves and other
+marine structures are constantly being destroyed or seriously
+injured by marine borers. Almost invariably they are confined to
+salt water, and all the woods commonly used for piling are
+subject to their attacks. There are two genera of mollusks,
+_Xylotrya_ and _Teredo_, and three of crustaceans, _Limnoria,
+Chelura_, and _Sphoeroma_, that do serious damage in many places
+along both the Atlantic and Pacific coasts.
+
+These mollusks, which are popularly known as "shipworms," are
+much alike in structure and mode of life. They attack the
+exposed surface of the wood and immediately begin to bore. The
+tunnels, often as large as a lead pencil, extend usually in a
+longitudinal direction and follow a very irregular, tangled
+course. Hard woods are apparently penetrated as readily as soft
+woods, though in the same timber the softer parts are preferred.
+The food consists of infusoria and is not obtained from the wood
+substance. The sole object of boring into the wood is to obtain
+shelter.
+
+Although shipworms can live in cold water they thrive best and
+are most destructive in warm water. The length of time required
+to destroy an average barked, unprotected pine pile on the
+Atlantic coast south from Chesapeake Bay and along the entire
+Pacific coast varies from but one to three years.
+
+Of the crustacean borers, _Limnoria_, or the "wood louse," is
+the only one of great importance, although _Sphoeroma_ is
+reported destructive in places. _Limnoria_ is about the size of
+a grain of rice and tunnels into the wood for both food and
+shelter. The galleries extend inward radially, side by side, in
+countless numbers, to the depth of about one-half inch. The thin
+wood partitions remaining are destroyed by wave action, so that
+a fresh surface is exposed to attack. Both hard and soft woods
+are damaged, but the rate is faster in the soft woods or softer
+portions of a wood.
+
+Timbers seriously attacked by marine borers are badly weakened
+or completely destroyed. If the original strength of the
+material is to be preserved it is necessary to protect the wood
+from the borers. This is sometimes accomplished by proper
+injection of creosote oil, and more or less successfully by the
+use of various kinds of external coatings.[38] No treatment,
+however, has proved entirely satisfactory.
+
+[Footnote 38: See Smith, C. Stowell: Preservation of piling
+against marine wood borers. Cir. 128, U.S. Forest Service, 1908,
+pp. 15.]
+
+
+
+FUNGOUS INJURIES[39]
+
+
+[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.]
+
+Fungi are responsible for almost all decay of wood. So far as
+known, all decay is produced by living organisms, either fungi
+or bacteria. Some species attack living trees, sometimes killing
+them, or making them hollow, or in the case of pecky cypress and
+incense cedar filling the wood with galleries like those of
+boring insects. A much larger variety work only in felled or
+dead wood, even after it is placed in buildings or manufactured
+articles. In any case the process of destruction is the same.
+The mycelial threads penetrate the walls of the cells in search
+of food, which they find either in the cell contents (starches,
+sugars, etc.), or in the cell wall itself. The breaking down of
+the cell walls through the chemical action of so-called
+"enzymes" secreted by the fungi follows, and the eventual
+product is a rotten, moist substance crumbling readily under the
+slightest pressure. Some species remove the ligneous matter and
+leave almost pure cellulose, which is white, like cotton; others
+dissolve the cellulose, leaving a brittle, dark brown mass of
+ligno-cellulose. Fungi (such as the bluing fungus) which merely
+stain wood usually do not affect its mechanical properties
+unless the attacks are excessive.
+
+It is evident, then, that the action of rot-causing fungi is to
+decrease the strength of wood, rendering it unsound, brittle,
+and dangerous to use. The most dangerous kinds are the so-called
+"dry-rot" fungi which work in many kinds of lumber after it is
+placed in the buildings. They are particularly to be dreaded
+because unseen, working as they do within the walls or inside of
+casings. Several serious wrecks of large buildings have been
+attributed to this cause. It is stated[40] that in the three
+years (1911-1913) more than $100,000 was required to repair
+damage due to dry rot.
+
+[Footnote 40: Dry rot in factory timbers, by Inspection Dept.
+Associated Factory Mutual Fire Insurance Cos., 31 Milk Street,
+Boston, 1913.]
+
+Dry rot develops best at 75 deg.F. and is said to be killed by a
+temperature of 110 deg.F.[41] Fully 70 per cent humidity is
+necessary in the air in which a timber is surrounded for the
+growth of this fungus, and probably the wood must be quite near
+its fibre saturation condition. Nevertheless _Merulius
+lacrymans_ (one of the most important species) has been found to
+live four years and eight months in a dry condition.[42]
+Thorough kiln-drying will kill this fungus, but will not prevent
+its redevelopment. Antiseptic treatment, such as creosoting, is
+the best prevention.
+
+[Footnote 41: Falck, Richard: Die Meruliusfauele des Bauholzes,
+Hausschwammforschungen, 6. Heft., Jena, 1912.]
+
+[Footnote 42: Mez, Carl: Der Hausschwamm. Dresden, 1908, p. 63.]
+
+All fungi require moisture and air[43] for their growth.
+Deprived of either of these the fungus dies or ceases to
+develop. Just what degree of moisture in wood is necessary for
+the "dry-rot" fungus has not been determined, but it is
+evidently considerably above that of thoroughly air-dry timber,
+probably more than 15 per cent moisture. Hence the importance of
+free circulation of air about all timbers in a building.
+
+[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.]
+
+Warmth is also conducive to the growth of fungi, the most
+favorable temperature being about 90 deg.F. They cannot grow in
+extreme cold, although no degree of cold such as occurs
+naturally will kill them. On the other hand, high temperature
+will kill them, but the spores may survive even the boiling
+temperature. Mould fungus has been observed to develop rapidly
+at 130 deg.F. in a dry kiln in moist air, a condition under which an
+animal cannot live more than a few minutes. This fungus was
+killed, however, at about 140 deg. or 145 deg.F.[44]
+
+[Footnote 44: Experiments in kiln-drying _Eucalyptus_ in
+Berkeley, U.S. Forest Service.]
+
+The fungus (_Endothia parasitica_ And.) which causes the
+chestnut blight kills the trees by girdling them and has no
+direct effect upon the wood save possibly the four or five
+growth rings of the sapwood.[45]
+
+[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.]
+
+
+
+PARASITIC PLANT INJURIES.[46]
+
+
+[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.]
+
+The most common of the higher parasitic plants damaging timber
+trees are mistletoes. Many species of deciduous trees are
+attacked by the common mistletoe (_Phoradendron flavescens_). It
+is very prevalent in the South and Southwest and when present in
+sufficient quantity does considerable damage. There is also a
+considerable number of smaller mistletoes belonging to the genus
+_Razoumofskya (Arceuthobium)_ which are widely distributed
+throughout the country, and several of them are common on
+coniferous trees in the Rocky Mountains and along the Pacific
+coast.
+
+One effect of the common mistletoe is the formation of large
+swellings or tumors. Often the entire tree may become stunted or
+distorted. The western mistletoe is most common on the branches,
+where it produces "witches' broom." It frequently attacks the
+trunk as well, and boards cut from such trees are filled with
+long, radial holes which seriously damage or destroy the value
+of the timber affected.
+
+
+
+LOCALITY OF GROWTH
+
+
+The data available regarding the effect of the locality of
+growth upon the properties of wood are not sufficient to warrant
+definite conclusions. The subject has, however, been kept in
+mind in many of the U.S. Forest Service timber tests and the
+following quotations are assembled from various reports:
+
+"In both the Cuban and longleaf pine the locality where grown
+appears to have but little influence on weight or strength, and
+there is no reason to believe that the longleaf pine from one
+State is better than that from any other, since such variations
+as are claimed can be found on any 40-acre lot of timber in any
+State. But with loblolly and still more with shortleaf this
+seems not to be the case. Being widely distributed over many
+localities different in soil and climate, the growth of the
+shortleaf pine seems materially influenced by location. The wood
+from the southern coast and gulf region and even Arkansas is
+generally heavier than the wood from localities farther north.
+Very light and fine-grained wood is seldom met near the southern
+limit of the range, while it is almost the rule in Missouri,
+where forms resembling the Norway pine are by no means rare. The
+loblolly, occupying both wet and dry soils, varies accordingly."
+Cir. No. 12, p. 6.
+
+" ... It is clear that as all localities have their heavy and
+their light timber, so they all share in strong and weak, hard
+and soft material, and the difference in quality of material is
+evidently far more a matter of individual variation than of soil
+or climate." _Ibid._, p.22
+
+"A representative committee of the Carriage Builders'
+Association had publicly declared that this important industry
+could not depend upon the supplies of southern timber, as the
+oak grown in the South lacked the necessary qualities demanded
+in carriage construction. Without experiment this statement
+could be little better than a guess, and was doubly unwarranted,
+since it condemned an enormous amount of material, and one
+produced under a great variety of conditions and by at least a
+dozen species of trees, involving, therefore, a complexity of
+problems difficult enough for the careful investigator, and
+entirely beyond the few unsystematic observations of the members
+of a committee on a flying trip through one of the greatest
+timber regions of the world.
+
+"A number of samples were at once collected (part of them
+supplied by the carriage builders' committee), and the fallacy
+of the broad statement mentioned was fully demonstrated by a
+short series of tests and a more extensive study into structure
+and weight of these materials. From these tests it appears that
+pieces of white oak from Arkansas excelled well-selected pieces
+from Connecticut, both in stiffness and endwise compression (the
+two most important forms of resistance)." Report upon the
+forestry investigations of the U.S.D.A. 1877-1898, p. 331. See
+also Rep. of Div. of For., 1890, p. 209.
+
+"In some regions there are many small, stunted hickories, which
+most users will not touch. They have narrow sap, are likely to
+be birdpecked, and show very slow growth. Yet five of these
+trees from a steep, dry south slope in West Virginia had an
+average strength fully equal to that of the pignut from the
+better situation, and were superior in toughness, the work to
+maximum load being 36.8 as against 31.2 for pignut. The trees
+had about twice as many rings per inch as others from better
+situations.
+
+"This, however, is not very significant, as trees of the same
+species, age, and size, growing side by side under the same
+conditions of soil and situation, show great variation in their
+technical value. It is hard to account for this difference, but
+it seems that trees growing in wet or moist situations are
+rather inferior to those growing on fresher soil; also, it is
+claimed by many hickory users that the wood from limestone soils
+is superior to that from sandy soils.
+
+"One of the moot questions among hickory men is the relative
+value of northern and southern hickory. The impression prevails
+that southern hickory is more porous and brash than hickory from
+the north. The tests ... indicate that southern hickory is as
+tough and strong as northern hickory of the same age. But the
+southern hickories have a greater tendency to be shaky, and this
+results in much waste. In trees from southern river bottoms the
+loss through shakes and grub-holes in many cases amounts to as
+much as 50 per cent.
+
+"It is clear, therefore, that the difference in northern and
+southern hickory is not due to geographic location, but rather
+to the character of timber that is being cut. Nearly all of that
+from southern river bottoms and from the Cumberland Mountains is
+from large, old-growth trees; that from the north is from
+younger trees which are grown under more favorable conditions,
+and it is due simply to the greater age of the southern trees
+that hickory from that region is lighter and more brash than
+that from the north." Bul. 80, pp. 52-55.
+
+
+
+SEASON OF CUTTING
+
+
+It is generally believed that winter-felled timber has decided
+advantages over that cut at other seasons of the year, and to
+that cause alone are frequently ascribed much greater
+durability, less liability to check and split, better color, and
+even increased strength and toughness. The conclusion from the
+various experiments made on the subject is that while the time
+of felling may, and often does, affect the properties of wood,
+such result is due to the weather conditions rather than to the
+condition of the wood.
+
+There are two phases of this question. One is concerned with the
+physiological changes which might take place during the year in
+the wood of a living tree. The other deals with the purely
+physical results due to the weather, as differences in
+temperature, humidity, moisture, and other features to be
+mentioned later.
+
+Those who adhere to the first view maintain that wood cut in
+summer is quite different in composition from that cut in
+winter. One opinion is that in summer the "sap is up," while in
+winter it is "down," consequently winter-felled timber is drier.
+A variation of this belief is that in summer the sap contains
+certain chemicals which affect the properties of wood and does
+not contain them in winter. Again it is sometimes asserted that
+wood is actually denser in winter than in summer, as part of the
+wood substance is dissolved out in the spring and used for plant
+food, being restored in the fall.
+
+It is obvious that such views could apply only to sapwood, since
+it alone is in living condition at the time of cutting.
+Heartwood is dead wood and has almost no function in the
+existence of the tree other than the purely mechanical one of
+support. Heartwood does undergo changes, but they are gradual
+and almost entirely independent of the seasons.
+
+Sapwood might reasonably be expected to respond to seasonal
+changes, and to some extent it does. Just beneath the bark there
+is a thin layer of cells which during the growing season have
+not attained their greatest density. With the exception of this
+one annual ring, or portion of one, the density of the wood
+substance of the sapwood is nearly the same the year round.
+Slight variations may occur due to impregnation with sugar and
+starch in the winter and its dissolution in the growing season.
+The time of cutting can have no material effect on the inherent
+strength and other mechanical properties of wood except in the
+outermost annual ring of growth.
+
+The popular belief that sap is up in the spring and summer and
+is down in the winter has not been substantiated by experiment.
+There are seasonal differences in the composition of sap, but so
+far as the amount of sap in a tree is concerned there is fully
+as much, if not more, during the winter than in summer.
+Winter-cut wood is not drier, to begin with, than
+summer-felled--in reality, it is likely to be wetter.[47]
+
+[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.]
+
+The important consideration in regard to this question is the
+series of circumstances attending the handling of the timber
+after it is felled. Wood dries more rapidly in summer than in
+winter, not because there is less moisture at one time than
+another, but because of the higher temperature in summer. This
+greater heat is often accompanied by low humidity, and
+conditions are favorable for the rapid removal of moisture from
+the exposed portions of wood. Wood dries by evaporation, and
+other things being equal, this will proceed much faster in hot
+weather than in cold.
+
+It is a matter of common observation that when wood dries it
+shrinks, and if shrinkage is not uniform in all directions the
+material pulls apart, causing season checks. (See Fig. 27.) If
+evaporation proceeds more rapidly on the outside than inside,
+the greater shrinkage of the outer portions is bound to result
+in many checks, the number and size increasing with the degree
+of inequality of drying.
+
+In cold weather, drying proceeds slowly but uniformly, thus
+allowing the wood elements to adjust themselves with the least
+amount of rupturing. In summer, drying proceeds rapidly and
+irregularly, so that material seasoned at that time is more
+likely to split and check.
+
+There is less danger of sap rot when trees are felled in winter
+because the fungus does not grow in the very cold weather, and
+the lumber has a chance to season to below the danger point
+before the fungus gets a chance to attack it. If the logs in
+each case could be cut into lumber immediately after felling and
+given exactly the same treatment, for example, kiln-dried, no
+difference due to the season of cutting would be noted.
+
+
+
+WATER CONTENT[48]
+
+
+[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.]
+
+Water occurs in living wood in three conditions, namely: (1) in
+the cell walls, (2) in the protoplasmic contents of the cells,
+and (3) as free water in the cell cavities and spaces. In
+heartwood it occurs only in the first and last forms. Wood that
+is thoroughly air-dried retains from 8 to 16 per cent of water
+in the cell walls, and none, or practically none, in the other
+forms. Even oven-dried wood retains a small percentage of
+moisture, but for all except chemical purposes, may be
+considered absolutely dry.
+
+The general effect of the water content upon the wood substance
+is to render it softer and more pliable. A similar effect of
+common observation is in the softening action of water on
+rawhide, paper, or cloth. Within certain limits the greater the
+water content the greater its softening effect.
+
+Drying produces a decided increase in the strength of wood,
+particularly in small specimens. An extreme example is the case
+of a completely dry spruce block two inches in section, which
+will sustain a permanent load four times as great as that which
+a green block of the same size will support.
+
+The greatest increase due to drying is in the ultimate crushing
+strength, and strength at elastic limit in endwise compression;
+these are followed by the modulus of rupture, and stress at
+elastic limit in cross-bending, while the modulus of elasticity
+is least affected. These ratios are shown in Table XV, but it is
+to be noted that they apply only to wood in a much drier
+condition than is used in practice. For air-dry wood the ratios
+are considerably lower, particularly in the case of the ultimate
+strength and the elastic limit. Stiffness (within the elastic
+limit), while following a similar law, is less affected. In the
+case of shear parallel to the grain, the general effect of
+drying is to increase the strength, but this is often offset by
+small splits and checks caused by shrinkage.
+
+|----------------------------------------------------------------------|
+|                               TABLE XV                               |
+|----------------------------------------------------------------------|
+|   EFFECT OF DRYING ON THE MECHANICAL PROPERTIES OF WOOD, SHOWN IN    |
+|       RATIO OF INCREASE DUE TO REDUCING MOISTURE CONTENT FROM        |
+|            THE GREEN CONDITION TO KILN-DRY (3.5 PER CENT)            |
+|                   (Forest Service Bul. 70, p. 89)                    |
+|----------------------------------------------------------------------|
+|      KIND OF STRENGTH      |  Longleaf   |   Spruce    |  Chestnut   |
+|                            |    pine     |             |             |
+|----------------------------+-------------+-------------+-------------|
+|                            |  (1)    (2) |  (1)    (2) |  (1)   (2)  |
+|                            |             |             |             |
+| Crushing strength parallel |             |             |             |
+|   to grain                 | 2.89   2.60 | 3.71   3.41 | 2.83   2.55 |
+| Elastic limit in           |             |             |             |
+|   compression              |             |             |             |
+|   parallel to grain        | 2.60   2.34 | 3.80   3.49 | 2.40   2.26 |
+| Modulus of rupture in      |             |             |             |
+|   bending                  | 2.50   2.20 | 2.81   2.50 | 2.09   1.82 |
+| Stress at elastic limit in |             |             |             |
+|   bending                  | 2.90   2.55 | 2.90   2.58 | 2.30   2.00 |
+| Crushing strength at right |             |             |             |
+|   angles to grain          |             | 2.58   2.48 |             |
+| Shearing strength parallel |             |             |             |
+|   to grain                 | 2.01   1.91 | 2.03   1.95 | 1.55   1.47 |
+| Modulus of elasticity in   |             |             |             |
+|   compression parallel to  |             |             |             |
+|   grain                    | 1.63   1.47 | 2.26   2.08 | 1.43   1.29 |
+| Modulus of elasticity in   |             |             |             |
+| bending                    | 1.59   1.35 | 1.43   1.23 | 1.44   1.21 |
+|----------------------------------------------------------------------|
+| NOTE.--The figures in the first column show the relative increase in |
+| strength between a green specimen and a kiln-dry specimen of equal   |
+| size. The figures in the second column show the relative increase of |
+| strength of the same block after being dried from a green condition  |
+| to 3.5 per cent moisture, correction having been made for shrinkage. |
+| That is, in the first column the strength values per actual unit of  |
+| area are used; in the second the values per unit of area of green    |
+| wood which shrinks to smaller size when dried.                       |
+|                                                                      |
+| See also Cir. 108, Fig. 1, p. 8.                                     |
+|----------------------------------------------------------------------|
+
+The moisture content has a decided bearing also upon the manner
+in which wood fails. In compression tests on very dry specimens
+the entire piece splits suddenly into pieces before any buckling
+takes place (see Fig. 9.), while with wet material the block
+gives way gradually, due to the buckling or bending of the walls
+of the fibres along one or more shearing planes. (See Fig. 14.)
+In bending tests on wet beams, first failure occurs by
+compression on top of the beam, gradually extending downward
+toward the neutral axis. Finally the beam ruptures at the
+bottom. In the case of very dry beams the failure is usually by
+splitting or tension on the under side (see Fig. 17.), without
+compression on the upper, and is often sudden and without
+warning, and even while the load is still increasing. The effect
+varies somewhat with different species, chestnut, for example,
+becoming more brittle upon drying than do ash, hemlock, and
+longleaf pine. The tensile strength of wood is least affected by
+drying, as a rule.
+
+In drying wood no increase in strength results until the free
+water is evaporated and the cell walls begin to dry[49]. This
+critical point has been called the _fibre-saturation point_.
+(See Fig. 24.) Conversely, after the cell walls are saturated
+with water, any increase in the amount of water absorbed merely
+fills the cavities and intercellular spaces, and has no effect
+on the mechanical properties. Hence, soaking green wood does not
+lessen its strength unless the water is heated, whereupon a
+decided weakening results.
+
+[Footnote 49: The wood of _Eucalyptus 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.]
+
+[Illustration: FIG. 24.--Relation of the moisture content to the
+various strength values of spruce. FSP = fibre-saturation
+point.]
+
+The strengthening effects of drying, while very marked in the
+case of small pieces, may be fully offset in structural timbers
+by inherent weakening effects due to the splitting apart of the
+wood elements as a result of irregular shrinkage, and in some
+cases also to the slitting of the cell walls (see Fig. 25).
+Consequently with large timbers in commercial use it is unsafe
+to count upon any greater strength, even after seasoning, than
+that of the green or fresh condition.
+
+[Illustration: FIG. 25.--Cross section of the wood of western
+larch showing fissures in the thick-walled cells of the late
+wood. Highly magnified. _Photo by U. S. Forest Service._]
+
+In green wood the cells are all intimately joined together and
+are at their natural or normal size when saturated with water.
+The cell walls may be considered as made up of little particles
+with water between them. When wood is dried the films of water
+between the particles become thinner and thinner until almost
+entirely gone. As a result the cell walls grow thinner with loss
+of moisture,--in other words, the cell shrinks.
+
+It is at once evident that if drying does not take place
+uniformly throughout an entire piece of timber, the shrinkage as
+a whole cannot be uniform. The process of drying is from the
+outside inward, and if the loss of moisture at the surface is
+met by a steady capillary current of water from the inside, the
+shrinkage, so far as the degree of moisture affected it, would
+be uniform. In the best type of dry kilns this condition is
+approximated by first heating the wood thoroughly in a moist
+atmosphere before allowing drying to begin.
+
+In air-seasoning and in ordinary dry kilns this condition too
+often is not attained, and the result is that a dry shell is
+formed which encloses a moist interior. (See Fig. 26.)
+Subsequent drying out of the inner portion is rendered more
+difficult by this "case-hardened" condition. As the outer part
+dries it is prevented from shrinking by the wet interior, which
+is still at its greatest volume. This outer portion must either
+check open or the fibres become strained in tension. If this
+outer shell dries while the fibres are thus strained they become
+"set" in this condition, and are no longer in tension. Later
+when the inner part dries, it tends to shrink away from the
+hardened outer shell, so that the inner fibres are now strained
+in tension and the outer fibres are in compression. If the
+stress exceeds the cohesion, numerous cracks open up, producing
+a "honey-combed" condition, or "hollow-horning," as it is
+called. If such a case-hardened stick of wood be resawed, the
+two halves will cup from the internal tension and external
+compression, with the concave surface inward.
