WEIRS. The constructions, whether of stonework, timber, or earth, by means of which the waters of a river are retained to a given height, or are diverted in any required direction, are known in the arts by the name of weirs, or dams ; and they are principally used fOr the purposes of creating mill-heads, artificial navigations, or head-waters for irriga tions. They may either be placed directly across a stream, or in the line of its flow; and when placed in the latter direction, they may be made to regulate the height of the stream by the level of their crown, in which case the weirs are called waste weirs, or tumbling bays, because the excess of the waters, beyond the quantity necessary to keep the surface up to the level of the crown, falls over the weir, and escapes through the byewash, without producing any useful effect.
The height of a weir is, of course, regulated by the depth of the water required to be upheld, and In many cases they are made in such a manner that the upper portion may be raised or lowered at will. In river navigations this variable height is often of great importance on account of the great differences which occur in the volume of the stream ; but in millworks the waste weirs are usually made to perform the function of regulators, thus leaving the water line near the shuttle at a constant level. Weirs across rivers are always accompanied by lock-chambers, or by flushing sluices, according to the nature of the traffic, in order to facilitate the passage from the level of the upper to the lower reach of the stream ; and whenever lock chambers are formed, it is important that their tail bays should fall again into the main stream at a point beyond the influence of the water falling over the crown of-the weir. In forming these structures it is necessary to guard agaiust the syphonic action of the water ponded up in the head bay, which has a great tendency to penetrate the materials composing the weir, and to blow them up, should their specific gravity not be such as to ensure the stability of the structure ; and also to guard against the undermining action of the water falling over the crown of the weir. On the former account it is desirable that the height of the head waters should be kept as low as possible, with regard to the convenience of the peculiar service considered; on account of the latter it is desirable that the down side of the weir should present a gradually inclined slope, so as to obviate the effects of the cataract, which would exist if the lower face wall were vertical. In many cases the weirs, which aro placed across rivers, are made to present an angle to the course of the stream, in order to offer a greater length, and consequently a smaller depth of water, on the crown ; but there can be no absolute rule in this matter, as in so many of the other practical details of hydraulic engineering.
The thickness of water falling over a weir is calculated by the formula in which x=the thickness sought; Q= the discharge per second; and L= the length of the weir. It is, however,
important to observe that although the effect of these weirs is to cause an accumulation of waters on their upper sides, the greatest depth is not found immediately upon them, but at a certain distance above them in the line of the stream. The surface of the fluid, in fact, assumes a convex form before arriving at the weir, the curve of which commences at a point on the up-stream dependent upon the velocity of arrival. It is considered that the outline of the curve is a portion of an hyperbola, whose summit ie above the weir before the waters begin to fall, and whose asymptote is the line of the natural meau fall of the waters before the establishment of the weir. According to d'Aubuisson Px and Guilhem, the equation to this curve is C1"4-/—. )I — = 1 ; in which x= the horizontal distance from any point1+ 4 u (px) 6 9 in the curve to the weir ; _y —the height to which the waters are heaped up at that point above the original level ; II= the greatest height to which they are raised ; and p= the fall of the river, in this case sup posed to have a straight course. Ae a general rule, the crown of a weir across a stream is kept at about S inches, or a foot, below the height calculated for the augmented depth.
Very good models of leading weirs have been executed by Smeatou, on the river Carron ; by Telford, on the Weaver ; and by Stevenson, on the Ribble and on the Deo. The American engineers have intro duced a very economical form of weir, consisting of what is called cribwork of timber filled in with loose rubble ; and the Dutch engineers execute with surprising skill weirs or dams of earthwork, bundles of reeds, and faseinea. A very ingenious description of moveable weirs is used upon the Seine, in order to close the passage of the subsidiary channels in the dry season ; it was invented by M. Poiree, and has been described by him in a pamphlet upon the subject. The waste weirs, and the head weirs of the majority of English mill streams, are rudely constructed earthen banks, which are too often undermined, and left in a very unsatisfactory state of repair : our canal weirs are, however, skilfully formed, as a general rule. Upon the great Indian irrigation canals the most extraordinary works of this description are to be found ; and the reader is referred, for a description of them, to General Baird Smith's, ' Indian Irrigation,' and to his Irrigation of the illadme Provinces.' Upon some of our rivers, structures composed of basket-work are placed for the purpose of arresting the progress of fish in their migra. tions up or down the stream, which are known by the name of fish weirs. These structures do not interfere with the conditions of flow of the rivers, and as their efficiency for the desired end must be regu lated by the habits of the fish they are designed to catch, the mode of their construction must be regulated by local conditions.
WELD. [CoLobnixo MATTERS.)