So they had developed a combined automatic flashlight and camera, that would, when set, take pictures of the animals as they came to the watering-place. The beasts themselves would, by breaking a thread, set the mechanism in motion.

"The flashlight powder--I wonder if we can get enough of that?"

spoke Joe. "It'll take quite a lot."

"We must get it--somehow," declared the captain. "I fancy we have some on hand, and perhaps you can make more. There is quite a chemical laboratory here at the dam. But we've got to hustle. The attempt is to be made some time after midnight."

"Hustle it is!" cried Blake. "Come on, Joe."

CHAPTER XXIV

THE TICK-TICK

"Put one camera here, Joe."

"All right, Blake. And where will you have the other?"

"Take that with you. Easy now. Don't make a noise, and don't speak above a whisper!" cautioned Blake Stewart. "You'll work one machine, and I'll attend to the other. We'll put the automatic between us and trust to luck that one of the three gets something when the flash goes off."

The two boys, with Captain Wiltsey, had made their way to a position near the spillway, below the great Gatun Dam. It was an intensely dark night, though off to the west were distant flashes of lightning now and then, telling of an approaching storm. In the darkness the boys moved cautiously about, planting their cameras and flashlight batteries to give the best results.

They had had to work quickly to get matters in shape before midnight. Fortunately they were not delayed by lack of magnesium powder, a large quant.i.ty having been found in one of the laboratories. 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 ident.i.ty 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 Ca.n.a.l 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.

"I'll just step over and see how Joe is," thought Blake. He judged it lacked half an hour yet of midnight.

He found Joe busy mending a broken wire that ran from the battery to the flashlight powder chamber.

"Just discovered it," Joe whispered. "Lucky I did, too, or it would have failed me just when I needed it."

"Is it fixed?" asked Blake, as his chum straightened up in the darkness.

"Yes, it'll do for a while, though it's only twisted together.

Say, but isn't it dark?"

"It sure is," agreed Blake.

Together they stood there near the great dam. There came to their ears the splashing of water over the spillway, for the lake was high, and much was running to waste.

"Well, I guess I'll be getting back," said Blake in a low voice.

"No telling when things will happen now."

As he started to go away Joe remarked:

"Where are you wearing your watch? I can hear it over here."

"Watch! I haven't mine on," Blake answered. "You can't see it in the dark, so I left it on the boat."

"Well, something is ticking pretty loud, and it isn't mine," Joe said, "for I did the same as you, and left it in my cabin. But don't you hear that noise?"

They both listened. Clearly to them, through the silence of the night, came a steady and monotonous tick--tick-tick--

"It's the clockwork of the automatic camera," Blake whispered.

"It can't be," answered Joe. "That's too far off. Besides, it's a different sound."

They both listened intently.

"Tick! Tick! Tick!" came to them through the dark silence.

CHAPTER XXV

MR. ALCANDO DISAPPEARS

For several seconds Blake and Joe stood there--without moving--only listening. And that strange noise they heard kept up its monotonous note.