The cylinder is marked exactly 200 millimeters below the surface of the water with a narrow strip of paper at _a_, stoppered, inverted and well shaken. The cylinder being again placed in normal position the soil particles under the influence of gravity tend to sink with greater or less rapidity according to their size. The siphon _a b c_ is filled with water, the cock at _c_ closed and the opened end _a_ placed in the cylinder A just at the mark 200 millimeters below the surface of the water, and the water thus transferred to B when desired. If the suspended matter is allowed to stand for 100 seconds the particles of more than two millimeters hydraulic value will have fallen below the open end of the siphon. If allowed to stand 1,000 seconds the silt value of the particles will be 0.2 millimeter per second. Whatever the number of seconds may be, the operation is continued until the water removed is practically clear. The open end of the siphon _a_ should be bent upwards so that no disturbing current may bring the particles below the line into the liquid discharged into B. [Illustration: FIGURE 26. SIPHON CYLINDER FOR SILT ANALYSIS. ] While the results obtained by this method are satisfactory as compared with other similar processes it cannot be highly recommended because of the time and trouble required to get a complete separation and by reason of the difficulty of collecting the separated silt. =202. Wolff’s Method.=[137]—As modified by Wolff the Knop process is conducted as follows: Fifty grams of fine earth are boiled with water and then the entire mixture is passed through three sieves with openings of one millimeter, 0.5 millimeter, and 0.25 millimeter in diameter, respectively. The finest part is mixed with water to a height of eighteen centimeters in a bottle twenty centimeters high and having a capacity of one liter and thoroughly agitated, after which it is left to rest, and finally the turbid liquid is drawn off with a siphon, the bottle refilled with water, agitated, and left to rest, and the process repeated as long as the water carries any suspended matter after a definite time. Wolff proposes for the first three periods of rest one hour, for the second three, a half an hour, for the third three, a quarter of an hour, and for the fourth three, five minutes. =203. Moore’s Modification of Knop’s Method.=[138]—The sample of soil is first passed through a sieve having round perforations three millimeters in diameter. The weight of the particles remaining on the sieve is then determined, and likewise that of the portion passing through, which is known as fine earth. The last named portion constitutes the material for all subsequent operations of mechanical and chemical analysis.