The positions of the outlets[16] having been settled after discussion with the cultivators, a table is prepared showing the chainage of the outlets, the probable head or difference between the F.S. level of the distributary and of the watercourse, and the F.S. discharge. From this the sizes of the outlets are calculated and shown in another column. If the length of the outlet barrel is not more than 5 or 6 times the diameter--in the case of a barrel whose cross section is not round or square, the mean diameter--the discharge can be calculated as for a “short tube,” but if longer the formula for flow in pipes should be used, allowance being, of course, made for the head lost at the entrance. The outlets generally consist at first of wooden “shoots” or long tubes, rectangular in cross section. This is because, after they have been tested by a year or two years’ working, the sizes nearly always require adjustment and the cultivators often wish to have the site shifted. [16] The positions can be slightly altered by the Engineers for any sufficient reason. [Illustration: FIG. 11.] The uncertainty as to the proper size of an outlet is due to several causes. If the command is very good there may be a clear fall from the outlet into the watercourse. In this case the discharge depends only on the depth of water in the distributary, and is known pretty accurately. But ordinarily the outlet is submerged, and its discharge depends on the difference between the water levels in the distributary and in the watercourse. The latter level is not fixed. The cultivators can lower it, to an extent which depends chiefly on the distance of the fields from the distributary, by deepening or widening the watercourse. In this way the discharge of the watercourse is increased except when a dam is temporarily made in it for the purpose of irrigating any comparatively high land. This uncertainty as to the discharge can in some cases be got over by building a cistern (Fig. 11). This has the same effect as raising the level of the barrel, the real outlet being no longer submerged, and the discharge depending on the depth of the crest of the overfall below the water in the distributary. But such cisterns add greatly to the cost of an outlet, and they can only be adopted when there is good command. A great cause of uncertainty as to the proper size of an outlet is the variability of the duty of the water on the watercourse. The soil may be clayey or sandy, the watercourse may be short or long, the crops grown may be ordinary ones or may be chiefly rice, which requires three or four times as much water as most other crops, and the cultivators may be careful or the opposite. Again, the people may, if the outlet gives a plentiful supply, often keep it closed, but there is no record of such closures nor would the people admit that they occur. These causes may all operate in one direction--on a whole distributary this cannot happen to the same extent--and thus