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.