+
+[Illustration: FIG. 26.--Progress of drying throughout the
+length of a chestnut beam, the black spots indicating the
+presence of free water in the wood. The first section at the
+left was cut one-fourth inch from the end, the next one-half
+inch, the next one inch, and all the others one inch apart. The
+illustration shows case-hardening very clearly. _Photo by U. S.
+Forest Service._]
+
+For a given surface area the loss of water from wood is always
+greater from the ends than from the sides, due to the fact that
+the vessels and other water-carriers are cut across, allowing
+ready entrance of drying air and outlet for the water vapor.
+Water does not flow out of boards and timbers of its own accord,
+but must be evaporated, though it may be forced out of very
+sappy specimens by heat. In drying a log or pole with the bark
+on, most of the water must be evaporated through the ends, but
+in the case of peeled timbers and sawn boards the loss is
+greatest from the surface because the area exposed is so much
+greater.
+
+The more rapid drying of the ends causes local shrinkage, and
+were the material sufficiently plastic the ends would become
+bluntly tapering. The rigidity of the wood substance prevents
+this and the fibres are split apart. Later, as the remainder of
+the stick dries many of the checks will come together, though
+some of the largest will remain and even increase in size as the
+drying proceeds. (See Fig. 27.)
+
+[Illustration: FIG. 27.--Excessive season checking. _Photo by U.
+S. Forest Service._]
+
+A wood cell shrinks very little lengthwise. A dry wood cell is,
+therefore, practically of the same length as it was in a green
+or saturated condition, but is smaller in cross section, has
+thinner walls, and a larger cavity. It is at once evident that
+this fact makes shrinkage more irregular, for wherever cells
+cross each other at a decided angle they will tend to pull apart
+upon drying. This occurs wherever pith rays and wood fibres
+meet. A considerable portion of every wood is made up of these
+rays, which for the most part have their cells lying in a radial
+direction instead of longitudinally. (See Frontispiece.) In
+pine, over 15,000 of these occur on a square inch of a
+tangential section, and even in oak the very large rays which
+are readily visible to the eye as flakes on quarter-sawed
+material represent scarcely one per cent of the number which the
+microscope reveals.
+
+A pith ray shrinks in height and width, that is, vertically and
+tangentially as applied to the position in a standing tree, but
+very little in length or radially. The other elements of the
+wood shrink radially and tangentially, but almost none
+lengthwise or vertically as applied to the tree. Here, then, we
+find the shrinkage of the rays tending to shorten a stick of
+wood, while the other cells resist it, and the tendency of a
+stick to get smaller in circumference is resisted by the endwise
+reaction or thrust of the rays. Only in a tangential direction,
+or around the stick in direction of the annual rings of growth,
+do the two forces coincide. Another factor to the same end is
+that the denser bands of late wood are continuous in a
+tangential direction, while radially they are separated by
+alternate zones of less dense early wood. Consequently the
+shrinkage along the rings (tangential) is fully twice as much as
+toward the centre (radial). (See Table XIV.) This explains why
+some cracks open more and more as drying advances. (See Fig.
+27.)
+
+Although actual shrinkage in length is small, nevertheless the
+tendency of the rays to shorten a stick produces strains which
+are responsible for some of the splitting open of ties, posts,
+and sawed timbers with box heart. At the very centre of a tree
+the wood is light and weak, while farther out it becomes denser
+and stronger. Longitudinal shrinkage is accordingly least at the
+centre and greater toward the outside, tending to become
+greatest in the sapwood. When a round or a box-heart timber
+dries fast it splits radially, and as drying continues the cleft
+widens partly on account of the greater tangential shrinkage and
+also because the greater contraction of the outer fibres warps
+the sections apart. If a small hardwood stem is split while
+green for a short distance at the end and placed where it can
+dry out rapidly, the sections will become bow-shaped with the
+concave sides out. These various facts, taken together, explain
+why, for example, an oak tie, pole, or log may split open its
+entire length if drying proceeds rapidly and far enough. Initial
+stresses in the living trees produce a similar effect when the
+log is sawn into boards. This is especially so in _Eucalyptus
+globulus_ and to a less extent with any rapidly grown wood.
+
+The use of S-shaped thin steel clamps to prevent large checks
+and splits is now a common practice in this country with
+crossties and poles as it has been for a long time in European
+countries. These devices are driven into the butts of the
+timbers so as to cross incipient checks and prevent their
+widening. In place of the regular S-hook another of crimped iron
+has been devised. (See Fig. 28.) Thin straps of iron with one
+tapered edge are run between intermeshing cogs and crimped,
+after which they may be cut off any length desired. The time for
+driving S-irons of either form is when the cracks first appear.
+
+[Illustration: FIG. 28.--Control of season checking by the use
+of S-irons. _Photo by U. S. Forest Service._]
+
+The tendency of logs to split emphasizes the importance of
+converting them into planks or timbers while in a green
+condition. Otherwise the presence of large checks may render
+much lumber worthless which might have been cut out in good
+condition. The loss would not be so great if logs were perfectly
+straight-grained, but this is seldom the case, most trees
+growing more or less spirally or irregularly. Large pieces crack
+more than smaller ones, quartered lumber less than that sawed
+through and through, thin pieces, especially veneers, less than
+thicker boards.
+
+In order to prevent cracks at the ends of boards, small straps
+of wood may be nailed on them or they may be painted. This
+method is usually considered too expensive, except in the case
+of valuable material. Squares used for shuttles, furniture,
+gun-stocks, and tool handles should always be protected at the
+ends. One of the best means is to dip them into melted
+paraffine, which seals the ends and prevents loss of moisture
+there. Another method is to glue paper on the ends. In some
+cases abroad paper is glued on to all the surfaces of valuable
+exotic balks. Other substances sometimes employed for the
+purpose of sealing the wood are grease, carbolineum, wax, clay,
+petroleum, linseed oil, tar, and soluble glass. In place of
+solid beams, built-up material is often preferable, as the
+disastrous results of season checks are thereby largely overcome
+or minimized.
+
+
+
+TEMPERATURE
+
+
+The effect of temperature on wood depends very largely upon the
+moisture content of the wood and the surrounding medium. If
+absolutely dry wood is heated in absolutely dry air the wood
+expands. The extent of this expansion is denoted by a
+coefficient corresponding to the increase in length or other
+dimensions for each degree rise in temperature divided by the
+original length or other dimension of the specimen. The
+coefficient of linear expansion of oak has been found to be
+.00000492; radial expansion, .0000544, or about eleven times the
+longitudinal. Spruce expands less than oak, the ratio of radial
+to longitudinal expansion being about six to one. Metals and
+glass expand equally in all directions, since they are
+homogeneous substances, while wood is a complicated structure.
+The coefficient of expansion of iron is .0000285, or nearly six
+times the coefficient of linear expansion of oak and seven times
+that of spruce[50].
+
+[Footnote 50: See Schlich's Manual of Forestry, Vol. V. (rev.
+ed.), p. 75.]
+
+Under ordinary conditions wood contains more or less moisture,
+so that the application of heat has a drying effect which is
+accompanied by shrinkage. This shrinkage completely obscures the
+expansion due to the heating.
+
+Experiments made at the Yale Forest School revealed the effect
+of temperature on the crushing strength of wet wood. In the case
+of wet chestnut wood the strength decreases 0.42 per cent for
+each degree the water is heated above 60 deg. F.; in the case of
+spruce the decrease is 0.32 per cent.
+
+The effects of high temperature on wet wood are very marked.
+Boiling produces a condition of great pliability, especially in
+the case of hardwoods. If wood in this condition is bent and
+allowed to dry, it rigidly retains the shape of the bend, though
+its strength may be somewhat reduced. Except in the case of very
+dry wood the effect of cold is to increase the strength and
+stiffness of wood. The freezing of any free water in the pores
+of the wood will augment these conditions.
+
+The effect of steaming upon the strength of cross-ties was
+investigated by the U.S. Forest Service in 1904. The conclusions
+were summarized as follows:
+
+"(1) The steam at pressure up to 40 pounds applied for 4 hours,
+or at a pressure of 20 pounds up to 20 hours, increases the
+weight of ties. At 40 pounds' pressure applied for 4 hours and
+at 20 pounds for 5 hours the wood began to be scorched.
+
+"(2) The steamed and saturated wood, when tested immediately
+after treatment, exhibited weaknesses in proportion to the
+pressure and duration of steaming. (See Table XVI.) If allowed
+to air-dry subsequently the specimens regained the greater part
+of their strength, provided the pressure and duration had not
+exceeded those cited under (1). Subsequent immersion in water of
+the steamed wood and dried specimens showed that they were
+weaker than natural wood similarly dried and resoaked."[51]
+
+[Footnote 51: Cir. 39. Experiments on the strength of treated
+timber, p. 18.]
+
+|------------------------------------------------------------------------------------------|
+|                                        TABLE XVI                                         |
+|------------------------------------------------------------------------------------------|
+|                EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE                 |
+|                                (Forest Service, Cir. 39)                                 |
+|------------------------------------------------------------------------------------------|
+|           |       Cylinder conditions       |                  Strength                  |
+|           |---------------------------------+--------------------------------------------|
+|           |            Steaming             |       Static        |  Impact  |           |
+|           |---------------------------------+---------------------+----------|  Average  |
+| Treatment |        |          |             | Bending  | Compres- |  Height  |  of the   |
+|           |        |          |             | modulus  |  sion    | of drop  |   three   |
+|           | Period | Pressure | Temperature |   of     | parallel | causing  | strengths |
+|           |        |          |             | rupture  | to grain | complete |           |
+|           |        |          |             |          |          | failure  |           |
+|-----------+--------+----------+-------------+----------+----------+----------+-----------|
+|           |        | Lbs. per |             | Per cent | Per cent | Per cent | Per cent  |
+|           |  Hrs.  | sq. inch |      deg.F.     |          |          |          |           |
+|           |        |          |             |           Untreated wood = 100%            |
+|           |        |          |             |          |          |          |           |
+| Steam,    |    4   |          |     230[a]  |   91.3   |   79.1   |   96.4   |    88.9   |
+| at        |    4   |     10   |     238     |   78.2   |   93.7   |   93.3   |    88.4   |
+| various   |    4   |     20   |     253     |   83.3   |   84.2   |   91.4   |    80.8   |
+| pressures |    4   |     30   |     269     |   80.4   |   78.4   |   89.8   |    82.9   |
+|           |    4   |     40   |     283     |   78.1   |   74.4   |   74.0   |    75.5   |
+|           |    4   |     50   |     292     |   75.8   |   71.5   |   63.9   |    70.4   |
+|           |    4   |    100   |     337     |   41.4   |   65.0   |   55.2   |    53.9   |
+|-----------+--------+----------+-------------+----------+----------+----------+-----------|
+| Steam,    |    1   |     20   |     257     |  100.6   |   98.6   |   86.7   |    95.3   |
+| for       |    2   |     20   |     267     |   88.4   |   93.0   |  107.0   |    96.1   |
+| various   |    3   |     20   |     260     |   90.0   |   93.6   |   84.1   |    89.2   |
+| periods   |    4   |     20   |     253     |   83.3   |   84.2   |   91.4   |    86.3   |
+|           |    5   |     20   |     253     |   85.0   |   78.1   |   84.2   |    82.4   |
+|           |    6   |     20   |     242     |   95.2   |   89.8   |   76.0   |    87.0   |
+|           |   10   |     20   |     255     |   73.7   |   82.0   |   76.0   |    77.2   |
+|           |   20   |     20   |     258     |   67.5   |   65.0   |   99.0   |    77.2   |
+|------------------------------------------------------------------------------------------|
+| [Footnote a: It will be noted that the temperature was 230 deg.. This is the maximum         |
+| temperature by the maximum-temperature recording thermometer, and is due to the handling |
+| of the exhaust valve. The average temperature was that of exhaust steam.]                |
+|------------------------------------------------------------------------------------------|
+
+"(3) A high degree of steaming is injurious to wood in strength
+and spike-holding power. The degree of steaming at which
+pronounced harm results will depend upon the quality of the wood
+and its degree of seasoning, and upon the pressure (temperature)
+of steam and the duration of its application. For loblolly pine
+the limit of safety is certainly 30 pounds for 4 hours, or 20
+pounds for 6 hours."[52]
+
+[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table
+5.]
+
+Experiments made at the Yale Forest School showed that steaming
+above 30 pounds' gauge pressure reduces the strength of wood
+permanently while wet from 25 to 75 per cent.
+
+
+
+PRESERVATIVES
+
+
+The exact effects of chemical impregnation upon the mechanical
+properties of wood have not been fully determined, though they
+have been the subject of considerable investigation.[53] More
+depends upon the method of treatment than upon the preservatives
+used. Thus preliminary steaming at too high pressure or for too
+long a period will materially weaken the wood, (See TEMPERATURE,
+above.)
+
+[Footnote 53: Hatt, W. K.: Experiments on the strength of
+treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]
+
+The presence of zinc chloride does not weaken wood under static
+loading, although the indications are that the wood becomes
+brittle under impact. If the solution is too strong it will
+decompose the wood.
+
+Soaking in creosote oil causes wood to swell, and accordingly
+decreases the strength to some extent, but not nearly so much so
+as soaking in water.[54]
+
+[Footnote 54: Teesdale, Clyde II.: The absorption of creosote by
+the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p.
+7.]
+
+Soaking in kerosene seems to have no significant weakening
+effect.[55]
+
+[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.]
+
+
+
+
+PART III TIMBER TESTING[56]
+
+
+
+[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.]
+
+
+
+WORKING PLAN
+
+
+Preliminary to making a series of timber tests it is very
+important that a working plan be prepared as a guide to the
+investigation. This should embrace: (1) the purpose of the
+tests; (2) kind, size, condition, and amount of material needed;
+(3) full description of the system of marking the pieces; (4)
+details of any special apparatus and methods employed; (5)
+proposed method of analyzing the data obtained and the nature of
+the final report. Great care should be taken in the preparation
+of this plan in order that all problems arising may be
+anticipated so far as possible and delays and unnecessary work
+avoided. A comprehensive study of previous investigations along
+the same or related lines should prove very helpful in outlining
+the work and preparing the report. (For sample working plan see
+Appendix.)
+
+
+
+FORMS OF MATERIAL TESTED
+
+
+In general, four forms of material are tested, namely: (1) large
+timbers, such as bridge stringers, car sills, large beams, and
+other pieces five feet or more in length, of actual sizes and
+grades in common use; (2) built-up structural forms and
+fastenings, such as built-up beams, trusses, and various kind of
+joints; (3) small clear pieces, such as are used in compression,
+shear, cleavage, and small cross-breaking tests; (4)
+manufactured articles, such as axles, spokes, shafts,
+wagon-tongues, cross-arms, insulator pins, barrels, and packing
+boxes.
+
+As the moisture content is of fundamental importance (see WATER
+CONTENT, above.), all standard tests are usually made in the
+green condition. Another series is also usually run in an
+air-dry condition of about 12 per cent moisture. In all cases
+the moisture is very carefully determined and stated with the
+results in the tables.
+
+
+
+SIZE OF TEST SPECIMENS
+
+
+The size of the test specimen must be governed largely by the
+purpose for which the test is made. If the effect of a single
+factor, such as moisture, is the object of experiment, it is
+necessary to use small pieces of wood in order to eliminate so
+far as possible all disturbing factors. If the specimens are too
+large, it is impossible to secure enough perfect pieces from one
+tree to form a series for various tests. Moreover, the drying
+process with large timbers is very difficult and irregular, and
+requires a long period of time, besides causing checks and
+internal stresses which may obscure the results obtained.
+
+On the other hand, the smaller the dimensions of the test
+specimen the greater becomes the relative effect of the inherent
+factors affecting the mechanical properties. For example, the
+effect of a knot of given size is more serious in a small stick
+than in a large one. Moreover, the smaller the specimen the
+fewer growth rings it contains, hence there is greater
+opportunity for variation due to irregularities of grain.
+
+Tests on large timbers are considered necessary to furnish
+designers data on the probable strength of the different sizes
+and grades of timber on the market; their coefficients of
+elasticity under bending (since the stiffness rather than the
+strength often determines the size of a beam); and the manner of
+failure, whether in bending fibre stress or horizontal shear. It
+is believed that this information can only be obtained by direct
+tests on the different grades of car sills, stringers, and other
+material in common use.
+
+When small pieces are selected for test they very often are
+clear and straight-grained, and thus of so much better grade
+than the large sticks that tests upon them may not yield unit
+values applicable to the larger sizes. Extensive experiments
+show, however, (1) that the modulus of elasticity is
+approximately the same for large timbers as for small clear
+specimens cut from them, and (2) that the fibre stress at
+elastic limit for large beams is, except in the weakest timbers,
+practically equal to the crushing strength of small clear pieces
+of the same material.[57]
+
+[Footnote 57: Bul. 108, U. S. Forest Service: Tests of
+structural timbers, pp. 53-54.]
+
+
+
+MOISTURE  DETERMINATION
+
+
+In order for tests to be comparable, it is necessary to know the
+moisture content of the specimens at the zone of failure. This
+is determined from disks an inch thick cut from the timber
+immediately after testing.
+
+In cases, as in large beams, where it is desirable to know not
+only the average moisture content but also its distribution
+through the timber, the disks are cut up so as to obtain an
+outside, a middle, and an inner portion, of approximately equal
+areas. Thus in a section 10" x 12" the outer strip would be one
+inch wide, and the second one a little more than an inch and a
+quarter. Moisture determinations are made for each of the three
+portions separately.
+
+The procedure is as follows:
+
+(1) Immediately after sawing, loose splinters are removed and
+each section is weighed.
+
+(2) The material is put into a drying oven at 100 deg. C. (212 deg. F.)
+and dried until the variation in weight for a period of
+twenty-four hours is less than 0.5 per cent.
+
+(3) The disk is again carefully weighed.
+
+(4) The loss in weight expressed in per cent of the dry weight
+indicates the moisture content of the specimen from which the
+specimen was cut.
+
+
+
+MACHINE FOR STATIC TESTS
+
+
+The standard screw machines used for metal tests are also used
+for wood, but in the case of wood tests the readings must be
+taken "on the fly," and the machine operated at a uniform speed
+without interruption from beginning to end of the test. This is
+on account of the time factor in the strength of wood. (See
+SPEED OF TESTING MACHINE, below.)
+
+The standard machines for static tests can be used for
+transverse bending, compression, tension, shear, and cleavage. A
+common form consists of three main parts, namely: (1) the
+straining mechanism, (2) the weighing apparatus, and (3) the
+machinery for communicating motion to the screws.
+
+The straining mechanism consists of two parts, one of which is a
+movable crosshead operated by four (sometimes two or three)
+upright steel straining screws which pass through openings in
+the platform and bear upward on the bed of the machine upon
+which the weighing platform rests as a fulcrum. At the lower
+ends of these screws are geared nuts all rotated simultaneously
+by a system of gears which cause the movable crosshead to rise
+and fall as desired.
+
+The stationary part of the straining mechanism, which is used
+only for tension and cleavage tests, consists of a steel cage
+above the movable crosshead and rests directly upon the weighing
+platform. The top of the cage contains a square hole into which
+one end of the test specimen may be clamped, the crosshead
+containing a similar clamp for the other end, in making tension
+tests.
+
+For testing long beams a special form of machine with an
+extended platform is used. (See Fig. 29.)
+
+The weighing platform rests upon knife edges carried by primary
+levers of the weighing apparatus, the fulcrum being on the bed
+of the machine, and any pressure upon it is directly transmitted
+through a series of levers to the weighing beam. This beam is
+adjusted by means of a poise running on a screw. In operation
+the beam is kept floating by means of another poise moved back
+and forth by a screw which is operated by a hand wheel or
+automatically. The larger units of stress are read from the
+graduations along the side of the beam, while the intermediate
+smaller weights are observed on the dial on the rear end of the
+beam.
+
+The machine is driven by power from a shaft or a motor and is so
+geared that various speeds are obtainable. One man can operate
+it.
+
+In making tests the operation of the straining screws is always
+downward so as to bring pressure to bear upon the weighing
+platform. For tests in tension and cleavage the specimen is
+placed between the top of the stationary cage and the movable
+head and subjected to a pull. For tests in transverse bending,
+compression, and cleavage the specimen is placed between the
+movable head and the platform, and a direct compression force
+applied.
+
+Testing machines are usually calibrated to a portion of their
+capacity before leaving the factory. The delicacy of the
+weighing levers is verified by determining the number of pounds
+necessary to move the beam between the stops while a load of
+1,000 pounds rests on the platform. The usual requirement is
+that ten pounds should accomplish this movement.
+
+The size of machine suitable for compression tests on 2" X 2"
+sticks or for 2" X 2" beams with 26 to 36-inch span has a
+capacity of 30,000 pounds.
+
+
+
+SPEED OF TESTING MACHINE
+
+
+In instructions for making static tests the rate of application
+of the stress, _i.e._, the speed of the machine, is given
+because the strength of wood varies with the speed at which the
+fibres are strained. The speed of the crosshead of the testing
+machine is practically never constant, due to mechanical defects
+of the apparatus and variations in the speed of the motor, but
+so long as it does not exceed 25 per cent the results will not
+be appreciably affected. In fact, a change in speed of 50 per
+cent will not cause the strength of the wood to vary more than 2
+per cent.[58]
+
+[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.]
+
+Following are the formulae used in determining the speed of the
+movable head of the machine in inches per minute (n):
+
+(1) For endwise compression          n = Z l
+
+                                          Z l^{2}
+(2) For beams (centre loading)       n = ---------
+                                            6h
+
+                                          Z l^{2}
+(3) For beams (third-pointloading)   n = ---------
+                                           5.4h
+
+  Z = rate of fibre strain per inch of fibre length.
+  l = span of beam or length of compression specimen.
+  h = height of beam.
+
+The values commonly used for Z are as follows:
+
+  Bending large beams                      Z = 0.0007
+  Bending small beams                      Z = 0.0015
+  Endwise compression-large specimens      Z = 0.0015
+  Endwise compression-small       "        Z = 0.003
+  Right-angled compression-large  "        Z = 0.007
+  Right-angled compression-small  "        Z = 0.015
+  Shearing parallel to the grain           Z = 0.015
+
+Example: At what speed should the crosshead move to give the
+required rate of fibre strain in testing a small beam 2" X 2" X
+30". (Span = 28".) Substituting these values in equation (2)
+above:
+          (0.0015 X 28^2)
+     n = ----------------- = 0.1 inch per minute.
+              (6 X 2)
+
+In order that tests may be intelligently compared, it is
+important that account be taken of the speed at which the stress
+was applied. In determining the basis for a ratio between time
+and strength the rate of strain, which is controllable, and not
+the ratio of stress, which is circumstantial, should be used. In
+other words, the rate at which the movable head of the testing
+machine descends and not the rate of increase in the load is to
+be regulated. This ratio, to which the name _speed-strength
+modulus_ has been given, may be expressed as a coefficient
+which, if multiplied into any proportional change in speed, will
+give the proportional change in strength. This ratio is derived
+from empirical curves. (See Table XVII.)
