3. =Materials carried in Suspension.=--It has long been known that the scouring and transporting power of a stream increases with its velocity. Observations made by Kennedy have shown that its power to carry silt decreases as the depth of water increases (_Min. Proc. Inst. C.E._, vol. cxix.). The power is probably derived from the eddies which are produced at the bed. Every suspended particle tends to sink, if its specific gravity is greater than unity. It is prevented from sinking by the upward components of the eddies. If V is the velocity of the stream and D its depth, the force exerted by the eddies generated on 1 square foot of the bed is greater as the velocity is greater, and is probably as V^2 or thereabouts. But, given the charge of silt, the weight of silt in a vertical column of water whose base is 1 square foot is as D. Therefore the power of a stream to support silt is as V^2 and inversely as D. The silt charge which a stream of depth D can carry is as V^{½}. V is called the “critical velocity” for that depth, and is designated as V_{0}. The full charge must be affected by the nature of the silt. The specific gravity of fine mud is not much greater than that of water, while that of sand is about 1·5 times as great. Moreover, the particles of sand are far larger than the particles of mud. If two streams of equal depths and velocities are fully charged, one with particles of mud and the other with particles of sand, the latter will sink more rapidly and will have to be more frequently thrown up. They will be fewer in number. From some observations referred to by Kennedy (_Punjab Irrigation Paper_, No. 9, “Silt and Scour in the Sirhind Canal,” 1904), it appears that in a fully charged stream which carried 1/3300 of its volume of a mixture of mud and sand of various grades, sand of a particular degree of coarseness formed only 1/35,000 of the volume of the water, but that when the same stream was clear and was turned on to a bed of the coarse sand it took up 1/15,000 of its volume. It would thus appear that the full charge of silt is less as its coarseness and heaviness are greater. This is in accordance with the laws mentioned above (_Art. 2_, par. 1). See also CHAP. V., _Art. 2_, last paragraph. It is probable that fine mud is carried almost equally into all parts of the stream, whereas sand is nearly always found in greater proportion near the bed and, as before remarked, some materials may be rolled and suspended alternately. The charge of mixed silt which a stream can carry is, no doubt, something between the charge which it can carry of each kind separately, but the laws of this part of the subject are not yet fully known. From the observations above referred to, Kennedy concludes that a canal with velocity V_{0} will carry in suspension 1/3300 to 1/5000 of its volume of silt, according as it is charged with sand of all classes or only with the heavier classes.