There is a concept known as homeostasis.
In its original sense, the term is most often used to describe the self-preservation functions of living organisms. It refers to the processes by which a body maintains internal stability—such as sweating to cool down when it is hot, shivering to generate heat when it is cold, or keeping the blood's pH level constant.
Occasionally, this concept is applied to artificial environments to describe the maintenance of constant temperature or humidity.
Jin’s thoughts had eventually led him to this very idea.
"Feedback, huh..."
If the temperature rises, lower it; if it drops too low, raise it. The same logic applied to humidity. If the oxygen concentration dipped, increase it; if it became too high, reduce it.
Jin was attempting to create Magi Machines equipped with these specific capabilities. Such technology could be applied not only to the internal environment of a spacesuit but also to a spaceship.
"I’ve already been using thermostats for temperature control..."
The linchpin of this technology was the sensors. He needed sensors capable of detecting temperature, humidity, oxygen levels, and carbon dioxide concentrations as accurately as possible. This was a significant challenge, even for someone of Jin’s caliber.
"I might be able to handle oxygen and carbon dioxide by adapting magic tools that use Analyze."
Jin figured he could likely detect hazardous gases using the same method.
"The real issues are temperature and humidity..."
Temperature was particularly tricky. Humidity only fluctuated between 0 and 100 percent, so he felt he could manage that with an application of Analyze. However, temperature was difficult to quantify because there was no inherent standard; it could only be perceived relatively.
To explain it to someone unable to use magic: with humidity, one could sense it as being "somewhere between 0 and 100." But since temperature lacked those natural bookends, the caster had to set the reference points themselves.
That "setting" was the crux of the problem. Because it could only be defined as "about this much," the precision required was staggering. Even for Jin, the odds of achieving the exact same value across ten different attempts were virtually zero.
"I suppose I should use 0°C and 100°C as my markers after all..."
"It is quite difficult, isn't it, Father?"
"...Lost in thought again?"
As Jin and Reiko agonized over the problem, Elsa walked in.
"Oh, Elsa. How did things go at Kaina Village?"
She was supposed to have been teaching the village children alongside Mine for the first time in a while.
"We finished....Jin-nii, don't tell me you forgot!?"
"Eh!?"
Jin instinctively glanced toward the workshop window. It was pitch black outside. It was certainly past 6:00 PM. Considering the time difference, Elsa must have left Kaina Village around 4:00 PM.
"...That was a close one."
Jin was the type to become so engrossed in his work that he would skip dinner entirely if left to his own devices, so her concern was warranted.
"Reiko-chan, you have to keep an eye on him."
"...No, I intended to take Father to dinner by force once it reached 7:00 PM."
It seemed Reiko was well aware of the time, but she had planned to wait until the last possible second to avoid interrupting Jin's train of thought.
Since Luna and Soleil had already prepared the meal, Elsa and Jin sat down at the table.
"The food on Hourai Island is always so good."
"Yeah, it really is."
"I am honored."
Soleil, who had been in charge of dinner, bowed.
The staple diet in Kaina Village consisted mostly of grains like wheat for bread or porridge, so rice was a luxury found only on Hourai Island. The menu also featured tofu made using bittern extracted from seawater and dashi crafted from kombu that the Mermaid Squad had discovered in the northern seas—delicacies found nowhere else.
For post-meal tea, they had a full selection: te-e, kahyi, peruhya, green tea, black tea, hojicha, and brown rice tea. This time, they chose hojicha.
"...Delicious."
Sipping the tea, which had been perfectly cooled to drinking temperature, Jin turned to Elsa.
"Regarding what I was thinking about earlier—I’ve been struggling with how to construct a temperature sensor."
He left out the part about what it was for, but Elsa seemed to have a vague intuition.
"...You were troubled by the lack of a reference point?"
She was sharp. Now a top-tier magi engineer in her own right, Elsa quickly grasped the nature of Jin’s dilemma.
"Temperature is hard to control without a standard to measure against."
"Yeah, that’s exactly it."
"...It is certainly a challenge."
Elsa fell silent as she, too, began to ponder the problem.
"Essentially, I just need a proper reference..."
Suddenly, an idea struck Jin.
"That's it! A reference! I can have a magi-crystal memorize the state of 20°C or 30°C and use that for comparison."
It was a concept similar to creating a reference voltage in an electrical circuit.
"That seems feasible."
"The next step is deciding how to record temperature as information..."
This was another hurdle. Humidity could be defined by the concentration of water molecules within a given space, but temperature was a different matter entirely. Jin and Elsa lapsed back into thought.
"Is a physical thermometer the only way...?"
Ultimately, that was the most straightforward conclusion.
"The simplest way would be to have a golem monitor a thermometer scale with its eyes, but..."
Jin felt that solution lacked elegance.
"Let’s see if we can create a thermometer using a different principle."
In modern Japan, there were many types of thermometers beyond those using mercury or alcohol. There were thermocouples, made by joining two different metals, and radiation thermometers that utilized infrared rays.
"Wait, infrared!"
Also known as heat rays, these were electromagnetic waves with wavelengths longer than 700 nanometers. Humans couldn't see them, but Reiko could. Jin couldn't even begin to imagine what those "colors" looked like to her.
However, Reiko and the other automata and golems were capable of "seeing" them.
"I can have them memorize the 'color' of a specific room temperature and then set the controls to maintain that color."
"...Jin-nii, that's too complicated. I don't understand."
Elsa couldn't quite follow Jin's explanation.
"Ah, right. Sorry....I suppose it's about time I gave you the final stage of knowledge."
Through several Trans Info sessions, Elsa had already acquired knowledge equivalent to a high school graduate in modern Japan. The final stage consisted of specialized technical knowledge and trivia that Jin had carefully curated. It included insights gained from sci-fi novels, anime, and manga, information from news and TV, and technical skills he had learned during his career in society.
Jin retrieved an all-attribute magi-crystal from the wall-mounted storage locker and presented it to Elsa along with a second crystal.
"Elsa, this is the final stage of knowledge. Will you accept it?"
It wasn't every single thing Jin knew, of course. He had filtered out any information that was unnecessary or irrelevant.
"...Yes, I would be happy to."
Believing this would allow her to assist Jin even further, Elsa accepted the magi-crystal without a moment's hesitation. She immediately began transferring the information to the empty crystal.
"Trans Info."
By converting the data into her own magic pattern, she could decompile it and integrate it directly into her own mind.
"Trans Info."
Elsa transferred the knowledge from the crystal into her brain.
"..."
Because the knowledge was so strictly curated, it wasn't an overwhelming burden, but the sudden influx of information still left her momentarily dazed. However, the feeling passed as the data settled into place.
"...Thank you, Jin-nii. I look forward to working with you."
"Yeah, Elsa. We'll be together forever from now on."
"...Mm."
At those words—unusually sentimental coming from Jin—Elsa nodded, her cheeks flushing a light pink.