On the surface, none of the letters seemed to hint at any particular clues. It was only when they reached the sixth letter, sent in October 1844, that things got more interesting:

“‘I so enjoyed the symposium last April and am eager to arrange another such trip. I am still mourning the loss of my gift, however,’” Theo read. “‘I was looking forward to solving the puzzle.’”

“The puzzle!” said Tess. “What puzzle?”

Theo read through the rest of the letter. “She doesn’t say. And she doesn’t give any specifics about this symposium, either.” He pushed the binder away, rubbed his forehead and the bridge of his nose, and caught the knowing eyes of Ada King, Countess of Lovelace, in the portrait on the wall.

Jaime said, “I know it’s just a picture, but it feels like she’s in the room, staring down at us.”

“It really does,” said Tess. “It’s as if she’s willing us to find something. Or knows we’re missing something.”

Theo glanced at the closed door, then got up to grab a couple of tissues from a box on top of a cabinet. He sat down again and pulled the binder closer. Using the tissues to cover his fingertips, he reached into the plastic pocket and slid the letter out. “Sorry, Dr. Little Crow,” he muttered.

The three of them held their breath, but there was nothing on the back of the letter, nothing lightly scratched between the lines.

“Maybe it’s written in invisible ink again?” Jaime suggested.

“We didn’t bring any solvents,” said Tess. “Some cipherists we are.”

Theo used the tissues to remove the other King–Babbage letters, but again, there was nothing on the back of any of them, nothing to indicate any clues. “Maybe we took a wrong turn somewhere,” he said. “Maybe the gown has nothing to do with Ada at all.” Starting with the first letter, he carefully returned each to their protective sleeves. But when he lifted the sixth letter, he paused.

“What?” said Jaime.

“The paper feels heavier than the rest.” He covered his palm with one of tissues and laid the paper on top of it to examine the edge. “It’s thicker, too.” He sighed when he realized what he had to do. “I’m really sorry, Dr. Little Crow.” He used the tip of his index finger to dig at the corner of the paper.

“Theo,” Tess warned.

“I know,” he said, but he kept digging, as gently as he could. Soon, a little flap of paper had come away from the original. Someone had pasted an extra piece of paper over the back of this letter so neatly that no one had detected it.

Until now.

When the flap was big enough, Theo peeled the two pieces of paper apart to reveal what had been written on the back of Ada’s sixth letter to Charles Babbage.

First, she had scrawled two small drawings: