Today, most countries in the world have cs. Some cs, such as the Suez or the Panama, (31) ships weeks of time by their voyage a thousand miles shorter. Other cs permit boats to reach cities that are not located on the coast. (32) other cs drain lands where there is too much water, help or (33) fields where there is not enough water. The (34) of a c depends on the kind of boats going through it. The c must be wide enough to permit two of the (35) boats using it to (36) each other easily. It must be deep enough to leave about two feet of water (37) the keel of the largest boat using the c. Some cs have sloping sides, while others have sides that are nearly (38) . Cs that are cut through rock can have nearly vertical sides. (39) , cs with earth banks may (40) if the angle of their sides is too steep. Some cs are lined with brick, stone, or concrete to keep the water (41) soaking into the mud. This also permits ships to go at (42) speeds, since they cannot make the banks fall in by (43) up the water. In small cs with mud banks, ships and barges (大平底船) must (44) their speed. When the c goes (45) different levels of water, the ships must be (46) or lowered from one level to the other. This is generally done by (47) 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 raised the level of the water in the lock (48) it is the same as the upper level of water. Now the upper gates can be opened to (49) the ship into the higher water. Sometimes a c contains a series of locks when the (50) in levels is very great.