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

"Hear it!" whispered Joe.

"Yes," answered Blake. "The brass box—the box—he had!"

"Yes," whispered Joe. All the suspicions he had had—all those he had laughed at Blake for harboring, came back to him in a rush. The brass-bound box contained clockwork. Was it an alarm after all? Certainly it had given an alarm now—a most portentous alarm!

"We've got to find it!" said Blake.

"Sure," Joe assented. "It may go off any minute now. We've got to find it. Seems to be near here."

They began looking about on the ground, as though they could see anything in that blackness. But they were trying to trace it by the sound of the ticks. And it is no easy matter, if you have ever tried to locate the clock in a dark room.

"We ought to give the alarm," said Blake.

"Before it is too late," assented Joe. "Where can it be? It seems near here, and yet we can't locate it."

"Get down on your hands and knees and crawl around," advised Blake. In this fashion they searched for the elusive tick-tick. They could hear it, now plainly, and now faintly, but they never lost it altogether. And each of them recognized the peculiar clicking sound as the same they had heard coming from the brass-bound box Mr. Alcando had said was his new alarm clock.

"Hark!" suddenly exclaimed Blake.

Off to the left, where was planted the automatic camera, came a faint noise. It sounded like a suppressed exclamation. Then came an echo as if someone had fallen heavily.

An instant later the whole scene was lit up by a brilliant flash—a flash that rivaled the sun in brightness, and made Blake and Joe stare like owls thrust suddenly into the glare of day.

"The dynamite!" gasped Joe, unconsciously holding himself in readiness for a shock.

"The flashlight—the automatic camera!" cried Blake. There was no need for silence now.

The whole scene was brilliantly lighted, and remained so for many seconds. And in the glare of the magnesium powder the moving picture boys saw a curious sight.

Advancing toward the dam was a solitary figure, which had come to halt when the camera went off with the flashlight. It was the figure of a man who had evidently just arisen after a fall.

"Mr. Alcando!" gasped Joe.

"The Spaniard!" fairly shouted Blake.

Then, as the two chums looked on the brilliantly lighted scene, knowing that the camera was faithfully taking pictures of every move of their recent pupil, the boys saw, rushing toward Alcando, a number of the men and soldiers who had been in hiding.

"He's surrounded—as good as caught," Blake cried. "So he's the guilty one."

"Unless there's a mistake," spoke Joe.

"Mistake! Never!" shouted his chum. "Look—the brass box!"

The glare of the distant flashlight illuminated the ground at their feet, and there, directly in front of them, was the ticking box. From it trailed two wires, and, as Blake looked at them he gave a start.

The next moment he had knelt down, and with a pair of pliers he carried for adjusting the mechanism of his camera severed the wires with a quick snap. The ticking in the box still went on, but the affair was harmless now. It could not make the electrical current to discharge the deadly dynamite.

"Boys! Boys! Where are you?" cried Captain Wiltsey.

"Here!" cried Blake. "We've stopped the infernal machine!"

"And we've got the dynamiter. He's your friend—"

The rest of the words died away as the light burned itself out. Intense blackness succeeded.

"Come on!" cried Joe. "They've got him. We won't have to work the hand cameras. The automatic did it!"

They stumbled on through the darkness. Lanterns were brought and they saw Mr. Alcando a prisoner in the midst of the Canal guards. The Spaniard looked at the boys, and smiled sadly.

"Well, it—it's all over," he said. "But it isn't as bad as it seems."

"It's bad enough, as you'll find," said Captain Wiltsey grimly. "Are you sure the wires are disconnected, boys?" he asked.

"Sure," replied Blake, holding out the brass box.

"Oh, so you found it," said the Spaniard. "Well, even if it had gone off there wouldn't have been much of an explosion."

"It's easy enough to say that—now," declared the captain.

But later, when they followed up the wires which Blake had severed, which had run from the brass-bound box to a point near the spillway of the dam, it was found that only a small charge of dynamite had been buried there—a charge so small that it could not possibly have done more than very slight damage to the structure.

"I can't understand it," said Captain Wiltsey. "They could just as well have put a ton there, and blown the place to atoms, and yet they didn't use enough to blow a boulder to bits. I don't understand it."

"But why should Mr. Alcando try to blow up the dam at all?" asked Blake, "That's what I can't understand."

But a little later they did, for the Spaniard confessed. He had to admit his part in the plot, for the moving pictures, made by the automatic camera, were proof positive that he was the guilty one.

