"Then a little instruction will be of service to you and to all of us," said the captain. "I noticed that you were sweeping the sea to the rear. That is not necessary, for at our speed a torpedo boat would not be able to catch us. All your time should be devoted to scanning that quadrant from straight ahead to a point but a little astern of your left quarter, as it is from that section, and the corresponding section on the right side of the vessel that we expect the enemy; do you understand what I mean?" "I think so," replied Ralph. "But suppose a submarine should be well ahead of us and submerge, and then wait until we have passed. In that case couldn't it again come up and send a torpedo into the stern of the ship?" "That might be possible, but not probable. A submarine is absolutely in the dark when completely submerged," said the captain. "It must come to the surface sufficiently near to bring its periscope out of the water, and that would reveal its presence to us. It would be a pretty hard job for a navigator in a submarine to calculate when the boat had passed sufficiently near to know the opportune time to come to the surface and give us the shot." "But couldn't they come near enough to take a chance? They might come up 500 feet away or 2,000. At either distance they could land a torpedo, couldn't they?" asked Alfred. "Quite true; but the submarine might not know whether we were armed or not, and it would not take the risk of exposure in that reckless manner," replied the captain. "But we are not armed, are we?" asked Ralph. "No; our guns will be ready for us on the return trip," answered the captain. After a moment he continued: "Let me also give you a hint as to the particular manner of using the glasses to get a correct view. Do not attempt to take in the entire field at one sweep. Sight at a point near the ship, say at a distance of a quarter of a mile; then slowly raise the glasses so that your view grows more and more distant and finally the focal point reaches the horizon. Then turn a point to the right or to the left, and bring down the forward end of the glasses until the view is again concentrated on the point nearest the ship." "That is something like making observations on a flying machine," replied Alfred, "only in that case the glass is held stationary, as the machine moves along, and in that way objects can be seen much better than by sweeping it around continuously. We learned that from Lieutenant Guyon." "Quite true; I see you are well qualified to observe. But to continue: after you have thus made the first observation as I have explained, the glasses should be held horizontally to take in the view at the horizon, and then swept around at that angle to the right or to the left, depressing it at each swing. That is called sweeping the sea." "I know two men who have glasses," said Ralph. "Shall I get them?" "Yes, if you can; this is the kind of service which is appreciated," said the captain.