“Footprints won’t count,” said Tom.  “There was a regular circus crowd out here yesterday.”

“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.”

“I see it now!” exclaimed Tom.  “While we were off over the river, watching for the smugglers, they were turning a trick here, and giving us a warning into the bargain.  We should have stayed around here.  I wonder if it was Andy’s airship that was used?”

“We can easily find that out,” said Mr. Whitford.  “I have a detective stationed in a house not far from where the Fogers live.  Andy came back from Shopton yesterday, just before you arrived here, and I can soon let you know whether he was out last night.  I’ll take this letter with me, and get right up to my office, though I’m afraid this won’t be much of a clew after all.  Print isn’t like handwriting for evidence.”

“And to think they sailed right over this place, and we weren’t home,” mourned Tom.  “It makes me mad!”

But there was no use in regretting what had happened, and, after a hot breakfast in the airship, with Mr. Damon presiding at the electrical stove, they all felt more hopeful.  Mr. Whitford left for his office, promising to send word to Tom as to whether or not Andy was abroad in the airship during the night.

“I wonder if that ‘Committee of Three’ is Andy and these two fellows with him in the airship?” asked Ned.

“Hard telling,” responded his chum.  “Now for a good sleep.  Koku, keep the crowd away while we have a rest,” for the giant had indulged in a good rest while the airship was on patrol during the night.

Not so much of a crowd came out as on the first day, and Koku had little trouble in keeping them far enough away so that Tom and the others could get some rest.  Koku walked about, brandishing a big club, and looking as fierce as a giant in a fairy tale.  It was afternoon when a message came from Mr. Whitford to the effect that Andy’s airship was not out the previous night, and that so far no clews had developed from the letter, or from any other source.