guessed too that, not finding them, we'd flash a light. That would
make us a good target to their confederates who had come to the
mouth of the alley, and they thought they could mow us down with
one volley. In other words the alley was a trap."

"By ginger, I believe you're right!" exclaimed Bart "The shots
came just after the light was flashed. It was a slick trick. You
have to hand it to them."

"But that doesn't explain where the men disappeared to who went
into the alley first," remarked Billy.

"No," admitted Frank. "And it doesn't explain either where the men
who fired the shots vanished to. But there's an answer to
everything, and I'm going to try to find the answer to this. I'm
not going to drop it. Of course, I suppose the secret service men
will take the thing up, but I'm going to do a little investigating
on my own account. I have a hunch that when I take a look at that
alley by daylight, I'll tumble to something."

And while the four chums, after their narrow escape, are cudgeling
their brains to solve the mystery, it may be well for the sake of
those who have not read the preceding volumes of this series to
trace briefly their adventures before this story opens.

Frank Sheldon, a vigorous, clean-cut, young fellow, was a resident
of Camport, a thriving and prosperous town of about twenty-five
thousand people. His father had died a few years before the war
broke out, and Frank lived in a little cottage with his mother, of
whom for some years he was the sole support. She was of French