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 midnight callers went on.  Joe and Blake worked in silence, making ready for their part in it.  All about the boys, though they could neither see nor hear them, were Uncle Sam’s men—­soldiers, some of them—­stationed near where, so rumor said, the attempt was to be made to explode the dynamite.

“We really ought to have another helper,” said Blake, thoughtfully.  “There is one place we can’t get in focus no matter how we try, with the three machines we have.  If we had another automatic it would be all right, but we have only the one.  Another hand camera would do, but we’d have to get someone to work it.  I would suggest we get Mr. Alcando, but you don’t seem to want him.  He could easily take charge of one.”

“It is better to have no foreigners,” replied the captain.  “Not that Mr. Alcando might not be all right, for he seems a nice chap.  But he is a Spaniard, or, rather a South American, and some of the South Americans haven’t any too much love for us; especially since the Canal was built.”

“Why?” asked Blake.

“Oh, for various reasons.  Some of them have lost trade because it shortens routes.  But there, I must go and see if all the men are in place.”  Captain Wiltsey left him, and once more the moving picture boy resumed his vigil.  All about him was silence and darkness.  As well as he could he looked to see that his camera was pointing in the right direction, and that it set firmly on the tripod, the legs of which were driven into the ground.