"I'm not looking for footprints," replied Mr. Whitford, "I have an idea--"
"Here's something!" interrupted Mr. Damon. "It looks like a lead weight for a deep-sea fishing line. Bless my reel. No one could do fishing here."
"Let me see that!" exclaimed Mr. Whitford eagerly. Then, as he looked at it, he uttered a cry of delight. "I thought so," he said. "Look at this bit of cord tied to the weight."
"What does that signify?" asked Tom.
"And see this little hole in the envelope, or, rather a place that was a hole, but it's torn away now."
"I'm not much the wiser," confessed Ned, with a puzzled look.
"Why, it's as plain as print," declared the government agent. "This warning letter was dropped from an airship, Tom."
"From an airship?"
"Yes. They sailed right over this place, and let the letter fall, with this lead weight attached, to bring it to earth just where they wanted it to fall."
"Bless my postage stamp!" cried Mr. Damon. "I never heard of such a thing."