"Yes, it was I who tried to blow up the dam," Alcando admitted, "but, as you have seen, it was only to be an attempt to damage it. It was never intended to really destroy it. It was an apparent attempt, only."

"But what for?" he was asked.

"To cause a lack of confidence in the Canal," was the unexpected answer. "Those I represent would like to see it unused. It is going to ruin our railroad interests."

Then he told of the plot in detail.

Alcando was connected, as I have told you, with a Brazilian railroad. The road depended for its profits on carrying goods across South America. Once the Canal was established goods could be transported much more cheaply and quickly by the water route. The railroad owners knew this and saw ruin ahead of them if the Canal were to be successful. Consequently they welcomed every delay, every accident, every slide in Culebra Cut that would put off the opening of the great waterway.

But the time finally came when it was finished, and a success. Then one of the largest stockholders of the railroad, an unprincipled man, planned a plot. At first his fellow stockholders would not agree to it, but he persuaded them, painting the ruin of their railroad, and saying only slight damage would be done to the Canal.

His plan was to make a slight explosion, or two or more of them, near Culebra Cut or at the great dam. This, he anticipated, would cause shippers to regard the Canal with fear, and refuse to send their goods through it. In that way the railroad would still hold its trade.

Alcando was picked for the work. He did not want to undertake it, but he was promised a large sum, and threats were made against him, for the originator of the plot had a certain hold over him.

"But I was to throw the blame on innocent parties if I could," the Spaniard went on, in his confession. "Also I was to select a means of causing the explosion that would not easily be detected. I selected moving pictures as the simplest means. I knew that some were to be made of the Canal for Government use, and I thought if I got in with the moving picture operators I would have a good chance, and good excuse, for approaching the dam without being suspected. After I had accomplished what I set out to do I could, I thought, let suspicion rest on the camera men.

"So I laid my plans. I learned that Mr. Hadley's firm had received the contract to make the views, and, by inquiries, through spies, I learned who their principal operators were. It was then I came to you boys," he said. "Ashamed as I am to confess it, it was my plan to have the blame fall on you."

Blake and Joe gasped.

"But when you saved my life at the broken bridge that time, of course I would not dream of such a dastardly trick," the Spaniard resumed. "I had to make other plans. I tried to get out of it altogether, but that man would not let me. So I decided to sacrifice myself. I would myself blow up the dam, or, rather, make a little explosion that would scare prospective shippers. I did not care what became of me as long as I did not implicate you. I could not do that.

"So I changed my plans. Confederates supplied the dynamite, and I got this clock-work, in the brass-bound box, to set it off by means of electrical wires. I planned to be far away when it happened, but I would have left a written confession that would have put the blame where it belonged.

"I kept the battery box connections and clockwork inside the small camera I carried. Tonight all was in readiness. The dynamite was planted, and I set the mechanism. But something went wrong with it. There was too much of a delay. I came back to change the timer. I broke the string connections you made, and—I was caught by the camera. The news had, somehow, leaked out, and I was caught. Well, perhaps it is better so," and he shrugged his shoulders with seeming indifference.

"But please believe me when I say that no harm would have come to you boys," he went on earnestly, "nor would the dam have been greatly damaged.

"It was all a terrible plot in which I became involved, not all through my own fault," went on the Spaniard, dramatically. "As soon as I met you boys, after you had saved my life, I repented of my part, but I could not withdraw. The plans of this scoundrel—yes, I must call him so, though perhaps I am as great—his plans called for finding out something about the big guns that protect the Canal. Only I was not able to do that, though he ordered me to in a letter I think you saw."

Blake nodded. He and Joe were beginning to understand many strange things.

"One of the secret agents brought me the box containing the mechanism that was to set off the dynamite," the Spaniard resumed. "You nearly caught him," he added, and Blake recalled the episode of the cigar smoke. "I had secret conferences with the men engaged with me in the plot," the conspirator confessed. "At times I talked freely about dynamiting the dam, in order to throw off the suspicions I saw you entertained regarding me. But I must explain one thing. The collision, in which the tug was sunk, had nothing to do with the plot. That was a simple accident, though I did know the captain of that unlucky steamer.

"Finally, after I had absented myself from here several times, to see that all the details of the plot were arranged, I received a letter telling me the dynamite had been placed, and that, after I had set it off, I had better flee to Europe."

