Shen Kuo was a Song dynasty official who had been a finance minister, a hydraulic engineer, a diplomat sent to negotiate a border with the Liao, and a military commander. In 1082 an army under his overall command was defeated at Yongle by the Western Xia, tens of thousands of men were lost, and he was demoted and eventually banished from public office.
He bought a garden estate near modern Zhenjiang that he had dreamed about years earlier, called it Dream Brook, and spent his remaining years writing down everything he knew. The result, the Mengxi Bitan or Dream Pool Essays, was finished around 1088 and contains about six hundred entries on whatever had caught his attention over a lifetime.
Several of those entries record things that Europe would not arrive at for another four hundred years.
The Needle Does Not Point South
Chinese writers had known for centuries that lodestone attracts iron, and a spoon shaped lodestone device used for divination goes back to the Han dynasty. What Shen Kuo did was describe the magnetised needle as a practical instrument and then record something nobody had noticed.
He wrote that magicians rub a needle with a lodestone so that it can point south, but that it is always displaced slightly to the east, and does not point due south.
That is magnetic declination, the angle between the direction a compass gives and true geographic north. It exists because the Earth’s magnetic field is generated by fluid motion in the core and is not aligned with the rotation axis, and it varies from place to place and drifts over decades.
In Europe the effect is usually credited to Christopher Columbus in 1492, and possibly to German instrument makers a few decades earlier. Shen Kuo wrote it down in 1088, which is a gap of four centuries.
He also tested four ways of mounting the needle: floating it on water, balancing it on a fingernail, resting it on the rim of a cup, and suspending it from a single silk fibre with a piece of wax. He preferred the silk thread, correctly, because it gives the least friction and the steadiest reading, and he noted the drawbacks of each of the others. This is not a mystical text. It is an instrument review.
Elsewhere he describes the compass being used at sea by navigators when the sky is overcast, one of the earliest such references anywhere.
Bamboo Where No Bamboo Grows
A landslide near Yanzhou, in the dry north of China, exposed a bank containing hundreds of petrified bamboo shoots. Bamboo does not grow in that region and, as far as anyone knew, never had.
Shen Kuo’s response was to work out what that implied. If the bamboo was there, the climate of that place must once have been wet and warm enough for bamboo, and therefore climates change over time.
He made a related argument from a different observation. Travelling through the Taihang mountains he found bands of shells and rounded pebbles embedded in the rock, hundreds of kilometres inland and high above the plain. He concluded that this had once been a shore, and that the great plain to the east had been built up over enormous periods by silt carried down by the rivers.
Both arguments are correct, and both are the same kind of reasoning that his contemporary Su Song applied to machinery: take an anomalous observation seriously, and follow it to whatever conclusion it forces, even if the conclusion is that the world used to be different.
Oil, Printing and a Lost Calendar
The Dream Pool Essays range far enough that any summary is a selection. Three entries in particular have outlived the rest.
- Petroleum. He described a black substance seeping from rocks in Shaanxi, gave it the name shiyou, rock oil, which is still the Chinese word for petroleum, and burned it to collect the soot for making ink, which he judged superior to pine soot. He predicted the stuff would come into great use in the world, and remarked that pine forests would be exhausted while this would not
- Movable type. The only surviving account of Bi Sheng’s invention of movable type, made from fired clay in the 1040s, is Shen Kuo’s. Without this passage the innovation would be unknown, since Bi Sheng was a commoner and no other record of him exists. Shen Kuo notes practical details, including that clay type is better than wood because wood absorbs moisture and warps, and that the type was stored sorted by rhyme
- The calendar. He proposed abandoning the lunar months entirely and using a purely solar calendar of twelve divisions marked by the seasonal terms. It was rejected as too radical. He wrote that men of the future would come to understand it, and something close to it was adopted in China in 1912
Optics, Sound and the Sky
He observed that a concave burning mirror produces an inverted image, located the focal point, and used a paper screen to find it. He described the pinhole image formed on a wall and compared the way a hand held between the sun and a wall reverses its shadow, giving a correct qualitative account of image inversion.
He demonstrated sympathetic resonance by cutting small paper figures and placing them on the strings of a lute, then plucking the corresponding string on a second instrument, so that the figures on the untouched string jumped while the others stayed still.
As director of the Bureau of Astronomy he spent five years observing the positions of the Moon, the Sun and the planets three times a night, and corrected the errors in the official charts. He explained retrograde motion of the planets, and described the shape of the Moon’s orbit relative to the ecliptic using the analogy of a willow leaf. He also had to fight the bureau’s own staff, who had grown used to filing observations they had not made.
Why the Line Stopped
The obvious question is why a civilisation that had the compass, the escapement, printing and gunpowder before anyone else did not produce the scientific revolution. It is Joseph Needham’s question, and it has no agreed answer.
What can be said about Shen Kuo specifically is that his work had no successors. The Dream Pool Essays is not a treatise. It is a set of observations, brilliant individually, that were not organised into a method anyone else could take up and continue. There was no equivalent of a scientific society, no journal, and no career in which a younger man could make his name by extending the compass observation or the fossil argument.
Shen Kuo’s declination measurement sat in a book of miscellaneous notes for eight hundred years. In Europe, the same observation arrived at a moment when ocean navigation had become a matter of national wealth, and within a century of Columbus, mariners were carrying declination tables and William Gilbert had published a book arguing that the Earth itself is a magnet. Mercator drew his 1569 chart with a small inset dealing with the problem of the magnetic pole.
The physical cause of the wandering was not understood until much later. That a compass needle can be disturbed by something happening on the Sun, which is the same field responding to a shock from outside, was only established after the storm of 1859 made it impossible to ignore.
The Disgraced Official
Shen Kuo died in 1095. The battle that ended his career is a footnote in Song military history, and the estate where he wrote is a park.
His own preface says that he had nobody left to talk to, and that the entries were conversations he was having with his writing brush. Almost everything he was first to notice was written in that condition, by a man who had been removed from every position he had held and had nothing else left to do.
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