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.