"How will it ever be possible to obtain a clew as to where they have gone?"
The ensign's answer appeared enigmatical.
"Could you describe the motor boat you saw off here yesterday?"
"Well, she was of a very remarkable color—a light green, with a signal mast sticking up amidships. Then, too, her cabin was unusually high."
"Good. Such a boat as that ought not to be very hard to locate."
"I don't quite understand."
"Well, then I'll explain. These waters are fairly well traveled, and by working our wireless we may be able to get into communication with some boat similarly equipped, which may have seen that green motor boat."
"Cracky, that's a good idea," cried the admiring boy; "let's go back and try it at once."
"Yes, it's small use our waiting about here. The rascals overreached us by getting away as quick as possible. I suppose they didn't want to run any chances of discovery."
The return to the Viper was quickly made, and the motor boat was driven back to the Submarine Island at top speed. Barton tried with all his might to overhear what was said in the bow of the boat where the Boy Scouts had gathered; but the ensign was careful to keep his voice low, and then, too, the noise of the engines precluded the machinist from catching a word, hard as he strained his ears.