Reform and the Celestial Army
Yi Yeon felt like he was about to lose his mind from sheer frustration.
He wanted to tell Yi I everything he knew. But there was no way to prove it, no coherent way to explain it, and it might cause enormous problems.
Who knows what kind of disaster that would bring.
So Yi Yeon decided to change the subject.
“Do you know how the Sexagenary Cycle is composed?”
“Yes, Your Majesty. I understand it consists of ten Heavenly Stems and twelve Earthly Branches combined to create sixty unique pairs.”
“Then do you know why the Stems are divided into ten and the Branches into twelve?”
“I have heard that the twelve Branches correspond to the twelve full moons in a year. However, I have never heard the reason for dividing the Stems into ten.”
“Is that so?”
“Yes, Your Majesty. Given that the Stems seem to describe the cycle of life, I have wondered whether they follow the Five Elements, but I remain uncertain.”
“I once thought the same, but I do not believe that is correct. Think carefully. The Branches, called yin, follow the movement of the moon, yet the Stems, called yang, do not follow the movement of the sun. How then could they relate to the Five Elements?”
When Yi Yeon spoke with such finality, Yi I closed his eyes tightly, either at a loss for words or lost in thought.
No! Don’t think about it!
Alarmed, Yi Yeon quickly spoke again.
“Something just occurred to me. Perhaps the ten Stems come from the fact that we have ten fingers.”
“Ah…”
Yi I stared blankly at his own fingers. Seeing this, Yi Yeon let out a sigh of relief.
Thank goodness.
It seemed he had successfully banished the word “revolution” from Yi I’s mind and replaced it with a harmless mystery about the Heavenly Stems.
Yi Yeon had once witnessed famous scholars in New York debating the Sexagenary Cycle.
None of them knew the true reason.
The most reasonable explanation among them had been remarkably simple.
“Perhaps the Heavenly Stems are based on the ten fingers of our hands. After all, everyone naturally uses their fingers to count and calculate.”
“Ah…”
Perfect!
When Yi I’s expression grew distant, as if pondering something profound, Yi Yeon quickly continued before the man could wander into dangerous territory.
“That is why I decided to divide one sijin into sixty units.”
Yi I tilted his head, clearly confused by the sudden leap in topic, then asked, “May I ask why Your Majesty chose to do so?”
“There is no special reason. I am the king of this nation, so I thought I could at least decide something of that scale. Is there any problem with it?”
“None at all, Your Majesty. There is absolutely no problem.”
“Truly?”
“Yes, Your Majesty. When you previously introduced the metric system, everyone praised it as convenient and marvelous. No one complained that it was inconvenient.”
The metric system Yi Yeon had worried about was spreading far faster than expected, reaching beyond Gyeonggi and Chungcheong all the way to Jeolla and Gyeongsang.
He reflected on why.
It was obvious. I worried for nothing.
The people who needed precise standards of length, weight, and volume most were merchants. Clear standards made trade smoother. The common people wanted them too. They were tired of being cheated.
So the court had displayed the new rulers, scales, and measuring vessels in front of government offices. They even posted explanatory signs written in Hangeul—similar to the vernacular script but far easier to read—detailing how to use them and how to convert measurements.
Alongside a stern warning: Anyone who violates or cheats with these standards, regardless of status or position, will be sentenced to hard labor.
As a result, the areas in front of government offices were constantly crowded. At first people came to resolve trade disputes. Then they returned to buy their own rulers, scales, and vessels. Eventually they came regularly to copy the designs and sell them. They were easy to use and clearly profitable.
The old cheok-geun system defined in the Gyeongguk Daejeon had caused constant confusion because it used the same terms as the previous system. Arguments broke out frequently. The metric system, however, used entirely new terminology. That was why it had taken root so quickly, replacing the old measurements.
Because of that success, Yi Yeon continued with even greater confidence.
“I have heard that in the distant western regions, they divide the day in two based on when the sun is highest—before noon and after noon. It seemed rational. If we split the day in two, wouldn’t it allow us to work more efficiently?”
