649 



CANAL. 



CANAL. 



650 



bay when it serves the additional purpose of a reservoir, must be such 

 as to ensure the fulfilment of all the above-mentioned requirements ; 

 but it is to be observed, that for many practical reasons connected with 

 the regularity in the disharge of the water, and with the equalisation 

 of the supply, it is preferable to make the reservoirs independent of 

 the portion of the canal devoted to traffic. As, indeed, it may often 

 be necessary to lay bare the whole bed of a canal, it must evidently be 

 important to avoid the loss of water in the summit level, if that should 

 happen to constitute a natural reservoir ; and, at any rate, it must be 

 more easy to control the passage of water from an artificial pond of 

 this description, than it can be to divert the flow of the outlet of the 

 natural drainage of a particular district. It would also be found, 

 generally speaking, to be the case, that the water flowing into the 

 summit level from the natural surface of the country would contain so 

 much alluvial matter as to produce a tendency to silt up the channel, 

 and that at times it might enter with sufficient violence to injure the 

 banks. If, however, any springs, or rivers of a constant volume, should 

 occur upon the line of the canal, they must of course be used as far as 

 possible, and artificial reservoirs must only be considered as costly 

 methods of supplying their absence, or of impounding the surface 

 waters, which would otherwise be wasted. There are many very 

 important practical questions connected with the execution of these 

 works, which are at the present day of quite as much importance to the 

 sanitary operations of some districts as they are even in canal opera- 

 tions; and it has therefore been considered advisable to treat them 

 collectively under the head RESERVOIR. 



Before entering upon the examination of the works of art upon a 

 line of canal, it may be advisable to dwell upon some of the questions 

 connected with the channel or bed. The first remark to be made is, 

 that in the execution of a canal it is desirable to set out its course, as 

 far as possible, upon that bank of the watercourse, whose direction it 

 follows, which has the smallest number of affluents. Any natural 

 streams which may exist must be either diverted into the canal, if 

 their waters should happen to be of a proper quality, or they must be 

 carried under its bed by means of culverts or of aqueducts. Again, it 

 is advisable to make any change of direction in the line of the canal 

 with as large a radius as possible ; and even to provide, in such posi- 

 tions, an increase of width in the canal which should allow the boats 

 to pass one another with greater ease, a precaution which, by the way, 

 it is advisable to repeat whenever, from any accidental circumstance, 

 there may be danger of the existence of any interference with the free 

 movement of the boats, as, for instance, in long deep cuttings, or at 

 the outlet of a tunnel, or at the end of a series of locks. In the 

 execution of small canals such ' turn-outa ' are of absolute necessity, 

 and they must be placed so that the bargemen can easily see from one 

 to the other. In large or in small canals alike, however, when inter- 

 mediate stations or wharves are established for the convenience of 

 local traffic, they must be formed beyond the line of the main water- 

 way, go as to allow the navigation to be carried on uninterruptedly. 

 If, therefore, there should be only one towing-path to the canal, the 

 wharves should be placed on the opposite side to that path, and beyond 

 the line of the main bed of the navigation. 



Formerly the impermeability of the bed of a canal depended entirely 

 upon the perfection with which the puddling had been performed; 

 and though at the present day the use of hydraulic lime concrete in 

 forming the bed has been largely applied by the French engineers 

 instead of the ancient system, the use of puddle is still so common that 

 it is necessary to allude to it. Puddle consists of clay, or loam, which 

 is reduced to a tenacious but plastic state, by working and chopping it 

 about with a spade, with the addition of a certain quantity of water, 

 until the whole mass becomes homogeneous and so much condensed that 

 it will not allow the passage of water through its texture. The best 

 description of earth for forming puddle is a light loam, with a mixture 

 of coarse sand or fine gravel ; very strong clay is unfit for the purpose, 

 on account of the great quantity of water it naturally holds, and of its 

 tendency to shrink and crack in proportion to the evaporation of 

 that water ; and vegetable soil is even more unfit still, for the roots 

 and leaves of plants it contains not only serve to render it porous by 

 their decay, but they likewise offer temptation to moles, rats, and 

 other burrowing animals. The ravages of these creatures are some- 

 time^ so serious in their effects upon the water-tightness of a canal 

 bed, that it is considered to be necessary, when the puddle stuff does 

 not contain gravel or sand, to mix those articles artificially in order to 

 resist their attacks ; care must, however, be taken that the size of the 

 stones in the gravel should not exceed that of musket-balls. Telford 

 used to consider that the operation of working the puddle consolidated 

 it to such an extent as to reduce its volume to about two-thirds of its 

 original bulk ; and both in his specifications and in his publications 

 upon the subject of canals he prescribes numerous precautions for its 

 preparation and use. Of late years, however, the thick bed of puddle, 

 which in always of difficult execution, and moreover always exposed to 

 accidents, has been replaced by a layer of concrete ; in the case of 

 the Can.il de St. Quentin, a lining of that material of only eight inches 

 in thickness was found successful in resisting the passage of water 

 through the banks, which it had not been possible to close by any of 

 the ordinary methods. Whether puddle or concrete be used, it is, 

 however, essential to protect the impermeable hearting of the bud by 

 covering it with a layer of earth, which should be of sufficient thick- 



ness to prevent the gaffs used by the bargemen from entering or 

 breaking its surface. This protecting coat is usually made about 

 one foot thick, and when concrete is used for the lining it is advisable 

 to form a step at the angles of the banks to prevent the protecting 

 coat from slipping. 



