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
flo