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.

|------------------------------------------------------

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|                             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   |