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—­”