"What does the envelope say?" Dan asked.

"'For A. & D. Cahill,'" Nellie read. '"From W. McIntyre.'"

"Mr. McIntyre!" Dan grabbed the package.

"Wait!" Amy yelled. "It could be a trap."

Dan rolled his eyes. "C'mon. It's from -- "

"It could be from anybody," Amy insisted. "It could blow up or something."

"Okay, whoa," Nellie said. "Why would somebody send a couple of kids a bomb? And who is this McIntyre dude?"

Dan grinned. "I say we let Nellie open it."

"Um, no!" Nellie said.

"You're the au pair! Aren't you supposed to defuse explosives for us and stuff?"

"I'm driving you, kiddo. That's enough!"

Amy sighed and snatched the package. She stepped into the parking lot, turned the flap of the envelope away from Nellie and Dan, and carefully peeled it open.

Nothing happened. Inside was a metal cylinder like a flashlight, except the light was a strip of purple glass running down one side. A note was attached in sloppy handwriting, like the writer had been in a hurry:

Meet me at Independence Hall this evening at eight, but only if you find the information

-- WM

(P.S. Thank you for calling the ambulance.)

"Find what information?" Dan asked, reading over her shoulder.

"The next clue, I guess."

"What clue?" Nellie demanded.

"Nothing," Dan and Amy said.

Nellie blew a tuft of black and blond hair out of her eyes. "Whatever. Stay right here.

I'll bring the car around."

She left them standing with the bags and Saladin in his new cat carrier. Saladin hadn't been too pleased with the cat carrier -- anymore than Nellie had been with the fresh red snapper they'd bought to keep him happy -- but Amy hadn't had the heart to leave him behind.

"Mrrp?"

Saladin asked.

Amy reached down and scratched his head through the bars. "Dan, maybe we shouldn't make that rendezvous. Mr. McIntyre told us not to trust anyone."

"But the note is from him!

""It could be a trick."

"That makes it even better! We've got to go!"

Amy twisted her hair. She hated it when Dan didn't take her seriously. And this could be dangerous.

"If we go, it says we have to find information first."

"But you know where to look, right? You're smart and stuff."

Smart and stuff.

Like that's all they needed to track down a clue in a huge city. Before they'd left Boston, she'd splurged and bought some books about Franklin and Philadelphia from her friends at the used bookshop. She'd spent the whole train trip reading, but still....

"I've got a few ideas," she admitted. "But I don't know where we're going in the long term. I mean -- have you thought about what this ultimate treasure could be?"

"Something cool."

"Oh, that's real helpful. I mean, what could make somebody the most powerful Cahill in history? And why thirty-nine clues?"

Dan shrugged. "Thirty-nine is a sweet number. It's thirteen times three. It's also the sum of five prime numbers in a row -- 3, 5, 7, 11, 13.

And if you add the first three powers of three, 3 (raised to the first power) plus 3 (raised to the second power) plus 3 (raised to the third power), you get thirty-nine."

Amy stared at him. "How did you know that?"

"What do you mean? It's obvious."

Amy shook her head in dismay. Dan acted like a doofus most of the time. Then he'd pull something like that -- adding prime numbers or powers of three that she'd never thought about. Their dad had been a mathematics professor, and Dan apparently had inherited all of his number sense. Amy had enough trouble remembering phone numbers.

She held up the weird metal cylinder Mr. McIntyre had sent them. She switched it on and the light glowed purple.

"What is that thing?" Dan asked.

"I don't know," Amy said. "But I have a feeling we'd better figure it out before eight o'clock."

Amy hated cars almost as much as she hated crowds. She promised herself that when she got older she'd live somewhere where she never had to drive. Part of that was because she'd been in the car with Nellie before.

Nellie had rented a Toyota hybrid. She said it was more environmental, which was fine with Amy, but it cost two hundred and fifty-eight dollars a day, and the way Nellie raced around corners and gunned the gas wasn't exactly "green."

They were on Interstate 95, heading into downtown, when Amy happened to look behind them. She wasn't sure why -- a prickling sensation on her neck like she was being watched. In fact, she was.

"We're being followed," she announced.

"What?" Dan said.

"Five cars back," Amy said. "Gray Mercedes. It's the Starlings."

"A Starbucks?" Nellie said excitedly. "Where?"

"Starlings," Amy corrected. "Our relatives. Ned, Ted, and Sinead."

Nellie snorted. "That's not really their names."

"I'm not joking," Amy said. "It's, um, part of the scavenger hunt. Nellie, we can't let them follow us. We have to lose them."

Nellie didn't need to be told twice. She yanked the wheel to the right and the Toyota careened across three lanes of traffic. Saladin yowled. Just as they were about to slam into the safety-impact barrels, Nellie slipped onto an exit ramp.

