“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.