grain sizes of one to eight millimeters hydraulic value lower velocities are sufficient; too low a velocity causes an indefinite duration of the operation and may be recognized by the increase of turbidity as the velocity is increased. As the whirling agitation caused by the rotation of the dasher would gradually communicate itself to the whole column of water and cause irregularities, a wire screen of 0.8 millimeter aperture is cemented to the lower base of the cylinder. The relay vessel should be a thick, conical test glass with foot; its object is to serve as a reservoir for the heavy sediments not concerned at the velocity used in the elutriator tube, and whose presence in the latter or in its base, the churn, would only cause abrasion of the grains and changes of current velocity, such as occur in the apparatus of Schöne, and compel the current measurement of the water delivered. It is connected above with the churn by a brass tube about ten millimeters in clear diameter, so as to facilitate the descent of the superfluous sediments, which the operator, knowing the proportion of area between the connecting tube and elutriator, can carry to any desired extent; thus avoiding the disturbance of the gauged current velocities, as well as all material abrasion. [Illustration: FIGURE 36. IMPROVED SCHÖNE’S APPARATUS WITH RELAY. ] A glass delivery tube should extend quite half way down the sides of the relay vessel, to insure a full stirring up of the coarse sediments when required. By means of a rubber hose, not less than twenty inches in length, this delivery tube connects with the siphon carrying the water from near the bottom of the Mariotte’s bottle, a ten-gallon acid carboy. A stop-cock provided with a long, stiff index lever, moving on an empirically graduated arc, regulates the delivery of water through the siphon. Knowing the area of the cross section of the elutriator tube, the number of cubic centimeters of water which should pass through it in one minute, at one millimeter velocity, is easily calculated, and from this the lever positions corresponding to other velocities are quickly determined and marked on the graduated arc. The receiving bottle for the sediments, also shown in the figure, must be wide and tall, so as to allow the sediment to settle while the water flows from the top into the waste pipe. The receiving funnel tube must dip nearly to the bottom of the bottle. Thus arranged, the instrument works very satisfactorily, and by its aid soils and clays may readily be separated into sediments of any hydraulic value desired. But in order to insure correct and concordant results, it is necessary to observe some precautions; _viz._, (1) The tube of the instrument must be as nearly cylindrical as possible and must be placed and maintained in a truly vertical position. A very slight variation from the vertical at once causes the formation of return currents, and hence of molecular aggregates on the lower side.