"The stones employed in making the abrasion test are about the size used in making macadam roads—between 6.31 cm. (2.5 inches) and 3.18 cm. (1.25 inches) in diameter. In making a test 5 kilogrammes (11 pounds) of perfectly clean stone of the above dimensions are placed in one of the cylinders; the cover is then \bolted on, and the cylinder rotated at the rate of 2,000 revolu tkons per hour for five hours. Four tests can be made at once by using all four cylinders. At each revolution of the shaft the fragments of stone are thrown twice from one end of the cylinder 4A) the other, which grinds them against one another and against the walls of the cylinder. After 10,000 revolutions the machine is stopped, the cylinder opened, and the contents placed on a sieve having 0.16 cm. inch) meshes. The material that passes through the sieve is put aside for the cementation test. The sieve and the remaining fragments of stone are then held under running water until all the adhering dust is washed off. After these re maining fragments have thoroughly dried they are carefully weighed, and their weight is subtracted from 5 kilogrammes (11 pounds)—the original weight of all the stone in the test. The difference obtained is the weight of the detritus under 0.16 cm. inch) worn off by the test." The relative resistance of stones in the abrasion test is expressed either (1) by the Per Cent of Wear, i. e., by the ratio of the weight of the dust worn off to the original weight of the stone, or (2) by the Co-efficient of Wear adopted by the National School of Roads and Bridges of France. The latter is represented by the formula:
in which w is the weight in grammes of detritus under 0.16 cm. inch) in size obtained per kilogramme (2.2 pounds) of stone. The French road engineers usually regard a co-efficient of 20 as indicating a stone excellent for road purposes, and one of 10 as sufficiently good. The larger the Co-efficient of Wear the better the stone; while a small Per Cent of Wear indicates a good stone. The Per Cent of Wear is equal to forty divided by the Co-efficient of Wear; and vice versa, the latter is equal to forty divided by the former.
Table 19, page 186, gives the Per Cent of Wear obtained by the Massachusetts Highway Commission; and Table 20, page 187, gives similar results obtained under the auspices of the U. S. Agri cultural Department—in both cases under the direction of Mr. Logan Waller Page. In Table 20 the Per Cent of Wear is the total percentage of all material less than 11 inches, while the French Co-efficient of Wear is based upon the percentage of material less than inch in diameter; and therefore in Table 20 these results are not related as stated in the preceding paragraph.
In 1901 the U. S. Road Material Laboratory introduced a new term to express the quality of a road-building material, but called it Co-efficient of Wear. In Table 18 this new term is des ignated Co-efficient of Wear (U. S. A. D.), the initials in the paren thesis standing for U. S. Agricultural Department, and distin guishes this Co-efficient from the similar French Co-efficient. The