“Indeed, Your Majesty.”
“Therefore, like them, I have decided to divide both morning and afternoon into twelve units each and call them ‘hours.’”
“Ah… How truly astonishing.”
“You think so?”
“Yes, Your Majesty. If we divide a day into twenty-four parts instead of twelve, wouldn’t we be able to tell time with even greater precision?”
“Correct. That is why I further divided each hour into sixty units called ‘minutes,’ and each minute into sixty units called ‘seconds.’”
“Then one sijin equals 7,200 seconds, and one hour equals 3,600 seconds?”
“Exactly. Well done.”
As expected of Yi I. He had calculated it instantly.
Because of moments like these, Yi Yeon took his time gathering his thoughts. He did not want to repeat the same mistake. He had also instructed that when the two of them were alone, court etiquette need not be strictly followed. Mathematics was a discipline of questioning and solving problems; excessive formality would only get in the way.
Yi Yeon turned to look behind him, then spoke again.
“I have often climbed Inwangsan and dropped apples and stones of identical size from the cliff countless times. The results confirmed my hypothesis. I also wanted to know exactly how fast they fell, so I repeated the experiment hundreds of times and discovered that the speed increases at a constant rate.”
“Your Majesty! Could you tell me that value?”
“Telling you the value is not difficult, but first you must understand the concepts of ‘kilometers per hour’ and ‘meters per second.’”
“I beg you to explain, Your Majesty.”
Yi Yeon smiled with satisfaction. Yi I’s uncontrollable curiosity had made him bow his head urgently again, causing Yi Yeon’s shoulders to rise with pride. Still, he let out a long breath to calm his nerves.
Whew. That was close.
Once again the cowardly King Seonjo had thrown a tantrum inside him, urging him to avoid the topic, but Yi Yeon had endured and overcome it. The result was a resounding success. Not a trace of suspicion could be seen on Yi I’s face.
Yi Yeon took out paper and a pencil from the royal study and wrote the following:
km/h = distance traveled per hour
m/s = distance traveled per second
1 kilometer (km) = 1,000 meters (m)
1 hour (h) = 3,600 seconds (s)
“As you can see, ‘kilometers per hour’ is the unit that expresses distance traveled in one hour, while ‘meters per second’ expresses distance traveled in one second. However, because one hour is 3,600 times longer than one second, I created a unit called the ‘kilometer,’ which is a thousand times longer than a meter. This makes calculations much more convenient.”
Yi I’s eyes widened at the explanation.
“Ah… To think there is such a simple way to express it. Your Majesty, how did you come up with these units?”
This time Yi Yeon answered immediately. He had prepared an excuse in case a genius like Yi I noticed something strange.
“When I was young, I read a book brought from the western regions and took inspiration from it.”
“Might this humble subject also be permitted to read that book?”
“Unfortunately, the dog we kept at home ate it.”
“Oh no…!”
Yi I looked genuinely heartbroken. He must have really wanted to see it. The young king who possessed knowledge Yi I did not was already astonishing. The fact that he kept creating new symbols had filled Yi I with respect. That was why he had wanted to read the book that seemed to have influenced the king so greatly.
And now he learned a dog had eaten it.
“Your Majesty! What a truly regrettable loss.”
Yi Yeon nodded as if he understood perfectly.
“Do not be too disappointed. If this nation continues to develop, we will eventually reach the western regions. Even if we do not, many people will come after hearing the rumors. You may ask them then.”
“Your words fill me with awe. I am constantly moved by Your Majesty’s wisdom. It is the greatest honor to be able to learn such new and astonishing knowledge.”
Yi Yeon could tell that Yi I’s words and behavior came from the heart.
He is not merely a genius.
His curiosity and passion for learning were truly remarkable. So Yi Yeon continued explaining with renewed energy.
“In any case, both the apples and the stones fell while accelerating at roughly 9.8 meters per second.”
“You said acceleration. Does that mean after two seconds they would be falling at 19.6 meters per second?”
