2. =General Design of a Weir.=--Unless the bed and sides of the channel are of rock, a weir has side walls and rests on a strong floor or “apron.” These need not extend far upstream, but must extend some way downstream because of the scouring action of the water.[11] A common type of weir is shown in fig. 32. The downstream face is made sloping, so that the water may not fall vertically and strike the floor below the weir. The thickness and length of the floor depend on the volume of water to be passed and on the height which it will fall and on the nature of the soil, and are generally matters of judgment, though rules regarding them, applicable to certain special cases, are given in the next article. [Illustration: FIG. 32.] The upper corners of the weir should be rounded. This prevents their being worn away; but the rounding of the upstream corner has another advantage. If the corner is sharp, the stream springs clear from it and the weir holds up the water higher, especially in floods. With small depths of water the difference is less, and it vanishes when there is only a trickle of water. Thus a crest rounded on the upstream side holds up low-water nearly as well as a sharp-edged crest, but lets floods pass more freely. Any batter given to the upstream face has a similar advantage. The rounding is of more importance as the batter is less. For similar reasons, the upstream wing walls should be splayed or even curved so as to be tangential to the side wall, and not built normally to the stream. These advantages are sometimes lost sight of. The downstream walls are splayed to reduce the swirl. The body of the weir may be of rubble and the face-work of dressed stone. In large weirs the stones are sometimes dowelled together. Where, as in many parts of India, stone is expensive, brick is used for small weirs, the crest and faces being brick on edge. Downstream of the floor, unless the channel is of very hard material, there is paving or pitching of the bed and pitching of the sides, and these may terminate in a curtain wall. The bank pitching may be of any of the kinds described in CHAP. VI., _Art. 3_, and the bed paving as described in CHAP. V., _Art. 6_, but downstream of a weir the eddying is continuous and the lap of the water on the bank is ceaseless, and good methods are necessary. Sometimes planking, laid over a wooden framing or attached to piles, is used instead of paving and pitching. In case the height of a weir is great relatively to its thickness, the danger of its being overturned must be considered. To be safe against overturning, the resultant of the pressure on the weir must pass through the middle third of its base (see fig. 62, CHAP. XIII.). [Illustration: FIG. 33.]