HI 



. ANAL. 



natural watercourse* so as to render both of them in fact lower 

 junction*. A careful examination of the beet works hitherto executed 

 (how* that it ia preferable to place the upper junction at rather an 

 acute angle with the natural flow of the rirer, awl to place the OMbM 

 toward* the down stream ; for unless the boat* are hauled back in this 

 manner, they are exposed to be carried by the itream with dangerous 

 violence anuut the entrance walla. If it were possible so to do, it 

 would be deairable to form a basin near the guard-locks ; but when 

 this cannot be done, the junction should be formed by a long curve of 

 considerable radius, and it should bo made rather wider than the 

 straight part of the canal. In order to prevent any inconvenient e 

 t'r-iu the iliflereaces of level in the river, it is necessary to construct a 

 lock with a pair of gates at each end ; and it is also advisable to make 

 a provision in the walls of the entrance channel for placing a dam, in 

 case it should be required at any time to close the canal. The prin- 

 cipal difficulty which occurs, however, in the construction of the' 

 upper junction of lateral canals, arises from the fact that they are 

 usually established upon rivers of a very variable regimen, anil that, 

 under such circumstances, it is often necessary to form a cross dam to 

 uphold the river waters. Very great precautions are required, in 

 these oases, to prevent any injurious action from the water falling 

 over the dam ; ami also to prevent the silting up of the pond of still 

 water thus formed, or of the entrance and basin which are connected 

 with it. In some tnntm the gates at the entrances are made in 

 separate portions, to allow of their being easily used under considerable 

 litTfrvmt* of level in the river; and in others, again, the first basin 

 of a lateral canal U so arranged that its level may be alternately either 

 above or below that of the river. Double seta of gates are required 

 when this system is adopted, which must be made to point, as neces- 

 sary, to the up or the down stream. 



The outfall, or bottom junction with the river, may he either 

 situated directly ujion the stream itself or upon some branch com- 



CANAL. Ml 



inunicating with it ; and this hitter course is adopted when there is 

 danger of the outfall becoming silted up, or of the formation of natural 

 ban, in consequence of the movement of the alluvial matters of the 

 river. However this may be, the principles to be observed in forming 

 such outfalls are firstly, that the passage of boats from the canal to the 

 river, or riet renA, should be easy ; secondly, that there should at all 

 times exist a sufficient depth of water ; thirdly, that it should be possible, 

 if required at any time, to place sluices for the purpose of scouring 

 the channel ; and fourthly, that there should be a basin of considerable 

 area, to receive the vessels about to pass from the one description of 

 navigation to the other. The quays of the lock chambers, basins, and 

 the platforms at the entrances must be kept above the level of the 

 highest tides, or of the land floods; and it is advisable to keep the 

 basin above mentioned open to the canal, so that all the lockage which 

 may be required may IKS effected in the entrance channel. In esta- 

 blishing the lower cills of the locks of ship or lateral canals which 

 terminate in tidal rivers, it is customary to moke the level of those 

 cills correspond with that of the mean low water of neap tides. 



2. Lock* and Lock-fhamben. Locks are, in fact, a species of pounds, 

 in which a portion of the water of a canal is retained within a structure 

 of such a nature as to allow its surface to correspond with the surfaces 

 of either of the reaches of the canal, situated at different levels, which 

 it may be desired to place in communication with one another. This 

 is usually effected by constructing in masonry a small entrance channel, 

 an upper pair of lock-gates, a lock-chamber, a lower pair of gates, and 

 a small exit channel similar to the one on the higher level ; and in the 

 gates, or in the side walls of the chambers, small sluices or paddles are 

 formed, through which the water may be passed from one level to the 

 other, so as to fill the chamber to the upper level when the low 

 of gates is closed, or to lower it when the upper gates are brought 

 into use. The operation of {Kissing a boat from one level to the other 

 is called ' locking up ' or ' locking down/ as the case may be. 



Ellcraicrc Canal. 



The number of locks required to compensate any given difference 

 of level must of course depend upon the fall given to them, and as 

 a genera] rule, it may lie asserted that all the falls of a canal should 

 be made equal, hi order to economise water in the passage of the 

 boats. It is to be observed that great falls entail the necessity for 

 more perfect execution of the work, because under such circumstances 

 the hydrostatic pressure upon the lowest part of the floor is increased 

 in proportion to the depth of the water; but evidently the greater 

 the fall the fewer need be the locks to overcome a given difference 

 of level, and therefore there will be an economy of time and of 



locks as possible within a short, dist.i 

 in such cases to observe particular 

 struction adopted, on account of the 



in- : luu of course it is necessary 

 precaution in the mode of con- 

 ncreaaed pressure upon the lower 



Lock. St. Lawrence Canal. 



raperintenrieuce when there are the smaller number of these machines. 

 For the same economical reasons it is desirable to concentrate as many 



lock floor, and of the danger of filtration. A numlwr of consecutive 

 locks, it is also to lie observed, consume more water than the same 

 nmiilx-r of locks separated by long reaches of canal would do. The 

 falls given to locks vary, according to circumstances, between 5 feet 

 and 10 feet, in canals with 6 feet of water; in ordinary canals, the fall 

 is usually made from 8 feet to 8 feet 6 inches; and it is usually 

 calculated that a boat would require as long a period to pass one such 

 lock as it would do to traverse the distance of one-third of a mile on a 

 level canal. 



The parts of a lock may be considered to consist of first, the upper 

 chamber or head, with its gates, groove for dam, clapping cill, and 

 breast wall; second, the lock-chamber, properly speaking, \\hidt 

 extends from the upper breast-wall to the extreme point of the pro- 

 jection of the lower gates ; and third, the breast-wall of the said lower 

 gates and the lower chamber. 



i in many canals the form (on plan) given to the lock-chamber is 

 made such as to give the side walls a concave direction to the water, 

 with a view thereby to increase the resistance to the pressure of the 



