“I can’t tell much by looking at it for the work has been cleverly concealed,” he admitted. “But from what you’ve told me, I am quite certain how the mine is made to explode.”
“How is it done?” Salt demanded.
“After the hole is made, a chemical—probably sodium—is inserted. Then another substance which melts slowly in water is used to seal up the opening.”
“Then that explains why Webb delayed so long in giving the signal after the mine had been dropped into the water!” Penny exclaimed from the doorway. “He was waiting for the substance to melt!”
“Exactly,” agreed the major. “If my theory is correct, only the action of water is required to explode this mine. The professor’s machine, of course, has nothing whatsoever to do with it.”
“Why don’t we explode the mine now?” Penny suddenly proposed. “That would put an end to the professor’s little scheme.”
“It might also prevent us from arresting him,” the major said. He debated a moment. Then he exclaimed: “It’s worth trying! We’ll load the mine on the boat and dump ’er in the lake!”
The men would not permit Penny to help with the dangerous work. Carefully they transported the mine to the boat. Salt was about to start the motor, but the major stopped him.
“No, we don’t want the sound of the engine to give us away,” he said. “We’ll row out into the lake.”
Penny was eager to accompany the men, but they would not hear of it. To her disappointment, she was compelled to remain on the beach.