"Pocket," explained Rippondene, speaking into the voice-tube that formed a means of communication between instructor and pupil. "You'll soon get used to them; carry on--up to four thousand." It was Derek's first "bump"--a vertical fall through fifty or a hundred feet, owing to the machine encountering a patch of thin air, or what is known to airmen as a pocket. "Look ahead!" came the warning. "There's another 'bus." Approaching each other at an aggregate speed of a hundred and fifty miles an hour the two biplanes swerved discreetly, for both were steered by quirks who took no risks. There are certain hard-and-fast rules of the air which have to be obeyed with as much precision as the mariner has to conform to the rule of the road at sea. They passed a good two hundred yards apart, but almost immediately Derek's 'bus started rocking and rolling in a disconcerting fashion as it encountered the backwash of air from the now rapidly receding biplane. Revelling in the novel situation, Derek held on, occasionally turning his machine in a wide circle and resisting any great inclination to bank. He felt as if he could carry on indefinitely, so exhilarating was the rush through the air, until the voice of his mentor sounded in his ear. "How about it?" it enquired brusquely. "I want my lunch even if you don't. Back you go, my festive." Derek swung the machine round until the needle of the compass showed that the Dromedary was flying in the reverse direction, but very soon the disconcerting truth became apparent. In his wild joy-ride he had neglected to take bearings and allow for the side-drift of the wind. He was lost. "Won't do to admit that," he soliloquized. "I'll bluff the old buffer, and trust to luck." For nearly ten minutes he flew by compass course, the while studying the expanse of ground three thousand feet below. Away to the south'ard he could discern the coast-line, quite forty miles distant. Evidently under the action of the south-westerly breeze the biplane had side-drifted more than thirty miles. Flecks of whitish vapour glided rapidly beneath the aeroplane. The sky was beginning to become overcast. Viewed from the ground those clouds would probably appear dark and semi-opaque. Viewed from above, and bathed in the brilliant sunshine, they were white as driven snow. Setting a compass course to counteract the current, Daventry flew steadily for twenty minutes. By the end of this time the ground was invisible. Reluctantly he resolved to dive through the clouds in order to verify his position. It seemed a thousand pities to plunge out of the sunshine, but his instructor was becoming impatient. The novelty of joy-riding in the air had long since worn off as far as Rippondene was concerned, whereas the pangs of hunger are not easily to be denied.