+
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XVII                                                                                TABLE XVII                                                                                            |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                            SPEED-STRENGTH MODULI AND RELATIVE INCREASE IN STRENGTH AT RATES OF FIBRE STRAIN INCREASING IN GEOMETRICAL RATIO. (Tiemann, _loc. cit._)                              |
+|                                                                            (Values in parentheses are approximate)                                                                              |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Rate of fibre strain.   |                       |                       |                       |                       |                       |                       |                       |
+|   Ten-thousandths inch  |          2/3          |            2          |            6          |           18          |           54          |          162          |          486          |
+|   per minute per inch   |                       |                       |                       |                       |                       |                       |                       |
+|-------------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| C | Speed of crosshead. |                       |                       |                       |                       |                       |                       |                       |
+| O |   Inches per minute |       0.000383        |         0.00115       |         0.00345       |         0.0103        |         0.0310        |         0.0931        |         .279          |
+| M |                     |                       |                       |                       |                       |                       |                       |                       |
+| P |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| R | Specimens           |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |
+| E |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
+| S | Relative            |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| S |   crushing          |                       | 100.0 | 100.0 | 100.0 | 103.4 | 100.8 | 101.5 | 107.5 | 102.7 | 103.8 | 113.9 | 105.5 | 107.9 | 121.3 | 108.3 | 116.4 | 128.8 | 110.0 |118.9  |
+| I |   strength          |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| O |                     |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| N | Speed-strength      |                       | 0.017 |(0.006)|(0.009)| 0.033 | 0.012 | 0.016 | 0.047 | 0.021 | 0.029 | 0.053 | 0.027 | 0.039 | 0.060 | 0.023 | 0.049 |(0.052)|(0.015)|(0.040)|
+|   |   modulus, _T_      |                       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|---+---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+|   | Speed of crosshead. |                       |                       |                       |                       |                       |                       |                       |
+|   |   Inches per minute |        0.0072         |         0.0216        |         0.0648        |         0.194         |         0.583         |         1.75          |         5.25          |
+| B |                     |                       |                       |                       |                       |                       |                       |                       |
+| E |---------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------+-----------------------|
+| N | Specimens           |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |  Wet  |  Dry  |  All  |
+| D |---------------------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------+-------|
+| I | Relative            |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+| N |   crushing          |  97.4 |  99.0 |  98.2 | 100.0 | 100.0 | 100.0 | 105.1 | 102.1 | 103.7 | 111.3 | 105.8 | 108.1 | 117.9 | 108.6 | 112.7 | 123.7 | 109.6 | 116.3 | 126.3 | 110.3 | 118.9 |
+| G |   strength          |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|   |                     |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|   | Speed-strength      |(0.014)|(0.005)| 0.012 | 0.033 | 0.014 | 0.026 | 0.049 | 0.026 | 0.037 | 0.053 | 0.033 | 0.038 | 0.049 | 0.014 | 0.035 | 0.038 | 0.006 | 0.025 |(0.023)|(0.004)|(0.014)|
+|   |   modulus, _T_      |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |       |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| NOTE.--The usual speeds of testing at the U.S. Forest Service laboratory are at rates of fibre strain                                                                                           |
+| of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively.                                                                                                     |
+|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+
+
+
+BENDING LARGE BEAMS
+
+
+_Apparatus_: A static bending machine (described above), with a
+special crosshead for third-point loading and a long platform
+bearing knife-edge supports, is required. (See Fig. 29.)
+
+[Illustration: FIG. 29.--Static bending test on large beam. Note
+arrangement of wire and scale for measuring deflection; also
+method of applying load at "third-points."]
+
+_Preparing the material_: Standard sizes and grades of beams and
+timbers in common use are employed. The ends are roughly squared
+and the specimen weighed and measured, taking the
+cross-sectional dimensions midway of the length. Weights should
+be to the nearest pound, lengths to the nearest 0.1 inch, and
+cross-sectional dimensions to the nearest 0.01 inch.
+
+_Marking and sketching_: The butt end of the beam is marked _A_
+and the top end _B_. While facing _A_, the top side is marked
+_a_, the right hand _b_, the bottom _c_, the left hand _d_.
+Sketches are made of each side and end, showing (1) size,
+location, and condition of knots, checks, splits, and other
+defects; (2) irregularities of grain; (3) distribution of
+heartwood and sapwood; and on the ends: (4) the location of the
+pith and the arrangement of the growth rings, (5) number of
+rings per inch, and (6) the proportion of late wood.
+
+The number of rings per inch and the proportion of late wood
+should always be determined along a radius or a line normal to
+the rings. The average number of rings per inch is the total
+number of rings divided by the length of the line crossing them.
+The proportion of late wood is equal to the sum of the widths of
+the late wood crossed by the line, divided by the length of the
+line. Rings per inch should be to the nearest 0.1; late wood to
+the nearest 0.1 per cent.
+
+Since in large beams a great variation in rate of growth and
+relative amount of late wood is likely in different parts of the
+section, it is advisable to consider the cross section in three
+volumes, namely, the upper and lower quarters and the middle
+half. The determination should be made upon each volume
+separately, and the average for the entire cross section
+obtained from these results.
+
+At the conclusion of the test the failure, as it appears on each
+surface, is traced on the sketches, with the failures numbered
+in the order of their occurrence. If the beam is subsequently
+cut up and used for other tests an additional sketch may be
+desirable to show the location of each piece.
+
+_Adjusting specimen in machine_: The beam is placed in the
+machine with the side marked _a_ on top, and with the ends
+projecting equally beyond the supports. In order to prevent
+crushing of the fibre at the points where the stress is applied
+it is necessary to use bearing blocks of maple or other hard
+wood with a convex surface in contact with the beam. Roller
+bearings should be placed between the bearing blocks and the
+knife edges of the crosshead to allow for the shortening due to
+flexure. (See Fig. 29.) Third-point loading is used, that is,
+the load is applied at two points one-third the span of the beam
+apart. (See Fig. 30.) This affords a uniform bending moment
+throughout the central third of the beam.
+
+[Illustration: FIG. 30.--Two methods of loading a beam, namely,
+third-point loading (upper), and centre loading (lower).]
+
+_Measuring the deflection_: The method of measuring the
+deflection should be such that any compression at the points of
+support or at the application of the load will not affect the
+reading. This may be accomplished by driving a small nail near
+each end of the beam, the exact location being on the neutral
+plane and vertically above each knife-edge support. Between
+these nails a fine wire is stretched free of the beam and kept
+taut by means of a rubber band or coiled spring on one end.
+Behind the wire at a point on the beam midway between the
+supports a steel scale graduated to hundredths of an inch is
+fastened vertically by means of thumb-tacks or small screws
+passing through holes in it. Attachment should be made on the
+neutral plane.
+
+The first reading is made when the scale beam is balanced at
+zero load, and afterward at regular increments of the load which
+is applied continuously and at a uniform speed. (See SPEED OF
+TESTING MACHINE, above.) If desired, however, the load may be
+read at regular increments of deflection. The deflection
+readings should be to the nearest 0.01 inch. To avoid error due
+to parallax, the readings may be taken by means of a reading
+telescope about ten feet distant and approximately on a level
+with the wire. A mirror fastened to the scale will increase the
+accuracy of the readings if the telescope is not used. As in all
+tests on timber, the strain must be continuous to rupture, not
+intermittent, and readings must be taken "on the fly." The
+weighing beam is kept balanced after the yield point is reached
+and the maximum load, and at least one point beyond it, noted.
+
+_Log of the test_: The proper log sheet for this test consists
+of a piece of cross-section paper with space at the margin for
+notes. (See Fig. 32.) The load in some convenient unit (1,000 to
+10,000 pounds, depending upon the dimensions of the specimen) is
+entered on the ordinates, the deflection in tenths of an inch on
+the abscissae. The increments of load should be chosen so as to
+furnish about ten points on the stress-strain diagram below the
+elastic limit.
+
+As the readings of the wire on the scale are made they are
+entered directly in their proper place on the cross-section
+paper. In many cases a test should be continued until complete
+failure results. The points where the various failures occur are
+indicated on the stress-strain diagram. A brief description of
+the failure is made on the margin of the log sheet, and the form
+traced on the sketches.
+
+_Disposal of the specimen_: Two one-inch sections are cut from
+the region of failure to be used in determining the moisture
+content. (See MOISTURE DETERMINATION, above.) A two-inch section
+may be cut for subsequent reference and identification, and
+possible microscopic study. The remainder of the beam may be cut
+into small beams and compression pieces.
+
+_Calculating the results_: The formulae used in calculating the
+results of tests on large rectangular simple beams loaded at
+third points of the span are as follows:
+
+              0.75 P
+     (1) J = --------
+               b h
+
+              l (P_{1} + 0.75 W)
+     (2) r = --------------------
+                   b h^{2}
+
+              l (P + 0.75 W)
+     (3) R = ----------------
+                 b h^{2}
+
+               P_{1} l^{3}
+     (4) E = ---------------
+              4.7 D b h^{3}
+
+              0.87 P_{1} D
+     (5) S = --------------
+                  2 V
+
+  b, h, l = breadth, height, and span of specimen, inches.
+  D = total deflection at elastic limit, inches.
+  P = maximum load, pounds.
+  P_{1} = load at elastic limit, pounds.
+  E = modulus of elasticity, pounds per square inch.
+  r = fibre stress at elastic limit, pounds per sq. inch.
+  R = modulus of rupture, pounds per square inch.
+  S = elastic resilience or work to elastic limit, inch-pounds
+        per cu. in.
+  J = greatest calculated longitudinal shear, pounds per square
+        inch.
+  V = volume of beam, cubic inches.
+  W = weight of the beam.
+
+In large beams the weight should be taken into account in
+calculating the fibre stress. In (2) and (3) three-fourths of
+the weight of the beam is added to the load for this reason.
+
+
+
+BENDING SMALL BEAMS
+
+
+_Apparatus_: An ordinary static bending machine, a steel I-beam
+bearing two adjustable knife-edge supports to rest on the
+platform, and a special deflectometer, are required. (See Fig.
+31.)
+
+[Illustration: FIG. 31.--Static bending test on small beam. Note
+the use of the deflectometer with indicator and dial for
+measuring the deflection; also roller bearings between beam and
+supports.]
+
+_Preparing the material_: The specimens may be of any convenient
+size, though beams 2" X 2" X 30" tested over a 28-inch span, are
+considered best. The beams are surfaced on all four sides, care
+being taken that they are not damaged by the rollers of the
+surfacing machine. Material for these tests is sometimes cut
+from large beams after failure. The specimens are carefully
+weighed in grams, and all dimensions measured to the nearest
+0.01 inch. If to be tested in a green or fresh condition the
+specimens should be kept in a damp box or covered with moist
+sawdust until needed. No defects should be allowed in these
+specimens.
+
+_Marking and sketching_: Sketches are made of each end of the
+specimen to show the character of the growth, and after testing,
+the manner of failure is shown for all four sides. In obtaining
+data regarding the rate of growth and the proportion of late
+wood the same procedure is followed as with large beams.
+
+_Adjusting specimen in machine_: The beam should be correctly
+centred in the machine and each end should have a plate with
+roller bearings between it and the support. Centre loading is
+used. Between the movable head of the machine and the specimen
+is placed a bearing block of maple or other hard wood, the lower
+surface of which is curved in a direction along the beam, the
+curvature of which should be slightly less than that of the beam
+at rupture, in order to prevent the edges from crushing into the
+fibres of the test piece.
+
+_Measuring the deflection_: The method of measuring deflection
+of large beams can be used for small sizes, but because of the
+shortness of the span and consequent slight deformation in the
+latter, it is hardly accurate enough for good work. The special
+deflectometer shown in Fig. 31 allows closer reading, as it
+magnifies the deflection ten times. It rests on two small nails
+driven in the beam on the neutral plane and vertically above the
+supports. The fine wire on the wheel at the base of the
+indicator is attached to another small nail driven in the beam
+on the neutral plane midway between the end nails. All three
+nails should be in place before the beam is put into the
+machine. The indicator is adjustable by means of a thumb-screw
+at the base and is set at zero before the load is applied.
+Deflections are read to the nearest 0.001 inch. For rate of
+application of load see SPEED OF TESTING MACHINE, above. The
+speed should be uniform from start to finish without stopping.
+Readings must be made "on the fly."
+
+_Log of the test_: The log sheets used for small beams (see Fig.
+32) are the same as for large sizes and the procedure is
+practically identical. The stress-strain diagram is continued to
+or beyond the maximum load, and in a portion of the tests should
+be continued to six-inch deflection or until the specimen fails
+to support a load of 200 pounds. Deflection readings for equal
+increments of load are taken until well beyond the elastic
+limit, after which the scale beam is kept balanced and the load
+read for each 0.1 inch deflection. The load and deflection at
+first failure, the maximum load, and any points of sudden change
+should be shown on the diagram, even though they do not occur at
+one of the regular points. A brief description of the failure
+and the nature of any defects is entered on the log sheet.
+
+[Illustration: FIG. 32.--Sample log sheet, giving full details
+of a transverse bending test on a small pine beam.]
+
+_Calculating the results_: The formulae used in calculating the
+results of tests on small rectangular simple beams are as
+follows:
+
+              0.75 P
+     (1) J = --------
+               b h
+
+              1.5 P_{1} l
+     (2) r = -------------
+                b h^{2}
+
+              1.5 P l
+     (3) R = ---------
+              b h^{2}
+
+              P_{1} l^{3}
+     (4) E = -------------
+              4 D b h^{3}
+
+              P_{1} D
+     (5) S = ---------
+                2 V
+
+The same legend is used as in BENDING LARGE BEAMS. The weight of
+the beam itself is disregarded.
+
+
+
+ENDWISE COMPRESSION
+
+
+_Apparatus_: An ordinary static testing machine and a
+compressometer are required. (See Fig. 33.)
+
+[Illustration: FIG. 33.--Endwise compression test, showing
+method of measuring the deformation by means of a
+compressometer.]
+
+_Preparing the material_: Two classes of specimens are commonly
+used, namely, (1) posts 24 inches in length, and (2) small clear
+blocks approximately 2" X 2" X 8". The specimens are surfaced on
+all four sides and both ends squared smoothly and evenly. They
+are carefully weighed, measured, rate of growth and proportion
+of late wood determined, as in bending tests. After the test a
+moisture section is cut and weighed. Ordinarily these specimens
+should be free from defects.
+
+_Sketching_: Sketches are made of each end of the specimens to
+show the character of the growth. After testing, the manner of
+failure is shown for all four sides, and the various parts of
+the failure are numbered in the order of their occurrence.
+
+_Adjusting specimen in machine_: The compressometer collars are
+adjusted, the distance between them being 20 inches for the
+posts and 6 inches for the blocks. If the two ends of the blocks
+are not exactly parallel a ball-and-socket block can be placed
+between the upper end of the specimen and the movable head of
+the machine to overcome the irregularity. If the blocks are true
+they can simply be stood on end upon the platform and the
+movable head allowed to press directly upon the upper end.
+
+_Measuring the deformation_: The deformation is measured by a
+compressometer. (See Fig. 33.) The latter registers to 0.001
+inch. In the case of posts the compression between the collars
+is communicated to the four points on the arms by means of brass
+rods; with short blocks, as in Fig. 33, the points of the arms
+are in direct contact with the collars. The operator lowers the
+fulcrum of the apparatus by moving the micrometer screws at such
+a rate that the set-screw in the rear end of the upper lever is
+kept barely touching the fixed arm below it, being guided by a
+bell operated by electric contact.
+
+_Log of the test_: The load is applied continuously at a uniform
+rate of speed. (See SPEED OF TESTING MACHINE, above.) Readings
+are taken from the scale of the compressometer at regular
+increments of either load or compression. The stress-strain
+diagram is continued to at least one deformation point beyond
+the maximum load, and in event of sudden failure, the direction
+of the curve beyond the maximum point is indicated. A brief
+description of the failure is entered on the log sheet. (See
+Fig. 34.)
+
+[Illustration: FIG. 34.--Sample log sheet of an endwise
+compression test on a short pine column.]
+
+In short specimens the failure usually occurs in one or several
+planes diagonal to the axis of the specimen. If the ends are
+more moist than the middle a crushing may occur on the extreme
+ends in a horizontal plane. Such a test is not valid and should
+always be culled. If the grain is diagonal or the stress is
+unevenly applied a diagonal shear may occur from top to bottom
+of the test specimen. Such tests are also invalid and should be
+culled. When the plane (or several planes) of failure occurs
+through the body of the specimen the test is valid. It may
+sometimes be advantageous to allow the extreme ends to dry
+slightly before testing in order to bring the planes of failure
+within the body. This is a perfectly legitimate procedure
+provided no drying is allowed from the sides of the specimen,
+and the moisture disk is cut from the region of failure.
+
+_Calculating the results:_ The formulae used in calculating the
+results of tests on endwise compression are as follows:
+
+              P
+     (1) C = -----
+              A
+
+              P_{1}
+     (2) c = -------
+                A
+
+              P_{1} l
+     (3) E = ---------
+                A D
+
+              P D
+     (4) S = -----
+              2 V
+
+  C = crushing strength, pounds per square inch.
+  c = fibre strength at elastic limit, pounds per square inch.
+  A = area of cross section, square inches.
+  l = distance between centres of collars, inches.
+  D = total shortening at elastic limit, inches.
+  V = volume of specimen, cubic inches.
+
+Remainder of legend as in BENDING LARGE BEAMS, above.
+
+
+
+COMPRESSION ACROSS THE GRAIN
+
+
+_Apparatus_: An ordinary static testing machine, a bearing
+plate, and a deflectometer are required. (See Fig. 35.)
+
+[Illustration: FIG. 35.--Compression across the grain. Note
+method of measuring the deformation by means of a
+deflectomoter.]
+
+_Preparing the material_: Two classes of specimens are used,
+namely, (1) sections of commercial sizes of ties, beams, and
+other timbers, and (2) small, clear specimens with the length
+several times the width. Sometimes small cubes are tested, but
+the results are hardly applicable to conditions in practice. In
+(2) the sides are surfaced and the ends squared. The specimens
+are then carefully measured and weighed, defects noted, rate of
+growth and proportion of late wood determined, as in bending
+tests. (See BENDING LARGE BEAMS, above.) After the test a
+moisture section is cut and weighed.
+
+_Sketching_: Sketches are made as in endwise compression tests.
+(See ENDWISE COMPRESSION, above.)
+
+_Adjusting specimen in machine_: The specimen is laid
+horizontally upon the platform of the machine and a steel
+bearing plate placed on its upper surface immediately beneath
+the centre of the movable head. For the larger specimens this
+plate is six inches wide; for the smaller sizes, two inches
+wide. The plate in all cases projects over the edges of the test
+piece, and in no case should the length of the latter be less
+than four times the width of the plate.
+
+_Measuring the deformation_: The compression is measured by
+means of a deflectometer (see Fig. 35), which, after the first
+increment of load is applied, is adjusted (by means of a small
+set screw) to read zero. The actual downward motion of the
+movable head (corresponding to the compression of the specimen)
+is multiplied ten times on the scale from which the readings are
+made.
+
+_Log of the test_: The load is applied continuously and at
+uniform speed (see SPEED OF TESTING MACHINE, above), until well
+beyond the elastic limit. The compression readings are taken at
+regular load increments and entered on the cross-section paper
+in the usual way. Usually there is no real maximum load in this
+case, as the strength continually increases as the fibres are
+crushed more compactly together.
+
+_Calculating the results_: Ordinarily only the fibre stress at
+the elastic limit (c) is computed. It is equal to the load at
+elastic limit (P_{1}) divided by the area under the plate (B).
+{      P_{1}  }
+{ c = ------- }
+{        B    }
+
+
+
+SHEAR ALONG THE GRAIN
+
+
+_Apparatus_: An ordinary static testing machine and a special
+tool designed for producing single shear are required. (See
+Figs. 36 and 37.) This shearing apparatus consists of a solid
+steel frame with set screws for clamping the block within it
+firmly in a vertical position. In the centre of the frame is a
+vertical slot in which a square-edged steel plate slides freely.
+When the testing block is in position, this plate impinges
+squarely along the upper surface of the tenon or lip, which, as
+vertical pressure is applied, shears off.
+
+[Illustration: Fig. 36.--Vertical section of shearing tool.]
+
+[Illustration: FIG. 37.--Front view of shearing tool with test
+specimen and steel plate in position for testing.]
+
+_Preparing the material_: The specimens are usually in the form
+of small, clear, straight-grained blocks with a projecting tenon
+or lip to be sheared off. Two common forms and sizes are shown
+in Figure 38. Part of the blocks are cut so that the shearing
+surface is parallel to the growth rings, or tangential; others
+at right angles to the growth rings, or radial. It is important
+that the upper surface of the tenon or lip be sawed exactly
+parallel to the base of the block. When the form with a tenon is
+used the under cut is extended a short distance horizontally
+into the block to prevent any compression from below.
+
+[Illustration: FIG. 38.--Two forms of shear test specimens.]
+
+In designing a shearing specimen it is necessary to take into
+consideration the proportions of the area of shear, since, if
+the length of the portion to be sheared off is too great in the
+direction of the shearing face, failure would occur by
+compression before the piece would shear. Inasmuch as the
+endwise compressive strength is sometimes not more than five
+times the shearing strength, the shearing surface should be less
+than five times the surface to which the load is applied. This
+condition is fulfilled in the specimens illustrated.
+
+Shearing specimens are frequently cut from beams after testing.
+In this case the specific gravity (dry), proportion of late
+wood, and rate of growth are assumed to be the same as already
+recorded for the beams. In specimens not so taken, these
+quantities are determined in the usual way. The sheared-off
+portion is used for a moisture section.
+
+_Adjusting specimen in machine_: The test specimen is placed in
+the shearing apparatus with the tenon or lip under the sliding
+plate, which is centred under the movable head of the machine.
+(See Fig. 39.) In order to reduce to a minimum the friction due
+to the lateral pressure of the plate against the bearings of the
+slot, the apparatus is sometimes placed upon several parallel
+steel rods to form a roller base. A slight initial load is
+applied to take up the lost motion of the machinery, and the
+beam balanced.
+
+[Illustration: FIG. 39.--Making a shearing test.]
+
+_Log of the test_: The load is applied continuously and at a
+uniform rate until failure, but no deformations are measured.
+The points noted are the maximum load and the length of time
+required to reach it. Sketches are made of the failure. If the
+failure is not pure shear the test is culled.
