The laws of silting and scouring and the principles on which channels should be designed are fully gone into in _River and Canal Engineering_. It is there explained that, for a channel of depth D, there is a certain critical velocity, V₀, which just prevents the deposit of the silt, consisting of heavy clay and fine sand, found in Indian rivers--this silt enters the canal in such immense quantities that the canal silt clearances would be impossible if much of it was deposited in the channels--that sand of grades heavier than [·1] may deposit in the head of a canal and well nigh threaten its existence, that the clear water entering the canal in winter may pick up and carry on some of the sand but that proper steps for preventing the deposit in the canal can be taken at the headworks. This last question has been referred to in Art. 1. The following additional rules for designing canals in Northern India are chiefly taken from those given by Kennedy in the explanatory notes to his Hydraulic Diagrams, which are in use in the Irrigation Branch in Northern India. (1) Near the hills where the bed is of shingle the velocity may exceed V₀. A few other soils will stand 1·1 V₀. (2) In ordinary channels any excess over V₀ will give much trouble lower down. (3) In the first four or five miles of a distributary, V₀ should be allowed and gradually be reduced to ·85 V₀ at the tail, the gradient being reduced if convenient, while a minor or branch distributary should have less than V₀ at its off-take and still less at the tail. The sand is drawn off by the outlets and in the lower part of a distributary it is often non-existent. (4) If there is efficient silt trapping at the head of the canal any figures arrived at by the preceding rules should be multiplied by ·9. (5) In the case of a canal having its head far from the hills, the sand is finer and any figures arrived at as above may be multiplied by, perhaps, about ·75, but further experience is needed to decide this. (6) If the soil is very poor, especially if the depth of water is more than 6 or 7 feet, the velocity should be less than V₀--say ·9 V₀--so as not to cause falling in of the banks. Depths of more than 9 or 9·5 feet should, as far as possible, be avoided for the same reason. (7) At a bifurcation, one branch channel may have no raised sill, and, owing to its smaller depth, it may draw off no surface water and get an undue share of rolling sand. Its velocity should be greater than V₀ and that of the other branch be less than V₀.