For half an hour Desmond Blake kept his understudy hard at it, showing him how to make the battleplane bank almost horizontally, and how to change the speed gear to enable the wings to overcome the force of gravity during the vertical flight.
"You'll do," declared the inventor admiringly. "Now back to your perch. We are going to have a shot at looping the loop."
Desmond Blake waited until Athol had regained the gimballed seat, then, depressing a lever that had the double effect of lowering the gearing of the engine and elevating the "aerilons," or wing-tips, he caused the battleplane to soar almost vertically upwards.
The lads wondered why the terrifically acute angle of ascent did not cause the fuel to flow to the rearmost of the four connected tanks, and thus affect the aircraft's lateral stability. The inventor, glancing over his shoulder, must have read their thoughts.
"Climbing to get a better chance in case she jibs," he called out. "No need to worry about the petrol. Each tank has a reserve valve that only operates when the angle of inclination exceeds fifteen degrees."
The arrangement of the tanks was another instance of Blake's forethought. At normal flying positions the petrol in each tank was practically at the same level in order to ensure constant trim of the machine. But directly the tilt of the battleplane tended to allow the volatile spirit to flow to the lowermost tank, automatic valves in the connecting pipes came into action, thus causing each tank to retain approximately the same weight of liquid fuel.
For three minutes the battleplane climbed steeply and at a high speed that had never yet been approached by the most daring aviator. Then, following a hasty caution from the pilot, the aeroplane began to describe a circle in a vertical plane. Although the seats retained their normal positions, the centrifugal force tended to throw Athol and Dick off their balance. The next moment their heads were within a few feet of the up-turned floor of the fuselage, while their feet were dangling in the space enclosed by the wind-screens. Five seconds later the battleplane had regained its normal position, having described a complete loop of a radius of less than a hundred feet.
"That's good!" exclaimed the inventor with pardonable pride. "Now look out to enjoy the sunshine."
To the lads' surprise the battleplane was bathed in bright wintry sunshine. The aeroplane had emerged above the bank of snow clouds and was cleaving her way through the clear air. Away to the south-west the sun was low in the heavens, for it was now within an hour of sunset.
"Time to get back," declared Blake briskly. "We've got to drop through the snow-clouds beneath, and trust to luck to pick up our bearings. 'Fraid I've overstepped the bounds of discretion, but it was jolly well worth it."