The cost of lining a million square feet of channel with oil, cement and puddle is estimated at Rs 30,000, Rs 27,500 and Rs 35,000 respectively. Allowance has to be made for the fact that watercourses flow intermittently, and that a lined channel gives no saving when it is not in flow, also that extensions of canals might have to be undertaken in order to utilise the water saved. After making these allowances it is estimated, in the paper above quoted, that the saving effected by lining a million square feet with oil, cement or puddle represents the interest on a capital sum of Rs 69,300, Rs 81,250 and Rs 81,250 respectively, or 2 or 3 times the sums sunk in constructing the linings. Hitherto the experiments have been carried out on a moderate scale, but extensive operations are now being undertaken on the Lower Chenab Canal, and possibly on others. In cases where it is not desired to incur much expenditure, it may be a good plan to construct watercourses to a cross section somewhat larger than that ultimately desired. The silt deposited on the bed and sides forms, in most cases, a more impervious lining than the original soil. The same plan can be adopted in the tail portion of a distributary. In a larger channel there would be less certainty that any deposit would take place unless short lengths, at frequent intervals, were excavated to the true or ultimate section, so as to form weirs and spurs; and even these might not stand. In Italy, in cases where the water naturally contains lime in suspension, the beds of canals have become gradually watertight by the deposit of lime in the channel.[53] In some cases lime has been artificially added. It appears that a considerable period of time is necessary for the process. [53] Min. Proc. Inst. C. E. Vol. CXVI. 3. =Modules.=--A module is an appliance which automatically gives a constant discharge through an aperture, however the water level on either the upstream or downstream side of the aperture may fluctuate. In an old and simple form of module there is a horizontal orifice in which works loosely a tapering rod attached to a float. The water passes through the annular space surrounding the rod. If the water level rises, the rise of the float brings a thicker part of the rod to the orifice and reduces the annular space. In another kind of module the water is discharged through a syphon. If the water level alters, the syphon moves in such a way that the head, or difference between the levels of its two ends, remains the same. The great objections to modules are that they are liable to get out of order or to be tampered with. A module recently invented and patented by Gibb[54] has no movable parts, and is not liable to these objections. [54] For description see Appendix H.