Today, most countries in the world have canals. Even in the twentieth century, goods can be moved more cheaply by boat than by any other means of transport. Some canals, such as the Suez or the Panama, save ships weeks of time by making their voyage a thousand miles shorter. Other canals permit boats to reach cities that are not located on the coast. Still other canals drain lands where there is too much water, help to irrigate fields where there is not enough water, and furnish water power for factories and mills.
3. The size of a canal depends on the kind of boats going through it. The canal must be wide enough to permit two of the largest boat using it to pass each other easily. It must be deep enough to leave about two feet of water beneath the keel of the largest boat using the canal.
4. Some canals have sloping sides, while others have sides that are nearly vertical. Canals that are cut through rock can have nearly vertical sides. However, canals with earth banks may crumble if the angle of their sides is too steep.
5. Some canals are lined with brick, stone, or concrete to keep the water from soaking into the mud. This also permits ships to go at greater speeds, since they cannot make the banks fall in by stirring up the water. In small canals with mud banks, ships and barges must limit their speed.
6. When the canal goes through different levels of water, the ships must be raised or lowered from one level to the other. This is generally done up by means of locks. If a ship wants to go up to higher water, the lower end of the lock opens to let the boat in. Then this gate closes, and the water is let into the lock chamber from the upper level. This raises the level of the water in the lock until it is the same as the upper level of water. Now the upper gates can be opened to release the ship into the higher water. Of course there must always be enough water on the upper level to allow for the flooding of the lock.Sometimes a canal contains a series of locks when the difference in levels is very great.
7. In places where it does not rain very often, irrigation canals drain water from rivers or lakes and carry it to fields. Sometimes artificial lakes, such as the lake behind the Aswan Dam on the Nile River, provide the irrigation water.
8. In places where there is too much water, canals can drain the water off the land for use in farming. In Holland, acres and acres of land have been drained in this way. Since much of this drained land is below sea level, the water in the canals has to be pumped up to sea level. Dikes have been built in Holland to keep the sea from covering the land, as it did in the past.
9. Sometimes canals have to be built across deep valleys. Bridges or aqueducts are constructed for this purpose. The Romans often brought water to cities from great distances by building such bridges, at the top of which were canals. Some canals go through mountains by means of tunnels. One such tunnel, near Marseille, France, is over four miles long.
10. Canals existed in Egypt thousands of years ago. The great canal at Babylon, between the Tigris and Euphrates, was built about 2000 B. C.. The Grand Canal of China, which is over 900 miles long, was begun about 2,500 years ago, and took centuries to finish. During the seventeenth century, France built many canals that are still in use today. However, they are not so heavily traveled as they were a hundred years ago, before railways were built. One such canal is a short-cut between the Atlantic Ocean and the Mediterranean. In Russia, there are canals reaching from Leningrad to the Caspian Sea. Canals in Germany permit boats to go from the Black Sea to the North Sea. The Kiel Canal provides a passageway between the North Sea and the Baltic. In America, the Great Lakes are all connected by canals, enabling ships to go from the Atlantic Ocean and the St. Lawrence River to Lake Superior. Since the lakes are at different levels, they are connected by locks.
11. Many countries have built canals near the coast, and parallel to the coast. These waterways make it possible for boats to travel between ports along the coast without being exposed to the dangers of the open sea.
（责任编辑:allen）
