Which Song dynasty polymath described the magnetic compass for navigation?

Shen Kuo, in his Dream Pool Essays of 1088 - a book of about six hundred entries written in retirement after a military disaster ended his political career, and one of the most remarkable documents in the history of science.

He gives the first clear account of the magnetic needle used for direction, and notices that it does not point at true north but slightly east of it - magnetic declination, four hundred years before it was recorded in Europe. He also explains how movable type printing worked, having known the man who invented it; describes petroleum and predicts it will be used in vast quantities; and works out from fossil bamboo found in a dry northern region that the climate there must once have been entirely different.

The reasoning is what stands out rather than the list. Finding marine shells in a mountain stratum, he concluded the land had been seabed and that the mountains were built by sediment and erosion over enormous time - the substance of geological uniformitarianism, arrived at in the eleventh century. He was also an astronomer, a hydraulic engineer, a diplomat and a general who lost a campaign against the Western Xia, which is what gave him the leisure to write it all down.