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Then lunch in the carriage. Claude, the butler, is eating outside, somewhat apologetic, but no matter how hard I try to persuade him, he just won't listen.

Lunch is served with bread and fruits. Then Jin took the opportunity to bring out a thermos.

"Oh, it's hot. Why?"

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Elsa is surprised to see that the tea poured from the thermos is still hot. Jin laughs,

"It's called a thermos," he explains.

Jin explains.

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"You use magic?"

Erza tilts her head and asks.

"No, it's called a thermos because it's hard to cool down as if by magic, but I don't use any particular magic. I've put in a magical formula for purification, but it has nothing to do with keeping it warm.

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"Then why?"

"Well, I'm interested too."

Not only Erza but also Reinhardt showed interest, so Jin sat back down to explain over a cup of tea (teehee).

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"This is another thing my teacher taught me," he began, "there are three different ways heat can be transferred.

He began. Reinhard and Elsa listened attentively.

The first one is conduction. This is when heat is transferred through an object.

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"Well, if you try to drink hot soup with a silver spoon, the handle gets hot right away.

Reinhard seems to swallow quickly.

"Next comes the radiation. When you hold your hand over the fire, it gets warm, doesn't it?"

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Erza replies this time.

"Yes. It's warm even from a distance. Is that what you mean by radiation?

Jin nodded his head in agreement with Erza and said the third thing.

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The third one is convection. When air or water gets warm, it becomes lighter and rises upward, while cold air is heavier and falls downward. This repeated cycle of air circulation is convection.

"I see, when you build a fire in a cold room, the ceiling is warm but the floor is cold," Reinhardt said.

Reinhardt said, and Erza shared her experience.

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A bath. If you fail to boil it, only the top is hot and the bottom is cold.

Jin clapped his hands,

"That's right.

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He praised them both.

"So, this thermos has a hollow wall structure that allows the air inside to escape. In this way, heat that should escape by convection and conduction does not escape. And since it is metal, it also reflects radiant heat and traps it inside.

Reinhardt's face lit up when Jin said this,

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"That's why it doesn't cool down easily!　That's great, Jin, what's the word for that?"

He was very excited.

"<>. It is a science that tries to explain all the phenomena in the world.

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When Jin explained this, Erza's face broke into a smile,

"I understand now. One of the reasons why Jin-kun is so great is because he knows the science. That's why all the magic tools you make make sense.

Jin said. Jin nodded his head,

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Erza, you are right. The science I learned from my teacher has been continued for thousands of years by my teacher and his teacher and his teacher's teacher, and so on.

He explained.

"Hmmm, I wish I could have learned from Jin's teacher!" said Reinhardt regretfully.

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Reinhardt said regretfully. Perhaps it was Reinhardt's own concern for Jin that he did not ask him about his teacher.

The night was over.

Night fell.

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The moon, which tells the time, is about to rise into the mid-sky. In other words, just before midnight.

"Well, let's go," Reinhardt whispered.

Reinhardt whispered.

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"Yes.

Jin nodded.

"............"

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Elsa was asleep in her reclining seat. So Reinhardt gives instructions to the butler, Claude.

"Claude stay here and watch over Erza. I and Jin are going to the ruins.

Yes, sir.

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Claude is not worried because Reiko will be his escort. Well, the > is watching over this carriage from the shadows.

Even though the road was taken during the day, it is not easy to walk at night. Even more so since we are walking without lights on as a precaution. However, Reiko, who does not mind the darkness, is leading the way, which makes it easier for Jin and the others.

Finally, we came to the entrance of the ruins. The light of the magic lamp was leaking from the inside.

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"Reiko, go and check if there is no one there.

"Yes."

At Jin's instruction, Reiko disappeared in stealth, approached the entrance of the ruins like the wind, peeked inside, and came back without making a sound.

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Neither Jin nor Reinhardt could see her.

"It's all right, there's no one here.

They stepped into the ruins again, reassured by the report that "It's okay, there is no one here.

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"Well, Jin, where should we start?　"Well, Jin, where should we start?

Reinhard said. Jin had lived in a house with a warp gate, albeit a small one, so he decided to follow your instructions.

Jin thinks for a moment and then replies.

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"Underground. An important warehouse would be built underground. And it must have been enchanted to conceal it.

Reinhardt agrees,

"Probably so. But a cloaking spell, huh? That's a problem.

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But it's a cloaking spell, isn't it? Jin, however, shakes his head and denies it.

"It's not good to try to detect them with magic."

What?

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"When there's a cavity behind the wall, if you tap on it, it makes a different sound, doesn't it?"

"Mm-hmm."

"In other words, we use sonar."

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When Jin explains this, Reinhardt is impressed and pats him on the shoulder.

"I see!　I hadn't noticed that!　Let's give it a try."

So Jin and Reinhardt split up and used their sonar magic to check the walls and floors.

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"I don't see it here. ......"

Jin was about to start on the next wall when Reinhardt shouted out.

"Jin!　Come here!"

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"What's wrong, did you find him?"

Jin rushed to Reinhardt's side.

"Yeah. Over there.

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So Jin also checked with the sonar and found that there was indeed a large cavity beyond that wall.

"Then the next question is how to get to the other side.

We could dig a hole with earth magic, but it would be bad to leave a mark on the wall for future use.

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In the end, they decide to dig another hole from the outside.

He goes outside for a while. He took Reiko, who was watching him, and went about 50 meters away.

"Okay, let's start digging from here.

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We chose a place where the rock outcropping would make it difficult to see from the outside, and finally started the work.

"<>"

Jin digs a hole,

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"<>"

Reinhardt strengthens the wall to make it harder to collapse.

Reiko disposed of the debris far away.

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After about an hour, the hole was finally penetrated.

"We made it!"

Beyond the hole was a wide open space.

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Jin and Reinhardt stepped carefully into the hole.