1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



283 



of condensation to be 100°, we shall have c = 112 + 1000 ; 

 the above equation may be put in the form, 



1112, and 



dif. X =^ 



1112 



If the pressure in the boiler be about 5 lb. above the atmosphere, we 

 shall not have a greater mean pressure than about 30 inches in the 

 cylinder, in which case 



if/. ^ = ^^ = 0-027. 

 -'1112 



This is about the difference between the elastic force of steam at 

 72 and 73 degrees, according to Dr. Dalton's latest experiments, and 

 the force at 73° is 0-95 inch; so that, when the mean pressure in the 

 cylinder is 30 inches, and the barometer without stands at 294 inches, 

 the condenser barometer should mark 28-55 inches. 



The calculation of the best proportion of power to tonnage (page 

 288) is so confused by errors (of the press?), that we have no leisure 

 at present to wade through it. 



The article on the immediate Mechanmn of Propnlsion is defective 

 in as much as the Archimedean Screw Propeller is not so much as 

 mentioned, and the author seems to have formed an erroneous idea of 

 the principle of Morgan's Paddle Wheel, in consequence of a trifling 

 resemblance which it bears to Oldham's Wheel. See the article On 

 Paddle Wlietls in the Appendix to the new edition of "Tredgold on 

 the Steam Engine." 



The Historical Sketch of 'Steam Locomotion, by Lieut. Lecount, 

 forms an interesting appendix to the work, which on the whole con- 

 tains much useful information on the subject of steam, perhaps more 

 than is to be found combined in any other volume- of its size, although 

 ■we do not think it does full justice to its title, particularly in what re- 

 gards Steam Navigation, as we have already observed. 



ON WIERS OR DAMS ON RIVERS. 



Observations on Hie Effect produced by erecting Weirs or Dams on 



Rivers, and on their efficacy for Navigation Purposes. 



By William Bull, Civil Engineer. 



Weiks are generally erected either for the purpose of raising a 

 head of water for the use of mills, or for the purpose of navigating 

 the channel of a river, and they cause in the first instance a permanent 

 elevation of the ordinary surface of the water. 



If a weir has the same length of top surface as the section of the 

 river at the place where it is erected, it will cause such flood waters 

 as would have been retained within the natural banks of the river, 

 before it was erected, to rise above them in proportion to its height, 

 and overflow the adjoining lands if artificial embankments of propor- 

 tional height are not erected to prevent it. 



When more water comes down the river than its banks could have 

 previously held, then, although the weir causes an increase of height, 

 the evil is less in proportion than in the former case. 



In extreme floods, when the water would rise far above the surface 

 of the valley, the small increase of height caused by the weir is of 

 little or no consequence, as other causes generally exist, such as em- 

 banked roads leading to bridges, and the contraction of the stream by 

 the bridges which obviates the effect of the weirs ; unless the latter 

 be situate at or close to the bridge. 



Weirs cause the beds of rivers to rise by retaining the sand and 

 gravel brought down by the stream, with much more rapidity than the 

 adjoining lands rise from the deposit of lighter silt first in the upper 

 portions of the rivers where they are erected, and ultimately through- 

 out their course as far as the weirs extend, by which the sectional 

 area of the rivers is diminished and of course the land adjoining ren- 

 dered more subject to floods. 



This is an evil that may be partially remedied by dredging and 

 embanking. I say partially, because, from the manner in wliich the 

 former is usually done, (?. e. only with a view to keep open a narrow 

 channel in the river for the use of boats,) it has very little tendency to 

 produce it, and if done to the whole extent of the river, would become 

 a very expensive operation, and the latter, even when well executed 

 at first, being constantly liable to delapidation, is for ever subjecting 

 the lands to inundation. This is particularly illustrated in Holland, 

 •where the rivers, having been dammed up and embanked, have been 

 permanently elevated above the adjoining lands, and where destruc- 

 tive inundations are by no means of rare occurrence. 



Where no weirs exist, rivers have generally a tendency to deepen 

 their beds (particularly if the water is confined to a channel of mode- 

 rate width) from their source to their confluence with the sea, or until 



they arrive at an estuary or low flat track of land, when the sand and 

 gravel or other material driven or borne down by the water, is either 

 deposited in such estuaries or on bars at the river mouths, or is dis- 

 persed along the shores of the sea by the tidal wave. 



