gazed at the envelope.

"It's pretty hard to identify plain print, done with a lead pencil,"
he murmured. "And this didn't came through the mail."

"I wonder how it got here?" mused Ned. "Maybe some of the crowd that
was here when we started off dropped it for the smugglers. Maybe the
smugglers were in that crowd!"

"Let's take a look outside," suggested Mr. Whitford. "We may be able
to pick up a clew there."

Although our friends were tired and sleepy, and hungry as well, they
forgot all this in the desire to learn more about the mysterious
warning that had come to them during the night. They all went
outside, and Ned pointed to where he had picked up the envelope.

"Look all around, and see if you can find anything more," directed
the custom agent.

"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 b