Dunlap replaced the receiver on the hook, and as he did so his eyes rested on the indicator board and he gasped sharply; for the indicator for Old Tower Number Three went wavering, trembling back to a vertical position on the time dial!
This unheard-of behavior of the indicator was the deepest mystery of all. The indicators, each controlled independent of the others by push buttons in each tower, were constructed mechanically to turn only from right to left.
The indicator for Old Tower Number Three had turned back from left to right!
CAPTAIN DUNLAP made no effort to solve the mystery.
Old Tower Number Three was securely locked and could not be approached except by crossing over the wall from New Tower Number Three on the Southeast corner of the wall, or from Tower Number Two on the Northeast corner of the wall. Dunlap himself had closed and locked the doors and windows of the tower. There was but one key to the tower doors, and that key was in Dunlap’s pocket.
Unlike the other towers, Old Tower Number Three could not be entered from the ground outside the wall. It was built solidly of stone from the ground up, and the only entrances were the two doors communicating with the top of the wall on either side of the tower.
Besides, strict orders had been given that no one enter the tower unless ordered there by a shift captain. And, too, in the glare of the arc lights near the wall, it would be impossible for anyone to cross the wall to the tower, without being seen by other wall guards.
Could the mysterious report have come from one of the other wall towers? Impossible for this reason: When the push button in one of the wall towers—say, that in Old Tower Number Three—was pressed by the man on duty there, the indicator on the board in the captain’s lookout turned to the left a quarter-turn on the time dial, the small bell on the board rang and all telephone connections with the other wall towers were automatically cut off until the captain had replaced the telephone receiver on the hook after receiving the report from Old Tower Number Three.
Dunlap said nothing to Bill Wilton when the latter returned to the yard lookout, after making his round in the yard. It would be best, he reasoned, to say nothing to anybody about the mysterious call. They would only laugh at him if he told them about it. If the indicator had not returned to a vertical position on the time dial he would have some proof on which to base his wild story of the ghost call. But the indicator had, before his own eyes, returned to its former position after the call.