board one-eighth inch thick (Pl. X, A, p. 48). The testing of the cylinders is described under the next heading.

MODULUS OF ELASTICITY IN COMPRESSION.

Apparatus.—The compressometer used in measuring the vertical deformations of the cylinders during testing is shown in Pl. IX, C. The gage length is exactly 12 inches. The micrometers measure to one ten-thousandth inch, and the contact is read by listening for the making and breaking of the electric circuit in the usual telephone operator's receivers (shown at the left in Pl. IX, C), which are held. to the ears of the operator by clamps.

Method. The ends of the cylinders are smoothed off with plaster of Paris at right angles to the axes a short time before testing. This insures an even distribution of the breaking load over the ends. For this purpose a rigid table is used, the top of which is horizontal and consists of a heavy piece of plate glass about 1 foot square. After the glass has been oiled, plaster of Paris is spread on it about onefourth inch thick, into which, before it sets, the cylinder is forced, so that all but a layer about one-sixteenth to one-eighth inch thick is squeezed out. The sides of the cylinder are made vertical by means of a spirit level. After the plaster at one end has set, the other end is treated in the same way. The cylinders thus prepared are placed in the testing machine without further bedding.

The load is applied continuously at a speed of about one-fortieth inch per minute, and readings of the gross deformations are taken at 5,000-pound intervals or at about 100 pounds per square inch. The load at first crack, the ultimate load, and the appearance of the ruptured cylinder are recorded, the latter in the form of a sketch. In Pl. IX, A and B, are shown typical forms of failure. In the example shown in B the rupture has occurred by shearing through the aggregate. Pl. IX, D, shows three typical cones from broken cylinders.

DENSITY TESTS.

Apparatus. The tests for density of concrete are made in cylinder molds of wrought-iron pipe similar to those used for the determination of the density of cement mortar, except that they are 8 inches in diameter and 9 inches long, or 10 inches in diameter and 12 inches long.

Method. The test is made in the same manner as that for density of mortar. In the series of tests now under way the proportions of cement and large and small aggregates are varied in order to obtain the mixture that will produce concrete with the greatest density. The ratio between the cement and the total aggregate is kept constant (1:9), while the ratio between the small and large aggregate is varied.

In another series about to be started this ratio of cement to total aggregate will be 1:6. Four or five trials are usually sufficient to determine the proportions giving a concrete of the greatest density. The results of these tests are recorded on Form D.

Form E is the log sheet upon which the micrometer readings in tests of modulus of elasticity are recorded, and Form F is used at the time of molding the test pieces. A record is made in the batch report of the basis upon which the proportioning was done, whether by volume, maximum density, or percentage of voids.

Time of placing in molds.... Batch used in specimens No....... Corresponding reports No.... Remarks.

Computations. The proportions by weight entered on the batch report are determined from the weight per cubic foot of the material and the weight of the moisture contained in it and correspond to the proportions by volume.

The percentage of water added in the mixer plus the percentage of moisture contained in the material is found in terms of the dry material.

Densities are calculated in the same way as for the sand mortar, except that in this case there are three ingredients instead of two.

The initial modulus of elasticity is determined from the slope of the tangent drawn at the origin of a curve whose abscissas are the average unit deformations obtained from the micrometer readings on either side of the cylinder and whose ordinates are the unit stresses. Forms G, H, I, and J are used for recording the results of these physical tests.

Apparatus. The large blocks of stone sent in by the field geologist, having been roughly dressed by hammer and chisel, are sawed into cubes by the gang saw (Pl. XI, C, p. 50) and finally ground to exactly 2 inches on an edge by rapidly revolving horizontal cast-iron disks about 18 inches in diameter.

Methods. The position in which the blocks were bedded in the quarry is indicated on the stone by the collector and this mark is transferred to the finished cubes. Half of the cubes sawed out of the blocks are then crushed with the bedding plane perpendicular and the other half with it parallel to the direction of application of the load,