【阅读选择】Two thousand years ago the Chinese had already developed a working compass. A piece of lodestone(天然磁石) would be cut into the shape of a ladle(长柄勺). Placed on a stone board with a smooth surface, the lodestone would move round until the "handle" of the ladle pointed south while the bulk(主体) of the ladle was attracted to magnetic north. This interestingly shaped device is mentioned in a book from about 80. Other ancient books may have dated it as far back as the fourth century Jade(玉石) finders played an important role in the invention of the "ladle". They travelled great distances to look for jade. In order not to lose directions, they would take a "ladle" with them. When did the Chinese invent the compass?