"What will we do with the cameras, Blake? The films, too, they will all be spoiled—we haven't enough waterproof cases!" cried Joe to his chum, as the boat, through some accident or failure, backed nearer and nearer to the closing steel gates.

"Will we really have to jump overboard?" asked the Spaniard. "I am not a very excellent swimmer."

But Blake, at whom these questions seemed directed, did not have to answer them. For, after a series of confused shouts on the top of the concrete wall above them the movement of the boat, as well as the slow motion of the lock gates, ceased. It was just in time, for the rudder of the tug was not more than a few feet away from the jaws of steel.

"You're all right now," a man called down to those on the tug, from the wall over their heads. "Something went wrong with the towing locomotives. There's no more danger."

"Well, I'm glad to know that," answered Captain Watson gruffly. "You might just as well kill a man as scare him to death. What was the matter, anyhow?"

"Well, all of our machinery isn't working as smoothly as we'll have it later," the canal engineer explained. "Some of our signals went wrong as you were being towed through, and you went backward instead of forward. Then it took a minute or so to stop the lock gates. But you're all right now, and you'll go on through."

Blake and Joe looked at each other and smiled in relief, and Mr. Alcando appeared to breathe easier. A little later the tug was again urged forward toward the front lock gates. Then the closing of those at her stern went on, until the vessel was in a square steel and concrete basin—or, rather, a rectangular one, for it was longer than it was wide, to lend itself to the shape of the vessels. As Blake had said, it was like a big swimming tank.

"Now we'll go up," Captain Watson said. "You can't get any pictures in here, I suppose?" he added.

"We can show the water bubbling up as it fills the lock," said Blake. "Water always makes a pretty scene in moving pictures, as it seems to move at just the right rate of speed. We'll take a short strip of film, Joe, I guess."

The tug did not occupy a whole section of the lock, for they are built to accommodate vessels a thousand feet long. To economize time in filling up such a great tank as that would be the locks are subdivided by gates into small tanks for small vessels.

"It takes just forty-six gates for all the locks," explained Captain Watson, while Blake and Joe were getting their camera in position, and the men at the locks were closing certain water valves and opening others. "Each lock has two leaves, or gates, and their weight runs anywhere from three hundred to six hundred tons, according to its position. Some of the gates are forty-seven feet high, and others nearly twice that, and each leaf is sixty-five feet wide, and seven feet thick."

"Think of being crushed between two steel gates, of six hundred tons each, eighty feet high, sixty-five feet wide and seven feet thick," observed Joe.

"I don't want to think of it!" laughed Blake. "We are well out of that," and he glanced back toward the closed and water-tight lock gates which had so nearly nipped the tug.

"Here comes the water!" cried the captain. There was a hissing and gurgling sound, and millions of bubbles began to show on the surface of the limpid fluid in which floated the Nama. The water came in from below, through the seventy openings in the floor of each lock, being admitted by means of pipes and culverts from the upper level.

As the water hissed, boiled and bubbled while it flowed in Blake took moving pictures of it. Slowly the Nama rose. Higher and higher she went until finally she was raised as high as that section of the lock would lift her. She went up at the rate of two feet a minute, though Captain Watson explained that when there was need of hurry the rate could be three feet a minute.

"And we have two more locks to go through?" asked Joe.

"Yes, two more here at Gatun, and three at Miraflores; or, rather, there is one lock at Pedro Miguel, where we go down thirty and a third feet, and then we go a mile to reach the locks at Miraflores.

"There we shall have to go through two locks, with a total drop of fifty-four and two-thirds feet," Captain Watson explained. "The system is the same at each place."

The tug was now resting easily in the basin, but some feet above the sea level. Blake and Joe had taken enough moving pictures of this phase of the Canal, since the next scenes would be but a repetition of the process in the following two locks that would lift the Nama to the level of Gatun Lake.

"But I tell you what we could do," Blake said to his chum.

"What's that—swim the rest of the way," asked Joe, "and have Mr. Alcando make pictures of us?"

"No, we've had enough of water lately. But we could get out on top of the lock walls, and take pictures of the tug going through the lock. That would be different."

"So it would!" cried Joe. "We'll do it!"

They easily obtained permission to do this, and soon, with their cameras, and accompanied by Mr. Alcando, they were on the concrete wall. From that vantage point they watched the opening of the lock gates, which admitted the Nama into the next basin. There she was shut up, by the closing of the gates behind her, and raised to the second level. The boys succeeded in getting some good pictures at this point and others, also, when the tug was released from the third or final lock, and steamed out into Gatun Lake. There was now before her thirty-two miles of clear water before reaching Miraflores.

"Better come aboard, boys," advised Captain Watson, "and I'll take you around to Gatun Dam. You'll want views of that."

"We sure will!" cried Blake.

