"Talking!" exclaimed Frank.
"Wigwagging!" Harry broke out. "Now, let us see what he says."
Slowly to the right and left, up and down, an electric bulb flashed in the sky. Harry counted.
"That's C;" he said, "and that's 'a,' and that's 'u,' and that's 't,' and now 'i,' and 'o,' and 'n.' 'Caution!' That means that we've got to stand pat for a time yet."
"It also means," Jack said, "that we've made no mistake about that being the Nelson, with a Boy Scout on board. Those wigwag signals show the supposition to be true."
"Well," Harry puzzled, "he wouldn't be sending us a warning from the sky if there wasn't some danger we were not aware of. There is something going on that we are not wise to."
There was a short silence on board and then Frank remarked:
"We must be nearer the falls than we thought, for the water seems to be a ripple about us. Rear it! I'm going to look out and see it looks like."
In a moment he was jamming the panel shut and springing the slides over the loopholes and the ventilators.
Jack sprang to the prow, not knowing what danger threatened, but obeying the sudden gestures of his chum to close every opening. Before he sprung the steel panel over the ventilator he glanced out on the river.