Is the compass evolved from the compass?

Zeng Sanyi, a scholar in the Southern Song Dynasty, said in Tongtan Lu: "Snails either have meridian needles or sew them with meridian needles."

The "snail" here is a snail, that is, a compass. This is a feng shui compass. At this time, the knowledge of magnetic declination has been applied to the compass.

This geomantic compass has not only the meridian positive needle, that is, the geomagnetic north and south directions determined by the magnetic needle, but also the sewing needle between the meridian and the geographical north and south directions determined by the solar shadow. There is an included angle between these two directions, which is the magnetic declination.

At that time, the compass was still a water compass, and the magnetic needles were all floating on the water through rushes. Xu Jing, a painter and calligrapher in the Northern Song Dynasty, said in the Map of Xuanhe's Mission to North Korea that when sailing at sea, he used a guide to float the needle when it was cloudy.

The drought compass probably appeared in the Southern Song Dynasty. Dry compass refers to a compass that does not use the "water float" to place the magnetic needle of the compass. Usually, a small hole is opened at the center of gravity of the magnetic needle as a support point, which is supported by a lower shaft. The friction resistance of the support point is very small, so that the magnetic needle can rotate freely. Obviously, the dry compass has more advantages than the water compass and is more suitable for navigation, because the magnetic needle has a fixed fulcrum and will not wander on the water.

The installation method of magnetic needle with fixed fulcrum of dry compass originated very early. Because Sina has a certain fulcrum, and Shen Kuo's magnetic needle device test also has a fixed fulcrum. Compass, as a pointing tool, played an important role in China's ancient military, production, daily life, topographic survey, especially in navigation.

China's compass was introduced to Europe through Arabia from the end of 12 to the beginning of 13, which played a positive role in the development of the world economy.

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