The headworks of the canal consist of a weir--which may be provided with sluices--across the river, and a head “regulator,” provided with gates, for the canal. There are however many canals, those for instance of the inundation canal class, which have no works in the river and these may go dry when the river is low. They usually have a regulator to prevent too much water from going down the canal during floods. If a canal is fed from a reservoir the headworks consist simply of a sluice or sluices. A canal must be so designed as to bring the water to within reasonable distance of every part of the area to be irrigated. Unless the area is small or narrow the canal must have “branches” and “distributaries.” A general sketch of a large canal is given in Fig. 1. On a large canal, irrigation is not usually done directly from the canal and branches. It is all done from the distributaries. [Illustration: FIG. 1.] From each distributary “watercourses” take off at intervals and convey the water to the fields. A small canal, say one whose length is not more than 15 miles or whose discharge is not more than 100 c. feet per second, may be regarded as a distributary and the word distributary will be used with this extended meaning. It is not always the case that the whole tract covered by a system of canal channels is irrigated. In the case of a canal fed from a river, the land near the river is often high or broken and the main canal runs for some distance before it reaches the tract to be irrigated. Again, within this tract there are usually portions of land too high to be irrigated. Those portions of the tract which can be irrigated are called the “commanded area.”