"I was at the indicator when an officer went up and the hatch was raised. As he didn't push it all the way down I had an idea he might soon return, so I moved up and stood between the twin tanks to the right of the steps. When the officer raised the hatch a streak of sunlight went right across the under side at the corner of the door, and I knew it couldn't come in at the front port hole," said Ralph, with a glow of pleasure in the discovery. The captain shook his head slowly, as he said: "I am afraid this will mean an additional source of worry to all of us; it is bad enough to be locked up and subjected to the guns of vessels and warships, but it will be doubly hazardous to pass through the mine fields, and avoid the nets." "Do you know anything about them, and how and where they are located?" asked Alfred. "Yes, I have a pretty good knowledge of their location, and how to avoid them, although they constantly change the nets, or provide new safety outlets," said the captain. [Illustration: _The Steel Nets_] "What do you mean by safety outlets?" asked Alfred. "Immense steel nets are stretched across the straits from Calais to Dover, two lines, in fact, between which the vessels plying between England and France go to and fro in safety. Furthermore, war vessels guard these nets on both sides, so that it would be a difficult matter to get near the nets," said the captain. "But submarines do seem to get through somewhere; do they not?" asked Ralph. "Yes; owing to their ability to make the trip under water, and taking advantage of the darkness, it is sometimes the case that they get through without being entangled in the nets," he replied. "But how do the ships that sail along the Channel get through?" asked Alfred. "That is just what I was referring to when I spoke of safety outlets. At a certain point there is an opening through the nets at one side, through which vessels can pass into the line between the two wire cordons. The opening in the other line of nets is not directly opposite, but a mile or so off to one side, so that in order to get to the opening in the other nets, it is necessary for the ship to sail along in the safety zone between the two nets, and make a turn at right angles to get out through the second opening. That method has been found to be most effective, and is called the safety lane," responded the captain. They were now in or near the most widely traveled part of the ocean on the western front of the continent. Thousands of ships pass and repass that zone which reaches from the southern part of Ireland to the western coast of France, and it was remarkable that the submarine was able to move along up to this time on the surface without being detected. Before the sun had gone down that night, however, they were compelled to submerge twice, and then the mantle of night shrouded the vessel and it moved along with more boldness. On this the fourth night of captivity, they were not locked in their prison.