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