A Midsummer's Equation

NINETEEN

“There are essentially two kinds of fireworks, but while they operate by the same basic principles, the method of propulsion is slightly different. The first, called an aerial shell, works a bit like a cannon. For example, take that straw.” Yukawa paused here to point at the straw in Kyohei’s glass of cola. “If you stuck a wad of tissue paper into one end of that straw and blew on the other, the wad of paper would go flying in the opposite direction, correct? With aerial shells, you place them onto a mortar tube launch pad, then stick a lifting charge underneath. The force and the gas pressure from the lifting charge is what sends the firework up into the sky. With the second type, a skyrocket, the rocket itself explodes, sending a spray of sparks beneath it, vaulting it into the air. It’s just like the water rocket, except with gunpowder in place of water and air pressure.”

Yukawa ate while he spoke, barely pausing for bites. The smoothness with which he was able to give his explanation even while swallowing impressed Kyohei more than what he was actually saying.

“So the fireworks you brought are skyrockets, not the aerial things?”

“Shells, yes. Real aerial shells aren’t purchasable without a license, and you need to be a registered pyrotechnician to get one of those.”

They had stopped by the convenience store on the way back from the ocean to buy the stash of fireworks. It hadn’t been Kyohei’s idea—he had only mentioned setting them off with his uncle the night before.

Kyohei had just finished his rice and was drinking his cola when one of the doors to the dining room opened and a man wearing a blue uniform and a hat poked his head in.

“Oh! Excuse me,” he said, immediately leaving and closing the door behind him.

Kyohei blinked. “Who do you think that was?”

“With that uniform, he’s in forensics. They must be back to do more investigating,” Yukawa said.

A few moments later, Narumi came with some tea. She apologized to Yukawa for the intrusion.

“Any idea what they’re looking for?” he asked.

“I’m not sure, but they seem interested in anything that can generate heat.”

“Such as?”

“Well, they wanted to make sure that all the burners in the kitchen worked, things like that.”

“That’s odd. I fail to see the connection with an accident down by the ocean.”

“Well, the police say it’s related, but they won’t tell me more than that.”

Yukawa sipped his tea and said only, “That is how they operate.”

* * *

They left the dining room after dinner and ran into several men dressed like the one before, wandering around the halls of the inn. Kyohei and Yukawa were stepping out the front door when the doorway to the basement stairs opened and Shigehiro appeared. “More fireworks?” he asked.

“Yeah. Mind if we borrow your bucket?”

“Not at all,” Shigehiro said, his eyes going to the plastic bag in Yukawa’s hands. “Looks like you got some shells in there,” he said.

“Technically they’re skyrockets—I hope it’s okay?”

Shigehiro grinned a little sheepishly and scratched his bald head. “Well, we got away with it last night, but really, the fire department only wants you to set off fireworks right by the ocean. Guess they’re worried about brushfires. Normally I wouldn’t give it a second thought, but with our guests tonight…”

“I understand completely,” Yukawa said. “Wouldn’t want any flying into the inn by mistake. We’ll hold off on the skyrockets tonight,” Yukawa said. Kyohei nodded.

They stepped outside and went around to the back of the inn where there was a small clearing between the building and the woods.

Kyohei was about to light a sparkler, but Yukawa stopped him. “Can you explain the basic principle of how fireworks work?”

“Well, it’s just gunpowder stuck on a stick, right?”

“If it were, it would explode as soon as you lit it.” Yukawa pulled something white from his pocket—a ball of cotton, which he placed on the ground. From his other pocket, he took a nail and some sandpaper. He began sanding the nail above the ball of cotton, so that little black specks of metal began to accumulate below.

“Now we light it,” Yukawa said, touching the flame of a disposable lighter to the ball.

The ball caught flame immediately, sending up tiny sparks. Kyohei shouted with surprise.

“Even metal that doesn’t normally burn will ignite under the proper conditions. Fireworks are essentially metal, several kinds mixed together.”

“Why do they use different kinds?”

“Good question. Let’s try lighting one of the sparklers,” Yukawa said, holding out his lighter.

Kyohei lit the sparkler in his hand and watched as multicolored sparks began to fly from the tip. As the sparkler burned, the color of the sparks changed.

“The blue sparks are copper, green is barium. Red sparks are strontium, and yellow are sodium. All metals. As you can see, each metal and metallic compound gives off a distinct color when it burns. This is called a flame reaction,” Yukawa explained, his quiet voice at odds with the noisily burning sparkler. “Fireworks use this effect to—” Yukawa’s voice trailed off as his eyes went upward.

Two forensics officers were coming down the fire escape on the backside of the inn. They looked in Kyohei and Yukawa’s direction and nodded their heads.

“I wonder where they were. I didn’t see them until just now.”

“Probably up on the roof. There’s a chimney up there.”

“Oh?” said Yukawa, raising an eyebrow.

One of the men, the one wearing glasses, walked over to them.

“Sorry to interrupt you. You’re the guest staying here?” he asked Yukawa.

“Yes, I am.”

“I was wondering if I could have a few words,” he said, pulling something out of his breast pocket.

“You don’t need to show me your badge, I know you’re a police officer. How can I help?” Yukawa asked.

“You’ve been staying here for the past two days?”

“That’s correct. I checked in the night before last.”

“Right. Has anything unusual happened during your stay at the inn?”

Yukawa made a face as though he didn’t understand the question. “If you mean what happened with the guest falling on the rocks, I did hear about that.”

“Not that, I meant anything unusual happening in the inn itself. Did you experience any strange physical sensations, smell any odd smells. Anything like that?”

“Sensations? Smells?” Yukawa shook his head. “I can’t say I noticed either of those, sorry.”

“I see. Thanks for your time,” the man said, turning to walk away.

“Aren’t you going to talk to him?” Yukawa said. The men turned and saw Yukawa pointing at Kyohei.

“Er, right,” the man said, looking a little bewildered. He took a step toward Kyohei. “How about you? Did you notice anything strange?”

Kyohei shook his head without saying anything.

The man nodded, bowed curtly to Yukawa, and left.

Yukawa looked back up at the inn for a moment, then turned back to Kyohei. “Where was I?”

“You just told me why fireworks change color.”

“Right. Let’s discuss the physics of the black snake firework, then,” Yukawa said, reaching back into his bag.