It is also to be observed, that in setting out a canal a longitudinal 

 fall must be given in order to ensure the flow of the water with 

 sufficient rapidity to compensate for the various sources of loss above 

 enumerated, but at the same time that velocity must never exceed 

 from twelve to fourteen inches per second. On some parts of the 

 Erie Canal a fall of one inch per mile was given, whilst in others the 

 rate of inclination was made only half an inch in that length, the 

 consequence of which was, that when the navigation was active great 

 attention was required to maintain the water at its proper level. 

 M. Girard, the engineer of the Canal de 1'Ourc, reasoning from the 

 longitudinal profiles of rivers, thought that it was advisable to make 

 the fall of that canal a portion of a large catenarian curve ; and he 

 therefore, in one part, made the inclination 0'0000625 m. per metre, 

 and in the other 0'0001236 m. per metre. The effect of this change 

 of inclination, however, was simply to produce a rapid current over 

 those parts of the canal where the fall was great, and a very sluggish 

 current in the more level portions ; so that it would appear to be 

 important, especially when the water in the canal is intended to have 

 a definite rate of flow, to maintain a perfect uniformity in the 

 inclination of its longitudinal section. 



With respect to the width and depth to be given to the channel or 

 bed, the ordinary dimensions of which have been previously described, 

 it may be advisable to state that the principles which should regulate 

 that width and depth are generally admitted to be that the bottom 

 or floor line should be twice the width of the widest barge likely to 

 enter the canal, with an allowance of from six to eight inches play on 

 each side. The banks of the canal must, if formed in earthwork, have 

 inclinations of one and a half to one ; and indeed it would be preferable 

 to make them two to one, unless the slopes are protected with masonry 

 or with rough pitching ; it is advisable also to form a small surf-beam 

 even in small canals, in order to prevent the barges from injuring the 

 banks. The depth of water should always be made at least from twelve 

 to eighteen inches greater than the draught of the largest barges navi- 

 gating the canal, in order to obviate any inconvenience from the 

 accumulation of silt, or from the growth of aquatic plants, or from 

 the occasional differences of level produced by the passage of the boats 

 through the locks or basins. In some cases the growth of aquatic 

 plants is encouraged as a protection to the banks ; but unless they are 

 kept within strictly defined limits they are susceptible of producing 

 much injury to the navigation. The effect of the unexplained intro- 

 duction of the Anacharig alrintulrum upon our canals has indeed been 

 so serious as to merit more than a passing notice. 



The towing-paths must be made at least from twelve to sixteen feet 

 in width when that operation is effected by horses, and they should be 

 made and maintained with almost as much care as would be observed 

 in the case of an ordinary turnpike road. When the haulage is 

 effected by men, the width of the paths need not exceed four feet ; 

 and upon some of the most frequented canals on the continent a small 

 berme is raised on the edge of the path, to protect it from the friction 

 of the tow-rope. When the quick boats upon the Scotch principle 

 are used, or when steam-vessels of any description ply upon canals, 

 especial precaution must be taken to prevent the banks from being 

 washed away by the waves thus produced. On the Scotch, and on 

 some of the French canals, the banks have been paved for this pur- 

 pose ; and on the great North Holland Canal a rude species of camp- 

 sheeting has been executed with the same intention. 



The philosophical considerations connected with the flow of water 

 in canals, and with the resistance of water to the displacement of 

 floating bodies, enter so intimately into the science of HYDRODYNAMICS, 

 that they are reserved for that head. 



In the construction of a canal, the most important works of art, 

 using that expression to designate the works which require the 

 employment of skilled labour, are, 1. the communications with the 

 natural watercourses or rivers ; 2. locks ; 3. aqueducts, siphons, and 

 culverts ; 4. over-bridges ; 5. tunnels ; 6. basins, docks, or entrep6ts, 

 and intermediate stations. As was before observed, the construction 

 of reservoirs will form the subject of a separate article. 



1. The communication with Rivers. In the cases of lateral and ship 

 canals it is necessary to consider the conditions likely to affect both 



Entrance from river. 



the upper and the lower junctions with the existing moans of navi- 

 gation; but in summit level canals the two ends are. placed up the 



