"Say! Is this as fast as this thing can go?" demanded Frank. "We won't ever get any place this way. Let her out a bit."
Jack did so and the little boat seemed literally to fly over the dark water. This terrific speed Jack kept up for some time and then slowed down.
"We'll bump into something at this rate," he said; "and that would settle the whole business. We must be cautious."
"Cautious!" repeated Frank. "We won't find that German being cautious."
"If we weren't cautious, it wouldn't do us any good if we did find him," argued Jack. "First thing you know we would be at the bottom."
Frank considered this point a moment.
"Guess you're right," he said at last.
"Swish!" went something at this moment, and, turning quickly, Frank saw a dark shape speeding away up the river.
"Hey! There went one the other way," he cried to Jack.
"That so?" replied Jack anxiously. "There is something up here, and I'm going to find out what it is."