"This leads me to say that the eye brings into your range something which may be familiar, that is, something of which you have seen before, and you say you have seen thus and so; or, on the other hand, you see something which is unknown, or strange. It is at this point where the value of observation is of service. If you cannot compare its size with something you have knowledge of, or have no gauge by which you can determine of what it is made, and no means which will enable you to judge of its use, or its purpose, you must depend on your own judgment to decide what it can possibly be. In course of time the man in an airship becomes a thinker and a reasoner, and does not depend so much on the eye, as upon a judgment aside from that which the eye tells him. Do you understand now what I mean when I say that the eye does not grow more acute, but that the mind becomes more active, and, through observation, enables the aviator to judge more accurately as time goes on." The captain's argument was unanswerable. It was a revelation to the boys, and, as the captain was about to leave, Ralph said: "We thank you, Captain, for the wonderful lesson you have taught us. I am sure we shall never forget it, and I know we shall profit by it." CHAPTER V THE CURTAIN OF FIRE Before the morning sun had lighted up the scene, they could hear the buzzing of airplanes overhead. That was a sound so familiar to them that they could, at times, distinguish even the motors that were used on most of them. "I'll bet that's a Morane," said Ralph, as they rolled around over the blankets, preparatory to getting up. "They are out pretty early in the morning," said Alfred. "Got to be out promptly before any of the Boches are able to come over and take observations," said the sergeant, in the adjoining van. "Oh, yes; I had forgotten about that," replied Alfred. "What time will the fireworks begin?" "As soon as the observations are completed," replied the sergeant. "Any particular set of fellows on the other side you are going to wipe out?" asked Ralph. "You bet! We're going to make Devil's Cut a promenade for a health resort," replied the sergeant. "Then I'm with you," said Alfred, springing out of his bunk. The ten machines in the air inspired the boys as they glanced aloft. "I rather have a longing to return to that business," said Alfred, pointing upwardly. Ralph walked away without replying.