WEIR, or WEAR—called also a dam, and in the n, of England and s. of Scotland a eauld—a structure placed across a river or stream for the purpose either of diverting the into a mill-lade, of raising the level of the surface of the river and thereby increas ing its depth for the purpose of navigation, or of providing the means of catching salmon and other fish. There is also the waste-weir, for the purpose of preventing a reservoir embankment being overtopped by floods; and the gauge-weir, for the purpose of comput ing the quantity of water flowing over it, from a measurement of the difference of level between the crest of the weir and the surface of the still above it. The word is also sometimes used, though perhaps not quite correctly, to denote a training-wall or other structure parallel with the general line of a river, for the purpose of remedying or preventing loops or sinnosities. A weir may—according to the purpose for which it is intended, to the nature of the materials at command, or to other circumstances—be formed either of stone, timber, or brushwood, or a combination of any two. It is gener ally placed obliquely across the stream, in order to make the length of its crest consider ably greater than the width of the channel, and thereby prevent the water in floods from rising to so great a height as it would do with a shorter crest, to the risk of damaging the adjoining low lands, and probably putting the mills above in backwater. In such cases the mill intake or the navigation lock, as the case may be, is generally placed at the downstream end of the weir. Much obliquity, however, makes the current to impigne against and cut into the side of the river opposite the lower face of the weir, to prevent that effect weirs pre sometimes made of the shape of two sides of a triangle, or rather of that of an hyperbola, with its apex pointing up stream, which arrangement, is peculiarly applicable to the case of there being an intake for a mill on each side of the river, and the apex is a very suitable place for a fish-pass or ladder. Not unfrequently, when at a wide part of the river, the weir is placed at right angles across, and with a slight curve upward; and a natural shelf of rock is often very advantageously made use of for either a mill or fishing weir, the low parts being made up where necessary with stone or timber.
The down-stream face of a weir is generally a pretty flat slope of stone "pitched" or set on edge, and with its toe, or lower edge, either sunk into rock, or protected from being underwashed by a row of timber-sheeting piles, and frequently also by an apron of The slope is either straight, or made with a hollow curve, so as to check the tendency of the water to acquire increasing velocity as it descends; and it is frequently divided into panels by timber-framing, so as, in the event of a portion of the pitching being washed out, to lessen the risk of the whole of it being carried away. The upstream face is generally a slope dipping into the water, and protected by stone pitch ing, but it is sometimes a perpendicular wall. In order to render an ordinary sloping weir water-tight, sometimes there is under the crest or coping a row of well-jointed and close-driven timber sheeting piles; but those being liable to decay, without their decay being visible, a better, though a more difficult and expensive arrangement, is to build a perpendicular wall of water-tight masonry under the crest. In either case, generally there is the additional precaution taken of having a wall of pounded clay on the up stream side of the wooden or stone barrier; and sometimes a mere wall of pounded clay alone, in the center of the weir, is trusted to, as the sole means of making it water tight; but the latter is not a satisfactory arrangement, unless the stone-work next to the clay be so closely compacted by an admixture of gravel and sand as to prevent any cur rent of water from reaching the clay and cutting into it. The down-stream face is some times made a nearly perpendicular wall, which, unless for the obstacle which it presents to the ascent of the salmon, is a very good arrangement, where the bottom of the channel is solid rock, so as not to be liable to be scooped out by the falling water; else it must have at its foot a level apron of heavy masonry for the water to fall on. The down stream face is also sometimes made of a series of steps, so forming a succession of levels and light falls which is a very good plan for breaking the force of the falling water; but it, like the perpendicular face, presents obstacles to the ascent of the salmon, unless a fish-pass or ladder be provided.