Let a stream be carrying a full charge of any kind of silt. Then if there is any reduction in velocity, a deposit will occur--unless there is also a reduction of depth--until the charge of silt is reduced again to the full charge for the stream. The deposit generally occurs slowly, and extends over a considerable length of channel. The heavier materials are, of course, deposited first. If a stream is not fully charged, it tends to become so by scouring its channel. It is generally believed that a stream fully charged with silt cannot scour silt from its channel, or bear any introduction of further silt. This seems to be correct in the main, but the remarks made in the latter part of the preceding paragraph must be taken into consideration. It has been stated (_Art. 2_) that a weir or a sudden rise in the bed does not necessarily cause an accumulation of rolled material. It never causes a deposit of suspended material unless it causes a heading up and reduction of velocity to below the critical velocity. 4. =Methods of Investigation.=--The quantity of silt in water is found by taking specimens of the water and evaporating it or, if the silt is present in great quantity, leaving it to settle for twelve hours--an ounce of alum can be added for every 10 cubic feet of water to accelerate settlement--drawing off the water by a syphon, and heating the deposit to dry it. The deposit is then measured or weighed. It is best to weigh it. If clay is filled into a measure, the volume depends greatly on the manner in which it is filled in. When silt deposits in large quantities in a channel, or when heavy scour occurs, the volume deposited or scoured is ascertained by taking careful sections of the channel. [Illustration: FIG. 1.] Silt is best classified by observing its rate of fall through still water. A sand which falls at ·10 feet per second is, in India, called class (·1), and mixed sand which falls at rates varying from ·1 to ·2 feet per second is called class ·1/·2. Fig. 1 shows a sand separator designed by Kennedy. The scale is ⅛. It has a syphon action, and the rate of flow can be altered by altering the length of the exit pipe. Suppose it is desired to measure the sand of class (·10) and all heavier kinds. The pipe is adjusted so as to give a velocity of ·1 foot per second to the upward flowing water, which then carries off all silt of class (·10) or finer. All heavier silt falls into the glass tube. It can be separated again by being mixed with water and passed through the instrument again, the velocity of flow through the instrument being increased.