A slight touch of the aileron actuating-gear and the descent began. Cutting out the engine, Derek let the machine vol-plane. It was a delicious, exciting, nerve-tingling sensation. In silence, save for the rush of the air past the struts and tension-wires, the huge fabric glided with great rapidity, momentarily nearing the extensive bank of snow-white clouds. Instinctively Derek shut his eyes as the dazzling mantle of vapour appeared to rise and envelop him. The next moment the biplane was plunging through the mist, in which the light gradually diminished until it was like being in a room in the twilight. No longer was the needle of the compass visible. Even the luminous point failed to show so much as a faint glow. Sense of stability, too, was lost. Whether the machine was banking steeply or volplaning naturally was a matter for conjecture. All Derek knew was that the 'bus was moving rapidly, not under its own volition, but solely under the unseen and unfelt force of gravity. Then, like an express train emerging from a tunnel, the old 'bus, rocking and plunging, shot out of the cloud-bank. Shaking the moisture from his goggles, Derek restarted his engine, and then looked somewhat anxiously over the side. Almost the first object that met his gaze was the Averleigh aerodrome at a distance of about two miles. "In sight of home," soliloquized the lad grimly; "but now comes the hardest part--landing. Hope I don't pancake or try to land below the ground." "Pancaking", it must be explained, consists in getting as much way off the machine as possible, and dropping practically vertically. Unless the correct height and drop be gauged normally about three feet--the machine is almost sure to "crash". Pancaking is only deliberately resorted to when one is forced to land in standing corn, stubble, or flooded ground. "Landing below the ground" is a term applied to an underestimation of the vertical distance when pancaking. Although of comparatively rare occurrence, its results are even more disastrous than overestimating the fall, and the crash almost invariably wrecks the machine completely and costs the pilot his life. Turning, so as to fly into the wind, Daventry made the plunge. Intent upon his task, he completely forgot the presence of his mentor, who, ready at an instant's notice to operate the "dual-control" mechanism, was silently yet critically watching his pupil. The ground appeared to be rising to greet the descending aeroplane--slowly at first, then with disconcerting acceleration. There was no time to stop and think; what had to be done must be done promptly, almost automatically. An error of judgment would certainly result in a crash of more or less seriousness.