Blake had accidentally seen that letter.

"I received instructions, the time we were starting off on the tug," went on Alcando, "that the original plot was to be changed, and that a big charge of dynamite was to be used instead of a small one.

"But I refused to agree to it," he declared. "I felt that, in spite of what I might do to implicate myself, you boys would be blamed, and I could not have that if the Canal were to suffer great damage. I would have done anything to protect you, after what you did in saving my worthless life," he said bitterly. "So I would not agree to all the plans of that scoundrel, though he urged me most hotly.

"But it is all over, now!" he exclaimed with a tragic gesture. "I am caught, and it serves me right. Only I can be blamed. My good friends, you will not be," and he smiled at Blake and Joe. "I am glad all the suspense is at an end. I deserve my punishment. I did not know the plot had been discovered, and that the stage was set to make so brilliant a capture of me. But I am glad you boys had the honor.

"But please believe me in one thing. I really did want to learn how to take moving pictures, though it was to be a blind as to my real purpose. And, as I say, the railroad company did not want to really destroy the dam. After we had put the Canal out of business long enough for us to have amassed a fortune we would have been content to see it operated. We simply wanted to destroy public confidence in it for a time."

"The worst kind of destruction," murmured Captain Wiltsey. "Take him away, and guard him well," he ordered the soldiers. "We will look further into this plot to-morrow."

But when to-morrow came there was no Mr. Alcando. He had managed to escape in the night from his frail prison, and whither he had gone no one knew.

But that he had spoken the truth was evident. A further investigation showed that it would have been impossible to have seriously damaged the dam by the amount of dynamite hidden. But, as Captain Wiltsey said, the destruction of public confidence would have been a serious matter.

"And so it was Alcando, all along," observed Blake, a few days later, following an unsuccessful search for the Spaniard.

"Yes, our suspicions of him were justified," remarked Blake. "It's a lucky thing for us that we did save his life, mean as he was. It wouldn't have been any joke to be suspected of trying to blow up the dam."

"No, indeed," agreed Blake. "And suspicion might easily have fallen on us. It was a clever trick. Once we had the Government permission to go all over with our cameras, and Alcando, as a pupil, could go with us, he could have done almost anything he wanted. But the plot failed."

"Lucky it did," remarked Joe. "I guess they'll get after that railroad man next."

But the stockholder who was instrumental in forming the plot, like Alcando, disappeared. That they did not suffer for their parts in the affair, as they should have, was rumored later, when both of them were seen in a European capital, well supplied with money. How they got it no one knew.

The Brazilian Railroad, however, repudiated the attempt to damage the Canal, even apparently, laying all the blame on the two men who had disappeared. But from then on more stringent regulations were adopted about admitting strangers to vital parts of the Canal.

"But we're through," commented Blake one day, when he and Joe had filmed the last views of the big waterway. "That Alcando was a 'slick' one, though."

"Indeed he was," agreed Joe. "The idea of calling that a new alarm clock!" and he looked at the brass-bound box. Inside was a most complicated electrical timing apparatus, for setting off charges of explosive. It could be adjusted to cause the detonation at any set minute, giving the plotter time to be a long way from the scene.

And, only because of a slight defect, Alcando would have been far from the scene when the little explosion occurred at Gatun Dam.

Once more the great Canal was open to traffic. The last of the slide in Culebra Cut had been taken out, and boats could pass freely.

"Let's make a trip through now, just for fun," suggested Blake to Joe one day, when they had packed up their cameras.

Permission was readily granted them to make a pleasure trip through to Panama, and it was greatly enjoyed by both of them.

"Just think!" exclaimed Blake, as they sat under an awning on the deck of their boat, and looked at the blue water, "not a thing to do."

"Until the next time," suggested Joe.

"That's right—we never do seem to be idle long," agreed Blake. "I wonder what the 'next time' will be?"

And what it was, and what adventures followed you may learn by reading the next volume of this series, to be called "The Moving Picture Boys Under the Sea; Or, The Treasure on the Lost Ship."

"Here you go, Blake!" cried Joe, a few days later. "Letter for you!"

"Thanks. Get any yourself?"

"Yes, one."

"Huh! How many do you want?" asked Blake, as he began reading his epistle. "Well, I'll soon be back," he added in a low voice, as he finished.

"Back where?" asked Joe.

"To New York."

And so, with these pleasant thoughts, we will take leave of the moving picture boys.