s 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 wh