For large works the advantages are generally with needles, but for small works, _e.g._ distributary heads and shallow water, with planks. Needles 14 feet long are not too long for trained men. Planks are more likely than needles to arrest rolling sand, and this can be taken into consideration in designing double regulators. See number 8 of Kennedy’s rules, Article 5. When planks are used there should be two sets of grooves. Planks are very suitable for escape heads which have only occasionally to be opened, earth being filled in between the two sets of planks. [Illustration: FIG. 16A] Regarding notched falls, in the case of small distributaries the notches are so narrow that they are extremely liable to be obstructed either accidentally by floating rubbish or wilfully by persons whose outlets are upstream of them. Weirs are not open to this objection, and are frequently adopted. There is not the least chance of their causing any silting worth mentioning. A simple weir if made of the proper height for the F.S. discharge, will cause a slight heading up with ³⁄₄ths of the F.S. discharge, and this unfairly benefits any outlets for a considerable distance upstream of the weir. This difficulty can be got over by making the weir as in Fig. 16A. [Illustration: BRIDGE AND NOTCH FALL. In this case the usual practice of placing the bridge downstream of the fall has not been followed. The gauge well is seen on the left bank. _To face p. 87._] For cisterns below falls the usual rule for the depth is K = H + ∛H √D where H is the depth of water in the upstream reach, and D is the difference between the upstream and downstream water levels. Another rule for distributaries is H + D K = ----- 3 the length of the cistern being 3 H and its width the bed width of the channel. At “incomplete” falls, i.e., where the tail water level is above the crest, it is not unusual to construct a low-level arch, which forms a syphon. The object is to allay the surging of the surface water. The question of skew bridges has been dealt with in Art. 3. Another question is that of the heights of bridges. Irrigation channels, especially the smaller ones, are very frequently at a high level, and bridges have ramps which are expensive to make and to maintain, and are inconvenient. The lowering of distributary bridges in such cases, so that they become syphons, or nearly so, has often been advocated and is frequently desirable. The bed should slope down to the floor and up again. The heading up can be reduced by giving ample waterway, but it will not be necessary to do this if there is head to spare. The fall in the water surface can be recognised and shown on the longitudinal section. The structure becomes one of the incomplete falls above described. The crown of the arch can, if desirable, be kept above F.S. level, so that floating rubbish will not accumulate.