"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?"