Canals are artificial channels cut in the ground, or built up above it, supplied with water from the sea, from rivers, or from springs, and forming waterways for inland navigation and goods traffic. They may also be employed to drain away the water from a district, or to supply water from a river to a region where it is scarce, and its want much felt for agricultural or other purposes.
Canals were known and appreciated by the ancients, both for navigation and irrigation. The Egyptians employed them extensively. Two still exist in Lincolnshire that were built by the Romans; and there are ancient canals in China where inclines were employed to transfer the boats from one cut to another at a different level, a method still used to solve the difficulty of traversing hilly country by a waterway. But it is only since the middle of the last century that canals have been taken up at all generally. Then Brindley designed and completed several in England, and canal schemes became popular. The introduction of railways considerably diminished the inland water-traffic in this country; though there are a few instances where canals still compete successfully with railways. The largest canals in Great Britain are the Gloucester and Berkeley, 17 miles long and 15 feet deep, enabling vessels of 600 tons to reach Gloucester from Sharpness; the Aire and Calder Navigation, 9 feet deep; the Forth and Clyde, 10 feet deep; and the Caledonian Canal, 60 miles long, 120 feet wide at the surface, 50 feet wide at the bottom, and 17 feet deep, which, by uniting a chain of lakes in Inverness, forms a waterway across Scotland for vessels of 300 tons.
In France there are 3,000 miles of canals and 2,000 miles of canalised rivers. Steps have been taken in that country to render all the principal waterways available for vessels of 300 tons with a draught of 6 feet. The flatness of Holland and certain parts of Belgium have rendered their canal traffic very flourishing for several centuries. The Amsterdam trade has been much improved recently by the construction of the ship canal, 16-1/2 miles long, between that town and the North Sea. It only involved the cutting of about three miles of canal, the rest being merely a channel dredged out of the Wyker Meer.
In Russia, the Volga and Neva canal connects those two rivers, and enables large vessels to pass from one to the other. A ship canal joins St. Petersburg with Cronstadt; its width is from 200 to 275 feet, and depth 22 feet; and it thus enables sea-going vessels to reach St. Petersburg, which the insufficient depth of the Gulf of Finland previously prevented.
In America the most important canals are the Erie, 370 miles long, joining the Hudson river to Lake Erie, for vessels of 250 tons; the Georgetown to Pittsburg, joining the Potomac with the Ohio, would be about the same length if completed, but not quite 200 miles have as yet been cut; the St. Lawrence system will enable vessels of from 1,000 to 1,500 tons to pass between Lake Erie and Montreal, the Welland canal connecting Lakes Erie and Ontario so as to avoid the Niagara river.
The Suez Canal, joining the Mediterranean with the Gulf of Suez, is of enormous importance, saving, as it does, the great detour round the Cape of Good Hope for vessels travelling between Europe and Australia or the south of Asia. It was begun in 1860 and finished in 1869 by M. de Lesseps, the French engineer, at a cost of £16,000,000. Its length is about 100 miles, bottom width 72 feet, surface width varying from 200 to 330 feet, and depth 26 feet. A service canal was cut for part of the way during the process of construction; and a fresh-water canal from the Nile to Suez was also formed, in order to give a supply to the waterless regions through which the works had to be conducted. The traffic has increased so enormously on the Suez Canal that it is shortly to be trebled in bottom width, and deepened to 28 feet.
The Panama Canal, as originally proposed by M. de Lesseps, was to join the Atlantic and Pacific Oceans by cutting across the Isthmus of Panama at its narrowest part, between Aspinwall and Panama. It was to be level throughout, traversing a range of hills by a cutting 300 feet deep at one part. This was commenced in 1882, but the difficulties in the work and the want of funds caused a change in the design, the plan of a locked canal being adopted to diminish the amount of cutting required. Natural difficulties of an exceedingly serious nature, which do not seem to have been foreseen by the engineer, put back the work continually, and in 1889 the company became insolvent. The report of a recent commission of French engineers sent to Panama seems to point to the impossibility of the success of the undertaking.
There is an American scheme for forming a waterway across the Nicaragua, by canalising the San Juan emissary of Lake Nicaragua. This seems much more feasible. A treaty has been signed between the United States and Nicaragua, and the Ship Canal Company formed in 1889. The canal route will have a total length of 170 miles, but only 28 miles of excavation will be necessary. There are to be three locks on each side of the lake, the minimum depth is to be 30 feet, and vessels are to pass from ocean to ocean in twenty-eight hours. The estimated cost is £12,000,000.
The Manchester Ship Canal allows large vessels to pass up from the Mersey to Manchester. It starts from the south side of the Mersey estuary at Eastham, runs near the shore to Runcorn, and then inland to Manchester, near the course of the Irwell. It is 35 miles long, with bottom width 120 feet and depth 26 feet. There are sets of locks at three different places, each set being arranged to accommodate vessels of different sizes. The docks at Manchester are 88 acres in extent. The work was completed in 1893, and the canal opened on 1st of January 1894.
Among many other canal schemes may be mentioned the Isthmus of Corinth Ship Canal to cut across the narrowest part of Greece; the Baltic Canal to traverse Holstein, and so join the Baltic directly with the North Sea; and the Isthmus of Perekop Canal to connect the Sea of Azov more directly with the Black Sea.
Drainage and irrigation canals are intended to lead water along from one place to another, and are therefore to be designed with a regular slope in the bed. If the slope is too slight, the current is not rapid enough to conduct the necessary amount of water without unduly increasing the sectional area of the canal; if too great, the rapid current induced will damage the canal bed. In this respect of slope such canals differ from navigation canals, which are laid in level reaches, and therefore require special means to conduct vessels from one reach to another at a different level. This transference is generally done by locks (q.v.). A lock is an enclosed space between two watertight gates that separate the two reaches of the canal. A boat passing from the lower level to the higher is first floated into the lock, from which water had been allowed to flow till the level was that of the lower reach. The upper gate is closed, and has to withstand the pressure of the water on its outside face. Then the lower gate is closed, and water from the higher level is allowed to enter gradually till the lock-level and that of the upper reach are the same. The upper gate is then opened, and the boat floated out.
If the difference in level is very great a series of locks may be employed, or a carriage may convey the vessel bodily up an incline from the one reach to the other, the carriage being drawn by a cable that is partially hauled by a descending load. The vessel may be taken out of the water, or it may be contained in a large tank or caisson. Hydraulic lifts are now much employed to effect the same result of changing levels.
The depth of a canal should be 1-1/2 feet greater than the draught of the vessel on it; its bottom width should be twice the breadth of beam; and the sides should slope from 1-1/2 to 2-1/2 feet per foot. though special circumstances may modify this rule considerably.
By the statute 8 and 9 Vic. c. 42. canal companies were entitled to become carriers on their canals; also to lease the same or to take leases of other canals, and by subsequent Acts the traffic and tolls over canals are regulated. Subject to the payment of tolls and the traffic rules, the public have a right of using the canal, and a canal company cannot confer an exclusive right to let boats for hire over their water so as to give the guarantee a right to sue a third party for the infringement of this right.
An Act of 40 and 41 Vic. c. 60 regulates the use and registration of canal boats as dwellings.