+
+The shearing strength per square inch is found by dividing the
+                                          {      P  }
+maximum load by the cross-sectional area. { Q = --- }
+                                          {      A  }
+
+
+
+IMPACT TEST
+
+
+_Apparatus_: There are several types of impact testing
+machines.[59] One of the simplest and most efficient for use
+with wood is illustrated in Figure 40. The base of the machine
+is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500
+pounds. Two upright columns, each 8 feet long, act as guides for
+the striking head. At the top of the column is the hoisting
+mechanism for raising or lowering the striking weights. The
+power for operating the machine is furnished by a motor set on
+the top. The hoisting-mechanism is all controlled by a single
+operating lever, shown on the side of the column, whereby the
+striking weight may be raised, lowered, or stopped at the will
+of the operator. There is an automatic safety device for
+stopping the machine when the weight reaches the top.
+
+[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.]
+
+[Illustration: FIG. 40.--Impact testing machine.]
+
+The weight is lifted by a chain, one end of which passes over a
+sprocket wheel in the hoisting mechanism. On the lower end of
+the chain is hung an electro-magnet of sufficient magnetic
+strength to support the heaviest striking weights. When it is
+desired to drop the striking weight the electric current is
+broken and reversed by means of an automatic switch and current
+breaker. The height of drop may be regulated by setting at the
+desired height on one of the columns a tripping pin which throws
+the switch on the magnet and so breaks and reverses the current.
+
+There are four striking weights, weighing respectively 50, 100,
+250, and 500 pounds, any one of which may be used, depending
+upon the desired energy of blow. When used for compression tests
+a flat steel head six inches in diameter is screwed into the
+lower end of the weight. For transverse tests, a well-rounded
+knife edge is screwed into the weight in place of the flat head.
+Knife edges for supporting the ends of the specimen to be
+tested, are securely bolted to the base of the machine.
+
+The record of the behavior of the specimen at time of impact is
+traced upon a revolving drum by a pencil fixed in the striking
+head. (See Fig. 41.) When a drop is made the pencil comes in
+contact with the drum and is held in place by a spring. The drum
+is revolved very slowly, either automatically or by hand. The
+speed of the drum can be recorded by a pencil in the end of a
+tuning fork which gives a known number of vibrations per second.
+
+[Illustration: FIG. 41.--Drum record of impact bending test.]
+
+One size of this machine will handle specimens for transverse
+tests 9 inches wide and 6-foot span; the other, 12 inches wide
+and 8-foot span. For compression tests a free fall of about 6.5
+feet may be obtained. For transverse tests the fall is a little
+less, depending upon the size of the specimen.
+
+The machine is calibrated by dropping the hammer upon a copper
+cylinder. The axial compression of the plug is noted. The energy
+used in static tests to produce this axial compression under
+stress in a like piece of metal is determined. The external
+energy of the blow (_i.e._, the weight of the hammer X the
+height of drop) is compared with the energy used in static tests
+at equal amounts of compression. For instance:
+
+Energy delivered, impact test            35,000 inch-pounds
+Energy computed from static test        .26,400   "    "
+Efficiency of blow of hammer            .75.3 per cent.
+
+_Preparing the material_: The material used in making impact
+tests is of the same size and prepared in the same way as for
+static bending and compression tests. Bending in impact tests is
+more commonly used than compression, and small beams with
+28-inch span are usually employed.
+
+_Method_: In making an impact bending test the hammer is allowed
+to rest upon the specimen and a zero or datum line is drawn. The
+hammer is then dropped from increasing heights and drum records
+taken until first failure. The first drop is one inch and the
+increase is by increments of one inch until a height of ten
+inches is reached, after which increments of two inches are used
+until complete failure occurs or 6-inch deflection is secured.
+
+The 50-pound hammer is used when with drops up to 68 inches it
+is reasonably certain it will produce complete failure or 6-inch
+deflection in the case of all specimens of a species; for all
+other species a 100-pound hammer is used.
+
+_Results_: The tracing on the drum (see Fig. 41) represents the
+actual deflection of the stick and the subsequent rebounds for
+each drop. The distance from the lowest point in each case to
+the datum line is measured and its square in tenths of a square
+inch entered as an abscissa on cross-section paper, with the
+height of drop in inches as the ordinate. The elastic limit is
+that point on the diagram where the square of the deflection
+begins to increase more rapidly than the height of drop. The
+difference between the datum line and the final resting point
+after each drop represents the set the material has received.
+
+The formulae used in calculating the results of impact tests in
+bending when the load is applied at the centre up to the elastic
+limit are as follows:
+
+               3 W H l
+     (1) r = -----------
+              D b h^{2}
+
+              F S l^{2}
+     (2) E = -----------
+                6 D h
+
+               W H
+     (3) S = -------
+              l b h
+
+  H = height of drop of hammer, including deflection, inches.
+  S = modulus of elastic resilience, inch-pounds per cubic inch.
+  W = weight of hammer, pounds.
+
+Remainder of legend as in BENDING LARGE BEAMS, above.
+
+
+
+HARDNESS TEST: ABRASION AND INDENTATION
+
+
+_Abrasion_: The machine used by the U.S. Forest Service is a
+modified form of the Dorry abrasion machine. (See Fig. 42.) Upon
+the revolving horizontal disk is glued a commercial sandpaper,
+known as garnet paper, which is commonly employed in factories
+in finishing wood.
+
+[Illustration: FIG. 42.--Abrasion machine for testing the
+wearing qualities of woods.]
+
+A small block of the wood to be tested is fixed in one clamp and
+a similar block of some wood chosen as a standard, as sugar
+maple, at 10 per cent moisture, in the opposite, and held
+against the same zone of sandpaper by a weight of 26 pounds
+each. The size of the section under abrasion for each specimen
+is 2" X 2". The conditions for wear are the same for both
+specimens. The speed of rotation is 68 revolutions a minute.
+
+The test is continued until the standard specimen is worn a
+specified amount, which varies with the kind of wood under test.
+A comparison of the wear of the two blocks affords a fair idea
+of their relative resistance to abrasion.
+
+Another method makes use of a sand blast to abrade the woods and
+is the one employed in New South Wales.[60] The apparatus
+consists essentially of a nozzle through which sand can be
+propelled at a high velocity against the test specimen by means
+of a steam jet.
+
+[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.]
+
+The wood to be tested is cut into blocks 3" X 3" X 1', and these
+are weighed to the nearest grain just before placing in the
+apparatus. Steam from the boiler at a pressure of about 43
+pounds per square inch is ejected from a nozzle in such a way
+that particles of fine quartz sand are caught up and thrown
+violently against the block which is being rotated. Only
+superheated steam strikes the block, thus leaving the wood dry.
+The test is continued for two minutes, after which the specimen
+is removed and immediately weighed.
+
+By comparison with the original weight the loss from abrasion is
+determined, and by comparison with a certain wood chosen as a
+standard, a coefficient of wear-resistance can be obtained. The
+amount of wear will vary more or less according to the surface
+exposed, and in these tests quarter-sawed material was used with
+the edge grain to the blast.
+
+_Indentation_: The tool used for this test consists of a punch
+with a hemispherical end or steel ball having a diameter of
+0.444 inch, giving a surface area of one-fourth square inch. It
+is fitted with a guard plate, which works loosely until the
+penetration has progressed to a depth of 0.222 inch, whereupon
+it tightens. (See Fig. 43.) The effect is that of sinking a ball
+half its diameter into the specimen. This apparatus is fitted
+into the movable head of the static testing machine.
+
+[Illustration: FIG. 43.--Design of tool for testing the hardness
+of woods by indentation.]
+
+The wood to be tested is cut square with the grain into
+rectangular blocks measuring 2" X 2" X 6". A block is placed on
+the platform and the end of the punch forced into the wood at
+the rate of 0.25 inch per minute. The operator keeps moving the
+small handle of the guard plate back and forth until it
+tightens. At this instant the load is read and recorded.
+
+Two penetrations each are made on the tangential and radial
+surfaces, and one on each end of every specimen tested.
+
+In choosing the places on the block for the indentations, effort
+should be made to get a fair average of heartwood and sapwood,
+fine and coarse grain, early and late wood.
+
+Another method of testing by indentation involves the use of a
+right-angled cone instead of a ball. For details of this test as
+used in New South Wales see _loc. cit._, pp. 86-87.
+
+
+
+CLEAVAGE TEST
+
+
+A static testing machine and a special cleavage testing device
+are required. (See Fig. 44.) The latter consists essentially of
+two hooks, one of which is suspended from the centre of the top
+of the cage, the other extended above the movable head.
+
+[Illustration: FIG. 44.--Design of tool for cleavage test.]
+
+The specimens are 2" X 2" X 3.75". At one end a one-inch hole is
+bored, with its centre equidistant from the two sides and 0.25
+inch from the end. (See Fig. 45.) This makes the cross section
+to be tested 2" X 3". Some of the blocks are cut radially and
+some tangentially, as indicated in the figure.
+
+[Illustration: FIG. 45.--Design of cleavage test specimen.]
+
+The free ends of the hooks are fitted into the notch in the end
+of the specimen. The movable head of the machine is then made to
+descend at the rate of 0.25 inch per minute, pulling apart the
+hooks and splitting the block. The maximum load only is taken
+and the result expressed in pounds per square inch of width. A
+piece one-half inch thick is split off parallel to the failure
+and used for moisture determination.
+
+
+
+TENSION TEST PARALLEL TO THE GRAIN
+
+
+Since the tensile strength of wood parallel to the grain is
+greater than the compressive strength, and exceedingly greater
+than the shearing strength, it is very difficult to make
+satisfactory tension tests, as the head and shoulders of the
+test specimen (which is subjected to both compression and shear)
+must be stronger than the portion subjected to a pure tensile
+stress.
+
+Various designs of test specimens have been made. The one first
+employed by the Division of Forestry[61] was prepared as
+follows: Sticks were cut measuring 1.5" X 2.5" X 16". The
+thickness at the centre was then reduced to three-eighths of an
+inch by cutting out circular segments with a band saw. This left
+a breaking section of 2.5" X 0.375". Care was taken to cut the
+specimen as nearly parallel to the grain as possible, so that
+its failure would occur in a condition of pure tension. The
+specimen was then placed between the plane wedge-shaped steel
+grips of the cage and the movable head of the static machine and
+pulled in two. Only the maximum load was recorded. (See Fig. 46,
+No. 1.)
+
+[Illustration: FIG. 46.--Designs of tension test specimens used
+in United States.]
+
+[Footnote 61: Bul. No. 8: Timber physics, Part II., 1893, p. 7.]
+
+The difficulty of making such tests compared with the minor
+importance of the results is so great that they are at present
+omitted by the U.S. Forest Service. A form of specimen is
+suggested, however, and is as follows: "A rod of wood about one
+inch in diameter is bored by a hollow drill from the stick to be
+tested. The ends of this rod are inserted and glued in
+corresponding holes in permanent hardwood wedges. The specimen
+is then submitted to the ordinary tension test. The broken ends
+are punched from the wedges."[62] (See Fig. 46, No. 2.)
+
+[Footnote 62: Cir. 38: Instructions to engineers of timber
+tests, 1906, p. 24.]
+
+The form used by the Department of Forestry of New South
+Wales[63] is as shown in Fig. 47. The specimen has a total
+length of 41 inches and is circular in cross section. On each
+end is a head 4 inches in diameter and 7 inches long. Below each
+head is a shoulder 8.5 inches long, which tapers from a diameter
+of 2.75 inches to 1.25 inches. In the middle is a cylindrical
+portion 1.25 inches in diameter and 10 inches long.
+
+[Illustration: FIG. 47.--Design of tension test specimen used in
+New South Wales.]
+
+[Footnote 63: Warren, W.H.: The strength, elasticity, and other
+properties of New South Wales hardwood timbers, 1911, pp.
+58-62.]
+
+In making the test the specimen is fitted in the machine, and an
+extensometer attached to the middle portion and arranged to
+record the extension between the gauge points 8 inches apart.
+The area of the cross section then is 1.226 square inches, and
+the tensile strength is equal to the total breaking load applied
+divided by this area.
+
+
+
+TENSION TEST AT RIGHT ANGLES TO THE GRAIN
+
+
+A static testing machine and a special testing device (see Fig.
+48) are required. The latter consists essentially of two double
+hooks or clamps, one of which is suspended from the centre of
+the top of the cage, the other extended above the movable head.
+The specimens are 2" X 2" X 2.5". At each end a one-inch hole is
+bored with its centre equidistant from the two sides and 0.25
+inch from the ends. This makes the cross section to be tested 1"
+X 2".
+
+[Illustration: FIG. 48.--Design of tool and specimen for testing
+tension at right angles to the grain.]
+
+The free ends of the clamps are fitted into the notches in the
+ends of the specimen. The movable head of the machine is then
+made to descend at the rate of 0.25 inch per minute, pulling the
+specimen in two at right angles to the grain. The maximum load
+only is taken and the result expressed in pounds per inch of
+width. A piece one-half inch thick is split off parallel to the
+failure and used for moisture determination.
+
+
+
+TORSION TEST[64]
+
+
+[Footnote 64: Wood is so seldom subjected to a pure stress of
+this kind that the torsion test is usually omitted.]
+
+_Apparatus_: The torsion test is made in a Riehle-Miller
+torsional testing machine or its equivalent. (See Fig. 49.)
+
+[Illustration: FIG. 49.--Making a torsion test on hickory.]
+
+_Preparation of material_: The test pieces are cylindrical, 1.5
+inches in diameter and 18 inches gauge length, with squared ends
+4 inches long joined to the cylindrical portion with a fillet.
+The dimensions are carefully measured, and the usual data
+obtained in regard to the rate of growth, proportion of late
+wood, location and kind of defects. The weight of the
+cylindrical portion of the specimen is obtained after the test.
+
+_Making the test_: After the specimen is fitted in the machine
+the load is applied continuously at the rate of 22 deg. per minute.
+A troptometer is used in measuring the deformation. Readings are
+made until failure occurs, the points being entered on the
+cross-section paper. The character of the failure is described.
+Moisture determinations are made by the disk method.
+
+_Results_: The conditions of ultimate rupture due to torsion
+appear not to be governed by definite mathematical laws; but
+where the material is not overstrained, laws may be assumed
+which are sufficiently exact for practical cases. The formulae
+commonly used for computations are as follows:
+
+              5.1 M
+     (1) T = -------
+              c^{3}
+
+              114.6 T f
+     (2) G = -----------
+                 a c
+
+  a = angle measured by troptometer at elastic limit, in
+        degrees.
+  c = diameter of specimen, inches.
+  f = gauge length of specimen, inches. _G_ = modulus of
+        elasticity in shear across the grain, pounds per square
+        inch.
+  M = moment of torsion at elastic limit, inch-pounds.
+  T = outer fibre torsional stress at elastic limit, pounds per
+        square inch.
+
+
+
+SPECIAL TESTS
+
+
+_Spike-pulling Test_
+
+Spike-pulling tests apply to problems of railroad maintenance,
+and the results are used to compare the spike-holding powers of
+various woods, both untreated and treated with different
+preservatives, and the efficiency of various forms of spikes.
+Special tests are also made in which the spike is subjected to a
+transverse load applied repetitively by a blow.
+
+For details of tests and results see:
+
+Cir. 38,  U.S.F.S.: Instructions to engineers of timber tests,
+p. 26. Cir. 46,  U.S.F.S.: Holding force of railroad spikes in
+wooden ties. Bul. 118, U.S.F.S,: Prolonging the life of
+cross-ties, pp. 37-40.
+
+
+_Packing Boxes_
+
+Special tests on the strength of packing boxes of various woods
+have been made by the U.S. Forest Service to determine the
+merits of different kinds of woods as box material with the view
+of substituting new kinds for the more expensive ones now in
+use. The methods of tests consisted in applying a load along the
+diagonal of a box, an action similar to that which occurs when a
+box is dropped on one of its corners. The load was measured at
+each one-fourth inch in deflection, and notes were made of the
+primary and subsequent failures.
+
+For details of tests and results, see:
+
+Cir. 47,  U.S.F.S.: Strength of packing boxes of various woods.
+Cir. 214, U.S.F.S.: Tests of packing boxes of various forms.
+
+
+_Vehicle and Implement Woods_
+
+Tests were made by the U.S. Forest Service to obtain a better
+knowledge of the mechanical properties of the woods at present
+used in the manufacture of vehicles and implements and of those
+which might be substituted for them. Tests were made upon the
+following materials: hickory buggy spokes (see Fig. 5); hickory
+and red oak buggy shafts; wagon tongues; Douglas fir and
+southern pine cultivator poles.
+
+Details of the tests and results may be found in:
+
+Cir. 142, U.S.F.S.: Tests on vehicle and implement woods.
+
+
+_Cross-arms_
+
+In tests by the U.S. Forest Service on cross-arms a special
+apparatus was devised in which the load was distributed along
+the arm as in actual practice. The load was applied by rods
+passing through the pinholes in the arms. Nuts on these rods
+pulled down on the wooden bearing-blocks shaped to fit the upper
+side of the arm. The lower ends of these rods were attached to a
+system of equalizing levers, so arranged that the load at each
+pinhole would be the same. In all the tests the load was applied
+vertically by means of the static machine.
+
+See Cir. 204, U.S.F.S.: Strength tests of cross-arms.
+
+
+_Other Tests_
+
+Many other kinds of tests are made as occasion demands. One kind
+consists of barrels and liquid containers, match-boxes, and
+explosive containers. These articles are subjected to shocks
+such as they would receive in transit and in handling, and also
+to hydraulic pressure.
+
+One of the most important tests from a practical standpoint is
+that of built-up structures such as compounded beams composed of
+small pieces bolted together, mortised joints, wooden trusses,
+etc. Tests of this kind can best be worked out according to the
+specific requirements in each case.
+
+
+
+
+APPENDIX
+
+
+
+SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE
+
+MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES
+
+Working Plan No. 124
+
+
+PURPOSE OF WORK
+
+It is the general purpose of the work here outlined to provide:
+
+(_a_) Reliable data for comparing the mechanical properties of
+various species;
+
+(_b_) Data for the establishment of correct strength functions
+or working stresses;
+
+(_c_) Data upon which may be based analyses of the influence on
+the mechanical properties of such factors as:
+
+Locality;
+
+Distance of timber from the pith of the tree;
+
+Height of timber in the tree;
+
+Change from the green to the air-dried condition, etc.
+
+The mechanical properties which will be considered and the
+principal tests used to determine them are as follows:
+
+Strength and stiffness--
+  Static bending;
+  Compression parallel to grain;
+  Compression perpendicular to grain;
+  Shear.
+
+Toughness--
+  Impact bending;
+  Static bending;
+  Work to maximum load and total work.
+
+Cleavability--
+  Cleavage test.
+
+Hardness--
+  Modification of Janka ball test for surface hardness.
+
+
+MATERIAL
+
+
+_Selection and Number of Trees_
+
+The material will be from trees selected in the forest by one
+qualified to determine the species. From each locality, three to
+five dominant trees of merchantable size and approximately
+average age will be so chosen as to be representative of the
+dominant trees of the species. Each species will eventually be
+represented by trees from five to ten localities. These
+localities will be so chosen as to be representative of the
+commercial range of the species. Trees from one to three
+localities will be used to represent each species until most of
+the important species have been tested.
+
+The 16-foot butt log will be taken from each tree selected and
+the entire merchantable hole of one average tree for each
+species.
+
+
+_Field Notes and Shipping Instructions_
+
+Field notes as outlined in Form--_a_ Shipment Description,
+Manual of the Branch of Products, will be fully and carefully
+made by the collector. The age of each tree selected will be
+recorded and any other information likely to be of interest or
+importance will also be made a part of these field notes. Each
+log will have the bark left on. It will be plainly marked in
+accordance with directions given under Detailed Instructions.
+All material will be shipped to the laboratory immediately after
+being cut. No trees will be cut until the collector is notified
+that the laboratory is ready to receive the material.
+
+
+DETAILED INSTRUCTIONS
+
+
+_Part of Tree to be Tested_
+
+(_a_) For determining the value of tree and locality and the
+influence on the mechanical properties of distance from the
+pith, a 4-foot bolt will be cut from the top end of each 16-foot
+butt log.
+
+(_b_) For investigating the variation of properties with the
+height of timber in the tree, all the logs from one average tree
+will be used.
+
+(_c_) For investigating the effect of drying the wood, the bolt
+next below that provided for in (_a_) will be used in the case
+of one tree from each locality.
+
+
+_Marking and Grouping of Material_
+
+The marking will be standard except as noted. Each log will be
+considered a "piece." The piece numbers will be plainly marked
+upon the butt end of each log by the collector. The north side
+of each log will also be marked.
+
+When only one bolt from a tree is used it will be designated by
+the number of the log from which it is cut. Whenever more than
+one bolt is taken from a tree, each 4-foot bolt or length of
+trunk will be given a letter (mark), _a, b, c,_ etc., beginning
+at the stump.
+
+All bolts will be sawed into 2-1/2" X 2-1/2" sticks and the
+sticks marked according to the sketch, Fig. 50. The letters _N,
+E, S,_ and _W_ indicate the cardinal points when known; when
+these are unknown, _H, K, L,_ and _M_ will be used. Thus, _N5,
+K8, S7, M4_ are stick numbers, the letter being a part of the
+stick number.
+
+[Illustration: FIG. 50.--Method of cutting and marking test
+specimens.]
+
+Only straight-grained specimens, free from defects which will
+affect their strength, will be tested.
+
+
+_Care of Material_
+
+No material will be kept in the bolt or log long enough to be
+damaged or disfigured by checks, rot, or stains.
+
+_Green material_: The material to be tested green will be kept
+in a green state by being submerged in water until near the time
+of test. It will then be surfaced, sawed to length, and stored
+in damp sawdust at a temperature of 70 deg.F. (as nearly as
+practicable) until time of test. Care should be taken to avoid
+as much as possible the storage of green material in any form.
+
+_Air-dry material_: The material to be air-dried will be cut
+into sticks 2-1/2" X 2-1/2" X 4'. The ends of these sticks will
+be paraffined to prevent checking. This material will be so
+piled as to leave an air space of at least one-half inch on each
+side of each stick, and in such a place that it will be
+protected from sunshine, rain, snow, and moisture from the
+ground. The sticks will be surfaced and cut to length just
+previous to test.
+
+
+_Order of Tests_
+
+The order of tests in all cases will be such as to eliminate so
+far as possible from the comparisons the effect of changes of
+condition of the specimens due to such factors as storage and
+weather conditions.
+
+The material used for determining the effect of height in tree
+will be tested in such order that the average time elapsing from
+time of cutting to time of test will be approximately the same
+for all bolts from any one tree.
+
+
+_Tests on Green Material_
+
+The tests on all bolts, except those from which a comparison of
+green and dry timber is to be gotten, will be as follows:
+
+_Static bending_: One stick from each pair. A pair consists of
+two adjacent sticks equidistant from the pith, as _N_7 and _N_8,
+or _H_5 and _H_6.
+
+_Impact bending_: Four sticks; one to be taken from near the
+pith; one from near the periphery; and two representative of the
+cross section.
+
+_Compression parallel to grain_: One specimen from each stick.
+These will be marked "1" in addition to the number of the stick
+from which they are taken.
