Today, most countries in the world have canals. Many countries have built canals near the coast, and parallel -1- the coast.Even in the twentieth century, goods can be moved more cheaply by boat than by any other -2- of transport. These
-3- make it possible for boats to travel -4- ports along the coast without being -5- to the dangers of the open.Some canals, such as the Suez and the Panama, save ships weeks of time by making their -6- a thousand miles shorter.Other canals permit boats to reach cities that are not -7- on the coast, still other canals -8- landsswheresthere is too much water, help to -9- fields wherethere is not enough water, and -10- water power for factories and mills.The size of a canal -11- on the kind of boats going through it.The canal must be wide enough to permit two of the largest boats using it to -12- each other easily.It must be deep enough to leave about two feet of water -13- the keel of the largest boat using the canal.When the planet Mars was first -14- through a telescope, people saw that the round disk of the planet was criss-crossed by a -15- of strangeblue-green lines. These were called "canals" -16- they looked the same as canals on earth -17- are viewed from an airplane.However, scientists are now -18- that the Martian phenomena are really not canals. The photographs -19- from space-ships have helped us to -20- the truth about the Martia "canals".