d "doped out" one or
two little things.

One thing will suggest another, and from Temple Camp, with its long
messboard and its clamoring, hungry scouts, and the tin dishes heaped
with savory hunters' stew, his thoughts wandered back across the ocean
to a certain particular mess plate, right here on this very ship--a mess
plate with a little black stain on it, where someone might have laid a
burning match-end.

He caught himself up and thought of Mr. Conne. But this was his time
off and he had the right to _think_ about anything he pleased. He could
not be reprimanded for just thinking. Nothing would tempt him to run the
risk of another encounter with one of those stern, brisk-speaking
officers, but he could _think_.

And he wondered whether that black spot _had_ been made by a match-end.
The spot would show plainly, of course, for he knew how shiny and clean
mess plates were kept. Had he not done his part in scouring and rubbing
them down there in the galley?

He wondered how the mess plate had happened to be in the stateroom,
anyway. Sherlock Nobody Holmes again! But the crew, as well as the
troops, carried their supper wherever they pleased to eat it. So there
was nothing so strange about that. If there had been, why, Uncle Sam's
all-seeing eye would not have missed it.

He fell to thinking of Bridgeboro again. And he thought of Adolf Schmitt
and----

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 br