+
+_Compression perpendicular to grain_: One specimen from each of
+50 per cent of the static bending sticks. These will be marked
+"2" in addition to the number of the stick from which they are
+cut.
+
+_Hardness_: One specimen from each of the other 50 per cent of
+the static bending sticks. These specimens will be marked "4."
+
+_Shear_: Six specimens from sticks not tested in bending or from
+the ends cut off in preparing the bending specimens. Two
+specimens will be taken from near the pith; two from near the
+periphery; and two that are representative of the average
+growth. One of each two will be tested in radial shear and the
+other in tangential shear. These specimens will have the mark
+"3."
+
+_Cleavage_: Six specimens chosen and divided just as those for
+shearing. These specimens will have the mark "5." (For sketches
+showing radial and tangential cleavage, see Fig. 45.)
+
+When it is impossible to secure clear specimens for all of the
+above tests, tests will have precedence in the order in which
+they are named.
+
+
+_Tests to Determine the Effect of Air-drying_
+
+These tests will be made on material from the adjacent bolts
+mentioned in "_c_" under Part of Tree to be Tested. Both bolts
+will be cut as outlined above. One-half the sticks from each
+bolt will be tested green, the other half will be air-dried and
+tested. The division of green and air-dry will be according to
+the following scheme:
+
+                     STICK  NUMBERS
+
+Lower bolt,  1,       4, 5,       8, 9,            } Tested
+                                            etc.   } green
+Upper bolt,     2, 3,       6, 7,       10,        }
+
+Lower bolt,     2, 3,       6, 7,       10,        } Air-dried
+                                            etc.   } and
+Upper bolt,  1,       4, 5,       8, 9,            } tested
+
+All green sticks from these two bolts will be tested as if they
+were from the same bolt and according to the plan previously
+outlined for green material from single bolts. The tests on the
+air-dried material will be the same as on the green except for
+the difference of seasoning.
+
+The material will be tested at as near 12 per cent moisture as
+is practicable. The approximate weight of the air-dried
+specimens at 12 per cent moisture will be determined by
+measuring while green 20 per cent of the sticks to be air-dried
+and assuming their dry gravity to be the same as that of the
+specimens tested green. This 20 per cent will be weighed as
+often as is necessary to determine the proper time of test.
+
+
+_Methods of Test_
+
+All tests will be made according to Circular 38 except in case
+of conflict with the instructions given below:
+
+_Static bending_: The tests will be on specimens 2" X 2" X 30"
+on 28-inch span. Load will be applied at the centre.
+
+In all tests the load-deflection curve will be carried to or
+beyond the maximum load. In one-third of the tests the
+load-deflection curve will be continued to 6-inch deflection, or
+till the specimen fails to support a 200-pound load. Deflection
+readings for equal increments of load will be taken until well
+past the elastic limit, after which the scale beam will be kept
+balanced and the load read for each 0.1-inch deflection. The
+load and deflection at first failure, maximum load and points of
+sudden change, will be shown on the curve sheet even if they do
+not occur at one of the regular load or deflection increments.
+
+_Impact bending_: The impact bending tests will be on specimens
+of the same size as those used in static bending. The span will
+be 28 inches.
+
+The tests will be by increment drop. The first drop will be 1
+inch and the increase will be by increments of 1 inch till a
+height of 10 inches is reached, after which increments of 2
+inches will be used until complete failure occurs or 6-inch
+deflection is secured.
+
+A 50-pound hammer will be used when with drops up to 68 inches
+it is practically certain that it will produce complete failure
+or 6-inch deflection in the case of all specimens of a species.
+For all other species, a 100-pound hammer will be used.
+
+In all cases drum records will be made until first failure. Also
+the height of drop causing complete failure or 6-inch deflection
+will be noted.
+
+_Compression parallel to grain_: This test will be on specimens
+2" X 2" X 8" in size. On 20 per cent of these tests
+load-compression curves for a 6-inch centrally located gauge
+length will be taken. Readings will be continued until the
+elastic limit is well passed. The other 80 per cent of the tests
+will be made for the purpose of obtaining the maximum load only.
+
+_Compression perpendicular to grain_: This test will be on
+specimens 2" X 2" X 6" in size. The bearing plates will be 2
+inches wide. The rate of descent of the moving head will be
+0.024 inch per minute. The load-compression curve will be
+plotted to 0.1 inch compression and the test will then be
+discontinued.
+
+_Hardness_: The tool shown in Fig. 43 (an adaptation of the
+apparatus used by the German investigator, Janka) will be used.
+The rate of descent of the moving head will be 0.25 inch per
+minute. When the penetration has progressed to the point at
+which the plate "_a_" becomes tight, due to being pressed
+against the wood, the load will be read and recorded.
+
+Two penetrations will be made on a tangential surface, two on a
+radial, and one on each end of each specimen tested. The choice
+between the two radial and between the two tangential surfaces
+and the distribution of the penetrations over the surfaces will
+be so made as to get a fair average of heart and sap, slow and
+fast growth, and spring and summer wood. Specimens will be 2" X
+2" X 6".
+
+_Shear_: The tests will be made with a tool slightly modified
+from that shown in Circular 38. The speed of descent of head
+will be 0.015 inch per minute. The only measurements to be made
+are those of the shearing area. The offset will be 1/8 inch.
+Specimens will be 2" X 2" X 2-1/2" in size. (For definition of
+offset and form of test specimen, see Fig. 38.)
+
+_Cleavage_: The cleavage tests will be made on specimens of the
+form and size shown in Fig. 45. The apparatus will be as shown
+in Fig. 44. The maximum load only will be taken and the result
+expressed in pounds per inch of width. The speed of the moving
+head will be 0.25 inch per minute.
+
+
+_Moisture Determinations_
+
+Moisture determinations will be made on all specimens tested
+except those to be photographed or kept for exhibit. A 1-inch
+disk will be cut from near the point of failure of bending and
+compression parallel specimens, from the portion under the plate
+in the case of the compression perpendicular specimens, and from
+the centre of the hardness test specimens. The beads from the
+shear specimens will be used as moisture disks. In the case of
+the cleavage specimens a piece 1/2 inch thick will be split off
+parallel to the failure and used as a moisture disk.
+
+
+RECORDS
+
+
+All records will be standard.
+
+
+PHOTOGRAPHS
+
+
+_Cross Sections_
+
+Just before cutting into sticks, the freshly cut end of at least
+one bolt from each tree will be photographed. A scale of inches
+will be shown in this photograph.
+
+
+_Specimens_
+
+Three photographs will be made of a group consisting of four 2"
+X 2" X 30" specimens chosen from the material from each
+locality. Two of these specimens will be representative of
+average growth, one of fast and one of slow growth. These
+photographs will show radial, tangential, and end surfaces for
+each specimen.
+
+
+_Failures_
+
+Typical and abnormal failures of material from each site will be
+photographed.
+
+
+_Disposition of Material_
+
+The specimens photographed to show typical and abnormal failures
+will be saved for purposes of exhibit until deemed by the person
+in charge of the laboratory to be of no further value.
+
+
+
+SHRINKAGE AND SPECIFIC GRAVITY
+
+Appendix to Working Plan 124
+
+
+PURPOSE OF WORK
+
+
+It is the purpose of this work to secure data on the shrinkage
+and specific gravity of woods tested under Project 124. The
+figures to be obtained are for use as average working values
+rather than as the basis for a detailed study of the principles
+involved.
+
+
+MATERIAL
+
+
+The material will be taken from that provided for mechanical
+tests.
+
+
+RADIAL AND TANGENTIAL SHRINKAGE
+
+
+_Specimens_
+
+_Preparation_: Two specimens 1 inch thick, 4 inches wide, and 1
+inch long will be obtained from near the periphery of each "_d_"
+bolt. These will be cut from the sector-shaped sections left
+after securing the material for the mechanical tests or from
+disks cut from near the end of the bolt. They will be taken from
+adjoining pieces chosen so that the results will be comparable
+for use in determining radial and tangential shrinkage. (When a
+disk is used, care must be taken that it is green and has not
+been affected by the shrinkage and checking near the end of the
+bolt.)
+
+One of these specimens will be cut with its width in the radial
+direction and will be used for the determination of radial
+shrinkage. The other will have its width in the tangential
+direction and will be used for tangential shrinkage. These
+specimens will not be surfaced.
+
+_Marking_: The shrinkage specimens will retain the shipment and
+piece numbers and marks of the bolts from which they are taken,
+and will have the additional mark _7_R or _7_T according as
+their widths are in the radial or tangential direction.
+
+
+_Shrinkage measurements_: The shrinkage specimens will be
+carefully weighed and measured soon after cutting. Rings per
+inch, per cent sap, and per cent summer wood will be measured.
+They will then be air-dried in the laboratory to constant
+weight, and afterward oven-dried at 100 deg.C. (212 deg.F.), when they
+will again be weighed and measured.
+
+
+VOLUMETRIC SHRINKAGE AND SPECIFIC GRAVITY
+
+
+_Specimens_
+
+_Selection and preparation_: Four 2" X 2" X 6" specimens will be
+cut from the mechanical test sticks of each "_d_" bolt; also
+from each of the composite bolts used in getting a comparison of
+green and air-dry. One of these specimens will be taken from
+near the pith and one from near the periphery; the other two
+will be representative of the average growth of the bolt. The
+sides of these specimens will be surfaced and the ends smooth
+sawn.
+
+_Marking_: Each specimen will retain the shipment, piece, and
+stick numbers and mark of the stick from which it is cut, and
+will have the additional mark "_S_."
+
+_Manipulation_: Soon after cutting, each specimen will be
+weighed and its volume will be determined by the method
+described below. The rings per inch and per cent summer wood,
+where possible, will be determined, and a carbon impression of
+the end of the specimen made. It will then be air-dried in the
+laboratory to a constant weight and afterward oven-dried at
+100 deg.C. When dry, the specimen will be taken from the oven,
+weighed, and a carbon impression of its end made. While still
+warm the specimen will be dipped in hot paraffine. The volume
+will then be determined by the following method:
+
+On one pan of a pair of balances is placed a container having in
+it water enough for the complete submersion of the test
+specimen. This container and water is balanced by weights placed
+on the other scale pan. The specimen is then held completely
+submerged and not touching the container while the scales are
+again balanced. The weight required to balance is the weight of
+water displaced by the specimen, and hence if in grams is
+numerically equal to the volume of the specimen in cubic
+centimetres. A diagrammatic sketch of the arrangement of this
+apparatus is shown in Fig. 51.
+
+[Illustration: FIG. 51.--Diagram of specific gravity apparatus,
+showing a balance with container (_c_) filled with water in
+which the test block (_b_) is held submerged by a light rod
+(_a_) which is adjustable vertically and provided with a sharp
+point to be driven into the specimen.]
+
+Air-dry specimens will be dipped in water and then wiped dry
+after the first weighing and just before being immersed for
+weighing their displacement. All displacement determinations
+will be made as quickly as possible in order to minimize the
+absorption of water by the specimen.
+
+
+
+STRENGTH VALUES FOR STRUCTURAL TIMBERS
+
+(From Cir. 189, U.S. Forest Service)
+
+
+The following tables bring together in condensed form the
+average strength values resulting from a large number of tests
+made by the Forest Service on the principal structural timbers
+of the United States. These results are more completely
+discussed in other publications of the Service, a list of which
+is given in BIBLIOGRAPHY, PART III.
+
+The tests were made at the laboratories of the U.S. Forest
+Service, in cooperation with the following institutions: Yale
+Forest School, Purdue University, University of California,
+University of Oregon, University of Washington, University of
+Colorado, and University of Wisconsin.
+
+Tables XVIII and XIX give the average results obtained from
+tests on green material, while Tables XX and XXI give average
+results from tests on air-seasoned material. The small
+specimens, which were invariably 2" X 2" in cross section, were
+free from defects such as knots, checks, and cross grain; all
+other specimens were representative of material secured in the
+open market. The relation of stresses developed in different
+structural forms to those developed in the small clear specimens
+is shown for each factor in the column headed "Ratio to 2" X
+2"." Tests to determine the mechanical properties of different
+species are often confined to small, clear specimens. The ratios
+included in the tables may be applied to such results in order
+to approximate the strength of the species in structural sizes,
+and containing the defects usually encountered, when tests on
+such forms are not available.
+
+A comparison of the results of tests on seasoned material with
+those from tests on green material shows that, without
+exception, the strength of the 2" X 2" specimens is increased by
+lowering the moisture content, but that increase in strength of
+other sizes is much more erratic. Some specimens, in fact, show
+an apparent loss in strength due to seasoning. If structural
+timbers are seasoned slowly, in order to avoid excessive
+checking, there should be an increase in their strength. In the
+light of these facts it is not safe to base working stresses on
+results secured from any but green material. For a discussion of
+factors of safety and safe working stresses for structural
+timbers see the Manual of the American Railway Engineering
+Association, Chicago, 1911. A table from that publication,
+giving working unit stresses for structural timber, is
+reproduced in this book, see Table XXII.
+
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XVIII                                                TABLE XVIII                                                            |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                                                  BENDING TESTS ON GREEN MATERIAL                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                 |      Sizes      |       |       |       |   F.S. at E.L.  |     M. of R.    |     M. of E.    |   Calculated    |
+|                 |-----------------| Num-  | Per   | Rings |                 |                 |                 |      shear      |
+|     Species     |          |      |  ber  | cent  |  per  |-----------------+-----------------+-----------------+-----------------|
+|                 |  Cross   | Span | of    | mois- | inch  | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio |
+|                 | Section  |      | tests |  ture |       | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" |
+|                 |          |      |       |       |       |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |
+|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------|
+|                 |          |      |       |       |       |         |       |         |       |  1,000  |       |         |       |
+|                 |  Inches  | Ins. |       |       |       |   Lbs.  |       |   Lbs.  |       |   lbs.  |       |   Lbs.  |       |
+|                 |          |      |       |       |       |         |       |         |       |         |       |         |       |
+| Longleaf pine   | 12 by 12 |  138 |    4  |  28.6 |   9.7 |  4,029  |  0.83 |  6,710  |  0.74 |  1,523  |  0.99 |    261  |  0.86 |
+|                 | 10 by 16 |  168 |    4  |  26.8 |  16.7 |  6,453  |   .85 |  6,453  |   .71 |  1,626  |  1.05 |    306  |  1.01 |
+|                 |  8 by 16 |  156 |    7  |  28.4 |  14.6 |  3,147  |   .64 |  5,439  |   .60 |  1,368  |   .89 |    390  |  1.29 |
+|                 |  6 by 16 |  132 |    1  |  40.3 |  21.8 |  4,120  |   .83 |  6,460  |   .71 |  1,190  |   .77 |    378  |  1.25 |
+|                 |  6 by 10 |  180 |    1  |  31.0 |   6.2 |  3,580  |   .72 |  6,500  |   .72 |  1,412  |   .92 |    175  |   .58 |
+|                 |  6 by  8 |  180 |    2  |  27.0 |   8.2 |  3,735  |   .75 |  5,745  |   .63 |  1,282  |   .83 |    121  |   .40 |
+|                 |  2 by  2 |   30 |   15  |  33.9 |  14.1 |  4,950  |  1.00 |  9,070  |  1.00 |  1,540  |  1:00 |    303  |  1.00 |
+| Douglas fir     |  8 by 16 |  180 |  191  |  31.5 |  11.0 |  3,968  |   .76 |  5,983  |   .72 |  1,517  |   .95 |    269  |   .81 |
+|                 |  5 by  8 |  180 |   84  |  30.1 |  10.8 |  3,693  |   .71 |  5,178  |   .63 |  1,533  |   .96 |    172  |   .52 |
+|                 |  2 by 12 |  180 |   27  |  35.7 |  20.3 |  3,721  |   .71 |  5,276  |   .64 |  1,642  |  1.03 |    256  |   .77 |
+|                 |  2 by 10 |  180 |   26  |  32.9 |  21.6 |  3,160  |   .60 |  4,699  |   .57 |  1,593  |  1.00 |    189  |   .57 |
+|                 |  2 by  8 |  180 |   29  |  33.6 |  17.6 |  3,593  |   .69 |  5,352  |   .65 |  1,607  |  1.01 |    171  |   .51 |
+|                 |  2 by  2 |   24 |  568  |  30.4 |  11.6 |  5,227  |  1.00 |  9,070  |  1.00 |  1,540  |  1.00 |    303  |  1.00 |
+| Douglas fir     |          |      |       |       |       |         |       |         |       |         |       |         |       |
+|   (fire-killed) |  8 by 16 |  180 |   30  |  36.8 |  10.9 |  3,503  |   .80 |  4,994  |   .64 |  1,531  |   .94 |    330  |  1.19 |
+|                 |  2 by 12 |  180 |   32  |  34.2 |  17.7 |  3,489  |   .80 |  5,085  |   .66 |  1,624  |   .99 |    247  |   .89 |
+|                 |  2 by 10 |  180 |   32  |  38.9 |  18.1 |  3,851  |   .88 |  5,359  |   .69 |  1,716  |  1.05 |    216  |   .78 |
+|                 |  2 by  8 |  180 |   31  |  37.0 |  15.7 |  3,403  |   .78 |  5,305  |   .68 |  1,676  |  1.02 |    169  |   .61 |
+|                 |  2 by  2 |   30 |  290  |  33.2 |  17.2 |  4,360  |  1.00 |  7,752  |  1.00 |  1,636  |  1.00 |    277  |  1.00 |
+| Shortleaf pine  |  8 by 16 |  180 |   12  |  39.5 |  12.1 |  3,185  |   .73 |  5,407  |   .70 |  1,438  |  1.03 |    362  |  1.40 |
+|                 |  8 by 14 |  180 |   12  |  45.8 |  12.7 |  3,234  |   .74 |  5,781  |   .75 |  1,494  |  1.07 |    338  |  1.31 |
+|                 |  8 by 12 |  180 |   24  |  52.2 |  11.8 |  3,265  |   .75 |  5,503  |   .71 |  1,480  |  1.06 |    277  |  1.07 |
+|                 |  5 by  8 |  180 |   24  |  47.8 |  11.5 |  3,519  |   .81 |  5,732  |   .74 |  1,485  |  1.06 |    185  |   .72 |
+|                 |  2 by  2 |   30 |  254  |  51.7 |  13.6 |  4,350  |  1.00 |  7,710  |  1.00 |  1,395  |  1.00 |    258  |  1.00 |
+| Western larch   |  8 by 16 |  180 |   32  |  51.0 |  25.3 |  3,276  |   .77 |  4,632  |   .64 |  1,272  |   .97 |    298  |  1.11 |
+|                 |  8 by 12 |  180 |   30  |  50.3 |  23.2 |  3,376  |   .79 |  5,286  |   .73 |  1,331  |  1.02 |    254  |   .94 |
+|                 |  5 by  8 |  180 |   14  |  56.0 |  25.6 |  3,528  |   .83 |  5,331  |   .74 |  1,432  |  1.09 |    169  |   .63 |
+|                 |  2 by  2 |   28 |  189  |  46.2 |  26.2 |  4,274  |  1.00 |  7,251  |  1.00 |  1,310  |  1.00 |    269  |  1.00 |
+| Loblolly pine   |  8 by 16 |  180 |   17  |  15.8 |   6.1 |  3,094  |   .75 |  5,394  |   .69 |  1,406  |   .98 |    383  |  1.44 |
+|                 |  5 by 12 |  180 |   94  |  60.9 |   5.9 |  3,030  |   .74 |  5,028  |   .64 |  1,383  |   .96 |    221  |   .83 |
+|                 |  2 by  2 |   30 |   44  |  70.9 |   5.4 |  4,100  |  1.00 |  7,870  |  1.00 |  1,440  |  1.00 |    265  |  1.00 |
+| Tamarack        |  6 by 12 |  162 |   15  |  57.6 |  16.6 |  2,914  |   .75 |  4,500  |   .66 |  1,202  |  1.05 |    255  |  1.11 |
+|                 |  4 by 10 |  162 |   15  |  43.5 |  11.4 |  2,712  |   .70 |  4,611  |   .68 |  1,238  |  1.08 |    209  |   .91 |
+|                 |  2 by  2 |   30 |   82  |  38.8 |  14.0 |  3,875  |  1.00 |  6,820  |  1.00 |  1,141  |  1.00 |    229  |  1.00 |
+| Western hemlock |  8 by 16 |  180 |   39  |  42.5 |  15.6 |  3,516  |   .80 |  5,296  |   .73 |  1,445  |  1.01 |    261  |   .92 |
+|                 |  2 by  2 |   28 |   52  |  51.8 |  12.1 |  4.406  |  1.00 |  7,294  |  1.00 |  1,428  |  1.00 |    284  |  1.00 |
+| Redwood         |  8 by 16 |  180 |   14  |  86.5 |  19.9 |  3,734  |   .79 |  4,492  |   .64 |  1,016  |   .96 |    300  |  1.21 |
+|                 |  6 by 12 |  180 |   14  |  87.3 |  17.8 |  3,787  |   .80 |  4,451  |   .64 |  1,068  |  1.00 |    224  |   .90 |
+|                 |  7 by  9 |  180 |   14  |  79.8 |  16.7 |  4,412  |   .93 |  5,279  |   .76 |  1,324  |  1.25 |    199  |   .80 |
+|                 |  3 by 14 |  180 |   13  |  86.1 |  23.7 |  3,506  |   .74 |  4,364  |   .62 |    947  |   .89 |    255  |  1.03 |
+|                 |  2 by 12 |  180 |   12  |  70.9 |  18.6 |  3,100  |   .65 |  3,753  |   .54 |  1,052  |   .99 |    187  |   .75 |
+|                 |  2 by 10 |  180 |   13  |  55.8 |  20.0 |  3,285  |   .69 |  4,079  |   .58 |  1,107  |  1.04 |    169  |   .68 |
+|                 |  2 by  8 |  180 |   13  |  63.8 |  21.5 |  2,989  |   .63 |  4,063  |   .58 |  1,141  |  1.08 |    134  |   .54 |
+|                 |  2 by  2 |   28 |  157  |  75.5 |  19.1 |  4,750  |  1.00 |  6,980  |  1.00 |  1,061  |  1.00 |    248  |  1.00 |
+| Norway pine     |  6 by 12 |  162 |   15  |  50.3 |  12.5 |  2,305  |   .82 |  3,572  |   .69 |    987  |  1.03 |    201  |  1.17 |
+|                 |  4 by 12 |  162 |   18  |  47.9 |  14.7 |  2,648  |   .94 |  4,107  |   .79 |  1,255  |  1.31 |    238  |  1.38 |
+|                 |  4 by 10 |  162 |   16  |  45.7 |  13.3 |  2,674  |   .95 |  4,205  |   .81 |  1,306  |  1.36 |    198  |  1.15 |
+|                 |  2 by  2 |   30 |  133  |  32.3 |  11.4 |  2,808  |  1.00 |  5,173  |  1.00 |    960  |  1.00 |    172  |  1.00 |
+| Red spruce      |  2 by 10 |  144 |   14  |  32.5 |  21.9 |  2,394  |   .66 |  3,566  |   .60 |  1,180  |  1.02 |    181  |   .80 |
+|                 |  2 by  2 |   26 |   60  |  37.3 |  21.3 |  3,627  |  1.00 |  5,900  |  1.00 |  1,157  |  1.00 |    227  |  1.00 |
+| White spruce    |  2 by 10 |  144 |   16  |  40.7 |   9.3 |  2,239  |   .72 |  3,288  |   .63 |  1,081  |  1.08 |    166  |   .83 |
+|                 |  2 by  2 |   26 |   83  |  58.3 |  10.2 |  3.090  |  1.00 |  5,185  |  1.00 |    998  |  1.00 |    199  |  1.00 |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                           |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                  |
+|    M. of E. = Modulus of elasticity.                                                                                              |
+|    M. of R. = Modulus of rupture.                                                                                                 |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                         |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                      |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XIX                                                         TABLE XIX                                                                   |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                                                 COMPRESSION AND SHEAR TESTS ON GREEN MATERIAL                                                 |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |                     Compression                      |                Compression                |           Shear          |
+|                 |                  parallel to grain                   |          perpendicular  to grain          |                          |
+|                 |------------------------------------------------------+-------------------------------------------+--------------------------|
+|                 |          |       |       | Cr.    |        | Cr.     |        |        |       |       | Cr.     |       |       |          |
+|     Species     |          |       | Per   | str.   | M. of  | str.    |        |        |       | Per   | str.    |       | Per   |          |
+|                 | Size of  |  No.  | cent  | at     | E.     | at max. | Stress |        |  No.  | cent  | at max. |  No.  | cent  |  Shear   |
+|                 | specimen |  of   | of    | E. L.  | per    | ld.,.   | area   | Height |  of   | of    | ld.,    |  of   | of    | strength |
+|                 |          | tests | mois- | per    | square | per     |        |        | tests | mois- | per     | tests | mois- |          |
+|                 |          |       |  ture | square | inch   | square  |        |        |       |  ture | square  |       |  ture |          |
+|                 |          |       |       | inch   |        | inch    |        |        |       |       | inch    |       |       |          |
+|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------|
+|                 |          |       |       |        |  1,000 |         |        |        |       |       |         |       |       |          |
+|                 |  Inches  |       |       |  Lbs.  |  lbs.  |  Lbs.   | Inches | Inches |       |       |   Lbs.  |       |       |   Lbs.   |
+|                 |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+| Longleaf pine   |  4 by 4  |   46  |  26.3 |  3,480 |        |  4,800  | 4 by 4 |     4  |   22  |  25.3 |    568  |   44  |  21.8 |     973  |
+|                 |  2 by 2  |   14  |  34.7 |        |        |  4,400  |        |        |       |       |         |       |       |          |
+| Douglas fir     |  6 by 6  |  515  |  30.7 |  2,780 |  1,181 |  3,500  | 4 by 8 |    16  |  259  |  30.3 |    570  |  531  |  29.7 |     765  |
+|                 |  5 by 6  |  170  |  30.9 |  2,720 |  2,123 |  3,490  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  902  |  29.8 |  3,500 |  1,925 |  4,030  |        |        |       |       |         |       |       |          |
+| Douglas fir     |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+|   (fire-killed) |  6 by 6  |  108  |  34.8 |  2,620 |  1,801 |  3,290  | 6 by 8 |    16  |   24  |  33.7 |    368  |   77  |  35.8 |     631  |
+|                 |  2 by 2  |  204  |  37.9 |        |        |  3,430  |        |        |       |       |         |       |       |          |
+| Shortleaf pine  |  6 by 6  |   95  |  41.2 |  2,514 |  1,565 |  3,436  | 5 by 8 |    16  |   12  |  37.7 |    361  |  179  |  47.0 |     704  |
+|                 |  5 by 8  |   23  |  43.5 |  2,241 |  1,529 |  3,423  | 5 by 8 |    14  |   12  |  42.8 |    366  |       |       |          |
