It has been known for fifty years that a beam or rod would fail at a relatively low stress if only repeated often enough. It has been found, however, that each material possesses a limiting stress, or endurance limit, within which it is safe, no matter how often the loading occurs. That limiting stress for all steels so far investigated causes fracture below 10 million reversals. In other words, a steel which will not break before 10,000,000 reversals can confidently be expected to endure 100,000,000, and doubtless into the billions. About the only way to test one piece such a large number of times is to fashion it into a beam, load it, and then turn the beam in its supports. Thus the stress in the outer fibers of the bar varies from a maximum stretch through zero to a maximum compression, and back again. A simple machine of this sort is shown in Fig. 10, where _B_ and _E_ are bearings, _A_ the test piece, turned slightly down in the center, _C_ and _D_ ball bearings supporting a load _W_. _K_ is a pulley for driving the machine and _N_ is a counter. [Illustration: FIG. 10.--Sketch of rotating beam machine for measuring endurance of metal.] HARDNESS TESTING The word "hardness" is used to express various properties of metals, and is measured in as many different ways. "Scratch hardness" is used by the geologist, who has constructed "Moh's scale" as follows: Talc has a hardness of 1 Rock Salt has a hardness of 2 Calcite has a hardness of 3 Fluorite has a hardness of 4 Apatite has a hardness of 5 Feldspar has a hardness of 6 Quartz has a hardness of 7 Topaz has a hardness of 8 Corundum has a hardness of 9 Diamond has a hardness of 10 A mineral will scratch all those above it in the series, and will be scratched by those below. A weighted diamond cone drawn slowly over a surface will leave a path the width of which (measured by a microscope) varies inversely as the scratch hardness. "Cutting hardness" is measured by a standardized drilling machine, and has a limited application in machine-shop practice. "Rebounding hardness" is commonly measured by the Shore scleroscope, illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter, 3/4 in. in length, and weighing about 1/12 oz. is dropped a distance of 10 in. upon the test piece. The height of rebound in arbitrary units represents the hardness numeral. [Illustration: FIG. 11.--Shore scleroscope.]