_Method_: In making an impact bending test the hammer is allowed to rest upon the specimen and a zero or datum line is drawn. The hammer is then dropped from increasing heights and drum records taken until first failure. The first drop is one inch and the increase is by increments of one inch until a height of ten inches is reached, after which increments of two inches are used until complete failure occurs or 6-inch deflection is secured. The 50-pound hammer is used when with drops up to 68 inches it is reasonably certain it will produce complete failure or 6-inch deflection in the case of all specimens of a species; for all other species a 100-pound hammer is used. _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 formulæ 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.