+|                 |  2 by 2  |  281  |  51.4 |        |        |  3,570  | 5 by 8 |    12  |   24  |  53.0 |    325  |       |       |          |
+|                 |          |       |       |        |        |         | 5 by 5 |     8  |   24  |  47.0 |    344  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  277  |  48.5 |    400  |       |       |          |
+| Western larch   |  6 by 6  |  107  |  49.1 |  2,675 |  1,575 |  3,510  | 6 by 8 |    16  |   22  |  43.6 |    417  |  179  |  40.7 |     700  |
+|                 |  2 by 2  |  491  |  50.6 |  3,026 |  1,545 |  3,696  | 6 by 8 |    12  |   20  |  40.2 |    416  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 6 |     6  |   53  |  52.8 |    478  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |   30  |  50.4 |    472  |       |       |          |
+| Loblolly pine   |  8 by 8  |   14  |  63.4 |  1,560 |    365 |  2,140  | 8 by 4 |     8  |   16  |  67.2 |    392  |  121  |  83.2 |     630  |
+|                 |  4 by 8  |   18  |  60.0 |  2,430 |    691 |  3,560  | 4 by 4 |     8  |   38  |  44.6 |    546  |       |       |          |
+|                 |  2 by 2  |   53  |  74.0 |        |        |  3,240  |        |        |       |       |         |       |       |          |
+| Tamarack        |  6 by 7  |    4  |  49.9 |  2,332 |  1,432 |  3,032  |        |        |       |       |         |   24  |  39.2 |     668  |
+|                 |  4 by 7  |    6  |  27.7 |  2,444 |  1,334 |  3,360  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  165  |  36.8 |        |        |  3,190  |        |        |       |       |         |       |       |          |
+| Western hemlock |  6 by 6  |   82  |  46.6 |  2,905 |  1,617 |  3,355  | 6 by 4 |     6  |   30  |  48.7 |    434  |   54  |  65.7 |     630  |
+|                 |  2 by 2  |  131  |  55.6 |  2,938 |  1,737 |  3,392  |        |        |       |       |         |       |       |          |
+| Redwood         |  6 by 6  |   34  |  83.6 |  3,194 |  1,240 |  3,882  | 6 by 8 |    16  |   13  |  86.7 |    473  |  148  |  84.2 |     742  |
+|                 |  2 by 2  |  143  |  36.8 |  3,490 |  1,222 |  3,980  | 6 by 6 |    12  |   14  |  83.0 |    424  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 7 |     9  |   13  |  74.7 |    477  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 3 |    14  |   13  |  75.6 |    411  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |    12  |   12  |  66.5 |    430  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |    10  |   11  |  55.0 |    423  |       |       |          |
+|                 |          |       |       |        |        |         | 6 by 2 |     8  |   12  |  56.7 |    396  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  186  |  75.5 |    569  |       |       |          |
+| Norway pine     |  6 by 7  |    5  |  29.0 |  1,928 |    905 |  2,404  |        |        |       |       |         |   20  |  26.7 |     589  |
+|                 |  4 by 7  |    8  |  28.4 |  2,154 |  1,063 |  2,652  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |  178  |  26.8 |        |        |  2,504  |        |        |       |       |         |       |       |          |
+| Red spruce      |  2 by 2  |   58  |  35.4 |        |        |  2,750  | 2 by 2 |     2  |   43  |  31.8 |    310  |   30  |  32.0 |     758  |
+| White spruce    |  2 by 2  |   84  |  61.0 |        |        |  2,370  | 2 by 2 |     2  |   46  |  50.4 |    270  |   40  |  58.0 |     651  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                                       |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                              |
+|    M. of E. = Modulus of elasticity.                                                                                                          |
+|    M. of R. = Modulus of rupture.                                                                                                             |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                                     |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XX                                                    TABLE XX                                                              |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                                              BENDING TESTS ON AIR-SEASONED MATERIAL                                               |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+|                 |      Sizes      |       |       |       |   F.S. at E.L.  |     M. of R.    |     M. of E.    |   Calculated    |
+|                 |-----------------| Num-  | Per   | Rings |                 |                 |                 |      shear      |
+|     Species     |          |      |  ber  | cent  |  per  |-----------------+-----------------+-----------------+-----------------|
+|                 |  Cross   | Span | of    | mois- | inch  | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio |
+|                 | Section  |      | tests |  ture |       | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" | per sq. | to 2" |
+|                 |          |      |       |       |       |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |  inch   | by 2" |
+|-----------------+----------+------+-------+-------+-------+---------+-------+---------+-------+---------+-------+---------+-------|
+|                 |          |      |       |       |       |         |       |         |       |  1,000  |       |         |       |
+|                 |  Inches  | Ins. |       |       |       |   Lbs.  |       |   Lbs.  |       |   lbs.  |       |   Lbs.  |       |
+|                 |          |      |       |       |       |         |       |         |       |         |       |         |       |
+| Longleaf pine   |  8 by 16 |  180 |    5  |  22.2 |  16.0 |  3,390  |  0.50 |  4,274  |  0.37 |  1,747  |  1.00 |    288  |  0.75 |
+|                 |  6 by 16 |  132 |    1  |  23.4 |  17.1 |  3,470  |   .51 |  6,610  |   .57 |  1,501  |   .86 |    388  |  1.01 |
+|                 |  6 by 10 |  177 |    2  |  19.0 |   8.8 |  4,560  |   .68 |  7,880  |   .68 |  1,722  |   .99 |    214  |   .56 |
+|                 |  4 by 11 |  180 |    1  |  18.4 |  23.9 |  3,078  |   .46 |  8,000  |   .69 |  1,660  |   .95 |    251  |   .66 |
+|                 |  6 by  8 |  177 |    6  |  20.0 |  13.7 |  4,227  |   .63 |  8,196  |   .71 |  1,634  |   .94 |    177  |   .46 |
+|                 |  2 by  2 |   30 |   17  |  15.9 |  13.9 |  6,750  |  1.00 | 11,520  |  1.00 |  1,740  |  1.00 |    383  |  1.00 |
+| Douglas fir     |  8 by 16 |  180 |   91  |  20.8 |  13.1 |  4,563  |   .68 |  6,372  |   .61 |  1,549  |   .91 |    269  |   .64 |
+|                 |  5 by  8 |  180 |   30  |  14.9 |  12.2 |  5,065  |   .76 |  6,777  |   .65 |  1,853  |  1.09 |    218  |   .52 |
+|                 |  2 by  2 |   24 |  211  |  19.0 |  16.4 |  6,686  |  1.00 | 10,378  |  1.00 |  1,695  |  1.00 |    419  |  1.00 |
+| Shortleaf pine  |  8 by 16 |  180 |    3  |  17.0 |  12.3 |  4,220  |   .54 |  6,030  |   .50 |  1,517  |   .85 |    398  |   .98 |
+|                 |  8 by 14 |  180 |    3  |  16.0 |  12.3 |  4,253  |   .55 |  5,347  |   .44 |  1,757  |   .98 |    307  |   .76 |
+|                 |  8 by 12 |  180 |    7  |  16.0 |  12.4 |  5,051  |   .65 |  7,331  |   .60 |  1,803  |  1.01 |    361  |   .89 |
+|                 |  5 by  8 |  180 |    6  |  12.2 |  22.5 |  7,123  |   .92 |  9,373  |   .77 |  1,985  |  1.11 |    301  |   .74 |
+|                 |  2 by  2 |   30 |   67  |  14.2 |  13.7 |  7,780  |  1.00 | 12,120  |  1.00 |  1,792  |  1.00 |    404  |  1.00 |
+| Western larch   |  8 by 16 |  180 |   23  |  18.3 |  21.9 |  3,343  |   .57 |  5,440  |   .53 |  1,409  |   .90 |    349  |   .96 |
+|                 |  8 by 12 |  180 |   29  |  17.8 |  23.4 |  3,631  |   .62 |  6,186  |   .60 |  1,549  |   .99 |    295  |   .81 |
+|                 |  5 by  8 |  180 |   10  |  13.6 |  27.6 |  4,730  |   .80 |  7,258  |   .71 |  1,620  |  1.04 |    221  |   .61 |
+|                 |  2 by  2 |   30 |  240  |  16.1 |  26.8 |  5,880  |  1.00 | 10,254  |  1.00 |  1,564  |  1.00 |    364  |  1.00 |
+| Loblolly pine   |  8 by 16 |  180 |   14  |  20.5 |   7.4 |  4,195  |   .81 |  6,734  |   .72 |  1,619  |  1.10 |    462  |  1.45 |
+|                 |  6 by 16 |  126 |    4  |  20.2 |   5.0 |  2,432  |   .47 |  4,295  |   .46 |  1,324  |   .90 |    266  |   .84 |
+|                 |  6 by 10 |  174 |    3  |  21.3 |   4.7 |  3,100  |   .60 |  6,167  |   .66 |  1,449  |   .99 |    173  |   .54 |
+|                 |  4 by 12 |  174 |    4  |  19.8 |   4.7 |  2,713  |   .52 |  5,745  |   .61 |  1,249  |   .85 |    185  |   .58 |
+|                 |  8 by  8 |  180 |    9  |  22.9 |   4.9 |  2,903  |   .56 |  4,557  |   .48 |  1,136  |   .77 |     93  |   .29 |
+|                 |  6 by  7 |  144 |    2  |  21.1 |   5.0 |  2,990  |   .58 |  4,968  |   .53 |  1,286  |   .88 |    116  |   .36 |
+|                 |  4 by  8 |  132 |    8  |  19.5 |   9.1 |  3,384  |   .65 |  6,194  |   .66 |  1,200  |   .82 |    196  |   .62 |
+|                 |  2 by  2 |   30 |  123  |  17.6 |   6.6 |  5,170  |  1.00 |  9,400  |  1.00 |  1,467  |  1.00 |    318  |  1.00 |
+| Tamarack        |  6 by 12 |  162 |    5  |  23.0 |  15.1 |  3,434  |   .45 |  5,640  |   .43 |  1,330  |   .82 |    318  |   .75 |
+|                 |  4 by 10 |  162 |    4  |  14.4 |   9.7 |  4,100  |   .54 |  5,320  |   .41 |  1,386  |   .84 |    252  |   .59 |
+|                 |  2 by  2 |   30 |   47  |  11.3 |  16.2 |  7,630  |  1.00 | 13,080  |  1.00 |  1,620  |  1.00 |    425  |  1.00 |
+| Western hemlock |  8 by 16 |  180 |   44  |  17.7 |  17.8 |  4,398  |   .69 |  6,420  |   .62 |  1,737  |  1.04 |    406  |  1.06 |
+|                 |  2 by  2 |   28 |  311  |  17.9 |  19.4 |  6,333  |  1.00 | 10,369  |  1.00 |  1,666  |  1.00 |    382  |  1.00 |
+| Redwood         |  8 by 16 |  180 |    6  |  26.3 |  22.4 |  3,797  |   .79 |  4,428  |   .57 |  1,107  |   .96 |    294  |  1.05 |
+|                 |  6 by 12 |  180 |    6  |  16.1 |  17.7 |  3,175  |   .66 |  3,353  |   .43 |    728  |   .64 |    167  |   .60 |
+|                 |  7 by  9 |  180 |    6  |  15.9 |  15.2 |  3,280  |   .69 |  4,002  |   .51 |  1,104  |   .96 |    147  |   .53 |
+|                 |  3 by 14 |  180 |    6  |  13.1 |  24.4 |         |       |  5,033  |   .64 |         |       |    291  |  1.04 |
+|                 |  2 by 12 |  180 |    5  |  13.8 |  14.4 |  3,928  |   .82 |  5,336  |   .68 |  1,249  |  1.09 |    260  |   .93 |
+|                 |  2 by 10 |  180 |    5  |  13.8 |  24.8 |  3,757  |   .79 |  4,606  |   .59 |  1,198  |  1.05 |    186  |   .67 |
+|                 |  2 by  8 |  180 |    6  |  13.7 |  20.7 |  4,314  |   .90 |  5,050  |   .65 |  1,313  |  1.15 |    166  |   .60 |
+|                 |  2 by  2 |   28 |  122  |  15.2 |  18.8 |  4,777  |  1.00 |  7,798  |  1.00 |  1,146  |  1.00 |    279  |  1.00 |
+| Norway pine     |  6 by 12 |  162 |    5  |  16.7 |   8.1 |  2,968  |   .56 |  5,204  |   .61 |  1,123  |   .97 |    286  |  1.02 |
+|                 |  4 by 10 |  162 |    5  |  13.7 |  12.0 |  5,170  |   .98 |  6,904  |   .82 |  1,712  |  1.48 |    317  |  1.13 |
+|                 |  2 by  2 |   30 |   60  |  14.9 |  11.2 |  5,280  |  1.00 |  8,470  |  1.00 |  1,158  |  1.00 |    281  |  1.00 |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                           |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                  |
+|    M. of E. = Modulus of elasticity.                                                                                              |
+|    M. of R. = Modulus of rupture.                                                                                                 |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                         |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                      |
+|-----------------------------------------------------------------------------------------------------------------------------------|
+
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XXI                                                         TABLE XXI                                                                   |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                                             COMPRESSION AND SHEAR TESTS ON AIR-SEASONED MATERIAL                                              |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |                     Compression                      |                Compression                |           Shear          |
+|                 |                  parallel to grain                   |          perpendicular  to grain          |                          |
+|                 |------------------------------------------------------+-------------------------------------------+--------------------------|
+|                 |          |       |       | Cr.    |        | Cr.     |        |        |       |       | Cr.     |       |       |          |
+|     Species     |          |       | Per   | str.   | M. of  | str.    |        |        |       | Per   | str.    |       | Per   |          |
+|                 | Size of  |  No.  | cent  | at     | E.     | at max. | Stress |        |  No.  | cent  | at max. |  No.  | cent  |  Shear   |
+|                 | specimen |  of   | of    | E. L.  | per    | ld.,.   | area   | Height |  of   | of    | ld.,    |  of   | of    | strength |
+|                 |          | tests | mois- | per    | square | per     |        |        | tests | mois- | per     | tests | mois- |          |
+|                 |          |       |  ture | square | inch   | square  |        |        |       |  ture | square  |       |  ture |          |
+|                 |          |       |       | inch   |        | inch    |        |        |       |       | inch    |       |       |          |
+|-----------------+----------+-------+-------+--------+--------+---------+--------+--------+-------+ ------+---------+-------+-------+----------|
+|                 |          |       |       |        |  1,000 |         |        |        |       |       |         |       |       |          |
+|                 |  Inches  |       |       |  Lbs.  |  lbs.  |  Lbs.   | Inches | Inches |       |       |   Lbs.  |       |       |   Lbs.   |
+|                 |          |       |       |        |        |         |        |        |       |       |         |       |       |          |
+| Longleaf pine   |  4 by 5  |   46  |  26.3 |  3,480 |        |  4,800  | 4 by 5 |     4  |   22  | 25.1  |    572  |   52  |  20.2 |     984  |
+| Douglas fir     |  6 by 6  |  259  |  20.3 |  3,271 |  1,038 |  4,258  | 4 by 8 |    16  |   44  | 20.8  |    732  |  465  |  22.1 |     822  |
+|                 |  2 by 2  |  247  |  18.7 |  3,842 |  1,084 |  5,002  | 4 by 8 |    10  |   32  | 18.1  |    584  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     8  |   51  | 20.2  |    638  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     6  |   49  | 24.0  |    613  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |   29  | 24.8  |    603  |       |       |          |
+| Shortleaf pine  |  6 by 6  |   29  |  15.7 |  4,070 |  1,951 |  6,030  | 8 by 5 |    16  |    4  | 17.8  |    725  |   85  |       |   1,135  |
+|                 |  2 by 2  |   57  |  14.2 |        |        |  6,380  | 8 by 5 |    14  |    3  | 16.3  |    757  |       |       |          |
+|                 |          |       |       |        |        |         | 8 by 5 |    12  |    5  | 15.1  |    730  |       |       |          |
+|                 |          |       |       |        |        |         | 5 by 5 |     8  |    6  | 13.0  |    918  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |   57  | 13.9  |    926  |       |       |          |
+| Western larch   |  6 by 6  |  112  |  16.0 |        |        |  5,445  | 8 by 6 |    16  |   17  | 18.8  |    491  |  193  |  15.0 |     905  |
+|                 |  4 by 4  |   81  |  14.7 |        |        |  6,161  | 8 by 6 |    12  |   18  | 17.6  |    526  |       |       |          |
+|                 |  2 by 2  |  270  |  14.8 |        |        |  5,934  | 5 by 4 |     8  |   22  | 13.3  |    735  |       |       |          |
+| Loblolly pine   |  6 by 6  |   23  |       |  3,357 |  1,693 |  5,005  | 8 by 5 |    16  |   12  | 19.8  |    602  |  156  |  11.3 |   1,115  |
+|                 |  5 by 5  |   10  |  22.4 |  2,217 |    545 |  2,950  | 8 by 5 |     8  |    7  | 22.9  |    679  |       |       |          |
+|                 |  4 by 8  |    8  |  19.4 |  3,010 |    633 |  3,920  | 4 by 5 |     8  |    8  | 19.5  |    715  |       |       |          |
+|                 |  2 by 2  |   69  |       |        |        |  5,547  |        |        |       |       |         |       |       |          |
+| Tamarack        |  6 by 7  |    3  |  15.7 |  2,257 |  1,042 |  3,323  | 2 by 2 |     2  |   57  | 16.2  |    697  |   60  |  14.0 |     879  |
+|                 |  4 by 7  |    3  |  13.6 |  3,780 |  1,301 |  4,823  |        |        |       |       |         |       |       |          |
+|                 |  4 by 4  |   57  |  14.9 |  3,386 |  1,353 |  4,346  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |   66  |  14.6 |        |        |  4,790  |        |        |       |       |         |       |       |          |
+| Western hemlock |  6 by 6  |  102  |  18.6 |  4,840 |  2,140 |  5,814  | 7 by 6 |    15  |   25  | 18.2  |    514  |  131  |  17.7 |     924  |
+|                 |  2 by 2  |  463  |  17.0 |  4,560 |  1,923 |  5,403  | 6 by 6 |     6  |   26  | 16.8  |    431  |       |       |          |
+|                 |          |       |       |        |        |         | 4 by 4 |     4  |    6  | 15.9  |    488  |       |       |          |
+| Redwood         |  6 by 6  |   18  |  16.9 |        |        |  4,276  | 8 by 6 |    16  |    5  | 25.4  |    548  |   95  |  12.4 |     671  |
+|                 |  2 by 2  |  115  |  14.6 |        |        |  5,119  | 6 by 6 |    12  |    6  | 14.7  |    610  |       |       |          |
+|                 |          |       |       |        |        |         | 7 by 6 |     9  |    5  | 14.8  |    500  |       |       |          |
+|                 |          |       |       |        |        |         | 3 by 6 |    14  |    2  | 12.6  |    470  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |    12  |    2  | 16.2  |    498  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |    10  |    4  | 14.3  |    511  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 6 |     8  |    2  | 13.2  |    429  |       |       |          |
+|                 |          |       |       |        |        |         | 2 by 2 |     2  |  145  | 13.8  |    564  |       |       |          |
+| Norway pine     |  6 by 7  |    4  |  15.2 |  2,670 |  1,182 |  4,212  | 2 by 2 |     2  |   36  | 10.0  |    924  |   44  |  11.9 |   1,145  |
+|                 |  4 by 7  |    2  |  22.2 |  3,275 |  1,724 |  4,575  |        |        |       |       |         |       |       |          |
+|                 |  4 by 4  |   55  |  16.6 |  3,048 |  1,367 |  4,217  |        |        |       |       |         |       |       |          |
+|                 |  2 by 2  |   44  |  11.2 |        |        |  7,550  |        |        |       |       |         |       |       |          |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+| _Note.--Following is an explanation of the abbreviations used in the foregoing tables:_                                                       |
+|    F.S. at E.L. = Fiber stress at elastic limit.                                                                                              |
+|    M. of E. = Modulus of elasticity.                                                                                                          |
+|    M. of R. = Modulus of rupture.                                                                                                             |
+|    Cr. str. at E.L. = Crushing strength at elastic limit.                                                                                     |
+|    Cr. str. at max. ld. = Crushing strength at maximum load.                                                                                  |
+|-----------------------------------------------------------------------------------------------------------------------------------------------|
+
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| TABLE XXII                                                                         TABLE XXII                                                                                    |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                                                                [b]WORKING UNIT-STRESSES FOR STRUCTURAL TIMBER[c]                                                                 |
+|                                                                       EXPRESSED IN POUNDS PER SQUARE INCH                                                                        |
+|                                                      (From Manual of the American Railway Engineering Assn., 1911, p. 153)                                                       |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| NOTE.--The working unit-stresses given in the table are intended for railroad bridges and trestles. For highway bridges and trestles the unit-stresses may be increased          |
+| twenty-five (25) per cent. For  buildings and similar structures, in which the timber is protected from the weather and practically free from impact, the unit-stresses may be   |
+| increased fifty (50) per cent. To compute the deflection of a beam under long-continued loading instead of that when the load is first applied, only fifty (50) per cent of the  |
+| corresponding modulus of elasticity given in the table is to be employed.                                                                                                        |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|                 |             BENDING             |                 SHEARING                |                      COMPRESSION                                        |          |
+|                 |---------------------------------+-----------------------------------------+-------------------------------------------------------------------------|  Ratio   |
+|                 |      Extreme       | Modulus of |    Parallel to     |    Longitudinal    |   Perpendicular    |    Parallel to     | For       | Formulae for       |    of    |
+|                 |       fibre        | elasticity |     the grain      |    shear in        |    to the grain    |     the grain      | columns   | working stress in |  length  |
+|    KIND OF      |       stress       |            |                    |    beams           |                    |                    | under 15  | long columns over |    of    |
+|    TIMBER       |--------------------+------------+--------------------+--------------------+--------------------+--------------------| diameters | 15 diameters      | stringer |
+|                 | Average  | Working |            | Average  | Working | Elastic  | Working | Elastic  | Working | Average  | Working | working   |                   |    to    |
+|                 | ultimate | stress  |  Average   | ultimate | stress  | limit    | stress  | limit    | stress  | ultimate | stress  | stress    |                   |  depth   |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Douglas fir     |   6100   |   1200  | 1,510,000  |   690    |   170   |    270   |   110   |    630   |   310   |   3600   |   1200  |    900    | 1200(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Longleaf pine   |   6500   |   1300  | 1,610,000  |   720    |   180   |    300   |   120   |    520   |   260   |   3800   |   1300  |    980    | 1300(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Shortleaf pine  |   5600   |   1100  | 1,480,000  |   710    |   170   |    330   |   130   |    340   |   170   |   3400   |   1100  |    830    | 1100(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| White pine      |   4400   |    900  | 1,130,000  |   400    |   100   |    180   |    70   |    290   |   150   |   3000   |   1000  |    750    | 1000(1-_l_/60_d_) |    10    |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Spruce          |   4800   |   1000  | 1,310,000  |   600    |   150   |    170   |    70   |    370   |   180   |   3200   |   1100  |    830    | 1100(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Norway pine     |   4200   |    800  | 1,190,000  |   590[d] |   130   |    250   |   100   |          |   150   |  2600[d] |    800  |    600    |  800(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Tamarack        |   4600   |    900  | 1,220,000  |   670    |   170   |    260   |   100   |          |   220   |  3200[d] |   1000  |    750    | 1000(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Western hemlock |   5800   |   1100  | 1,480,000  |   630    |   160   |   270[d] |   100   |    440   |   220   |   3500   |   1200  |    900    | 1200(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Redwood         |   5000   |    900  |   800,000  |   300    |    80   |          |         |    400   |   150   |   3300   |    900  |    680    |  900(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Bald cypress    |   4800   |    900  | 1,150,000  |   500    |   120   |          |         |    340   |   170   |   3900   |   1100  |    830    | 1100(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| Red cedar       |   4200   |    800  |   800,000  |          |         |          |         |    470   |   230   |   2800   |    900  |    680    |  900(1-_l_/60_d_) |          |
+|-----------------+----------+---------+------------+----------+---------+----------+---------+----------+---------+----------+---------+-----------+-------------------+----------|
+| White oak       |   5700   |   1100  | 1,150,000  |   840    |   210   |    270   |   110   |    920   |   450   |   3500   |   1300  |    980    | 1300(1-_l_/60_d_) |    12    |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| These unit-stresses are for a green condition of timber and are                                                                                     _l_ = Length in inches.      |
+| to be used without increasing the live load stresses for impact.                                                                                    _d_ = Least side in inches.  |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+|[Footnote b: Adopted, Vol. 1909, pp. 537, 564, 609-611.]                                                                                                                          |
+|[Footnote c: Green timber in exposed work.]                                                                                                                                       |
+|[Footnote d: Partially air-dry]                                                                                                                                                   |
+|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+
+
+
+
+BIBLIOGRAPHY
+
+
+
+  Part I: Some general works on mechanics, materials of
+            construction, and testing of materials.
