A phrase from one of those letters ran through his mind—It’s the same idea as a periscope.
For a moment Tom Slade felt just as so often he had felt when he had found an indistinct footprint along a woodland trail. What was the same idea as a periscope? What was a periscope, anyway?
Why, a thing on a submarine by means of which you could look two ways at once—you could look up through the ocean and across the ocean—all with one look.
He wondered whether Mr. Conne had noticed that rather puzzling phrase and whether the people on this ship had seen that letter. Mr. Conne had seemed to think that one the least important of the lot. Perhaps he had just told the ship’s people to look out for spies. And they would do that anyway. The names of uniformed spies in the army cantonments—names in black and white—that was the important thing—the big discovery.
But Tom Slade was only a humble Sherlock Nobody Holmes and he couldn’t get that phrase out of his head.
It’s the same idea as a periscope.
A periscope is a kind of a—a kind of a——
Tom’s brow was knit, just as when he used carefully and anxiously to move the grass away from an all but obliterated footprint, and his eyes were half closed and keen.
“I know what it is,” he said to himself, suddenly. “It means how light can be passed through a room even while the room is dark all the time—kind of reflected—and you wouldn’t have to use any match.”
He stood still, almost frightened at his own conclusion. The clean, shiny mess plate and the phrase out of that letter seemed to fit together like the sections of a picture puzzle. The black spot and the match-end (if there was any match-end) meant just nothing at all. The dim light out in the passageway down below hardly reached the dark staterooms, but——