This was quickly made up into flashlight cartridges,
to be exploded at once, or in a series, by means of a high
voltage storage battery.

The moving picture cameras had been put in place, Blake to work
one and Joe the other, while the automatic, which was operated by
clockwork, once the trigger-string was broken, also setting off
the continuous flashlight, was set between the two boys, to
command a good view of the dam, and of whoever should approach to
blow it up.

It now lacked an hour of midnight when, so the rumors said, the
attempt was to be made. Of the nature of these rumors, and of how
much truth there was in them, the boys could only guess. They did
not ask too much, knowing that there might be Government secrets
it would not be wise for them to know.

But that certain level-headed men did "take stock" in those rumors
was evident, for elaborate preparations had been made to protect
the dam. The preparations were conducted with as much secrecy as
possible in order that the conspirators might not become aware of
them.

"We don't want to scare them off," explained Captain Wiltsey.
"That may seem a strange thing to say," he went on, "but it is the
truth. Of course we don't want the dam blown up, or even slightly
damaged, but it will be better to let them make the attempt, and
catch them red-handed, than just to scare them off before they
make a try. Because, if we do that they may only come back again,
later, when we're not ready for them. But if we let them see we
are prepared and can catch some of them at work, it will end the
conspiracy."

"That's right!" agreed Blake. "Well, we'll do all we can to help
make the capture. We'll capture their likenesses on the films,
anyhow, and you'll know who they are."

"Which will be something," the captain said. "We haven't been able
as yet to discover the identity of any of them. They have kept
very secret, and worked very much in the dark."

It had been arranged, among Captain Wiltsey and his helpers, that
they were to give a certain signal when they discovered the
dynamiters at work, and then the boys would set off their
flashlights and begin to work their hand cameras. The automatic
one, of course, would need no attention, provided the miscreants
went near enough the net-work of strings to break one and so set
the mechanism in motion. But that was problematical, and, as Joe
said, they would have to "trust to luck."

And so the preparations for receiving the