The magnetic compass is used for navigation at sea

Chinese scholars had known for centuries that lodestone aligns itself north-south, and used it in geomancy. In the Pingzhou Table Talks, written in 1119 by Zhu Yu, the compass is mentioned for navigation by sea, describing practice from the late eleventh century. The Song navy and merchant ships used a needle floated in water or hung from a thread. Arab and European sailors adopted the compass by the late twelfth century, and Alexander Neckam of England mentions it about 1187. It allowed ships to sail in cloudy weather and out of sight of land, and made the age of ocean voyages possible.

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