Along a main canal and its branches there is nearly always a “canal dak” or system of conveyance of bags containing correspondence for the officials stationed on the canal or touring along it. Along the main line, and most of the way down the branches, there is a line of telegraph for the special use of the canal officials. The telegraph offices are at the chief regulators, with tapping stations, for the use of officials on tour, at the rest houses near to which the line runs. However carefully a canal has been designed, alterations in the channels from silting and scour soon take place and they go on more or less without cessation. In a distributary, especially if the gradient has of necessity been made somewhat flat, there is quite likely to be a deposit in the upper reach. The deposit is generally greatest at the head and decreases, in going downstream, at a fairly uniform rate. It may extend for half-a-mile or less or more. Or a deposit may occur on the sides, which grow out and contract the channel. This often occurs over a great length of a distributary or even over the whole of it. Sometimes a distributary scours its bed, or the sides may fall in somewhat. Clearances of the silt and cutting of the berms are effected at intervals. Falling in of the sides may be stopped by means of bushing, and scour of the bed may be stopped by raising the crest of a fall or by introducing a weir, but in the meantime the changes cause the discharge tables for the distributary to become more or less erroneous. In many cases silt deposits in the upper part of the distributary during the summer months when the river water is heavily silted and scours away again in the winter, the régime of the channel being, on the whole, permanent. The changes which occur in the branches and main canal are similar to the above and the remedies adopted are similar. On some of the older canals the scour was so serious that many intermediate weirs had to be constructed. The remarkable silting in the head reach of the Sirhind Canal has been described in _River and Canal Engineering_, Chapter V. The remedy consisted in keeping the gates of the under-sluices properly closed so that a pond was formed in which the river silt deposited. When necessary the canal is closed, the sluices opened, and the silt scoured away. For a plan of the headworks see fig. 24. In working a canal, it is necessary to arrange so that the water sent down any channel is as nearly as possible in accordance with the demand. The zilladar supplies the Subdivisional Officer, every week or ten days, with an “indent” showing how much water is required in each distributary and the Subdivisional Officer makes indents on the subdivision next above. The officer in charge of the headworks thus knows what the demand is. When it is more than the supply available, the water is dealt out to the various divisions according to rules approved of by the Superintending Engineer of the canal. [Illustration: FIG. 24.]