1 have observed many instances of the gradual lowering of the I)ed3 

 of rivers, but more particularly one which has recently come under 

 my observation, where the out-fall of a mill-goit has been lowered two 

 feet in about four years. This phenomenon is, of course, most obvious 

 where the fall in the river is greatest, all other circumstances being 

 equal. 



By making a weir of greater length than the general section of the 

 river, and by widening the river above and below the same, a part of 

 the injury to adjoining lands by raising the surface of ths water may 

 certainly be avoided, so long as the river is continued of the increased 

 width, and by extending the length of the weir and the widening of 

 the river to a great or almost indefinite extent, both in line of the 

 current as well as in width, the injury to adjoining lands may be 

 nearly if not entirely obviated for a limited time, but it can only be 

 for a limited time without constant care and attention, and a con- 

 siderable periodical outlay, (much more of each than is usual or likely 

 to be devoted to such purpose,) because, from the surface of the 

 water being extended, the stream will become proportionally sluggish, 

 particularly towards one or both sides, where, as well as in the natural 

 bed of the river, the matter brought down by the stream ivill be de- 

 posited, and the river will again assume its original width or nearly 

 so, and render the increased length of the weir of little or no avail. 

 The time which will expire before the river assumes its original 

 width will depend materially on the quantity of matter held in sus- 

 pension by the water, or driven forward by the impetus of the stream, 

 and on the velocity of the stream above and at the point where the 

 weit is erected. 



In such instances as where, for some distance before arriving at the 

 weir, the fall and consequent velocity of the stream is of a moderate 

 degree, and where the upper surface of the river has been enlarged, 

 and thereby the velocity of the current diminished, there being only a 

 light alluvial matter and sand held in suspension, or driven forward 

 by the stream, such matter will be rapidly deposited on the sides of 

 the river, until the sectional area is again so contracted as to increase 

 the rapidity of the current to its original rate. 



If, instead of the fall and velocity being of that moderate degree 

 which will only carry forward the lighter matter, it is of such a degree 

 as to force down gravel and other heavy matter, the length of the 

 pond first caused by the erection of the weir will be gradually di- 

 minished, until the whole of the original bed of the river is filled up 

 to the weir, so as to form the inclination of the new bed at nearly the 

 same angle as it was before the weir was erected; but in this case the 

 contraction of the stream to its original width will go on much more 

 slowly than in the former case, arising from the filling up of the bed, 

 causing the velocity of the stream to be reduced in a less ratio than 

 it would have been if the heavier matter had not been brought down 

 into the original bed of the river. 



If the bed of the river is constantly dredged, so as to keep it of its 

 original deoth to the upper end of the pond formed by the weir, then 

 the top surface will contract much more rapidly than in the last case, 

 until it arrives at or approximates to its former width, as in the first 

 case by reason of the heavy material being prevented by the dredging 

 from raising the body of the stream so much as it would have done 

 had no dredging been used, and the consequent less velocity of the 

 stream allowing the lighter matter to be deposited at the sides. 



From the foregoing observations it will follow that the increased 

 length of the weir, and accompanying width of the river beyond its 

 former dimensions, renders but a partial and temporary advantage, in 

 diminishing the damage to adjoining lands arising from the erection 

 of such weir, and that such erection, whether of greater length than 

 the section of the river or not, does not in itself provide a pernianent 

 means of navigation. 



Many instances of the inadequacy of weirs for supplying a perma- 

 nent means of navigating natural rivers may, I have no doubt, be ad- 

 duced. In the instance of the Calder and Hebble navigation, where 

 I have been practically acquainted with the subject for the last eight 

 years, they are abundantly manifest, as well as in the adjoining navi- 

 gation of the Aire and Calder, with which I am well acquainted, the 

 proprietors of both of which have been for many years adopting means 

 to avoid the natural stream by substituting cuts or canals. 



When first the River Calder was made navigable, it was divided 

 into pools by weirs of sufficient height to give the required depth of 

 water and these weirs were passed by means of locks ; but it was 

 soon found that the matter brought down by the stream was rapidly 

 filling up the pools, and consequently diminishing the depth of water, 

 whereby the navigation was much impeded; this was first most ap- 



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