He opened the small bit of paper. It had some writing in lead pencil. Once it had doubtless been plain enough, but the dampness must have caused it to become faint. Still, Jack could make it out without much difficulty. This was what he read aloud, so that Tom and the other pilot could hear:
"Look carefully to your planes; examine every part. There is treachery in the air!"
"That's all, fellows," said Jack, much puzzled, as he turned the paper over and over, looking for some signature.
"No name attached, Jack?" asked his chum.
"Nothing whatever to tell who wrote that warning. Here, take a look at it, Tom. Your eyes may be sharper than mine and see something I've missed."
But Tom and the other pilot both failed to throw any light on the matter after examining the paper thoroughly. They exchanged stares. Then Jack laughed, a little queerly.
"This is certainly a mystery," he went on to say, trying to take the thing as a joke. "Some kind friend sends me a solemn warning, and then neglects to sign his name. Do you think any of the fellows of the escadrille could be up to a prank?"
Tom shook his head. The other pilot also exhibited positive signs of doubt in connection with such a thing.
"The boys often have their little jokes, and we are a merry bunch much of the time, just to change off from the nervous strain we're living under," the man observed. "But I'm sure not one of them would dream of doing a thing like this. It would be a mean trick."
"Then both of you are inclined to believe this warning was meant in all seriousness, are you?" continued Jack, no longer grinning as before.