The most important use of drifting (漂流) bottles is to find ocean currents. When the position and direction of currents are known, ships can use the forward movement of a current or stay away from currents that would carry them off their course. Benjamin Franklin was one of the first to use bottles in the study of currents. He wondered why British mall ships needed a week or two longer than U.S. ships needed in order to cross the Atlantic Ocean. Franklin thought the Gulf Stream (墨西哥湾流) might explain this difference.Franklin talked with captains of U.S. ships. He found that they knew each turn of the Gulf Stream. They used the current in every possible way. From his talks with the captains. Franklin made his first map of the Gulf Stream. Then he checked his map by using sealed (密封的) bottles. The map that he finally made is still used, with only a few changes, today.Why are drifting bottles used?
