The method of distribution from a distributary to the watercourses varies. In many modern canals there is, at each watercourse head, a sluice which is adjusted at frequent intervals according to the supply and the demand. One method, which is excellent because it fulfils in the highest degree the principle of keeping the water in bulk, is to have very large watercourses and, by means of regulators which are built at frequent intervals, to turn the whole of the water of the distributary into a few watercourses at a time, beginning with those nearest the head of the distributary and working downstream. But a system which seems eminently suitable may be impracticable because of local circumstances. In India, any such arrangement would need an army of officials and would lead to unbounded corruption. In India the water from a distributary enters the watercourses through “outlets” which are small masonry tubes passing through the banks of the distributary. There is no easy way of closing these outlets or at least of keeping them closed if the cultivators choose to open them, but it is easy to close a whole distributary and so regulate the supply. This is the chief reason why watercourses in India do not usually take off direct from the canals. The presence of silt in the water of a river from which a canal is drawn is often spoken of as being a great evil. If it is an evil at all it is a very mixed evil. The deposits of silt in the channels have been enormously reduced by the application of scientific principles of design. The clayey silt which remains in the water and reaches the fields, brings to them greatly increased fertility. In India the fertility of the soil is often reduced or destroyed by the formation on the surface of the ground of an efflorescence called “reh.” It consists of various salts or compounds of sodium and occurs chiefly where there is an impervious layer of subsoil. The salts exist as an ingredient of the upper soil. This becomes saturated with rain or canal water and as the water evaporates the salts are left on the surface. Remedies are drainage, or flooding the soil and running the water off, or deep tilling, or chemical treatment with lime or gypsum. (_Indian Engineering, 8th Jan., 1910_). The inundation canals of the Punjab have been described in _Punjab Rivers and Works_. All descriptions and remarks in the present book regarding Indian canals must be assumed to refer to perennial canals unless the contrary is stated or implied.