It was, indeed, a sad sight to witness, knowing that the shots meant the death of thirty or more human beings. "Well, I am awfully sorry for them, even if they had no sympathy for us, and didn't wait to see whether or not we were put into safety before they sent our ship down," said Alfred reflectively, as he turned and entered the conning tower. The scene had its fascination for Ralph, although he felt the horror of it all as he stood leaning over the railing, gazing at the patrol boats which were sailing back and forth in and around the spot where the petroleum was fast covering the surface of the water in all directions. "You can understand now, can't you, why flying machines are such good spotters for submarines?" remarked the captain. "Do you mean the oil that comes on top of the water?" asked Ralph. "Yes," was the reply. "But does oil arise at all times when a submarine is submerged?" asked Ralph. "More or less oil is constantly detaching itself from the body of the hull, at the discharge ports, and it can't be helped because all of the gas discharge ports are under water at all times, whether the vessel is running on or under the water, hence, as it moves along it will leave a trail of oil which can be easily detected by a machine in flight above the surface of the water," said the captain. "But doesn't a machine, when it is under the water, leave a ripple that is easily seen by a flying machine?" asked Ralph. "Yes; I was going to refer to that," replied the captain. "An aviator has a great advantage over an observer on a vessel, for the reason that the slightest movement of the surface of the sea, even though there may be pronounced waves, can be noted. If the submarine is moving along near the surface, the ripple is very pronounced, and the streak of oil which follows is very narrow. Should the submarine stop, the oil it discharges accumulates on top of the water at one place, and begins to spread out over the surface of the water and this makes it a mark for the watchful eye of the airmen of the sea patrols," answered the captain. "I heard one of the officers at the aviation camp say that a submarine could be seen easily through fifty feet of water by an airman," remarked Alfred. "Do you think that is so?" he asked. "I know it is possible," replied the captain. "But why is it that when you are on a ship it is impossible to see through the water that depth?" "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.