" It is most important that these briquettes should be compressed in a uniform manner, and for this a special machine has been designed. The die is placed on an iron platform supported by a piston rod, which is connected directly with a hydraulic piston below. Water from a tank is admitted to the hydraulic .cylinder through a small orifice in the pipe. As the piston rises the platform and die are carried up with it, the plug of the latter coming in contact with a yoke attached to a properly weighted lever arm. When the lever arm is raised one-eighth of an inch it closes an electric circuit which trips a right angle cock, shutting off the water and opening the exhaust. One minute is required to compress a briquette, and the maximum load is applied only for an instant. By this device practically uniform conditions are obtained.
"The height of the briquette is measured, and if it is not exactly 25 millimeters' the required amount of material is added or subtracted to make the next briquette the required height. Five briquettes are made from each test sample, and allowed to dry twelve hours in air and twelve hours in a steam bath. After cooling in a desiccator they are tested by impact in a machine especially designed for the purpose." The machine commonly used for this purpose is known as the Page-Johnson Impact Machine. It was designed by Mr. L. W. Page and afterward modified by Mr. A. N. Johnson. The blow is delivered by a hammer weighing one kilogram striking upon a flat-end plunger, which is pressed upon the briquette by two light spiral springs. The standard fall of the hammer for a test is i centimeter (0.39 inch), and this blow is repeated until the bond of cementation of the material is destroyed. The number of blows required is noted and the average obtained upon five briquettes is given as the cementing value.
In making this test the results may be considerably affected by slight differences in manipulating the material. It is important that the same amount of kneading be used in all tests and that the dough should be allowed to stand at least 24 hours before forming the cylinders.
Grinding Test. The test for abrasion by grinding is sometimes used in France, where it is known as the Dorry test. It has also been used by the Office of
Public Roads at Washington. The object of the test is to give a measure of the hardness of the rock. It gives interesting information concerning the material, but is not of special value in testing road material. The test is made as follows: " The test piece in the form of a cylinder about 3 inches in length by i inch in diameter is prepared by an annular core drill and placed in the grinding machine in such a manner that the base of the cylinder rests on the upper surface of a circular grinding disk of cast iron, which is rotated in a horizontal plane by a crank movement. The specimen is weighted so as to exert a pressure of 25o grams per square centimeter against the disk, which is fed from a funnel with sand of about i + millimeters in diameter. After moo revolutions the loss in weight of the sample is determined and the coefficient of wear obtained by deducting one-third of this loss from 2o." Impact Test. This test is intended as a measure of the toughness of the material. It is frequently made, although not of special value as a test for road material. It is made as follows: "The test piece is a cylindrical rock core similar to that used in determining and the test is made with an impact machine con structed on the principle of the pile driver. .The blow is delivered by a hammer weighing 2 kilograms, which is raised by a sprocket chain and released automatically by a concentric electromagnet. The test consists of a centimeter fall of the hammer for the first blow and an increased fall of I centimeter for each succeeding blow until the failure of the test piece occurs. The number of blows required to cause this failure repre sents the toughness. " The abrasion and cementation tests are frequently employed for the purpose of comparing the properties of various road stones, and afford a means by which a judgment may be formed as to the probable relative values of various materials for road construction. No fixed standard for comparison has been devised, and the relative importance of the various properties depends upon the character of the road to be constructed.