+
+ Part II: Publications and articles on the mechanical properties
+            of wood, and timber testing.
+
+Part III: Publications of the U.S. Government on the mechanical
+            properties of wood, and timber testing.
+
+
+
+PART 1. SOME GENERAL WORKS ON MECHANICS, MATERIALS OF
+          CONSTRUCTION, AND TESTING OF MATERIALS
+
+
+ALLAN, WILLIAM: Strength of beams under transverse loads. New
+York, 1893.
+
+ANDERSON, SIR JOHN: The strength of materials and structures.
+London, 1902.
+
+BARLOW, PETER: Strength of materials, 1st ed. 1817; rev. 1867.
+
+BURR, WILLIAM H.: The elasticity and resistance of the materials
+of engineering. New York, 1911.
+
+CHURCH, IRVING P.: Mechanics of engineering. New York, 1911.
+
+HATFIELD, R.G.: Theory of transverse strain. 1877.
+
+HATT, W.K., and SCOFIELD, H.H.: Laboratory manual of testing
+materials. New York, 1913.
+
+JAMESON, J.M.: Exercises in mechanics. (Wiley technical series.)
+New York, 1913.
+
+JAMIESON, ANDREW: Strength of materials. (Applied mechanics and
+mechanical engineering, Vol. II.) London, 1911.
+
+JOHNSON, J.B.: The materials of construction. New York, 1910.
+
+KENT, WILLIAM: The strength of materials. New York, 1890.
+
+KOTTCAMP, J.P.: Exercises for the applied mechanics laboratory.
+(Wiley technical series.) New York, 1913.
+
+LANZA, GAETANO: Applied mechanics. New York, 1901.
+
+MERRIMAN, MANSFIELD: Mechanics of materials. New York, 1912.
+
+MURDOCK, H.E.: Strength of materials. New York, 1911.
+
+RANKINE, WILLIAM J.M.: A manual of applied mechanics. London,
+1901.
+
+THIL, A.: Conclusion de l'etude presentee a la Commission des
+methodes d'essai des materiaux de construction. Paris, 1900.
+
+THURSTON, ROBERT H.: A treatise on non-metallic materials of
+engineering: stone, timber, fuel, lubricants, etc. (Materials of
+engineering, Part I.) New York, 1899.
+
+UNWIN, WILLIAM C.: The testing of materials of construction.
+London, 1899.
+
+WATERBURY, L.A.: Laboratory manual for testing materials of
+construction. New York, 1912.
+
+WOOD, DEVOLSON: A treatise on the  resistance of materials. New
+York, 1897.
+
+
+
+PART II. PUBLICATIONS AND ARTICLES ON THE MECHANICAL PROPERTIES
+           OF WOOD, AND TIMBER TESTING
+
+
+ABBOT, ARTHUR V.: Testing machines, their history, construction
+and use. Van Nostrand's Eng. Mag., Vol. XXX, 1884, pp. 204-214;
+325-344; 382-397; 477-490.
+
+ADAMS, E.E.: Tests to determine the strength of bolted timber
+joints. Cal. Jour, of Technology, Sept., 1904.
+
+ALVAREZ, ARTHUR C.: The strength of long seasoned Douglas fir
+and redwood. Univ. of Cal. Pub. in Eng., Vol. I, No. 2,
+Berkeley, 1913, pp. 11-20.
+
+BARLOW, PETER: An essay on the strength and stress of timber.
+London, 1817; 3d ed., 1826.
+
+----: Experiments on the strength of different kinds of wood
+made in the carriage department, Royal Arsenal, Woolwich. Jour.
+Franklin Inst., Vol. X, 1832, pp. 49-52. Reprinted from
+Philosophical Mag. and Annals of Philos., No. 63, Mch., 1832.
+
+BATES, ONWARD: Pine stringers and floorbeams for bridges. Trans.
+Am. Soc. C.E., Vol. XXIII.
+
+BAUSCHINGER, JOHANN: Untersuchungen ueber die Elasticitaet und
+Festigkeit von Fichten- und Kiefernbauhoelzern. Mitt. a. d.
+mech.-tech. Laboratorium d. k. techn. Hochschule in Muenchen, 9.
+Hft., Muenchen, 1883.
+
+----: Verhandlungen der Muenchener Conferenz und der von ihr
+gewaehlten staendigen Commission zur Vereinbarung einheitlicher
+Pruefungsmethoden fuer Bau- und Constructions-material. _Ibid._,
+14. Hft., 1886.
+
+----: Untersuchungen ueber die Elasticitaet und Festigkeit
+verschiedener Nadelhoelzer. _Ibid._, 16. Hft., 1887.
+
+BEARE, T. HUDSON: Timber: its strength and how to test it.
+Engineering, London, Dec. 9, 1904.
+
+BEAUVERIE, J.: Le bois. I. Paris, 1905, pp. 105-185.
+
+----: Les bois industriels. Paris, 1910, pp. 55-77. Bending
+tests with wood, executed at the Danish State Testing
+Laboratory, Copenhagen. Proc. Int. Assn. Test. Mat., 1912,
+XXIII_{2}, pp. 17. See also Eng. Record, Vol. LXVI, 1912, p.
+269.
+
+BERG, WALTER G.: Berg's complete timber test record. Chicago,
+1899. Reprint from Am. By. Bridges and Buildings. BOULGER, G.S.:
+Wood. London, 1908, pp. 112-121.
+
+BOUNICEAU,--: Note et experiences sur la torsion des bois.
+[N.p., n.d.]
+
+BOVEY, HENRY T.: Results of experiments at McGill University,
+Montreal, on the strength of Canadian Douglas fir, red pine,
+white pine, and spruce. Trans. Can. Soc. C.E., Vol. IX, Part I,
+1895, pp. 69-236.
+
+BREUIL, M. PIERRE: Contribution to the discussion on the testing
+of wood. Proc. Int. Assn. Test. Mat., 1906, Disc, 1_e_, pp. 2.
+
+BROWN, T.S.: An Account of some experiments made by order of
+Col. Totten, at Fort Adams, Newport, R.I., to ascertain the
+relative stiffness and strength of the following kinds of
+timber, viz.: white pine (_Pinus strobus_), spruce (_Abies
+nigra_), and southern pine (_Pinus australis_), also called
+long-leaved pine. Jour. Franklin Inst., Vol. VII (n.s.), 1831,
+pp. 230-238.
+
+BUCHANAN, C.P.: Some tests of old timber. Eng. News, Vol. LXIV,
+No. 23, 1910, p. 67.
+
+BUSGEN, M.: Zur Bestimmung der Holzhaerten. Zeitschrift f. Forst-
+und Jagdwesen. Berlin, 1904, pp. 543-562.
+
+CHEVANDIER, E., et WERTHEIM, G.: Memoire sur les proprietes
+mecaniques du bois. Paris, 1846.
+
+CIESLAR, A.: Studien ueber die Qualitaet rasch erwachsenen
+Fichtenholzes. Centralblatt f. d. ges. Forstwesen, Wien, 1902,
+pp. 337-403.
+
+CLINE, McGARVEY: Forest Service investigations of American woods
+with special reference to investigations of mechanical
+properties. Proc. Int. Assn. Test. Mat., 1912, XXIII_{5}, pp.
+17.
+
+----: Forest Service tests to determine the influence of
+different methods and rates of loading on the strength and
+stiffness of timber. Proc. Am. Soc. Test. Mat., Vol. VIII, 1908,
+pp. 535-540.
+
+----: The Forest Products Laboratory: its purpose and work.
+Proc. Am. Soc. Test. Mat., Vol. X, 1910, pp. 477-489.
+
+----: Specifications and grading rules for Douglas fir timber:
+an analysis of Forest Service tests on structural timbers. Proc.
+Am. Soc. Test. Mat., Vol. XI, 1911, pp. 744-766.
+
+Comparative strength and resistance of various tie timbers.
+Elec. Traction Weekly, Chicago, June 15, 1912.
+
+DAY, FRANK M.: Microscopic examination of timber with regard to
+its strength. 1883, pp. 6.
+
+DEWELL, H.D.: Tests of some joints used in heavy timber framing.
+Eng. News, Mch. 19, 1914, pp. 594-598; _et seq._
+
+DOeRR, KARL: Die Festigkeit von Fichten- und Kiefernholz.
+Deutsche Bauzeitung, Berlin, Aug. 17, 1910. See also Zeitschrift
+d. ver. deutsch. Ing., Bd. 54, Nr. 36, 1910, p. 1503.
+
+DUPIN, CHARLES: Experiences sur la flexibilite, la force, et
+l'elasticite des bois. Jour, de l'Ecole Polytechnique, Vol. X,
+1815.
+
+DUPONT, ADOLPHE, et BOUQUET DE LA GRYE: Les bois indigenes et
+etrangers. Paris, 1875, pp. 273-352.
+
+ESTRADA, ESTEBAN DUQUE: On the strength and other properties of
+Cuban woods. Van Nostrand's Eng. Mag., Vol. XXIX, 1883, pp.
+417-426; 443-449.
+
+EVERETT, W.H.: Memorandum on mechanical tests of some Indian
+timbers. Govt. Bul. No. 6 (o.s.), Calcutta.
+
+EXNER, WILHELM FRANZ: Die mechanische Technologie des Holzes.
+Wien, 1871. (A translation and revision of Chevandier and
+Wertheim's Memoire sur les proprietes mecaniques du bois.)
+
+----: Die technischen Eigenschaften der Hoelzer. Lorey's Handbuch
+der Forstwissenschaft, II. Bd., 6. Kap., Tuebingen, 1903.
+
+FERNOW, B.E.: Scientific timber testing. Digest of Physical
+Tests, Vol. I, No. 2, 1896, pp. 87-95.
+
+FOWKE, FRANCIS: Experiments on British colonial and other woods.
+1867.
+
+GARDNER, ROLAND: I. Mechanical tests, properties, and uses of
+thirty Philippine woods. II. Philippine sawmills, lumber market
+and prices. Bul. 4, Bu. For., P.I., 1906. (2d ed., 1907,
+contains tests of 34 woods.)
+
+GAYER, KARL: Forest utilization. (Vol. V, Schlich's Manual of
+Forestry. Translation of Die Forstbenutzung, Berlin, 1894.)
+London, 1908.
+
+GOLLNER, H.: Ueber die Festigkeit des Schwarzfoehrenholzes. Mitt.
+a. d. forstl. Versuchswesen Oesterreichs. II. Bd., 3. Hft.,
+Wien, 1881.
+
+GOTTGETREU, RUDOLPH: Physische und chemische Beschaffenheit der
+Baumaterialien. 3d ed., Berlin, 1880.
+
+GREEN, A.O.: Tasmanian timbers: their qualities and uses.
+Hobart, Tasmania, 1903, pp. 63.
+
+GREGORY, W.B.: Tests of creosoted timber. Trans. Am. Soc. C.E.,
+Vol. LXXVI, 1913, pp. 1192-1203. See also _ibid._, Vol. LXX, p.
+37.
+
+GRISARD, JULES, et VANDENBERGHE, MAXIMILIEN: Les bois
+industriels, indigenes et exotiques; synonymie et description
+des especes, proprietes physiques des bois, qualites, defauts,
+usages et emplois. Paris, 189-. From Bul. de la Societe
+nationale d'acclimatation de France, Vols. XXXVIII-XL.
+
+Hardwoods of Western Australia. Engineering, Vol. LXXXIII, Jan.
+11, 1907, pp. 35-37.
+
+HATT, WILLIAM KENDRICK: A Preliminary program for the timber
+test work to be undertaken by the Bureau of Forestry, United
+States Department of Agriculture. Proc. Am. Soc. Test. Mat.,
+Vol. III, 1903, pp. 308-343. Appendix I: Method of determining
+the effect of the rate of application of load on the strength of
+timber, pp. 325-327; App. II: A discussion on the effect of
+moisture on strength and stiffness of timber, together with a
+plan of procedure for future tests, pp. 328-334.
+
+HATT, WILLIAM KENDRICK: Relation of timber tests to forest
+products. Proc. Int. Assn. Test. Mat., 1906, C 2 _e_, pp. 6.
+
+----: Structural timber. Proc. Western Ry. Club, St. Louis, Mch.
+17, 1908.
+
+----: Abstract of report on the present status of timber tests
+in the Forest Service, United States Department of Agriculture.
+Proc. Int. Assn. Test. Mat., 1909, XVL, pp. 10.
+
+---- and TURNER, W.P.: The Purdue University impact machine.
+Proc. Am. Soc. Test. Mat., Vol. VI, 1906, pp. 462-475.
+
+HAUPT, HERMAN: formulae for the strain upon timber. Center of
+gravity of an ungula and semi-cylinder. Jour. Franklin Inst.,
+Vol. XIX, 3d series, 1850, pp. 408-413.
+
+HEARDING, W.H.: Report upon experiments ... upon the compressive
+power of pine and hemlock timber. Washington, 1872, pp. 12.
+
+HOWE, MALVERD A.: Wood in compression; bearing values for
+inclined cuts. Eng. News, Vol. LXVIII, 1912, pp. 190-191.
+
+HOYER, EGBERT: Lehrbuch der vergleichenden mechanischen
+Technologie. 1878.
+
+IHLSENG, MANGUS C.: On the modulus of elasticity of some
+American woods as determined by vibration. Van Nostrand's Eng.
+Mag., Vol. XIX, 1878, pp. 8-9.
+
+----: On a mode of measuring the velocity of sounds in woods.
+Am. Jour. Sci. and Arts, Vol. XVII, 1879.
+
+JACCARD, P.: Etude anatomique des bois comprimes. Mitt. d. Schw.
+Centralanstalt f. d. forst. Versuchswesen. X. Bd., 1. Hft.,
+Zurich, 1910, pp. 53-101.
+
+JANKA, GABRIEL: Untersuchungen ueber die Elasticitaet und
+Festigkeit der oesterreichischen Bauhoelzer. I. Fichte Suedtirols;
+II. Fichte von Nordtirol vom Wienerwalde und Erzgebirge; III.
+Fichte aus den Karpaten, aus dem Boehmerwalde, Ternovanerwalde
+und den Zentralalpen. Technische Qualitaet des Fichtenholzes im
+allgemeinen; IV. Laerche aus dem Wienerwalde, aus Schlesien,
+Nord- und Suedtirol. Mitt. a. d. forst. Untersuchungswesen
+Oesterreichs, Wien, 1900-13.
+
+----: Untersuchungen ueber Holzqualitaet. Centralblatt f. d. ges.
+Forstwesen. Wien, 1904, pp. 95-115.
+
+----: Ueber neuere holztechnologische Untersuchungen. Oesterr.
+Vierteljahresschrift fuer Forstwesen, Wien, 1906, pp. 248-269.
+
+----: Die Haerte des Holzes. Centralblatt f. d. ges. Forstwesen,
+Wien, 1906, pp. 193-202; 241-260.
+
+JANKA, GABRIEL: Die Einwirkung von Suess- und Salzwaessern auf die
+gewerblichen Eigenschaften der Hauptholzarten. I. Teil.
+Untersuchungen u. Ergebnisse in mechanisch-technischer Hinsicht.
+Mitt. a. d. forst. Versuchswesen Oesterreichs, 33. Hft., Wien,
+1907.
+
+----: Results of trials with timber carried out at the Austrian
+forestry testing-station at Mariabrunn. Proc. Int. Assn. Test.
+Mat., 1906, Disc. 2 _e_, pp. 7.
+
+----: Ueber die an der k. k. forstlichen Versuchsanstalt
+Mariabrunnen gewonnenen Resultate der Holzfestigkeitspruefungen.
+Zeitschrift d. Oesterr. Ing. u. Arch. Ver., Wien, Aug. 9, 1907.
+
+----: Ueber Holzhaerteprufueng. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 443-456.
+
+----: Testing the hardness of wood by means of the ball test.
+Proc. Int. Assn. Test. Mat., 1912, XXIII_{3}.
+
+JENNY, K.: Untersuchungen ueber die Festigkeit der Hoelzer aus den
+Laendern der ungarischen Krone. Budapest, 1873.
+
+JOHNSON, J.B.:  Time tests of timber in endwise compression.
+Paper before Section D, Am. Assn. for Adv. of Sci., Aug., 1898.
+
+JOHNSON, WALTER B.: Experiments on the adhesion of iron spikes
+of various forms when driven into different species of timbers.
+Jour. Franklin Inst., Vol. XIX (n.s.), 1837, pp. 281-292.
+
+JULIUS, G.A.: Western Australia timber tests, 1906. The physical
+characteristics of the hardwoods of Western Australia. Perth,
+1906, pp. 36.
+
+----: Supplement to the Western Australia timber tests, 1906.
+The hardwoods of Australia. Perth, 1907, pp. 6.
+
+KARMARSH, CARL: Handbuch der mechanischen Technologie. I. Aufl.,
+1837; V. Aufl., 1875; verm. von H. Fisher, 1888.
+
+KIDDER, F.E.: Experiments on the transverse strength of southern
+and white pine. Van Nostrand's Eng. Mag., Vol. XXII, 1880, pp.
