"For this reason," answered the captain: "if you are on a ship, and you are looking even from the topmast of the vessel, the line of vision from the eye strikes the surface of the water at an angle. The result is that the surface of the water acts as a reflector, exactly the same as when the line of sight strikes a pane of glass."
"Do you mean that the sight is reflected just as it is when you are outside of a house and try to look into the window at an angle?" asked Ralph.
"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.