“Correct.”
“Ah… So the speed increases by 9.8 meters every second, which is why you called it acceleration.”
“Exactly.”
Yi Yeon could not help but be impressed. Even before he could add further explanation, Yi I had perfectly grasped the concept of gravitational acceleration.
To think they worked such a genius to death as a mere administrator.
Anger surged within him. Not only had the previous king encouraged the factional strife that led to the nation’s ruin, but he had used a ballistic missile to kill a chicken. Yi Yeon wanted to curse him thoroughly, but since that was impossible, he could only swallow his rage.
He asked Yi I, who was busily writing something down, “Have you obtained what you wanted?”
“Yes, Your Majesty. I am truly grateful.”
Yi I prostrated himself deeply in thanks, then prepared to leave.
“Your Majesty, I shall organize the values you have given me and return with my findings.”
What? He’s leaving already?
Yi Yeon had not even reached the main topic. Yet Yi I was trying to slip away like a fish that had swallowed the bait and was now fleeing. Yi Yeon hurriedly cast his net.
“Wait. I have something to ask you.”
“Yes, Your Majesty.”
“Why did you create this formula?”
“I visited Seogang Naru some time ago.”
“Did you?”
“Yes, Your Majesty. I went both to enjoy the breeze and to see if I could find practical applications for the differential calculus you taught me. While I was there, I witnessed the wasteful use of valuable gunpowder.”
After the reign of King Sejong, black powder had grown increasingly scarce. These days, however, there was a surplus. And that surplus had led to a major incident—one that everyone in Hanyang and the surrounding provinces knew about. Everyone except Yi I, who had been completely absorbed in mathematics.
The head of the Fire Bureau, having learned the method of manufacturing black powder from Yi Yeon, had been able to mass-produce saltpeter by mixing chicken droppings supplied from Inghwado and Ganghwado with dried willow branches.
When nitrogen-rich chicken droppings and potassium-rich willow branches were placed in jars and left to age, a solution containing potassium nitrate could be obtained. During the aging process, organic matter in the droppings was broken down by microorganisms and converted into ammonia and other nitrogen compounds. These nitrogen compounds then underwent nitrification to become nitric acid. At the same time, the nitric acid reacted with potassium ions extracted from the willow branches to form potassium nitrate—saltpeter.
The entire process required careful control of temperature, humidity, and airflow, which was why jars were used. Holes were also drilled in the bottom of the jars so that only the solution that seeped out could be collected and dried to easily obtain saltpeter.
This saltpeter was then mixed with charcoal and sulfur in the proper ratios to create sticky lumps of black powder. The lumps were ground through a sieve into small granules and dried in the shade, producing usable black powder.
Aside from the method of making saltpeter, the process was not very different from traditional black powder production. The performance, however, had been mediocre. So Yi Yeon had added the method used by the people of Yadeok.
“Create a barrel coated on the inside with lead. Mix a very small amount of graphite powder into the black powder, add lead beads, and rotate the barrel.”
“Yes, Your Majesty.”
“You must use lead, and do not keep any flint or iron objects nearby. A spark could cause an explosion and kill you.”
“I will keep it in mind, Your Majesty.”
The black powder produced this way had greatly improved moisture resistance and could be stored for long periods. The particles were also uniform, increasing both stability and explosive power.
If I added a little potassium perchlorate, sodium benzoate, dicyandiamide, dextrin, and water, I could create a safe substitute for black powder.
But Yi Yeon had no intention of making substitutes. Why bother when we can simply move on to smokeless powder?
Once high-quality black powder was being mass-produced, Yi Yeon had ordered the gunpowder stored at the Military Weapons Office to be used for practice with the new rifles and cannons.
Leaving it unused would only let it degrade.
During the inventory inspection, however, an enormous corruption scandal had been uncovered.
“What? They were selling saltpeter to the tile makers?”
Historical Note
The blue roof tiles of the palace first appeared during the reign of King Sejong. They were made by mixing clay with saltpeter, firing it once, applying glaze, and firing it again.