od.

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 s