4. =Other Artificial Channels.=--The method of calculating the discharges of channels in which water is to flow is a question of hydraulics. The principles and rules to be followed, in the design of earthen channels, have been stated in CHAP. IV., _Art. 6_, and in CHAP. VIII., _Art. 5_. The design of banks has been dealt with in _Art. 1_ of this Chapter. For conveying water for the supply of towns, or for other purposes, masonry conduits are often used. A usual form is shown in fig. 30. The curving of the profile of the cross-section gives an increased sectional area and hydraulic radius, and hence an increased discharge. [Illustration: FIG. 30.] CHAPTER X WEIRS AND SLUICES 1. =Preliminary Remarks.=--Every structure which interferes at all with a stream causes an abrupt change in the stream (CHAP. IV., _Art. 1_). At an abrupt change there are always eddies, and these have a peculiar scouring effect. This effect is greatest where the velocity of the stream is abruptly reduced as where, for instance, after being contracted by an obstruction, it expands again or where it falls over a weir or issues from a sluice opening. In all cases of this kind the protection of the structure from scour is of primary importance. The site of a weir or other permanent structure should, if the stream is unstable, be in a fairly straight reach, or at least not be immediately downstream of a bend. This is because of the tendency of bends to shift downstream (CHAP. IV., _Art. 8_). There is no particular advantage in selecting a narrow place. A narrow place is likely to be deep or it may be liable to widen. In a hard and stable stream there is no restriction as to site. Weirs are frequently constructed for purposes of navigation, as mentioned in CHAP. VIII. They are also used in streams which are not navigable in order that the gradient may not be too steep, and in irrigation canals for the same reason. They are used both in rivers and canals in order that the water-level may be raised and water drawn off by branch channels for purposes of manufactures, water-power or irrigation. Upstream of a weir there is more or less tendency for silt to deposit, but it by no means follows that there will be deposit (CHAP. IV., _Art. 2_, last par., and _Art. 3_, last par.). When deposit of sand or mud is feared, small horizontal passages, known as “weep holes,” may be left in the weir at the level of the upstream bed. In the old Nile barrages iron gratings were provided, but they were needlessly large. [Illustration: FIG. 31.]