"What's that?" asked Harris.
"I don't know," replied Jerrold; "it's nothing of mine and wasn't in the safe, to my recollection, at the time the blue prints were taken."
"Well, it may be yours, for all that. If it was in the roll, it stands to reason it must have been in the safe. Better open it. Probably you can tell from the contents whether it is yours or not."
Harris picked up the envelope and handed it to Jerrold. The latter took it from him with a puzzled expression on his face.
"I'm pretty sure this isn't mine," said he, turning the envelope over and over.
"Well, you've got to be absolutely sure," returned Harris.
Jerrold, thus urged, tore open the envelope, drew out the sheet and cast his eyes over it.
"No," he declared, "it doesn't belong to me. The thieves must have put it in with the blue prints."
"Let's have a look at it," said the officer.
Drawing closer to the candle, Harris proceeded to read the letter. While he read, his face brightened and a look of surprise and exultation rose in his eyes.