S67 



CANAL. 



CANAL. 



58 



canals having locks of 17 feet clear width, a height of 13 feet from 

 the water-line to the under side of the keystone is sufficient ; but that 

 if the barges frequenting the canal are so large as to require the locks 

 to be 26 feet in width, the clear height as above must be made 18 feet. 

 Under these bridges, however, the water-way is usually made about 

 from 1 foot 4 inches to 2 feet wider than it is made in the locks, 

 because the boats do not stop to pass under them as they do to pass 

 through the locks ; for this reason also it is desirable, wherever it is 

 possible so to do, to carry the towing-paths under the bridges, in order 

 to avoid the necessity for casting off the tow-ropes. Over-bridges 

 should always be placed in positions where the canal has a straight 

 run above and below them of from 200 to 300 yards. 



In very flat districts, aa in the fen-lands of the east of England, in 

 Holland, or in Belgium, it is often necessary to execute lifting, or 

 swing bridges, in order to avoid the necessity for raising the level of 

 the approaches. There are many ingenious modifications of this class 

 of structures, such as the lifting bridges upon the Surrey canal, wherein 

 the roadway is raised bodily by means of counterpoises ; the ordinary 

 Dutch lifting bridges worked by long levers fixed above the ground ; 

 the ordinary turning bridges, like those introduced upon the Regent's 

 canal ; or the rolling bridges recently applied upon some of the railway 

 lines. But upon whatever principle these moveable bridges may be 

 constructed, they are always costly in their first execution ; and their 

 subsequent maintenance, working, and repair always entails a con- 

 siderable outlay. If possible, therefore, the erection of this description 

 of works should be avoided, and permanent bridges with proper 

 approaches substituted for them, especially in the neighbourhood of 

 towns. 



5. Tumtelt. When the cutting required for the purpose of carrying 

 a canal at a given level becomes too expensive, from the mass of earth 

 to be removed, and it is not considered advisable to diminish the depth 

 of excavation by the introduction of locks ; or when it is considered 

 that there is a probability of meeting a copious supply of water by 

 passing through the heart of a hill ; tunnelling should be resorted to. 

 Local considerations must at all times seriously affect the economical 

 bearings of these questions, but, as a general rule, it is thought that 

 when the excavation for a canal would exceed 50 ft. in depth, there 

 would be an advantage in executing a tunnel rather than in executing 

 such a cutting. Some engineers, in order to avoid the construction of 

 tunnels, which certainly entail many inconveniences upon the naviga- 

 tion of a canal, reduce the width of the water-way in deep cuttings to 

 that which is absolutely necessary for the passage of one boat ; and, 

 indeed, it is to be observed that the majority of canal tunnels also are 

 made of this width. But the economy thus attained in a cutting is far 

 less than might have been expected, because the cubical quantity of 

 earth-work in it is mainly affected by the angle of the slopes, and the 

 saving produced is, in fact, only equal to the amount resulting from the 

 multiplication of the diminution of the width by the depth. There is 

 also an objection to diminishing the width of a canal in euch positions, 

 arising from the obstruction thus offered to the passage of the boats 

 from the smaller area of the water surface ; and for the same reason it 

 in not desirable to reduce the section of the channel even in tunnels. 



Whether the tunnel be executed to allow the passage of one or two 

 boat* simultaneously, it is customary to give a play of about 2 ft. ou 

 either side beyond the width theoretically required. On the Canal de 

 St. Quentiu the tunnels are executed with towing paths on either side 

 of 4 ft. in width : in the Paddington and the Hareca&tle tunnels, there 



Harecastlc Tunnel. 



ar towing-paths only on one side, and of smaller dimensions than is 

 desirable; whilst in the Blisworth tunnel there is no towing-path, and 

 the boats are propelled by a process technically called " legging," in 

 which the bargemen lie on the top of the barge, and push it forward 

 by pressing then- feet against the top of the arch. In some cases the 

 boats are hauled through the tunnels by means of tow-ropes fixed at 

 the respective ends; and again there is, in other canals, special 

 machinery for the purpose of haulage. 



The height to be given to the underside of the lining of a tunnel 

 above the water-line is usually made between 11 ft. and 16 ft. : in the 

 case of the canal between the Chesapeake and Ohio, this height was 

 made 1 7 ft. 3 in., and in that of the canal St. Quentin it was made 

 20 ft. In all these cases it is supposed that the masts of the barges 

 .we either struck or lowered ; but in the project for the great inter- 

 oceanic canal, by M. Oarella, for uniting the Atlantic and Pacific, it 

 was suggested that the clear height of the tunnel, near the summit 

 level, should not be less than 122 ft. At all times, however, the clear 



height to be given must depend greatly upon the transverse section of 

 the tunnel, and this will in its turn be affected by the nature of the 

 strata traversed. For instance, in the Thames and Medway tunnel 

 the chalk was so fissured, and the strata were inclined at such an angle, 

 that it became desirable to throw out the tunnel to the form of a Gothic 

 arch, rising occasionally as much as 31 ft. 8 in. above the water-line : 

