"What's that big, long affair, jutting out so far from the locks?" asked Blake, when the tug had approached nearer.

"That's the central pier," the captain informed him. "It's a sort of guide wall, to protect the locks. You know there are three locks at this end; or, rather, six, two series of three each. And each lock has several gates. One great danger will be that powerful vessels may ram these gates and damage them, and, to prevent this, very elaborate precautions are observed. You'll soon see. We'll have to tie up to this wall, or we'll run into the first protection, which is a big steel chain. You can see it just ahead there."

Joe and Blake, who had gotten all the pictures they wanted of the approach to the lock, stopped grinding away at the handle of the camera long enough to look at the chain.

These chains, for there are several of them, each designed to protect some lock gate, consist of links made of steel three inches thick. They stretch across the locks, and any vessel that does not stop at the moment it should, before reaching this chain, will ram its prow into it.

"But I'm not taking any such chances," Captain Watson informed the boys. "I don't want to be censured, which might happen, and I don't want to injure my boat."

"What would happen if you did hit the chain?" asked Blake. They had started off again, after the necessary permission to enter the locks had been signaled to them. Once more Blake and Joe were taking pictures, showing the chain in position.

"Well, if I happened to be in command of a big vessel, say the size of the Olympic, and I hit the chain at a speed of a mile and a half an hour, and I had a full load on, the chain would stop me within about seventy feet and prevent me from ramming the lock gate."

"But how does it do it?" asked Joe.

"By means of machinery," the captain informed him. "Each end of the chain fender goes about a drum, which winds and unwinds by hydraulic power. Once a ship hits the chain its speed will gradually slacken, but it takes a pressure of one hundred tons to make the chain begin to yield. Then it will stand a pressure up to over two hundred and fifty tons before it will break. But before that happens the vessel will have stopped."

"But we are not going to strike the chain, I take it," put in Mr. Alcando.

"Indeed we are not," the captain assured him. "There, it is being lowered now."

As he spoke the boys saw the immense steel-linked fender sink down below the surface of the water.

"Where does it go?" asked Blake.

"It sinks down in a groove in the bottom of the lock," the captain explained. "It takes about one minute to lower the chain, and as long to raise it."

"Well, I've got that!" Blake exclaimed as the handle of his camera ceased clicking. He had sufficient views of the giant fender. As the tug went on Captain Watson explained to the boys that even though a vessel should manage to break the chain, which was almost beyond the bounds of possibility, there was the first, or safety gate of the lock. And though a vessel might crash through the chain, and also the first gate, owing to failure to stop in the lock, there would be a second gate, which would almost certainly bring the craft to a stop.

But even the most remote possibility has been thought of by the makers of the great Canal, and, should all the lock-gates be torn away, and the impounded waters of Gatun Lake start to rush out, there are emergency dams that can be put into place to stop the flood.

These emergency dams can be swung into place in two minutes by means of electrical machinery, but should that fail, they can be put into place by hand in about thirty minutes.

"So you see the Canal is pretty well protected," remarked Captain Watson, as he prepared to send his tug across the place where the Chain had been, and so into the first of the three lock basins.

"Say! This is great!" cried Blake, as he looked at the concrete walls, towering above him. They were moist, for a vessel had recently come through.

Now the tug no longer moved under her own steam, nor had it been since coming alongside the wall of the central pier. For all vessels must be towed through the lock basins, and towed not by other craft, but by electric locomotives that run alongside, on the top of the concrete walls.

Two of these locomotives were attached to the bow of the tug, and two to the stern. But those at the stern were not for pulling, as Joe at first supposed, for he said:

"Why, those locomotives in back are making fast to us with wire hawsers. I don't see how they can push with those."

"They're not going to," explained Captain Watson. "Those in the stern are for holding back, to provide for an emergency in case those in front pull us too fast."

"Those who built the Canal seem to have thought of everything," spoke Blake with much enthusiasm.

"You'll think so, after you've seen some more of the wonders," the tug captain went on with a smile. "Better get your cameras ready," he advised, "they'll be opening and closing the gates for us now, and that ought to make good pictures, especially when we are closed in the lock, and water begins to enter."

"How does it come in?" asked Joe. "Over the top?"

"No, indeed. They don't use the waterfall effect," answered Blake, who had been reading a book about the Canal. "It comes in from the bottom; doesn't it, Captain Watson?"

"Yes, through valves that are opened and closed by electricity. In fact everything about the lock is done by electricity, though in case of emergency hand power can be used. The water fills the lock through openings in the floor, and the water itself comes from Gatun Lake. There, the gate is opening!"

The boys saw what seemed to be two solid walls of steel slowly separated, by an unseen power, as the leaves of a book might open. In fact the gates of the locks are called "leaves." Slowly they swung back out of the way, into depressions in the side walls of the locks, made to receive them.

"Here we go!" cried the captain, the tug began to move slowly under the pull of the electric locomotives on the concrete wall above them. "Start your cameras, boys!"

Blake and Joe needed no urging. Already the handles were clicking, and thousands of pictures, showing a boat actually going through the locks of the Panama Canal, were being taken on the long strip of sensitive film.

"Oh, it is wonderful!" exclaimed Mr. Alcando. "Do you think—I mean, would it be possible for me to—"

"To take some pictures? Of course!" exclaimed Blake, generously. "Here, grind this crank a while, I'm tired."

The Spaniard had been given some practice in using a moving picture camera, and he knew about at what speed to turn the handle. For the moving pictures must be taken at just a certain speed, and reproduced on the screen at the same rate, or the vision produced is grotesque. Persons and animals seem to run instead of walk. But the new pupil, with a little coaching from Blake, did very well.

"Now the gates will be closed," said the tug captain, "and the water will come in to raise us to the level of the next higher lock. We have to go through this process three times at this end of the Canal, and three times at the other. Watch them let in the water."

The big gates were not yet fully closed when something happened that nearly put an end to the trip of the moving picture boys to Panama.

For suddenly their tug, instead of moving forward toward the front end of the lock, began going backward, toward the slowly-closing lock gates.

"What's up?" cried Blake.

"We're going backward!" shouted Joe.

"Yes, the stern locomotives are pulling us back, and the front ones seem to have let go!" Captain Watson said. "We'll be between the lock gates in another minute. Hello, up there!" he yelled, looking toward the top of the lock wall. "What's the matter?"

Slowly the tug approached the closing lock gates. If she once got between them, moving as they were, she would be crushed like an eggshell. And it seemed that no power on earth could stop the movement of those great, steel leaves.

"This is terrible!" cried Mr. Alcando. "I did not count on this in learning to make moving pictures."

"You'll be in tighter places than this," said Blake, as he thought in a flash of the dangers he and Joe had run.

"What'll we do?" asked Joe, with a glance at his chum.

"Looks as though we'd have to swim for it if the boat is smashed," said Blake, who remained calm. "It won't be hard to do that. This is like a big swimming tank, anyhow, but if they let the other water in—"

He did not finish, but they knew what he meant. Slowly and irresistibly the great lock gates were closing and now the tug had almost been pulled back between them. She seemed likely to be crushed to splinters.