"Well, that's lost them some gas, anyhow," murmured Lawless as he brought the head of the machine round, ready to swoop down a second time. As they approached again they could hear shouts from the airship, and the next moment a small projectile whistled past them. "They're using a pneumatic gun," ejaculated the Lieutenant. Before another shot could reach them, the knife attached to the aeroplane was again tearing its way through the fabric of the balloon, making a tremendous slit right across the upper part of one side. As Lawless brought the machine round once more he saw the airship heel, and judged from this that she must have lost a considerable quantity of gas, several compartments having probably been ripped open. "Three or four more attacks like the last, and she'll collapse," he said. He steered as before straight at the swaying bulk of the airship, which, after the first attack, had reduced her speed to about twenty miles an hour. Then, as the knife thrust itself into the gas vessel, there was a sharp, cracking sound. The aeroplane seemed to stop with a terrific jerk, tilting forward to such an extent that both pilot and mechanic were nearly flung from their seats. Then, recovering its balance with equal suddenness, it darted forward and passed the dirigible. "Saints preserve us!" ejaculated Mike. "Wus it a cyclone or a church staple that struck us, yer honour?" As a matter of fact the knife had struck one of the aluminium girders which formed the framework of the balloon structure, and snapped off. "Be ready to take the steering wheel!" shouted Lawless. He bent forward, and, picking up a long sheath-knife which formed part of the aeroplane's equipment, placed it in the belt of his leather jacket. Then, turning the machine, he wheeled round once more, rose above the dirigible, and then descended until only a few feet separated the aeroplane from the gigantic gas-bag immediately beneath it. "Take the wheel now!" he cried, and then, to Mike's amazement and horror, stood up and leapt out of the monoplane, alighting on the broad surface of the airship's gas envelope. Finding himself safe, he drew his knife and started to rip and hack at the skin of the balloon. He had known, before making his perilous leap, that he was courting almost certain death, for, should he succeed in deflating the gas vessel, he himself would share in the disaster which must follow. Yet he knew that if the dirigible reached the Forth Bridge her crew would be able to drop explosives without serious interruption. There were no anti-aircraft guns in the neighbourhood at that time, while an attempt to beat off the aerial monster with rifle fire would be hopeless. He alone could avert the threatened disaster. The monoplane, now in charge of Mike, had disappeared in the darkness, and Lawless supposed that the mechanic had gone off to give the alarm.