The U. S. Bureau of Soils uses for the separation of clay, instead of subsidence for 24 hours, the more expeditious process of centrifuging the turbid soil water in appropriate glass cylinders, by the aid of an electric motor; and thus in a relatively short time obtains “clay” in which the upper limit of size is one-half of that mentioned above, viz., .005 mm. But for the costliness of the appliances required, including the entire time of an operator, this method of separating the clay would undoubtedly be preferable to the elimination by subsidence; the more as a more minute grain-size for the clay group is thus secured. [Illustration: FIG. 6.—Schöne’s Elutriator.] The separation of the clay having been accomplished, the various sizes of silt and sand may be separated by again suspending them in water; and interrupting the settling process at stated times, the grain-sizes corresponding to definite velocities in settling may be segregated and weighed. When this process of settling and decanting is carefully and repeatedly carried out, very good results are obtained. _Hydraulic Elutriation._—The sedimentation (or “beaker”) method, long practiced in the arts is, however, quite tedious, requiring the constant close attention of a skilled observer. The desired results may, in the writer’s judgment, be more conveniently obtained by the hydraulic method, whenever no very large volume of work of this kind is required to be done at once. When instead of allowing the soil to settle in quiet water, the latter is used as an ascending current of regularly graded velocities, it is clear that the soil particles will be carried off by this current in exact conformity with their several sizes (or strictly speaking, volume-weights); and when maintained in such a current for a sufficient length of time, the entire quantity of the sediment corresponding to the prevailing velocity will be carried away. It is of course easy to ascertain to what grain-sizes certain velocities of the upward current (regulated by a stopcock with arm moving on a graduated scale) correspond, and to regulate accordingly the intervals between the different velocities to greater or less detail, as may be desired. A number of instruments have been devised for this purpose.