+166-168.
+
+----: Experiments on the strength and stiffness of small spruce
+beams. _Ibid._, Vol. XXIV, 1881, pp. 473-477.
+
+----: Experiments on the fatigue of small spruce beams. Jour.
+Franklin Inst., Vol. CXIV, 1882, pp. 261-279.
+
+KIDWELL, EDGAR: The efficiency of built-up wooden beams. Trans.
+Am. Inst. Min. Eng., Feb., June, 1898.
+
+KIRKALDY, WM. G.: Illustrations of David Kirkaldy's system of
+mechanical testing. London, 1891.
+
+KUMMER, FREDERICK A.: The effects of preservative treatment on
+the strength of timber. Proc. Am. Soc. Test. Mat., Vol. IV,
+1904, pp. 434-438.
+
+LABORDERE, P., and ANSTETT, F.: Contribution to the study of
+means for improving the strength of wood for pavements. Proc.
+Int. Assn. Test. Mat., 1912, XXIII_{4}, pp. 12.
+
+LANZA, GAETANO: An account of certain tests on the transverse
+strength and stiffness of large spruce beams. Trans. Am. Soc.
+Mech. Eng., Vol. IV, 1882, pp. 119-135. See also Jour. Franklin
+Inst., Vol. XCV, 1883, pp. 81-94.
+
+LASLETT, T.: Properties and characteristics of timber. Chatham,
+1867.
+
+----: Timber and timber trees, native and foreign. (2d ed.
+revised and enlarged by H. Marshall Ward.) London and New York,
+1894.
+
+LEA, W.: Tables of strength and deflection of timber. London,
+1861.
+
+LEDEBUR, A.: Die Verarbeitung des Holzes auf mechanischem Wege.
+1881.
+
+LORENZ, N. VON: Analytische Untersuchung des Begriffes der
+Holzhaerte. Centralblatt f. d. ges. Forstwesen, Wien, 1909, pp.
+348-387.
+
+LUDWIG, PAUL: Die Regelprobe. Ein neues Verfahren zur
+Haertebestimmung von Materialien. Berlin. 1908.
+
+MACFARLAND, H.B.: Tests of longleaf pine bridge timbers. Bul.
+149, Am. Ry. Eng. Assn., Sept., 1912. See also Eng. News, Dec.
+12, 1912, p. 1035.
+
+McKAY, DONALD: On the weight and strength of American
+ship-timber. Jour. Franklin Inst., Vol. XXXIX (3d series), 1860,
+p. 322.
+
+MALETTE, J.: Essais des bois de construction. Revue Technique,
+Apr. 25, 1905.
+
+MANN, JAMES: Australian timber: its strength, durability, and
+identification. Melbourne, 1900.
+
+MARTIN, CLARENCE A.: Tests on the relation between cross-bending
+and direct compressive strength in timber. Railroad Gazette,
+Mch. 13, 1903.
+
+Methods of testing metals and alloys ... Recommended by the
+Fourth Congress of the International Association for Testing
+Materials, held at Brussels, Sept. 3-6, 1906. London, 1907, pp.
+54. Methods of testing wood, pp. 39-49.
+
+MIKOLASCHEK, CARL: Untersuchungen ueber die Elasticitaet und
+Festigkeit der wichtigsten Bau- und Nutzhoelzer. Mitt. a. d.
+forstl. Versuchswesen Oesterreiches, II. Bd., 1. Hft., Wien,
+1879.
+
+MOELLER, JOSEPH: Die Rohstoffe des Tischler- und
+Drechslergewerbes. I. Theil: Das Holz. Kassel, 1883, pp. 68-122.
+
+MOLESWORTH, G.L.: Graphic diagrams of strength of teak beams.
+Roorke, 1881.
+
+MORGAN, J.J.: Bending strength of yellow pine timber. Eng.
+Record, Vol. LXVII, 1913, pp. 608-609.
+
+MOROTO, K.: Untersuchungen ueber die Biegungselasticitaet und
+-Festigkeit der japanischen Bauhoelzer. Centralblatt f. d. ges.
+Forstwesen, Wien, 1908, pp. 346-355.
+
+NORDLINGER, H.: Die technischen Eigenschaften der Hoelzer fuer
+Forst- und Baubeamte, Technologen und Gewerbetreibende.
+Stuttgart, 1860.
+
+----: Druckfestigkeit des Holzes. 1882.
+
+----: Die gewerblichen Eigenschaften der Hoelzer. Stuttgart,
+1890.
+
+NORTH, A.T.: The grading of timber on the strength basis.
+Address before Western Society of Engineers. Lumber World
+Review, May 25, 1914, pp. 27-29.
+
+NORTON, W.A.: Results of experiments on the set of bars of wood,
+iron, and steel, after a transverse stress. Van Nostrand's Eng.
+Mag., Vol. XVII, 1877, pp. 531-535.
+
+PACCINOTTI E PERI: [Investigations into the elasticity of
+timbers.] Il Cimento, Vol. LVIII, 1845.
+
+PALACIO, E.: Tensile tests of timber. La Ingenieria, Buenos
+Aires, May 31, 1903, _et seq._
+
+PARENT,--: Experiences sur la resistance des bois de chene et de
+sapin. Memoires de l'Academie des Sciences, 1707-08.
+
+Propositions relatives a l'etablissement d'un precede uniforme
+pour l'essai des qualites techniques des bois. Proc. Int. Assn.
+Test. Mat., 1901, Annexe, pp. 13-28.
+
+ROGERS, CHARLES G.: A manual of forest engineering for India.
+Vol. I, Calcutta, 1900, pp. 50-91.
+
+RUDELOFF, M.: Der heutige Stand der Holzuntersuchungen. Mitt. a.
+d. koeniglichen tech. Versuchsanstalt, Berlin, IV, 1899.
+
+----: Principles of a standard method of testing wood. Proc.
+Int. Assn. Test. Mat., 1906, 23 C, pp. 16.
+
+----: Large _vs._ small test-pieces in testing wood. Proc. Int.
+Soc. Test. Mat., 1912, XXIII_{1}, pp. 7.
+
+SARGENT, CHARLES SPRAGUE:  Woods of the United States, with an
+account of their structure, qualities, and uses. New York, 1885.
+
+SCHNEIDER, A.: Zusammengesetzte Traeger. Zeitschrift d. Oesterr.
+Ing. u. Arch. Ver., Nov. 24; Dec. 9, 1899.
+
+SCHWAPPACH, A.F.: Beitraege zur Kenntniss der Qualitaet des
+Rotbuchenholzes. Zeitschrift f. Forst- und Jagdwesen, Berlin,
+1894, pp. 513-539.
+
+----: Untersuchungen ueber Raumgewicht und Druckfestigkeit des
+Holzes wichtiger Waldbaeume. Berlin, 1897-98.
+
+----: Etablissement de methodes uniformes pour l'essai a la
+compression des bois. Proc. Int. Assn. Test. Mat., 1901, Rapport
+23, pp. 28.
+
+SEBERT, H.: Notice sur les bois de la Nouvelle Caledonie suivie
+de considerations generates sur les proprietes mecaniques des
+bois et sur les precedes employes pour les mesurer. Paris.
+
+SHERMAN, EDWARD C.: Crushing tests on water-soaked timbers. Eng.
+News, Vol. LXII, 1909, p. 22.
+
+SNOW, CHARLES H.: The principal species of wood: their
+characteristic properties. New York, 1908.
+
+STAUFFER, OTTMAR: Untersuchungen ueber specifisches
+Trockengewicht, sowie anatomisches Verhalten des Holzes der
+Birke. Muenchen, 1892.
+
+STENS, D.: Ueber die Eigenschaftenimpraegnierter Grubenholzer,
+insbesondere ueber ihre Festigkeit. Glueckauf, Essen, Mch. 6,
+1907.
+
+Strength of wood for pavements. Can. Eng., Toronto, Sept. 12,
+1912.
+
+STUeBSCHEN-KISCHNER: Karmarsch-Heerins technisches Wroeterbuch. 3.
+Aufl., 1886.
+
+TALBOT, ARTHUR N.: Tests of timber beams. Bul. 41, Eng. Exp.
+Sta., Univ. of Ill., Urbana, 1910.
+
+Tests of wooden beams made at the Massachusetts Institute of
+Technology on spruce, white pine, yellow pine, and oak beams of
+commercial sizes. Technology Quarterly, Boston, Vol. VII, 1894.
+
+TETMAJER, L. v.: Zur Frage der Knickungsfestigkeit der
+Bauhoelzer. Schweizerische Bauzeitung, Bd. 11, Nr. 17.
+
+----: Methoden und Resultate der Pruefung der schweizerischen
+Bauhoelzer. Mitt. d. Anstalt z. Pruefung v. Baumaterialien am
+eidgenoessischen Polytechnicum in Zuerich. 2. Hft., 1884.
+
+----: Methoden und Resultate der Pruefung der schweizerischen
+Bauhoelzer. Mitt. d. Materialpruefungs-Anstalt am Schweiz.
+Polytechnikum in Zuerich. Landesaustellungs-Ausgabe, 2. Hft.,
+Zuerich, 1896.
+
+THELEN, ROLF: The structural timbers  of the Pacific Coast.
+Proc. Am. Soc. Test. Mat., Vol. VIII, 1908, pp. 558-567.
+
+THURSTON, R.H.: Torsional resistance of materials determined by
+a new apparatus with automatic registry. Jour. Franklin Inst.,
+Vol. LXV, 1873, pp. 254-260.
+
+----: On the strength of American timber. _Ibid._, Vol. LXXVIII,
+1879, pp. 217-235.
+
+----: Experiments on the strength of yellow pine. _Ibid._, Vol.
+LXXIX, 1880, pp. 157-163.
+
+----: Influence of time on bending strength and elasticity.
+Proc. Am. Assn. for Adv. Sci., 1881. Also Proc. Inst. C.E., Vol.
+LXXI.
+
+----: On the effect of prolonged stress upon the strength and
+elasticity of pine timber. Jour. Franklin Inst., Vol. LXXX,
+1881, pp. 161-169.  THURSTON, R.H.: On Flint's investigations of
+Nicaraguan woods. _Ibid._, Vol. XCIV, 1887, pp. 289-315.
+
+TIEMANN, HARRY DONALD: The effect of moisture and other
+extrinsic factors upon the strength of wood. Proc. Am. Soc.
+Test. Mat., Vol. VII, 1907, pp. 582-594.
+
+----: The effect of the speed of testing upon the strength of
+wood and the standardization of tests for speed. _Ibid._, Vol.
+VIII, 1908, pp. 541-557.
+
+----: The theory of impact and its application to testing
+materials. Jour. Franklin Inst., Vol. CLXVIII, 1909, pp.
+235-259; 336-364.
+
+----: Some results of dead load bending tests of timber by means
+of a recording deflectometer. Proc. Am. Soc. Test. Mat., Vol.
+IX, 1909, pp. 534-548.
+
+TJADEN, M.E.H.: Het Indrukken van Paalkoppen in Kespen. De
+Ingenieur, Sept. 11, 1909.
+
+----: Weerstand van Hout loodrecht op de Vezelrichting. _Ibid._,
+May, 1911.
+
+----: Buigvastheid van Hout. _Ibid._, May 31, 1913.
+
+TRAUTWINE, JOHN C.: Shearing strength of some American woods.
+Jour. Franklin Inst., Vol. CIX, 1880, pp. 105-106.
+
+TREDGOLD, THOMAS: Elementary principles of carpentry. London,
+1870.
+
+TURNBULL, W.: A practical treatise on the strength and stiffness
+of timber. London, 1833.
+
+Untersuchungen ueber den Einfluss des Blauwerdens auf die
+Festigkeit von Kiefernholz. Mitt. a. d. koenig. techn.
+Versuchsanstalten, I, 1897.
+
+Verfahren zur Pruefung v. Metallen und Legierungen, von
+hydraulischen Bindemitteln, von Holz, von Ton-, Steinzeug- und
+Zementroehren. Empfohlen v. dem in Bruessel v. 3-6, IX, 1906,
+abgeh. IV. Kongress des internationalen Verbandes f. die
+Materialpruefungen der Technik. Wien, 1907.
+
+WARREN, W.H.: Australian timbers. Sydney, 1892.
+
+----: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Sydney, 1911.
+
+----: The strength, elasticity, and other properties of New
+South Wales hardwood timbers. Proc. Int. Assn. Test. Mat., 1912,
+XXIII_{6}, pp. 9.
+
+----: The properties of New South Wales hardwood timbers.
+Builder, London, Nov. 1, 1912.
+
+----: The hardwood timbers of New South Wales, Australia. Jour.
+Soc. of Arts, London, Dec. 6, 1912.
+
+WELLINGTON, A.M.: Experiments on impregnated timber. Railroad
+Gazette, 1880.
+
+WIJKANDER, ----: Untersuchung der Festigkeitseigenschaften
+schwedischer Holzarten in der Materialpruefungsanstalt des
+Chalmers'schen Institutes ausgefuehrt. 1897.
+
+WING, CHARLES B.: Transverse strength of the Douglas fir. Eng.
+News, Vol. XXXIII, Mch. 14, 1895.
+
+
+
+PART III. PUBLICATIONS OF THE U.S. GOVERNMENT ON THE MECHANICAL
+            PROPERTIES OF WOOD, AND TIMBER TESTING
+
+
+MISCELLANEOUS
+
+
+House Misc. Doc. 42, pt. 9, 47th Cong., 2d sess., 1884. (Vol.
+IX, Tenth Census report.) Report on the forests of North America
+(exclusive of Mexico). Part II, The Woods of the United States.
+
+House Report No. 1442, 53d Cong., 2d sess. Investigations and
+tests of American timber. 1894, pp. 4.
+
+War Dept. Doc. 1. Resolutions of the conventions held at Munich,
+Dresden, Berlin, and Vienna, for the purpose of adopting uniform
+methods for testing construction materials with regard to their
+mechanical properties. By J. Bauschinger. Translated by O.M.
+Carter and E.A. Gieseler. 1896, pp. 44.
+
+War Dept. Doc. 11. On tests of construction materials.
+Translations from the French and from the German. By O.M. Carter
+and E.A. Gieseler. 1896, pp. 84.
+
+House Doc. No. 181, 55th Cong., 3d sess. Report upon the
+forestry investigations of the U.S. Department of Agriculture,
+1877-1898. By B.E. Fernow, 1899, pp. 401. Contains chapter on
+The work in timber physics in the Division of Forestry, by
+Filibert Roth, pp. 330-395.
+
+
+FOREST SERVICE
+
+
+Cir. 7--The Government timber tests [189-], pp. 4.
+
+Cir. 8--Strength of "boxed" or "turpentine" timber. 1892, pp. 4.
+
+Bul. 6--Timber Physics. Pt. I. Preliminary report. 1. Need of
+the investigation. 2. Scope and historical development of the
+science of "timber physics." 3. Organization and methods of
+timber examinations in the Division of Forestry. By B.E. Fernow,
+1892, pp. 57.
+
+Unnumbered Cir.--Instructions for the collection of test pieces
+of pines for timber investigations [1893], pp. 4.
+
+Cir. 9--Effect of turpentine gathering on the timber of longleaf
+pine. By B.E. Fernow [1893], p. 1.
+
+Bul. 8--Timber physics. Pt. II. Progress report. Results of
+investigations on longleaf pine. 1893, pp. 92.
+
+Bul. 10--Timber: an elementary discussion of the characteristics
+and properties of wood. By Filibert Roth. 1895, pp. 88.
+
+Bul. 12--Economical designing of timber trestle bridges. By A.L.
+Johnson, 1896, pp. 57.
+
+Cir. 12--Southern pine, mechanical and physical properties.
+1896, pp. 12.
+
+Cir. 15--Summary of mechanical tests on thirty-two species of
+American woods. 1897, pp. 12.
+
+Cir. 18--Progress in timber physics. 1898, pp. 20.
+
+Cir. 19--Progress in timber physics: Bald cypress (_Taxodium
+distichum_). By Filibert Roth, 1898, pp. 24.
+
+Y.B. Extr. 288--Tests on the physical properties of woods. By
+F.E. Olmstead, 1902, pp. 533-538.
+
+Unnumbered Cir.--Timber tests. [1903], pp. 15.
+
+Unnumbered Cir.--Timber preservation and timber testing at the
+Louisiana Purchase Exposition. 1904, pp. 6.
+
+Cir. 32--Progress report on the strength of structural timber.
+By W.K. Hatt, 1904, pp. 28.
+
+Bul. 58--The red gum. By Alfred Chittenden. Includes a
+discussion of The mechanical properties of red gum wood, by W.K.
+Hatt. 1905, pp. 56.
+
+Cir. 38--Instructions to engineers of timber tests. By W.K.
+Hatt, 1906, pp. 55. Revised edition, 1909, pp. 56.
+
+Cir. 39--Experiments on the strength of treated timber. By W.K.
+Hatt, 1906, pp. 31. Revised edition, 1908.
+
+Bul. 70--Effect of moisture upon the strength and stiffness of
+wood. By H.D. Tiemann, 1906, pp. 144.
+
+Cir. 46--Holding force of railroad spikes in wooden ties. By
+W.K. Hatt, 1906, pp. 7.
+
+Cir. 47--Strength of packing boxes of various woods. By W.K.
+Hatt, 1906, pp. 7.
+
+Cir. 108--The strength of wood as influenced by moisture. By
+H.D. Tiemann, 1907, pp. 42.
+
+Cir. 115--Second progress report on the strength of structural
+timber. By W.K. Hatt, 1907, pp. 39.
+
+Cir. 142--Tests of vehicle and implement woods. By H.B. Holroyd
+and H.S. Betts, 1908, pp. 29.
+
+Cir. 146--Experiments with railway cross-ties. By H.B. Eastman,
+1908, pp. 32.
+
+Cir. 179--Utilization of California eucalypts. By H.S. Betts and
+C. Stowell Smith, 1910, pp. 30.
+
+Bul. 75--California tanbark oak. Part II, Utilization of the
+wood of tanbark oak, by H.S. Betts, 1911, pp. 24-32.
+
+Bul. 88--Properties and uses of Douglas fir. By McGarvey Cline
+and J.B. Knapp, 1911, pp. 75.
+
+Cir. 189--Strength values for structural timbers. By McGarvey
+Cline, 1912, pp. 8.
+
+Cir. 193--Mechanical properties of redwood. By A.L. Heim, 1912,
+pp. 32.
+
+Bul. 108--Tests of structural timbers. By McGarvey Cline and
+A.L. Heim, 1912, pp. 1231.
+
+Bul. 112--Fire-killed Douglas fir: a study of its rate of
+deterioration, usability, and strength. By J.B. Knapp, 1912, pp.
+18.
+
+Bul. 115--Mechanical properties of western hemlock. By O.P.M.
+Goss, 1913, pp. 45.
+
+Bul. 122--Mechanical properties of western larch. By O.P.M.
+Goss, 1913, pp. 45.
+
+Cir. 213--Mechanical properties of woods grown in the United
+States. 1913, pp. 4.
+
+Cir. 214--Tests of packing boxes of various forms. By John A.
+Newlin, 1913, pp. 23.
+
+Review Forest Service Investigations. 1913. [Outline of
+investigations.] Vol. I, pp. 17-21. A microscopic study of the
+mechanical failure of wood, by Warren D. Brush. Vol. II, pp.
+33-38.
+
+Bul. 67, U.S.D.A.--Tests of Rocky Mountain woods for telephone
+poles. By Norman deW. Betts and A.L. Heim, 1914, pp. 28.
+
+Bul. 77, U.S.D.A.--Rocky Mountain mine timbers. By Norman deW.
+Betts, 1914, pp. 34.
+
+Bul. 86, U.S.D.A.--Tests of wooden barrels. By J.A. Newlin,
+1914, pp. 12.
+
+
+REPORTS OF TESTS ON THE STRENGTH OF STRUCTURAL MATERIAL, MADE AT
+THE WATERTOWN ARSENAL, MASS.
+
+
+House Ex. Doc. No. 12, 47th Cong., 1st sess., 1882. Strength of
+wood grown on the Pacific slope, pp. 19-93.
+
+Senate Ex. Doc. No. 1, 47th Cong., 2d sess., 1883. Resistance of
+white and yellow pines to forces of compression in the direction
+of the fibers, as used for columns, or posts, pp. 239-395.
+
+Senate Ex. Doc. No. 5, 48th Cong., 1st sess., 1884. Tests of
+California laurel wood by compression, indentation, shearing,
+transverse tension, pp. 223-236. Tests of North American woods
+(under supervision of Prof. C.S. Sargent in charge of the
+forestry division of the Tenth Census), with 16 photographs of
+fractures of American woods, pp. 237-347.
+
+Senate Ex. Doc. No. 35, 49th Cong., 1st sess., 1885. Adhesion of
+nails, spikes, and screws in various woods. Experiments on the
+resistance of cut nails, wire nails (steel), wood screws, lag
+screws in white pine, yellow pine, chestnut, white oak, and
+laurel, pp. 448-471.
+
+House Ex. Doc. No. 14, 51st Cong., 1st sess., 1890. Adhesion of
+spikes and bolts in railroad ties, pp. 595-617.
+
+House Ex. Doc. No. 161, 52d Cong., 1st sess., 1892. Adhesion of
+nails in wood, pp. 744-745.
+
+House Ex. Doc. No. 92, 53d Cong., 3d sess., 1895.
+Woods--compression tests (endwise compression), pp. 471-476.
+
+House Doc. No. 54, 54th Cong., 1st sess., 1896. Compression
+tests on Douglas fir wood, pp. 536-563. Expansion and
+contraction of oak and pine wood, pp. 567-574.
+
+House Doc. No. 164, 55th Cong., 2d sess., 1898. Compression
+tests of timber posts, pp. 405-411. New posts of yellow pine and
+spruce, pp. 413-450; Old yellow pine posts from Boston Fire
+Brick Co. building, No. 394 Federal St., Boston, Mass., pp.
+451-473.
+
+House Doc. No. 143, 55th Cong., 3d sess., 1899. Fire-proofed
+wood (endwise and transverse tests), pp. 676-681.
+
+House Doc. No. 190, 56th Cong., 2d sess., 1901. Cypress wood for
+United States Engineer Corps; compression and transverse tests,
+pp. 1121-1126. Old white pine and red oak from roof trusses of
+Old South Church, Boston, Mass., pp. 1127-1130. Compression of
+rubber, balata, and wood buffers, pp. 1149-1158.
+
+House Doc. No. 335, 57th Cong., 2d sess., 1903. Douglas fir and
+white oak woods. Transverse and shearing tests; also
+observations on heat conductivity of sticks over wood fires and
+a stick exposed to low temperature. Expansion crosswise the
+grain of wood after submersion, pp. 519-561. Adhesion of lag
+screws and bolts in wood, pp. 563-578.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Mechanical Properties of Wood
+by Samuel J. Record
+
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