 whilst in the more uniform strata met with in the Harecastle tunnel, 

 the height could at all times be kept within the limits originally 

 intended. Whatever the form of the sides and of the vault may be, 

 the bottom should be formed by an invert. The thickness of the 

 lining (and it may here be observed that it is very rarely indeed that 

 the lining of a tunnel can be dispensed with) must depend upon the 

 nature of the ground traversed, its hydroscopic powers, and the incli- 

 nation of the strata. It appears, from an examination of the most 

 successful examples of such works, that the extreme thickness to be 

 given to the vaults need not exceed 3 ft. 4 in. or 3 ft. 6 in. even in the 

 largest tunnels, and the worst descriptions of soil ; the sides should be 

 made about from 6 in. to 12 in. thicker. [TUNNELS.] 



6. Basins, Docks, and intermediate Stations. At the termini of an 

 important line of navigation it is necessary to provide accommodation 

 for the transaction of the business to which it gives rise, or to pro- 

 vide wharves where the barges may unload or receive their cargo, and 

 where the goods they transport may be stored for sale, or transshipped 

 at once into seagoing vessels if required. It must, therefore, be evident 

 that the nature of the terminal arrangements must depend greatly 

 upon the local conditions under which they are placed ; and that the 

 principles which would be applicable to a ship canal would be very 

 different from those applicable to a mere summit level one. The only 

 invariable rules which apply to this class of works are, that the area of 

 the basins should be sufficient to allow the barges or vessels, which are 

 being loaded or unloaded, to lie alongside the wharves without inter- 

 fering with the movement upon the canal itself ; and that the level of 

 the water in the basins should be maintained independently of that in 

 the inner reaches : in the best canals there are therefore looks at 

 the junction of the canal with the basins, for the express purpose of 

 isolating the level of the former if it should be requisite so to do. In 

 the cases of canals terminating in a tidal river, the observance of this 

 precaution is more than usually imperative. 



The area of a terminal basin depending, as was above stated, on the 

 character of the vessels resorting to it, it follows that the basin 

 intended to serve the purposes of both the canal and the deep sea 

 vessels must be much larger than the terminal basin which is exclu- 

 sively devoted to the purposes of the inland navigation. For ordinary 

 working operations, it would appear that it is not desirable to allow 

 more than two tiers of vessels to lie alongside of the wharves of a dock 

 at the same time ; and it is also known that it is essential to leave a 

 space between the vessels moored alongside the quays, which should 

 allow at least one of the longest of the vessels using the dock to move 

 freely in any direction. The minimum width of a basin must, then, 

 be equal to at least four times the width, plus once the length, of the 

 largest vessel it is intended to receive, and plus the space to be left 

 between each of the elements of the calculation ; and its length should 

 on the average be equal to at least ten times that of the longest vessel 

 using the basin, plus four times the width of the said vessels, in order 

 to allow some boats to moor at the ends. The unloading machinery, 

 cranes, staiths, derricks, and timber slips, must be regulated by the 

 precise nature of the traffic of the canal, and they cannot give rise to 

 any very general observations. The terminal basin of the Regent's 

 Canal is one of the most perfect structures of this description in exist- 

 ence ; the Basin de la Villette, in Paris, is a veiy fair specimen of an 

 inland basin in a great town ; whilst the details of the Entrepot des 

 Marais may be advantageously studied, on account of the ingenious 

 combinations therein effected to facilitate the vexatious operations of 

 the French Custom House. 



Intermediate stations upon a canal must be established mainly upon 

 the same conditions aa have been above traced for terminal stations ; 

 observing, simply, that precautions must be taken to prevent the 

 barges lying alongside the wharves from interfering in any manner 

 with the through traffic. It would, indeed, be advisable to place any 

 important intermediate stations upon lateral branches from the main 

 line of the canal. 



Statistics. It would be difficult at the present day, when so many 

 canals are being transformed into railways, to give any very accurate 

 statistics of the lengths of canal actually in existence. From the 

 observations of Mr. O. Rennie, it would however appear, that in the 

 years 1838 and 1839, before this description of change had taken place 

 to any notable extent, there had been formed in Great Britain about 

 2236 miles of improved river navigation, at a cost of 6,269,0007. ; and a 

 total length of 2477 miles of artificial canal navigation, at a cost of 

 28,406,389/. In the United States there had been executed up to tho 

 same period about 2000 miles of canal, at a cost of about 9,200,000^. ; 

 and in France there were about 1974 miles of canal, which had cost 

 about 12,257,184J. The average annual cost ef maiutainance varies 

 from 1801. to 601. per mile of canal, according to the nature and activity 

 of the traffic. As a general rule, it may be assumed that the cost of 

 a canal may in England be estimated to be about 41. per yard lineal, 

 with an addition of 1000/. per yard vertical of lockage, or rather with 

 an addition of about 30002. for every lock of ordinary dimensions. The 



