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.