For an instant the moving picture boys could hardly grasp the meaning of the fateful words spoken by Captain Wiltsey. But it needed only a look at his face to tell that he was laboring under great excitement.

"The Gatun Dam to be destroyed," repeated Joe. "Then we'd better get—"

"Do you mean by an earthquake?" asked Blake, breaking in on his chum's words.

"No, I don't take any stock in their earthquake theories," the captain answered. "That's all bosh! It's dynamite."

"Dynamite!" cried Joe and Blake in a breath.

"Yes, there are rumors, so persistent that they cannot be denied, to the effect that the dam is to be blown up some night."

"Blown up!" cried Blake and Joe again.

"That's the rumor," continued Captain Wiltsey. "I don't wonder you are astonished. I was myself when I heard it. But I've come to get you boys to help us out."

"How can we help?" asked Blake. "Not that we won't do all we can," he added hastily, "but I should think you'd need Secret Service men, detectives, and all that sort of help."

"We'll have enough of that help," went on the tug boat commander, who was also an employee of the commission that built the Canal. "But we need the peculiar help you boys can give us with your cameras."

"You mean to take moving pictures of the blowing up of the dam?" asked Joe.

"Well, there won't be any blowing up, if we can help it," spoke the captain, grimly. "But we want to photograph the attempt if it goes that far. Have you any flashlight powder?"

"Yes," Blake answered. "Or, if not, we can make some with materials we can easily get. But you can't make more than a picture or two by flashlight."

"Couldn't you if you had a very big flashlight that would last for several minutes?"

"Yes, I suppose so."

"Well, then, figure on that."

"But I don't understand it all," objected Blake, and Joe, too, looked his wonder. Both were seeking a reason why the captain had said he was glad Mr. Alcando had gone out to get the camera he had forgotten.

"I'll explain," said Mr. Wiltsey. "You have no doubt heard, as we all have down here, the stories of fear of an earthquake shock. As I said, I think they're all bosh. But of late there have been persistent rumors that a more serious menace is at hand. And that is dynamite.

"In fact the rumors have gotten down to a definite date, and it is said to-night is the time picked out for the destruction of the dam. The water of the Chagres River is exceptionally high, owing to the rains, and if a breach were blown in the dam now it would mean the letting loose of a destructive flood."

"But who would want to blow up the dam?" asked Blake.

"Enemies of the United States," was the captain's answer. "I don't know who they are, nor why they should be our enemies, but you know several nations are jealous of Uncle Sam, that he possesses such a vitally strategic waterway as the Panama Canal.

"But we don't need to discuss all that now. The point is that we are going to try to prevent this thing and we want you boys to help."

"With a flashlight?" asked Blake, wondering whether the captain depended on scaring those who would dare to plant a charge of dynamite near the great dam.

"With a flashlight, or, rather, with a series of them, and your moving picture cameras," the captain went on. "We want you boys to get photographic views of those who will try to destroy the dam, so that we will have indisputable evidence against them. Will you do it?"

"Of course we will!" cried Blake. "Only how can it be done? We don't know where the attempt will be made, nor when, and flashlight powder doesn't burn very long, you know."

"Yes, I know all that," the captain answered. "And we have made a plan. We have a pretty good idea where the attempt will be made—near the spillway, and as to the time, we can only guess at that.

"But it will be some time to-night, almost certainly, and we will have a sufficient guard to prevent it. Some one of this guard can give you boys warning, and you can do the rest—with your cameras."

"Yes, I suppose so," agreed Blake.

"It will be something like taking the pictures of the wild animals in the jungle," Joe said. "We did some of them by flashlight, you remember, Blake."

"Yes, so we did. And I brought the apparatus with us, though we haven't used it this trip. Now let's get down to business. But we'll need help in this, Joe. I wonder where Alcando—?"

"You don't need him," declared the captain.

"Why not?" asked Joe. "He knows enough about the cameras now, and—"

"He's a foreigner—a Spaniard," objected the captain.

"I see," spoke Blake. "You don't want it to go any farther than can be helped."

"No," agreed the captain.

"But how did you and the other officials hear all this?" Joe wanted to know.

"In a dozen different ways," was the answer. "Rumors came to us, we traced them, and got—more rumors. There has been some disaffection among the foreign laborers. Men with fancied, but not real grievances, have talked and muttered against the United States. Then, in a manner I cannot disclose, word came to us that the discontent had culminated in a well-plotted plan to destroy the dam, and to-night is the time set.

"Just who they are who will try the desperate work I do not know. I fancy no one does. But we may soon know if you boys can successfully work the cameras and flashlights."

"And we'll do our part!" exclaimed Blake. "Tell us where to set the cameras."

"We can use that automatic camera, too; can't we?" asked Joe.

"Yes, that will be the very thing!" cried Blake. They had found, when making views of wild animals in the jungle, as I have explained in the book of that title, that to be successful in some cases required them to be absent from the drinking holes, where the beasts came nightly to slake their thirst.

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."