中国古代四大发明怎么说？(4)

Early in the Spring and Autumn Period （770-476BC）, while mining ores and melting copper and iron, Chinese people chanced upon a natural magnetite that attracted iron and pointed fixedly north. In the Warring States Period （206BC-23AD）, after constant improvement the round compass came into being. Referred to as a "South-pointer", the spoon- or ladle- shaped compass is of magnetic lodestone, and the plate is of Bronze. The circular center represents Heaven, and the square plate represents Earth. The handle of the spoon points south. The spoon is a symbolic representation of the Great Bear. The plate bears Chinese characters which denote the eight main directions of north, north-east, east, etc. This type of compass has been scientifically tested and found to work tolerably well.

By the time of the Tang dynasty （618-907） and the beginning of the Northern Song Dynasty （960-1127）, Chinese scholars had devised a way to magnetize iron needles, by rubbing them with magnetite, and then suspending them in water. They also observed that needles cooled from red heat and held in the north-south orientation （the earth's axis） would become magnetic. These more refined needle compasses could then be floated in water （wet compass）, placed upon a pointed shaft （dry compass） or suspended from a silk thread, etc. Consequently, they were much more useful for navigation purposes since they were much more portable.

During the Song Dynasty （960-1279） many trading ships were then able to sail as far as Saudi Arabia without getting lost. The compass was introduced to the Arab world and Europe during the Northern Song Dynasty. The spread of the compass to Europe opened the oceans of the world to travel and led to the discovery of the New World