"Isn't it all wonderful!" exclaimed Joe, who was deeply impressed by all he saw.

"It is, indeed!" agreed the Spaniard. "Your nation is a powerful and great one. It is a tremendous achievement."

Aboard the tug they went around toward the great dam that is really the key to the Panama Canal. For without this dam there would be no Gatun Lake, which holds back the waters of the Chagres River, making a big lake eighty-five feet above the level of the ocean. It is this lake that makes possible the operation of a lock canal. Otherwise there would have to be a sea-level one, and probably you boys remember what a discussion there was, in Congress and elsewhere, about the advantages and disadvantages of a sea-level route across the Isthmus.

But the lock canal was decided on, and, had it not been, it is probable that the Canal would be in process of making for many years yet to come, instead of being finished now.

"Whew!" whistled Joe, as they came in sight of the dam. "That sure is going some!"

"That's what it is!" cried Captain Watson, proudly, for he had had a small part in the work. "It's a mile and a half long, half a mile thick at the base, three hundred feet through at the waterline, and on top a third of that."

"How high is it?" asked Joe, who always liked to know just how big or how little an object was. He had a great head for figures.

"It's one hundred and five feet high," the captain informed him, "and it contains enough concrete so that if it were loaded into two-horse wagons it would make a procession over three times around the earth."

"Catch me! I'm going to faint!" cried Blake, staggered at the immensity of the figure.

"That dam is indeed the key to the whole lock," murmured Mr. Alcando, as he looked at the wonderful piece of engineering. "If it were to break—the Canal would be ruined."

"Yes, ruined, or at least destroyed for many years," said Captain Watson solemnly. "But it is impossible for the dam to break of itself. No waters that could come into the lake could tear it away, for every provision has been made for floods. They would be harmless."

"What about an earthquake?" asked Joe. "I've read that the engineers feared them."

"They don't now," said the captain. "There was some talk, at first, of an earthquake, or a volcanic eruption, destroying the dam, but Panama has not been visited by a destructive earthquake in so long that the danger need not be considered. And there are no volcanoes near enough to do any harm. It is true, there might be a slight earthquake shock, but the dam would stand that. The only thing that might endanger it would be a blast of dynamite."

"Dynamite!" quickly exclaimed Mr. Alcando. "And who would dare to explode dynamite at the dam?"

"I don't know who would do it, but some of the enemies of the United States might. Or someone who fancied the Canal had damaged him," the captain went on.

"And who would that be?" asked Blake in a low tone.

"Oh, someone, or some firm, who might fancy that the Canal took business away from them. It will greatly shorten certain traffic and trade routes, you know."

"Hardly enough to cause anyone to commit such a crime as that, do you think?" asked the Spaniard.

"That is hard to answer," went on the tug commander. "I know that we are taking great precautions, though, to prevent the dam, or the locks, from being damaged. Uncle Sam is taking no chances. Well, have you pictures enough?"

"I think so," answered Blake. "When we come back we'll stop off here and get some views from below the dam, showing the spillway."

"Yes, that ought to be interesting," the captain agreed.

The tug now steamed on her way out into Gatun Lake, and there a series of excellent views were obtained for the moving picture cameras. Mr. Alcando was allowed to do his part. He was rapidly learning what the boys could teach him.

"Of course it could never happen," the Spaniard said, when the cameras had been put away, for the views to be obtained then were of too much sameness to attract Joe or Blake, "it would never happen, and I hope it never does; but if it did it would make a wonderful picture; would it not?" he asked.

"What are you talking about?" asked Blake.

"The Gatun Dam," was the answer. "If ever it was blown up by dynamite it would make a wonderful scene."

"Too wonderful," said Joe grimly. "It would be a terrible crime against civilization to destroy this great canal."

"Yes, it would be a great crime," agreed the Spaniard in a low voice. A little later he went to his stateroom on the tug, and Blake and Joe remained on deck.

"Queer sort of a chap; isn't he?" said Joe.

"He sure is—rather deep," agreed his chum.

"Are you boys going into the jungle?" asked the tug captain that afternoon.

"Yes, we want to get a few views showing life in the woods," answered Blake. "Why?"

"Well, the reason I asked is that I can take you to the mouth of the Chagres River and from there you won't have so much trouble penetrating into the interior. So if you're going—"

"I think we had better go; don't you?" asked Blake of his chum.

"Surely, yes. We might get some fine pictures. They'll go well with the Canal, anyhow; really a sort of part of the series we're taking."

"All right, then, I'll leave you in the jungle," the captain said.

A day or so later, stops having been made to permit the boys to film certain scenes they wanted, the tug reached Gamboa, where they stopped, to plan a trip into the interior.

Then, one morning, with their cameras loaded with film, they started off for a brief trip into the jungle.