听力原文： I spoke yesterday about the construction of ancient Viking ships. Today I'd like to discuss the transocean voyages that the Norse made in these open boats-voyages made without compasses or charts. Somehow the Vikings managed to get across the North Atlantic and back home again. Although the shortest distance between the coast of Norway and Green- land is about 900 miles, the Vikings preferred to take a longer route south of Iceland and there-by avoid pack-ice. This was a voyage of well over one thousand miles. How did Norse sailors find land after days of sailing out of sight of land? well, experienced sailors used the relative position of the stars to help them navigate. The sun's position could also be noted, but it moves across the sky and its position alters a little every day, so it was not easy for the Vikings to use. However, even when out of sight of land, an experienced sailor could find in- formation. As there are landmarks on land, so there are at sea. Whales gathered in large numbers to feed at an area half a day's sail south of Iceland. Migrating birds on their annual flight were also helpful because they always followed the same route. So, geese flying between Britain and Iceland were of particular use to the Vikings. One Icelander also took ravens with him, releasing them until one day they didn't return. He followed their direction and found land. In 900 A. D, ingenuity had to take the place of technology. What is the main topic of the talk?