653 



CANAL. 



CANAL. 



651 



earth ; and this pressure, it is necessary to observe, is greater than that 

 of ordinary earth on account of the filtration which always takes place 

 through the masonry of the locks, or from the upper reach of the 

 canal. It is, however, more than questionable whether there be any 

 real advantage in the adoption of this particular form; because the 

 waste of water it entails is a permanent source of expense, and more- 

 over it is tolerably certain that the extra thickness it will be necessary 

 to give to the lock-floor will more than counterbalance the economy 

 produced in the side walls. For these reasons, therefore, in the best 

 modern canals the lock-chambers are made with parallel sides ; and 

 unless the chambers should be formed in the earthwork itself they are 

 also made with vertical sides. The materials usually employed are 

 masonry, whether of stone or of brick ; but in Holland, the United 

 States, our own colonies, &c., it may occasionally be advantageous to 

 substitute timber, and even in some instances in our own country, 

 cast-iron has been advantageously employed instead of masonry. Of 

 whatever materials, however, the locks may be formed, the top of the 

 side walls should be kept at least two feet above the maximum water- 

 line of the upper baain. 



: It is usual to form grooves in the upper and lower chambers, to 

 receive the ends of the temporary dams which may occasionally be 

 required for the purpose of repairing the locks. These grooves should 

 be made about 8 inches square, and of the whole height of the side 

 walls ; they must be placed at such a distance from the return walls as 

 to allow a sufficient resistance to the various actions upon the foot of the 

 dam. A reveal must also be formed on each side to receive the gates 

 when open, and it should be made about 4 inches deeper than the 

 gates, and 6 inches longer than their development. In order effectually 

 to protect the gates when shut back in these reveals, the side walls 

 must be carried from 6 feet to 7 feet beyond them on the upper side, 

 before arriving at the groove last mentioned ; on the lower side, the 

 walls are prolonged beyond the reveals according to the width of the 

 opening. Thus, for a 15 feet lock, they should be prolonged 12 feet ; 

 for an 18 feet lock, 14 feet ; for a 20 feet lock, 16 feet ; and so on. 



In calculating the thickness to be given to the side walls of a lock it 

 is advisable to consider that the earth at the back thereof is a semi- 

 fluid denser than water, and to make the walls of a thickness adequate 

 to resist its action. Some continental engineers make the thickness 

 equal to 0'50 of the height ; but in our own country this dimension 

 ranges from 0'25 to 0'50, and the usual proportion adopted is 0'40, for 

 the lock-chambers at least ; because it is customary to strengthen the 

 walls below the tail-gates, and when a lock is joined to a dam (as is 

 very often the case in river navigation works), the end wall to the up 

 stream is also strengthened, in order to enable it to resist the weight of 

 water and the filtrations which take place. The floor of a lock is 

 usually executed with an invert, to enable it to resist more effectually 

 the hydrostatic pressure of these filtrations ; and it is usual to calculate 

 its dimensions on the supposition that it must be able to resist a head 

 of water equal to the total depth of the lock. The thickness usually 

 given to the floor of a 16 feet wide lock is, therefore, 2 feet 8 inches ; 

 and that of a 20 feet wide lock is 3 feet 4 inches. But whatever 

 thickness be given, precautions should be taken to intercept filtrations 

 from the higher level, either by driving a row of close piling or by 

 stepping the foundations below the level of the ordinary parts of the 

 chambers, so as to break the thread of any stream which might find 

 its way under them. 



The clapping cills are laid with a projection towards the upper level 

 of the water equal to between one-ninth and one-third of the width of 

 the opening ; in the most modern canals the proportion is made be- 

 tween one-fifth and one-sixth, but Professor Barlow states that a pro- 

 jection whose sides would form an angle of about nineteen to twenty- 

 four, or between one-eighth and one-ninth, is the most advantageous. 

 It is customary to allow the cills to project about one foot above the 

 floor of the lock, to form as it were a species of shoulder against which 

 the lock-gates should be able to bear ; and when the floor is executed 

 in stone or brickwork, wooden or iron cills are introduced to receive 

 the immediate shock of the gates. It is often desirable to execute 

 the masonry at the back of the cills as a portion of an arch abutting 

 against the side walls of the chambers. 



