“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!”