The last view Amy got of the Starlings was Sinead's freckled face pressed against the window of the Mercedes, her jaw hanging open as she watched Amy and Dan get away "Is that lost enough?" Nellie asked.

"Mrrp!"

Saladin protested.

"You could've killed us!" Dan had a big grin on his face. "Do that again!"

"No!" Amy said. "Locust Street. And hurry!"

Their first stop was the Library Company of Philadelphia, a big redbrick building in the middle of downtown. Amy and Dan asked Nellie to wait in the car with Saladin. Then they walked up the front steps.

"Oh, boy, another library," Dan said. "We have such great luck with libraries."

"Franklin founded this place," Amy told him. "It's got a lot of books from his personal collection. If we can convince the librarians -- "

"What's the big deal with Benjamin Franklin, anyway? I mean, so the guy invented electricity or whatever. That was hundreds of years ago."

"He didn't invent electricity," Amy said, trying not to sound too annoyed. "He discovered that lightning was the same stuff as electricity. He invented lightning rods to protect buildings and experimented with batteries and-"

"I do that. Have you ever put one on your tongue?"

"You're an idiot. The thing is Franklin was famous for a lot of reasons. He started out getting rich with his printing business. Then he became a scientist and invented a bunch of stuff. Later he helped write the Declaration of Independence and the Constitution. He was even an ambassador to England and France. He was brilliant. World famous. Everybody liked him, and he lived until he was, like, in his eighties."

"Superman," Dan said.

"Pretty much."

"So do you think he knew what it was -- this treasure we're looking for?"

Amy hadn't thought of that. Franklin had been one of the most influential people in history. If he was a Cahill, and he knew about this secret family treasure...

"I think," she said, "we'd better find out."

She pushed open the doors and led Dan inside.

Fortunately, the librarians were having a slow day, and Amy wasn't shy around them at all. She loved librarians. When she told them she was doing a summer research project on Benjamin Franklin and needed to use historical documents, they fell all over themselves to help her.

They made Amy and Dan wear latex gloves and sit in a climate-controlled reading room while they brought out old books to look at.

The librarian set the first one down and Amy gasped. "This is Franklin's first cartoon!"

Dan squinted at it. The picture showed a snake, cut into thirteen pieces, each one labeled with the name of an American colony.

"Not very funny for a cartoon," Dan said.

"It's not supposed to be funny," Amy said. "Back then, cartoons made a point. Like, he's saying if the colonies don't get together, Britain will cut them apart."

"Uh-huh." Dan turned his attention to his computer. They'd been in the library maybe five minutes, and here he was, already looking bored, clacking away on his laptop rather than helping her.

Amy pored over the other artifacts: a newspaper that had been printed on Franklin's own printing press, a copy of Pilgrim's Progress that Franklin had owned. So much amazing stuff ... but what was she looking for?

Amy felt pressured, and she didn't do well under pressure.

"Find what you need?" the librarian asked. She had frizzy hair and bifocals and looked sort of like a friendly witch.

"Um, maybe some more, please. Anything that was ... important to Franklin."

The librarian thought for a moment. "Franklin's letters were important to him. He wrote many, many letters to his friends and family because he lived in Europe so long.

I'll bring you some." She adjusted her glasses and left the room.

"Franklin invented those, too," Amy said absently. Dan frowned. "Librarians?"

"No, bifocals! He cut up two sets of lenses and pasted them half-and-half, so he could see long distance and short distance with the same pair."

"Oh." Dan didn't look impressed. He went back to playing on his laptop. He had the mystery flashlight from Mr. McIntyre in front of him, and he kept switching it on and off.

The librarian brought them a stack of new stuff, including old letters preserved in plastic sheets. Amy read through them but felt more hopeless than ever. Nothing jumped out at her. Nothing screamed "clue."

Suddenly, Dan sat up straight. "I found it!"

"You found what?" She'd assumed Dan was playing games, but when he turned the laptop to face her, there was a picture of a flashlight just like the one Mr. McIntyre had sent them.

"It's a black light reader," Dan announced.

"Oh!" the librarian said. "Very ingenious. We have one of those for our collection."

Amy looked up. "Why? What do they do?"

"They reveal secret writing," the librarian said. "During the Revolutionary War, spies would use invisible ink to send messages on documents that seemed harmless, like love letters or orders for merchants. The receiver would use heat or a special chemical wash to make secret words appear between the lines. Of course we can't damage our documents by spraying chemicals on them, so we use black light to check for secret messages instead." Amy held up the black light reader. "Can we -- "

"I can save you time, my dear," the librarian said.

"We check all our colonial documents as a matter of course. There are no secret messages, unfortunately."

Amy's heart sank. They'd wasted their time here, and she still didn't know what she was looking for. She had a mental list of other places to visit, but it was very long.

There was no way they could hit them all before eight tonight.

Secret messages.