Captain Daventry still hesitated. An experienced and thoroughly up-to-date naval officer, he understood his own profession from top to bottom. The navy, notwithstanding rapid and recent developments, was a long-established firm. There was, in his opinion, something substantial in a battleship, in spite of U-boats and mines. But the wear and tear of an airman, the fragile nature of his craft, and above all the uncertain moods of the aerial vault made flying, in his estimation, a short-lived and highly-dangerous profession, albeit men look to it with all the zest of amateurs following a new form of pastime. "Hang it all, Pater!" continued Derek, warming to his subject; "the Boche has to be knocked out in the air as well as on the sea. Someone's got to do it; so why can't I have a hand in the game?" "I'm not thinking of the war, but after," replied his father. "Since you're keen on it, carry on, and good luck. The after-the-war problem must wait, I suppose." And so it happened that in due course Derek Daventry presented himself for an interview at the Reception Depot of the newly-constituted Air Ministry. That ordeal successfully passed, and having satisfied the Medical Board, after a strenuous examination, that he was thoroughly sound in mind and body, the lad found himself an R.A.F. cadet at a large training-centre on the south coast. Here his experience was varied and extensive. In a brief and transitory stage, the mere soldiering part of which he tackled easily, thanks to his school cadet training, he was initiated into the mysteries of the theory of flight, the air-cooled rotary engines, wireless telegraphy, aerial photography, and a score of subjects indispensable to the science of war in the air. Then, punctuated by regular medical examinations--for in no branch of the service is the precept _mens sana in corpore sano_ held in higher esteem--came additional courses in the arts of destructive self-defence: machine-guns, their construction, use, and defects; bombs of all sizes and varieties; aerial nets, their use and how to avoid them; the composition of poison-gas and "flaming onions"; how to avoid anti-aircraft fire; and a dozen other problems that have arisen out of the ashes of the broken pledges of the modern Hun.