When the upper chambers terminate with a breast-wall, that part of 

 the structure is made concave (on plan) to the lock chamber, in order 

 to* economise masonry, and to receive the prows of the boats lying 

 between the gates. Formerly these breast-walls were built vertical, 

 and their upper surface was made to range exactly with those of the 



clapping-cilU and the upper part of the chamber ; but of late years the 

 upper portion of the breast-wall has been made with an incline towards 



the chamber, and the face has been formed with a concave front 

 towards the same direction, in order to break the fall of the water 

 from the sluices in the gates ; whilst iu some of the most modern works, 

 the breast-walls have been suppressed altogether. It is more than 

 questionable whether there is any advantage in this latter system ; and 

 certainly it entails considerable trouble and difficulty in the original 

 construction, and in the subsequent management of the lock-gates and 

 chambers. Owing to the agitation produced by the escape of the 

 water from the lower gates, it is advisable to prolong the floor of the 

 lower chamber for some distance into the bed of the canal ; and in 

 many cases the entry to the upper chamber is also paved, in order to 

 prevent the excavations which might otherwise be occasioned by the 

 velocity given to the water at the upper gate. 



Especial precautions must be observed in working the grooves 

 intended to receive the heel-posts, as it is upon the correctness with 

 which they fit that the water-tightness of the lock mainly depends. 

 In some cases these grooves are executed in stone, in others of hard 

 wood, and in others of cast-iron ; but whatever be the material em- 

 ployed, the greatest attention must be paid to their execution. Care 

 must also be taken that no parts of a lock project, and in fact that 

 there shall be nothing in the respective chambers likely to catch the 

 boats, or their loads, during their passage through the locks. Mooring- 

 posts and rings must be provided occasionally, and every provision 

 must be made to facilitate the passage. 



In almost all the important canals the lock-gates consist of two 

 leaves, fitting close, and bearing against the cills at the bottom, and 

 against the grooves at the sides. Generally speaking they are made of 

 wood, but of late years many gates have been executed iu cast, or even 

 in wrought-iron, or in a compound system of cast and wrought-iron 

 and wood. These gates are composed of the upright posts, distinguished 



Upper Gate. 



Lower Gate. 



by the names of the heel-posts, and the mitre or meeting-posts ; and of 

 the horizontal rails, which support the weight of the water, and vary in 

 number according to the height of the gate. Against these rails the 

 planking, or the wrought-iron plates, as the case may be, are applied. 

 The posts are kept about two inches clear of the floor of the chambers 

 in which they work, so as not to bear immediately upon it in their 

 revolutions ; they are made from eight to ten inches above the highest 

 water h'ne, when they are opened by machinery, but are somewhat 

 longer when that operation is performed by means of a long lever on 

 the top of the gates, as is usually the case. The lowest rail, generally 

 speaking, terminates at four inches from the floor, and the upper rail 

 finishes about four inches above the water line ; the intermediate rails 

 are spaced according to the effort they are intended to resist. A raking 

 brace, or strut, is introduced between the rails, and a wrought-iron tie 

 connects the foot of the mitre-post to the top of the heel-post ; these, 

 together with the squares let in at the junction of the rails with the 

 upright posts, serve to maintain the rigidity of the framing ; but even 

 with these precautions it is advisable, in preparing the gates, to keep 

 the extremity of the framing at least two inches above the level it will 

 occupy in execution, in order to allow for its shrinkage. In the most 

 modern locks, the heel-posts turn upon a metal pivot let into the cill, 

 and working in a reversed socket let into the post itself and at the 

 top they usually work in a metal collar, attached to the walls of the 

 chamber. 



The water is passed from one level in the lock to the other, by means 

 of sluices formed on the gates themselves, or by means of small 

 culverts in the side walls, or by siphons. The usual proportion of the 

 water-way of the sluices to the width of the chamber is about 0'002 or 

 0-0013 to 1 ; the conditions which, however, regulate their dimensions 

 are, that they should be made as large as possible, in order to allow 

 the locks to be filled in the shortest space of time, without giving rise 

 to a dangerous current or fall in the lower basin. The orifices should 

 be placed, when vanes on the gates are used, as nearly as may be on 

 the centre line of the leaves of the gates, in order that their jets may 

 meet, and mutually destroy their effects. When side-culverts are 

 used, the sluices are always placed on the upper side of the lock. It 

 has, however, been found that there are so many practical objections to 

 these side tunnels, that at the present day engineers have reverted to 

 the old system of vanes on the gates. The dimensions of the vaues are 

 ascertained by the formula Q = m S-/2gH ; in which Q= the quantity 

 discharged; m= a coefficient expressing the relation between the 

 real and the theoretical discharge, which is equal to 0'625 when 



