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.

"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 slowl