"What's that, I wonder?" thought the lad as, after a seemingly interminable lapse of time, a faint hissing, bubbling noise caught his ear. For some seconds he listened intently. Then came the unmistakable odour of the fumes of a powerful acid, mingled with the spluttering of the drifting flakes as they came in contact with the hot metal.
The miscreant, whoever he might be, had fought shy of the task of picking the lock, and was employing either sulphuric or nitric acid.
Athol knelt up, gripping the coaming of the aperture and straining his ears. Then, just as he was about to steal softly to his companions, he felt a hand laid lightly upon his shoulder.
Desmond Blake had also detected the signs of the miscreant's attempt.
Without trusting himself even to whisper, Blake began to apply a series of light touches to his assistant's arm. Athol, quick to grasp the significance, understood. The inventor was employing the Morse system of communication.
"No action till I give the word," he tapped out. "Wake Dick."
Although his chum was sound asleep Athol succeeded in rousing him in silence, and the three airmen gathered round the aperture of the fuselage, awaiting developments.
Quite half an hour passed; then came the rending of the chemically-treated corrugated metal sheeting. A muffled exclamation of pain followed by a guttural oath plainly indicated that the fellow had burnt himself with the powerful corrosive.
Crawling through the opening the intruder hung a great coat over the hole, to trap any rays of light from passing without, and switched on an electric torch. For some seconds he stood gazing at the mechanical marvel he meant to destroy. His scientific curiosity made him temporarily set aside his purpose, for still holding the torch he began to swing himself up the girder-ladder communicating with the interior of the apparently untenanted battleplane.
The reflected glare of the upturned torch made it easy for the lads to follow the inventor's unspoken directions. Cautiously they backed until they had placed the motor space between them and the aperture towards which the fellow was climbing.