"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.