rtant they are. I am the only man in the world who knows of the
flight you two are to begin in a few minutes. At least, I pray to God
that I'm the only one. However, in view of the fact that absolutely
nothing is _sure_ in this war, I must give you this order: Under no
circumstances, not even under the threat of the most horrible kind of
death, is either of you to permit a single one of these letters to get
into the hands of anybody but the American officer whose name is typed
on the front of each envelope. Do I make myself clear?"

"Yes, sir," Dawson said with a nod.

"Definitely, sir," Freddy Farmer echoed.

"And I'm sure you do," the colonel said. "I can't help, though, but
stress that point. _Don't let any of these envelopes out of your sight
until each has been delivered to the proper person._ Make doubly sure
that each of those proper persons understands what he is to do.
Naturally, you'll be asked questions by those officers as to what the
envelopes contain. That is one reason why I'm not even telling you, so
that you can truthfully reply that you do not know. Just remember, this
is the most secret mission you have ever undertaken. Guard these
envelopes with your lives and see that they are delivered to the proper
parties. If the commanding officer does not happen to be there when you
arrive, do _not_ give the envelope to the next in command. Stay there
until the commanding officer does arrive. If you have a forced landing,
play up the fact you are on a survey flight. If your plane is damaged, a
wire sent to me in Washington will get an immediate reply ordering the
commandant of the airfield nearest the scene of your crash to turn over
a plane to you. If one of you happens to be injured in the crash, the
other will carry on alone as soon as possible, without creating
suspicion that the flight is not for survey purposes."

The colonel suddenly stopped talking and drew from his pocket two small
vials containing a brownish liquid. He placed them beside the pile of
envelopes, and looked at the two air aces again.

"If both of you are hurt badly," he said, "or if--and I pray God it will
not happen--you should fall into the hands of enemy agents, or
force-land on the water and be approached by a lurking U-boat, you are
to take the caps from these vials and pour the contents over the
envelopes. The powerful acid they contain will completely destroy the
envelopes and their contents in a matter of seconds. In short, i