water absorbed is determined by weighing the material after surface drying with blotters and filter paper, and again when dry.

Computations. The weight in grams of the flask is known and the following quantities are measured and recorded, the weights being in grams and the volumes in cubic centimeters: (1) Weight of flask and sand; (2) weight of flask, sand, and water; (3) weight of surface-dried sand; (4) weight of dry sand; (5) volume of sand and water, read on the neck of the flask.

From these data the original weight in grams of the dry sand, the weight in grams of water absorbed per gram of dry sand, and the total weight in grams of water absorbed are computed. The difference between the volume of the sand and water read on the neck of the flask and the volume of the water in cubic centimeters in the flask corrected for absorption gives the volume of the sand in cubic centimeters. Finally, the ratio between the weight in grams and the volume in cubic centimeters of the dry sand gives the specific gravity.

PERCENTAGE OF SILT.

Apparatus.-The apparatus used in this determination is shown in Pl. VI, B. The glass percolator (a) is 13 inches long, with an upper inside diameter of 3 inches and a lower inside diameter of 2 inches. The vessel (b) is placed with its outlet (c) 3 feet above the top of the percolator. The opening (d) at the bottom of the percolator is one-half inch in diameter, and is fitted with a perforated porcelain disk to prevent the passage of the material placed in the vessel. The glass tube (e) is placed with its lower end 10 inches above the surface of the material.

Method. The vessel (b) and the percolator (a) are filled with clean water, and a 100- to 200-gram sample, depending upon the probable amount of silt present, is put in the latter. The upper stopper of the percolator is put in place, and the current of water is started. The silt is carried by the current of water over into the vessel f. The flow of water is continued until the effluent is clear, when the upper stopper is removed, the material stirred, and the current again started. This operation is repeated until the effluent is clear immediately after the material is stirred. The water containing the silt is evaporated on a water bath, when the silt is scraped and brushed onto a watch glass and permitted to remain uncovered until it attains the temperature of the room so that it will be under the same conditions as the original sample. It is then weighed. The silt thus obtained is chemically analyzed.

Computations. The percentage of silt is the weight of the silt divided by the weight of the material placed in the percolator and multiplied by 100.

GRANULOMETRIC ANALYSIS.

Apparatus. Two sets of screens are used for the granulometric analysis; one for material larger than one-fourth inch and one for smaller material. The set of large-mesh screens (openings 1, 1, 1, 1, 1, 11, 12, and 2 inches) are 2 feet wide, 4 feet long, and 6 inches deep, and are provided with hooks at the corners by which they may be suspended by ropes passing through pulleys at the ceiling (Pl. VII). A shallow wooden box (a, Pl. VII, A) fits beneath each of the two finest screens so that material passing these screens will be caught in the box with the loss of as little fine material as possible. When the material has been placed upon the screen, the latter is raised to a convenient height by means of the ropes and is repeatedly pulled out about 18 inches from the post at b and released, so as to bump against the post. The impact violently jars the screen and aids the material in passing through.

The set of smaller-mesh screens (openings 10, 20, 30, 40, 50, 80, 100, and 200 to the linear inch) are brass circular hand sieves, 8 inches in diameter, and fit one upon another, each set being provided with a cover and a bottom pan. A Howard & Morse power sifter is used, which gives the nest of sieves a rotary motion, violently reversed after a part revolution, and at the same time a vertical bumping motion.

Method. A 100-pound sample is dried to such a degree that it will not clog the screens. It is placed upon the 1-inch screen and the material passing is reserved. That portion remaining on the screen is then run onto the screen of next larger mesh (Pl. VII, B) and the operation repeated. Each sifting is continued until the material ceases to pass through the sieve. The material passing each screen is weighed. A 500-gram sample is then taken from the material that passes the 1-inch screen and is placed upon the upper of the nest of small sieves and sifted for fifteen minutes, when the residue on each sieve and the material passing the No. 200 sieve are weighed.

Computations. The percentages recorded represent the residues on each of the fine and the coarse sieves, and are given in terms of the weight of the original sample.

TESTS OF MORTAR.

DESCRIPTION.

The sand and screenings received at the laboratories are made into mortar, using different proportions and sizes of material. This mortar is then investigated as to tensile, compressive, and transverse strength, and as to density. Should the material contain particles larger than one-fourth inch, these are removed by the use of a 1-inch screen.

The

mortars are made with one part typical Portland cement in proportions of 1:3 and 1:4, and in addition a 1:3 mortar is made from sand screened to one size between Nos. 30 and 40 sieves and from stone screenings sifted to one size between Nos. 10 and 20 sieves. These mortars are molded into tensile briquets, 2-inch cubes, and transverse test pieces of 1-inch cross section 13 inches long.

STRENGTH TESTS.

Apparatus. An improved Fairbanks shot machine is used for the tension tests, and a 40,000-pound capacity oil-pressure hand-operated machine, shown in Pl. III, A (p. 22), is used for the compression tests. The transverse tests are made either on the 10,000-pound wire tester, which is fitted with transverse tools, or on a long-lever 2,000pound Tinius Olsen & Co. machine with a special bearing made by the same company, as shown in Pl. III, C. When transverse specimens are being tested, the heavy counterpoise is replaced by a light wooden one loaded with shot. This requires a greater movement of the poise to balance the same load, and thus permits the small loads sustained by the beams to be more accurately measured than with the original counterpoise.

Methods. The methods recommended by the special committee on uniform methods of the American Society of Civil Engineers are used in the mixing and molding. The test pieces are placed in the testing machine upon their sides (with reference to the position in which they are molded).

Computations. The only computations necessary are those for obtaining the unit strengths from the gross loads read at the machine and the averages of the three tests in each case.

DENSITY TESTS.

Apparatus.-The implements used in the density tests are shown in Pl. VIII, A. Two sizes of molds are used in order to determine which size gives the more uniform results. After this has been decided, it is proposed to abandon the one giving the less consistent results. The molds are sections of wrought-iron pipe capped at one end. One is 31⁄2 inches in diameter and 41⁄2 inches deep, while the other is 4 inches in diameter and 5 inches deep, inside dimensions. The diameter of the tamper head is one-half the diameter of the cylinder in each case, so that when the tamper is moved around the inner circumference of the cylinder and kept in contact with the mold the entire surface of the mortar in the cylinder will be tamped. The table top is of slate.

Method. The required proportions of the dry material are carefully weighed out, and water is added to form a normal consistency. The