The day after the Cygnus reached completion.
Just as he had the day before, Marcia headed out to her part-time fishing job, while Jin returned to Hourai Island.
"Now, let's see if my theory holds water."
The idea had first struck him the previous night while he lay in bed at the Sea Roar Pavilion.
"Blower."
Jin cast a basic wind spell, the kind typically used to clear away dust. A gust of wind gusted from his palm for about five seconds before tapering off.
"Hmm. As I thought, there's no recoil."
Jin had realized that most magic lacked any sort of kickback. Usually, the law of action and reaction was a fundamental rule of physics, and it seemed to remain valid even in this world.
However, magic appeared to be the exception. Jin set out to analyze why.
"Alright, Reiko. When I give the signal, I want you to use Blower."
"Yes, Father."
As a Super Automata, Reiko was programmed with high-level attack magic of every attribute—spells far beyond the reach of even her creator—all for the sake of his protection.
"Three, two, one... now!"
"Blower."
"Trace."
As a powerful gale erupted from Reiko’s hand, Jin tracked the resulting flow of mana.
"Next, give me Wind. Three, two, one... now!"
"Wind."
"Inspection."
This time, Jin scrutinized the interplay between the mana and the magic’s effects. After repeating the process three times, he finally had his answer.
"I’ve got it! It’s exactly as I suspected."
"Father?"
Finding it rare for Jin to be so excited he was nearly jumping, Reiko tilted her head in confusion.
"Thanks, Reiko. I finally understand why only the target gets blown away by wind magic."
Jin began to explain his findings to her.
"Take the Blower spell, for instance. It looks like the wind is being generated from the palm, but in reality, the spell gathers the surrounding air and then fires it."
Since it was a relatively weak spell, Jin surmised that this was simply how its structure was composed.
"Normally, there should be a recoil when that air is launched, but the caster never feels it. That’s because a secondary spell to cancel the recoil is triggered at the same time."
"Is that so?"
"Yeah. There’s no doubt. I confirmed it by examining the airflow, the mana movement, and the overall structure of the magic."
Jin took a breath before continuing.
"So, all I have to do is take the spell formulas for Blower or Wind and strip out the part that cancels the recoil."
As Jin understood it, the act of magic began by recording a "flow" of procedures into the world itself. This was followed by the incantation—the words of power—and finally the activation of mana.
To use an analogy with magic tools, magic was the act of recording or memorizing a flow into the "world"—much like writing onto a magi-plate—and then triggering that flow using words of power and mana.
To record a new flow into the fabric of the world was, in essence, the act of creating a new magic. This act of "recording" was not something just anyone could do, and it was this capacity that defined the difference in skill between various magi.
Setting that aside, Jin pressed on.
"That means I should be able to create a magic tool based on that principle."
"I believe I understand the theory. But Father, what use would such a magic tool have?"
Jin gave her a mischievous, boyish grin.
"Just you watch."
He fetched some light silver from the warehouse and began to work. Even for Jin, creating something entirely from scratch took a bit of time, but it wasn't long before he was finished.
"Done."
The result was a cylindrical object, roughly ten centimeters in diameter and thirty centimeters long.
"Father, what is this?"
Jin opened a lid on the top of the cylinder, slotted a small magi-stone inside, and snapped it shut.
"Reiko, hold this cylinder firmly. Be careful not to crush it."
"Understood."
As Reiko gripped the device, Jin gave the command.
"Wake up."
The moment the device activated, a blast of wind erupted from the cylinder. The force was enough that it nearly dragged Reiko along with it.
"Father, this...!"
"Yeah, it’s a total success. I’m calling it the Magi Jet Engine."
Jin’s engineering-minded naming sense was, for once, relatively sensible.
"Keep holding it like that until the magi-stone runs out of mana."
The engine continued to roar for approximately twenty minutes.
"Mm-hmm. This is also exactly as I calculated."
"How so, Father?" Reiko asked as Jin nodded to himself in satisfaction.
"In a standard Wind spell, the part of the magic that prevents recoil consumes more than half of the total mana. By getting rid of that part, the spell requires significantly less power to maintain."
"Which means... if you don't cancel the recoil, it can run for much longer on the same amount of mana?"
Jin nodded happily.
"Exactly. And with this, we’ve got a clear path toward flight—and we can build high-speed boats, too."
Jin had the distinct feeling he might end up building a jet engine before he ever bothered with a propeller.
"Whoops, it’s almost noon. Let’s head back to Port Rock for now."
Emerging from the sea cave and heading toward the town, Jin noticed something off.
"Father! A fire!" Reiko cried out at the same moment.
Heavy black smoke was billowing up from the direction of the harbor.
"Wait, isn't that where the docks are?"
Driven by a sudden, sinking feeling, Jin broke into a run, with Reiko right on his heels.
"Oh no, the dock!"
Even from a distance, the situation was clear. The fire had broken out at the very dock Marcia was renting. Crimson flames licked the interior, billowing thick smoke that suggested the blaze was already out of control.
"Reiko, can you put it out?"
Before Jin could even finish his sentence, a massive wall of seawater swirled upward and slammed into the dock. Thick white steam hissed into the air, and the overwhelming volume of water extinguished the flames in an instant.
"What was that?"
"That was Water Strom," Reiko noted.
"Reinhard..."
The one wielding the magic was the magi engineer from the Shouro Empire they had met just the day before.
The dock had been set ablaze! What had become of the Cygnus? And who was behind this?