Then, remembering certain smooth blurs on the polished machinery he had recently examined, he took the microscope and made another examination of the spots. Presently he called Frank to his side.
"Look through the glass," he said, handing the instrument to Frank, "and tell me what you see."
"Rubber!" cried the boy, after a short examination. "There are a few traces here of the same rubber composition I found on the waste. Can you tell me what it means?"
"Quite simple," Ned replied, as the boys gathered about him. "The use of rubber composition by men engaged in nefarious undertakings dates back to the time of the utilization of the whorls and lines of the human fingers as aids in the detection of crime."
"I guess I know what you are going to say," cried Frank.
"When the thumb- and finger-print experts got busy with their photographs and their enlarged reproductions, the criminals began studying on methods to offset this dangerous aid to detective work."
"I knew it," cried Frank.
"And so," Ned went on, "they conceived the idea of filling the lines on the fingers and hands and making them perfectly smooth. This is rubber paint," he went on. "The man who was hidden in here when we came in did not care to leave any finger marks behind him."
"But he did leave smooth blurs on the machines where his fingers touched them!" said Jack.
"Certainly, and so pointed out the location of his efforts. Still, I do not think he meditated disabling the Sea Lion. It is more probable that he believed Lieutenant Scott to be the expert in charge of the boat and sought to kill or disable him."