M, WELD. 



The height of a weir if, of coune, regulated by the depth of the 

 water required to be upheld, and in many cues they are made in such 

 a manner that the upper portion may be raised or lowered at will. In 

 river navigation* thu variable height it often of great importance on 

 account of the great difference* which occur in the volume of the 

 ctream ; 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 sluice*, according to the nature of tl..- 

 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 orHhe weir. In forming these structures it is necessary to 

 guard against 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 conveni 

 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 are 

 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 x = 0'64 . /( ) , in which x the thickness sought; Q = tha 



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 is above the weir before the waters begin 

 to fall, and whose asymptote is the line of the natural mean fall of the 

 waters before the establishment of the weir. According to d'Aubuisson 



\VKI.LS. 



-11 



and Guilhem, the equation to this curve is I y+P x ] jT = 



V II / ii 



; in which .'- the horizontal distance from any point 



l-r4_H 



9 B 



in the curve to the weir ; y = the height to which the waters are heaped 

 up at that point above the original level ; 11 = the greatest height to 

 which they are raised ; and p = the fall of the river, in this case sup- 

 posed to nave a straight course. As a general rule, the crown of a 

 weir across a stream is kept at about 8 inches, or a foot, below the 

 height calculated for the augmented depth. 



Very good models of leading weirs have been executed by Sineaton, 

 on the river Carron ; by Telford, on the Weaver ; and by Stevenson, 

 on the Kibble and ou the Dee. 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 fascines. A very ingenious description of moveable weirs 

 is used upon the Seine, in order to close the passage of the subsidiary 

 fhapnl in the dry season ; it was invented by M. Poire'e, 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 tin in. ( 

 General Baird Smith's, ' Indian Irrigation,' and to his ' Irrigation of 

 the Madras Provinces.' 



a 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. 



WKU>. [COLOUBIKO MATTERS.] 



\VKLD1NO is the process of uniting pieces of metal by hammering 

 when hot. It depends on properties chiefly possessed by iron and the 

 allied group of metals; and is illustrated by the details given under 



CCTLKRT ; lR(r ftc. 



\\KLLS. The term Well is, generally speaking, applied to any 



excavation sunk in the ground, for the purpose of obtaining, or of 

 i water ; the technical distinction between the two kinds 

 of wells being that the former are known by the simple name of 

 the latter by the names of dead veto, or of ulaorbiag wtUt, accordingly 

 as the water escapes by deep seated, or by superficial permeable beds. 

 The distinction between Artesian and common wi-lU 1ms already been 

 n-tVm-d to under ARTESIAN WKLLS ; and that between a thaft and a 

 tnl I consists in the fact that shafts are generally dry, and arr in' 

 to give access to galleries, tunnelx, or subterranean workings, whereas 

 wells are intended to act as reservoir* nf water. 



Common wells are inadi- by finking an excavation through an upr r 

 permeable, or partially permeable, stratum, which lies uiion nn 

 impermeable one, in such a manner as to allow the water falling 

 the surface to percolate through, and to accumulate in, the up| . r 

 stratum. The depth to which it may be necessary to sink a 

 well will then depend on 1, the permeability of the water-bi 

 stratum; 2, on its area; 8, on ita thickness; 4, on the relief of the 

 upholding stratum; and 5, on the existenc. "i otln-r \\vll*, or places 

 of draught upon the source of supply. The diameter of the well will 

 depend upon the rapidity with which the water will enter fro; 

 sides, or bottom, to replace the consumption ; it being always observed 

 that there are d priori advantages in making the well of large dimen- 

 sions, in order to obviate the effects of the stagnation of the 

 which affects small quantities more rapidly than it does larger ones. 

 The construction of the sides, and of the margins of the wcuV 

 depend entirely upon the nature of the ground traversed, and upon it* 



i or less tendency to cave in. 



<;ni, Tally speaking, the margin of a well intended to supph 

 for domestic use is executed of watertight masonry, and the well 

 is lined with masonry of the same description to some considerable 

 depth, in order to shut out from the well the infiltrations from lainl- 

 drains, and from surface waters. In England this part of the w> 

 done in brickwork and cement, or in cast iron ; abroad it is conin 

 done in ashlar masonry. Bulow the level to which these waters may 

 reach, the sides of the wells only require to be lined in such a mamu r 

 as to resist the tendency of the sides to collapse; and this ..l.jivt may 

 in many cases be effected by what is called dr>, kwork 



laid without mortar. It is customary in modem will sinking to 

 execute the steining by sinking the ground to the full diameter 

 outaide of the work to depths of about 6 feet each, and tli. 

 what is called a wooden curb, or a frame circular in plan, of from 8 to 

 6 inches in thickness, at the level of the floor ; upon this curb the 

 stt'ining is then built regularly, as close to the side of the well as 

 possible. The ground is excavated from under the uj 

 are maintained in their positions either by the friction upon the side 

 of the steining, or by means of shores, struts, &c. hi traversing 

 running sands, or formations containing much water, close tubes 

 be used ; and great precautions are required to prevent such sands 

 from bloating, that is to say, from rising in the excavation. In the 

 lower part* of all wells, whatever may be the nature of the material 

 used for lining the sides, means must be provided for the ingress of the 

 water; when iron cylinders are used, or close planking of wood is 

 introduced, a series of holes in the circumference will effect this object. 

 Great care must also be taken that the materials employed be of such 

 nature as not to act upon the quality of the waters ; aud whilst every 

 precaution must be taken to protect the waters from the light, and 

 from atmospheric impurities, it is desirable that an efficient ventilation 

 should take place in the well. Local considerations of economy must, 

 of course, regulate to a great extent the selection of the materials to be 

 used in steining a well; but theoretically it would be preferable to 

 employ none but the hardest fire bricks and lioman cement, or cast- 

 iron. Copper tubes are sometimes used in Artesian wells ; but in 

 ordinary wells the cost of copper pipes of the size required would 

 render the use of that material impossible. Wrought-irou tubes are far 

 too liable to rust, to allow of their being used in wells, without their 

 being galvanised ; cast-iron pipes are therefore most generally used. 



In such formations as the gravels when they hold much water, it 

 usually is sufficient to sink a well in a vertical direction so far 

 maintain a good depth of water at all times of the year, provided there 

 Vic not many wells drawing their supply from the same bed. It rarely 

 happens that the permeable gravel strata of large towns are able to 

 supply the demand made upon them, and it is therefore almost always 

 the case that after the wells have been gradually deepened, their use is 

 forcedly abandoned in such places : the area of supply is in 

 limited ; the demands of a large population practically are unbounded. 

 But there is another reason of far more serious im|"( m,, why the 

 ordinary wells of towns are at the present day being gradually 

 abandoned, in the contamination to which they are exposed from the 

 infiltrations from drains, sewers, cesspools, and dr.,.l v.rlls. whirh 

 eventually introduce so large a proportion of organic matters (in the 

 form of the nitrates) into the waters, as to render them ulifit for 

 human consumption. The infiltrations from the sewers, or the leakage 

 from the water pipes of a town distribution may, it is true, provide an 

 unexpected quantity of water in the superficial wells ; but the earth 

 traversed by it is so charged with organic matters, under all t 

 that the quality of the water must be of the most objectionable 

 nature. In towns situated upon the chalk even it has been found that 

 unless the wells are sunk to a great depth, they are subject to con- 





