Regarding the distribution of the silt at various depths, in water 5 to 17 feet deep, the quantity of silt near the bed may, when the charge is heavy and consists of mixed silt, be 1¼ to 3 times that at the surface. If the charge is fine mud, there is likely to be as much silt at the surface as near the bed, if sand, there may be none at the surface and little in the upper part of the stream. In all cases single observations are likely to show extraordinarily discordant results; a number of observations must be made at each point and averaged. 6. =Practical Formulæ and Figures.=--A stream which carries silt generally rolls materials along its bed. The proportion between the quantities of material rolled and carried is never known, and this makes it impossible to frame an exact formula applicable to such cases, but Kennedy, from his observations on canals fully charged with the heavy silt and fine sand usually found in Indian rivers near the hills, arrived at the empirical formula for critical velocities V = ·84 D^{·64} The observations were made on the Bari Doab Canal and its branches, the widths of the channels varying from 8 feet to 91 feet, and the depths of water from 2·3 feet to 7·3 feet. The beds of these channels have, in the course of years, adjusted themselves by silting or scouring, so that there is a state of permanent régime, each stream carrying its full charge of silt, and the charges in all being about equal. From further observations referred to above (_Art. 3_, par. 2) it appears that this kind of silt forms about 1/3300 of the volume of the water, and that on the Sirhind Canal, sand coarser than the (·10) class, formed 1/35,000 of the volume of water. The formula gives the following critical velocities for various depths:-- D = 1 2 3 4 5 6 7 V_{0} = ·84 1·30 1·70 2·04 2·35 2·64 2·92 D = 8 9 10 V_{0} = 3·18 3·43 3·67. In Indian rivers not near the hills the silt carried is not so heavy, and the critical velocities are supposed to be about three-fourths of the above. Thrupp (_Min. Proc. Inst. C.E._, vol. clxxi.) gives the following ranges of velocities as those which will enable streams to carry different kinds of silt. It does not appear that the streams would be fully charged except at the higher figure given for each case. D = 1·0 10·0 V = 1·5 to 2·3 3·5 to 4·5 (Coarse sand). V = ·95 to 1·5 2·3 to 3·5 (Heavy silt and fine sand). V = ·45 to ·95 1·2 to 2·3 (Fine silt).