Another thing the boy realized, as he waited with anxiety for the next move. The man, whoever he was, was thoroughly familiar with the plan of the Sea Lion.
The grooves in the edge of the door had been planned so as to give entrance to visitors who were not receiving their air from the Sea Lion. No one was believed to know anything about this arrangement—no one save the builders and the Secret Service men.
While Jimmie watched, the intruder moved the lever and the water in the chamber began to lower. When the water was forced out fresh air was automatically forced in.
Before long the intruder disconnected his hose with his helmet and threw the end over a hook provided for that purpose. When the water was all out he knocked heavily on the door leading to the room where Jimmie stood.
"There'll be doings here directly," the boy thought.
Again and again the visitor beat upon the door, but Jimmie gave no sign. He could not well observe the man now, for, with the water out of the chamber, the light carried by the man inside shone brightly against the glass panel, and the boy would have been observed had he stood close to it.
Jimmie grew more anxious as the seconds passed. He was trying to put away the thought that the intruder had cut the air-hose attached to the helmets of his friends.
For all he knew all three boys might be lying drowned, on the floor of the ocean. The thought was unbearable, and he resolved to banish it in action.
His first impulse was to disconnect the exhaust and fill the chamber with water. The man in there had disconnected his air-hose and would soon drown.
But the brutality of such a course soon presented itself, and Jimmie cast about for some other method of meeting the dangerous situation. He could hear the visitor fumbling at the door, and wondered if he knew the secret of opening it.