"Exactly; that is one explanation. The other is this: sea water is clear and transparent. By looking down directly on the water, a dark object, unless too far below the surface, will be noted for the reason that it makes a change in the coloring from the area surrounding it, and a cigar-shaped object at fifty feet below, whether it should be black or white, would quickly be detected," explained the captain. "I remember that Lieutenant Winston, who has flown across the channel many times, told me that he could tell when he was nearing land, in a fog, by sailing close to the water, even though the land couldn't be seen. Do you know how he was able to do that?" asked Ralph. "That is one of the simplest problems," replied the captain. "The shallower the water the lighter the appearance to an observer in an airship. As the water grows deeper the color seems to grow greener and bluer, the bluest being at the greatest depth." The chaser was now under way, and described a circle to the right. The captain, after saluting the officer on the bridge of _l'Orient_, gave the signal "Forward," and slowly the submarine sheered about and followed. The second line of buoys appeared a quarter of a mile to the east of the one they had just left. In a half-hour the two vessels passed through the gateway and turned to the north. "We can't be very far from England," remarked Alfred. "I judge we are fifteen miles from Dover," replied the captain. "Do you intend to go to Dover?" asked Ralph. "No; there are no stations there that can receive crafts of this kind. I do not know to what point they may take us; possibly to the mouth of the Thames, and from there to some point where the vessel will be interned," answered the captain. "How deep is the channel here?" asked Ralph. "Probably not to exceed 120 feet," was the reply. "Not more than that in the middle of the Channel,--half way between England and France?" asked Alfred in surprise. "No; the Channel is very shallow," answered the captain. "No wonder then," said Alfred, "that the submarines are having such a hard time getting through, even though they don't have the nets!" Having passed the cordon of nets the chaser turned and slowly steamed past the submarine. The lieutenant stepped to the side of the bridge and said: "I suppose, Captain, you can now make the pier-head at Ramsgate, where you will get a ship to convoy you to the harbor. Good luck to you! Adieu!" The boys waved their caps in salute, as the chaser began to move, and the crew lined up to give the final goodbye. The captain smiled and replied: "I think I have ample assistance on board; give my regards to the admiral." "How far is it to Ramsgate?" asked Ralph. "It cannot be more than twenty-five miles, and at the rate we are now going we should reach the head at five this evening. That will be the end of our troubles, as the naval officials will take care of this vessel from that point," said the captain.