For the best part of an hour Derek continued his flight, describing huge figures-of-eight in order to keep in touch with the aerodrome. In vain he maintained a sharp look-out for any lurid bursts of flame on the distant horizon that would indicate that the Boche was setting to work, and that the anti-aircraft guns were giving the raiders a hot tonic.
He was on the point of discharging his signal-pistol in order to inform the aerodrome that he was about to make a landing when a dark, indistinct mass shot by a hundred feet below him, and then vanished in the darkness.
"By Jove! I wonder if that's a Fritz?" ejaculated the young pilot. "I'll try and find out."
Almost before the Dromedary began to rock in the eddies in the wake of the mysterious aeroplane Derek swung his 'bus round, banking steeply ere he steadied her on her course. A glance at the altimeter showed him that the height was eight thousand five hundred feet, quite enough manoeuvring space for the work in hand, provided he could find his quarry.
It was almost like looking for a needle in a bottle of hay. Even taking into consideration the superior speed of the Dromedary, the initial start obtained by the Hun (supposing that Derek's surmise proved to be correct) and a slight divergence of courses would result in the two aeroplanes being separated by miles of darkness.
Still keenly on the alert, Derek held on, at the same time putting a tray of ammunition to each of the two Lewis guns, the heels of which were within a few inches of the pilot's face.
"I've missed the beggar," declared Daventry, after continuing the phantom pursuit for nearly a quarter of an hour. "Hard lines if the fellow were a Boche. I'll give myself another five minutes——By smoke! now what's that?"
Right ahead, but on a slightly-lower level, was something gaunt, indistinct, and moving. For a few seconds Derek could hardly credit his good fortune, thinking that in the stress and strain of the night-flight he was the victim of a hallucination. Another minute, however, removed all cause for doubt. It was a 'plane; more, it was a Boche, for the black crosses of infamy were discernible in the cold starlight.
The Dromedary was rocking in the tail-stream of the Hun machine. Gently Derek brought his 'bus up, until it was flying in comparatively still air. Eighty yards away was the Boche, flying serenely in blissful ignorance of the fact that a British machine was literally sitting on its tail.
Deliberately, and without the faintest compunction—for the night-raider had none when dropping his powerful bombs upon the civilian population of London and other cities and towns—Derek brought the sights of the right-hand gun to bear upon the back of the Hun pilot. A burst of vivid flashes, and the deed was done.