Jin and the group returned to the front of the laboratory.
"I still have a bit of time..." Jin muttered to himself before giving an order. "Laojun, bring out Titan 3."
"Understood," Laojun replied immediately. A second later, Titan 3 appeared before the laboratory.
"Whoa!"
"Th-That startled me..."
"Sorry about that. Since we had a moment, I thought I’d explain a few things that have been on my mind lately."
"Things on your mind?" Rubina tilted her head.
"Yeah. The issues with your giant golem."
"Pl-Please do!" Rubina’s eyes sparkled. Seeing her so thrilled, Jin thought it might be a good idea to offer similar practical advice to the others as well.
"So, when I was designing Titan 3, the things I focused on were..." Jin began his lecture.
He explained how the ratio of strength to weight shifts as an object increases in size. He also discussed how joint movement is measured by angular velocity—how many seconds it takes to move a certain number of degrees—which meant that at the same angular velocity, the speed at the extremities increases the larger the golem is.
"I see. Surface area increases in proportion to the square of the dimensions. The cross-sectional area follows the same rule. But the weight increases in proportion to the cube."
"Exactly."
At Rubina’s level, she probably couldn't fully grasp concepts like the second moment of area yet, and since Jin had only memorized the basic concepts and formulas himself, he couldn't offer a truly exhaustive explanation anyway. However, despite the simplicity of the lesson, Rubina seemed to digest the essential points.
"I'm going to try improving it when I get home!"
"Yeah, give it your best." Jin smiled at Rubina, who was now practically vibrating with motivation.
Then came lunch.
The menu was tempura soba. They were given the choice of having the buckwheat noodles hot or cold.
"The broth is excellent; the flavor really comes through."
"The tempura batter is so light and crispy."
"Whoa, that wasabi has a real kick!"
Everyone seemed used to the dish and thoroughly enjoyed the Hourai Island-style flavors.
After lunch, Jin decided to show them around the laboratory.
Since it wasn't fundamentally different from the facility on Onogoro Island, the tour was brief. He merely explained the Predecessor's study from outside the door, though he did let them see the inside of his own room.
Eventually, Jin decided to show them the photograph he had been debating whether to reveal.
"Here, these are photographs of your ancestors."
"What!"
It was the group photo taken four hundred years ago.
"I've never seen this before!"
Since they had moved to Onogoro Island after the dissolution of the First Generation Jin Family, Jin assumed none of them possessed photos of their ancestors from that era. He figured the originals were likely kept in what one might call the "head family" estates scattered across various nations, and it seemed his intuition was correct.
Incidentally, the photos of the aged family members—including Jin himself—were kept in a private collection out of Laojun’s consideration for his master.
"This is Beena, and next to her is Louis. They are your ancestors in the direct line, Rubina."
"Ah, she kind of looks like me."
Strictly speaking, it was Rubina who resembled her.
"And this is Marcia."
"She has such a lovely figure..." Natalia sighed softly.
"Reinhard and Berche."
"So this is them..." The couple, Trihart and Susanne, looked on with deep emotion.
"And this is Elsa. My wife."
"She's a beautiful person..."
Looking at the photo of his beloved wife from days gone by, Jin felt a sharp twinge in his chest. However, perhaps because he was using a duplicate body, he managed to keep the emotion from showing on his face.
"N-Now then, let's move on."
Next, they headed to the workshop.
"Wow, incredible!"
"So this is Jin-sama's workshop..."
"This is where so many legends were born..."
Everyone was starstruck. Jin felt a bit bashful.
"Ah! There are some unusual materials here!" Rubina cried out.
Driven by curiosity, she had been wandering through the workshop until she stumbled upon the material shelves.
"Those are the material samples," Jin explained.
Even Jin had moments where he struggled to decide which material to use. In those times, looking at the physical samples would often trigger a flash of inspiration.
Rubina had discovered the metal sample shelf. It contained plates one millimeter thick and ten centimeters square, as well as round bars one centimeter in diameter and ten centimeters long. They were all neatly organized with name tags.
"...Pure silver, 925 silver, 950 silver, copper, cupronickel, brass, bronze, gunmetal, nickel silver, beryllium copper, pure iron, mild steel, hard steel, carbon tool steel, nickel steel, chrome steel, chrome vanadium steel, nickel chrome vanadium steel, chrome molybdenum steel, nickel chrome molybdenum steel, molybdenum vanadium steel, Invar, tungsten steel, nichrome, KS magnet steel, MK magnet steel, high-speed steel, alloy tool steel, silicon steel, 18-8 stainless steel, 18-10 stainless steel, 18-12 stainless steel, pure aluminum, magi-duralumin, super magi-duralumin, extra super magi-duralumin, magnesium alloy, solder, type metal, pewter, light silver, 64 light silver, mithril silver, magi-mithril, adamantite, magi-adamantite, hyper adamantite, iridosmine, orichalcum..."
Rubina read the names off the shelves in a single breath.
"...Phew... that's a lot of metals I've never heard of."
"I-It's staggering..."
Back in modern Japan, Jin’s profession had been metal-related. He had created as many alloys as he could remember and displayed them as samples before the knowledge faded. Once he started, he had gotten carried away, resulting in this massive collection.
"Jin-sama, what is 'pewter'?"
"Ah, that's an alloy of tin and antimony. Historically, it was used for tableware as a substitute for silver."
In Earth's history, at least. Poor-quality pewter used to contain lead instead of antimony, which made it toxic, and some modern versions mixed in copper instead. Jin had never used the metal for tableware and didn't intend to, but he had included it in the collection for completeness.
Other questions followed, such as the difference between super magi-duralumin and extra super magi-duralumin or the meaning of "high-speed steel," and Jin answered them all.
"Noooooooooooooooo!!"
Natalia's scream tore through the air.
It happened when Jin showed them the ground spiders and Ground Caterpillars in the underground farm. Natalia turned white as a sheet and bolted back toward the surface. Arrow, Alco, and Veil scrambled to catch up with her.
Rubina, meanwhile, remained perfectly calm. She seemed to have no issues with insects. Shu, Trihart, and Susanne pulled back a little, but they didn't show the same paralyzing level of revulsion.
"So this is where that incredibly strong silk comes from."
Rubina was fascinated even here. Jin admired her attitude—viewing everything as a new discovery and passionately absorbing knowledge.
"Understanding the nature of your materials is vital. To reach your goals, you have to know how to bring out a material’s characteristics to their fullest potential."
"Yes, I'll keep studying!"
Rubina had been much more agreeable lately, which was a relief to Jin. Teaching a rebellious child was a difficult task.
After that, they took a brief look at the underground mining area where the Stone Golems worked, then it was time for three o’clock tea.
Natalia, who had fled the underground farm, had finally regained her color.
"Ah, this is lovely."
It was gyokuro tea produced on Hourai Island. It had a different depth of flavor compared to sencha. To go with it, they were served chestnut yokan—a pure version made with an abundance of maruon fruit.
As neither the tea nor the sweets existed on Onogoro Island, the entire group found them to be quite a treat.