Although the figures of duty take no account of the number of days a canal was closed, they are the most convenient standard for judging generally of the work likely to be done by a projected canal. It will readily be seen that figures of duty are not exact and are only an approximate guide. The delta figures are, on the perennial canals of the Punjab, also worked out for each month of the crop, the volume of water used from the beginning of the crop up to the end of the month being divided by the area irrigated up to the end of the month. But when irrigation is in full swing, some little delay occurs in booking the fields. Moreover the same field is watered a number of times during the crop and much depends on whether waterings have just been given or are just about to be given. The figures are useful to some extent for comparison. The figures for the rabi crop of 1908-09 were as follows, the figure for March being the final figure for the crop. Up to end of Oct., Nov., Dec., Jan., Feb., March. Progressive value of delta. 1·69 1·34 1·29 1·47 1·71 2·05. One great principle to be followed in order to obtain a high duty is to restrict the supply of water. A cultivator whose watercourse is always running full may waste great quantities of water, but if he knows that it is only to run for a few days out of a fortnight he will use the water carefully. It is not, of course, meant that the water kept back is run into escapes and wasted. It goes to irrigate other lands. The available supply of water should be spread over as large an area of land as just, and only just, to suffice.[6] Other methods of improving the duty are the reduction in the number of watercourses, the apportionment of the sizes of outlets, watercourses and distributaries to the work that they have to do, careful attention to the distribution of the water and the prevention of wastage due to carelessness. [6] A system of lavish supply is in most cases likely to lead to harm by water-logging of the soil or its exhaustion by over-cropping or to raising of the spring level and injury to the public health. The following information concerning duties is taken from Buckley’s Irrigation Pocket Book:--