They had lunch in the carriage after that. Claude, the butler, politely abstained and ate outside; Jin felt somewhat guilty about it, but the man stood firm no matter how much they tried to persuade him, so there was nothing to be done.
For lunch, they had the bread and fruit they had brought along. It was then that Jin finally brought out his secret weapon: a magic bottle.
"Ah, it’s hot. Why?"
Elsa was surprised to find that the Te-e poured from the magic bottle was still steaming. Jin smiled and explained.
"It’s called a 'magic bottle.'"
"Are you using magic?" Elsa asked, tilting her head slightly.
"No, it’s only called a magic bottle because it keeps things hot as if by magic; I’m not actually using any spells for that. Though, I did include a Cleanup magi-formula inside, but that has nothing to do with heat retention."
"Then, how?"
"Yes, I’m quite curious as well."
Since both Elsa and Reinhard were showing interest, Jin sat back to explain while they sipped their Te-e.
"This is something my teacher taught me. There are three ways that heat is transferred," he began. Reinhard and Elsa both listened intently.
"First is conduction. This is when heat travels through an object."
"I see. Like when you try to eat hot soup with a silver spoon and the handle gets hot right away," Reinhard replied, catching on immediately.
"Next is radiation. When you hold your hands toward a fire, it feels warm, right?"
Elsa answered this time.
"Mm. It’s warm even from a distance. Is that what 'radiation' is?"
Jin nodded at Elsa’s response and moved on to the third.
"The third is convection. When air or water warms up, it becomes lighter and rises, while cold air is heavier and sinks. Convection is the circulation caused by this repeating process."
"I see. If you light a fire in a cold room, the ceiling gets warm but the floor stays cold. That’s it, then," Reinhard noted.
Elsa added an example from her own experience. "Like a bath. If it’s not heated right, the top is scalding but the bottom is cold."
Jin clapped his hands. "Exactly. You both have sharp minds."
He praised them and continued. "So, in this magic bottle, the wall of the flask is a hollow structure from which the air has been removed. By doing that, the heat that would normally escape through convection and conduction stays trapped inside. And since it’s made of metal, it also reflects radiant heat, keeping it contained."
Reinhard’s face lit up with excitement. "So that’s why it doesn't cool down! Incredible, Jin. What do you call this field of study?"
"It’s 'science.' It’s a discipline that seeks to explain every phenomenon in the world."
As Jin explained, Elsa’s face broke into a rare smile. "I understand now. Part of why Jin-kun is so amazing is because he knows this science. That’s why the magic tools he makes are all so logical."
Jin nodded. "You’re right, Elsa. The science I learned from my teacher is a discipline that has lasted for thousands of years, passed down from teacher to student through the ages."
"I wish I could have been taught by your teacher as well!" Reinhard said regretfully. Perhaps out of consideration for Jin, he didn't pry any further into the details of who this teacher was.
＊ ＊ ＊
Night fell.
The moon, which served as their clock, was nearing its zenith. It was just before midnight.
"Well then, shall we go?" Reinhard said in a low voice.
"Yeah," Jin nodded.
"……"
Elsa was asleep, leaning back in her reclined seat. Reinhard gave instructions to his butler.
"Claude, stay here and watch over Elsa. Jin and I are going to the ruins."
"Yes, I understand."
Since Reiko was coming along as a guard, Claude wasn't particularly worried. Of course, the Secret Mobile Unit (SP) was also watching over the carriage from the shadows.
Even though it was a path they had taken during the day, walking at night was difficult. This was especially true because they were walking without lights as a precaution. However, since Reiko was leading the way—unaffected by the darkness—Jin and Reinhard felt at ease.
Finally, they reached the entrance of the ruins. Light from magi-lamps leaked out from the inside; they were likely left on around the clock.
"Reiko, go see if anyone is there."
"Yes, Father."
At Jin’s command, Reiko used Stealth to vanish. She approached the entrance like a breeze, peered inside, and returned just as silently. Neither Jin nor Reinhard had actually seen her move.
"It is safe. There is no one inside," she reported. Relieved, the two stepped into the ruins once more.
"Now then, Jin, where should we start? You probably know more about this than I do, so I’ll follow your lead," Reinhard said. He figured Jin would be more knowledgeable about such things, given that he lived in the Mansion equipped with a Warp Gate.
Jin thought for a moment before answering. "The basement. If there’s an important storehouse, they would build it underground. And there’s undoubtedly a concealment spell placed on it."
Reinhard nodded. "That’s likely true. However, a concealment spell… That’s troublesome."
But Jin shook his head. "That’s because you’re trying to detect it with magic."
"Eh?"
"When there’s a hollow space behind a wall, it makes a different sound when you tap it, right?"
"Indeed."
"In other words, we’ll use 'Sonar.'"
When Jin explained this, Reinhard was impressed and patted him on the shoulder. "I see! I didn't think of that! Let’s try it right away."
Jin and Reinhard split up and used Sonar magic to inspect the walls and floors.
"Nothing here…"
Just as Jin was about to start on the next section, Reinhard called out.
"Jin! Come here!"
"What is it? Did you find something?" Jin hurried over to him.
"Yeah. It’s behind this wall."
When Jin checked with Sonar himself, he confirmed that there was indeed a large hollow space behind the surface.
"The next question is how we get in there."
They could use earth magic to dig a hole, but leaving obvious traces behind would be problematic for their future plans. In the end, they decided to dig a separate tunnel from the outside.
They stepped out of the ruins for a moment. Taking Reiko, who had been on watch, they moved about fifty meters away.
"Alright, let’s start digging from around here."
Choosing a spot hidden from view by a rocky protrusion, they finally began the work.
"Dig."
Jin excavated the path.
"Hardening."
Reinhard reinforced the walls to prevent them from collapsing. Reiko took the resulting rock debris and carried it far away to dispose of it.
After about an hour of labor, the tunnel finally broke through.
"We did it!"
A wide space opened up on the other side. Jin and Reinhard cautiously stepped inside.