With the Eve Festival over, Jin and Elsa relaxed in the Royal Palace’s state guest suite.
"…Today was reasonably fun."
"Mm."
A knock sounded at the door.
"Yes?"
When Reiko opened the door, she found Prince Ernest III standing there, accompanied by his maid Lyla and the golem Lotte.
"Your Highness."
Jin and Elsa rose from their seats to welcome Ernest.
"Jin, I wanted to talk to you for just a little bit. Is now a good time?"
"Yeah, sure."
"In that case, excuse me for dropping in."
Prince Ernest III remained perfectly frank. However, his height had grown to nearly match Jin’s, and his facial features had taken on a more mature cast.
"Again, Jin, Elsa—congratulations on your marriage."
"Father, Elsa-sama, congratulations on your marriage."
"Ji-Jin-sama, Elsa-sama, congratulations on your marriage—mashu!"
The three offered their blessings. Jin and the others politely ignored the fact that one of them had tripped over her tongue.
"Thank you."
"Thank you very much."
Jin took a moment to observe Lotte.
"…Right, Lotte seems to be in good shape, too."
"Yes, thanks to you."
Since she had received a thorough tuning at Jin’s own hands, she was essentially maintenance-free under normal operating conditions.
Not wanting to conduct their business while standing, Jin offered them seats. Ernest sat down while Lyla and Lotte took their positions diagonally behind him to his left and right.
Sitting directly across from Jin, Ernest wore a slightly bashful expression before he spoke.
"I mentioned this briefly before, but there are a few things I’d like to ask you regarding magi-craft."
"Go ahead."
"Last time… back at the World Conference, I asked about control cores. Today, I want to ask about skeletons."
"Okay."
Whether through self-study or by consulting the palace’s magi craftsmen, Prince Ernest was remarkably knowledgeable for his age.
"It’s common to make them out of metal, but that’s heavy, isn't it? And if you make them thin, they get weak… I’m pretty sure Lotte’s skeleton is 64 light silver now, though it used to be iron. Was it hollow back then, by any chance?"
Having watched from nearby while Jin repaired the damaged parts after the incident at the Golem Party, Ernest had noticed that Lotte was built differently from most common golems.
"You’re right…"
At Ernest's own request, Jin used a casual tone with him outside of official settings.
"This is getting a little away from magi-craft, though," Jin prefaced. "I’d like to start by thinking about the strength of a rod."
"Eh… okay." Ernest looked perplexed, not yet grasping Jin’s intention.
"First… ah, this will do."
Jin pulled a piece of wood-paper used for notes out of his luggage and cut it into strips roughly twelve centimeters long and seven centimeters wide.
"Now, I’m going to make a bridge with this paper. We’ll place coins on top and experiment to see how much weight it can withstand."
Jin first demonstrated by folding a sheet of paper over itself four times.
"Like this."
He laid the bridge made of that paper across piers made of stacked coins—roughly ten copper coins high.
"Now, how many will it hold?"
As it turned out, the paper bridge buckled as soon as the second coin was placed on it.
"So, two coins were too much. But if I do this…"
He took another piece of paper, this time folding it into a U-shape, and placed it across the piers with the open side facing down. This time, it held twenty-five coins.
"Wow, it gets that much stronger?"
"Now, what if we flip it upside down?"
He placed the paper so the open side of the U-shape faced upward.
"…Isn't it the same?" Ernest asked.
"Is it?" Jin countered, beginning to place coins.
"Ah."
The paper bridge buckled at the ninth coin.
"…Why?"
Jin smiled. "Exactly. That 'why' is the important part. You experiment, learn the results, and apply them. What lies beyond that is the 'why'."
"What do you mean?"
"I mean, if you can understand the reason 'why,' you'll eventually be able to predict these kinds of results without having to experiment at all."
"Ah, I see!"
"That said, the physics behind this bridge are complicated, and even I can't explain the math fully."
Structural mechanics involved concepts like the second moment of area and integrals; as a high school graduate, Jin unfortunately couldn't provide an academic lecture to that level.
"I went on a bit of a tangent, but by repeating experiments like this, you can discover structures that are both strong and light."
"Yeah."
"And so, the conclusion. In Lotte’s case, the majority of her skeleton has a hollow structure."
"You mean like a pipe?"
"Exactly. In this case, the resistance to bending isn't all that different from when the rod is solid. Well, assuming we're talking about a circular cross-section with the same outer diameter."
"I see…"
Jin's memory was a bit fuzzy on the finer details, but he continued.
"Returning to magi-craft, I suspect there aren't many magi craftsmen who can process these hollow pipes."
"Is that so?"
"Yeah. From what I’ve seen, there are almost no golems or automata built that way."
The skeleton of Elena, the doll who had once led the Unifilar, had been hollow, but he couldn't exactly bring her up as an example.
"Maybe just Tia, who stays with Lies?" Jin recalled that almost every other unit he had seen featured a solid structure rather than a hollow one. "The strength of a rod—or a skeleton—isn't just about bending, of course. There’s torsion, tension, compression, shear, and combinations of all those."
For strengths other than bending, one could generally consider them proportional to the cross-sectional area, Jin concluded. While that conclusion wasn't perfectly accurate, there was no one present who could challenge him on it.
Ernest then raised a hand. "Jin, what is 'shear'?"
"Ah, was that confusing? Shear is… right, it's a force like the one used when cutting something with scissors. You can think of it like being slashed by a sword."
"Ah, I get it. That makes sense."
"The point is, the foundation of structural design is making something as light as possible without sacrificing strength."
Jin added that this principle applied not just to golems and automata, but also to bridges, buildings, carriages, and ships.
"I see… that was fascinating, Jin! Thank you."
Knowing he couldn't overstay his welcome, Ernest thanked Jin, stood up, and left the room.
"Jin-nii, that was interesting."
Once the three had left, Elsa finally spoke up.
"An experiment like that seems like it would be fun to show at the Technology Expo, too."
"Ah, certainly."
Because the World Conference had been held this year, the Technology Expo was reportedly scheduled for sometime after autumn.
"I wonder if we should participate…"
Jin had completely forgotten about tomorrow's busy schedule.