It was not difficult to fix the spar securely to the machine so that the business end protruded about six feet beyond the driver's seat, somewhat after the manner of a steel-pointed ram. This done, and it being time to set off, the machine was taken out of the hangar and a few minutes later Lawless and Mike were high above the flight station. Lawless now steered straight for the Forth Bridge, as he wanted to find out first of all whether the luminous marks were still visible. Having satisfied himself that they showed up as vividly as ever, he swept round in an easterly direction, making straight for the open sea. They had just passed above the Bass Rock, when the Lieutenant, who had been gazing expectantly into the darkness ahead, shouted a question to Mike: "Do you see anything in front of us?" The Irishman, who possessed remarkably keen eyes, raised himself a little and peered over the Lieutenant's shoulder. "B the powers, if 'tis not a dirigible, 'tis the ghost av wan!" he exclaimed. "I thought so," replied Lawless in a tone of grim satisfaction. "And, what's more," he added under his breath, "she's making straight for the Forth." Both the aeroplane and the airship must have been travelling at a very considerable speed, for even as the Lieutenant spoke the dirigible changed from a blurred outline into a well-defined shape. There was no mistaking her now--a gigantic Zeppelin, armed, no doubt, with bombs and machine guns. "Now we'll have a chance of testing this little invention of mine," murmured the Lieutenant, almost with a chuckle; and he gave a glance of satisfaction at the weird apparatus attached to the machine. As a preliminary to the duel the Lieutenant rose in the air, circled above the dirigible, and then swooped down and passed abreast of the car, so close that the crew could be distinctly seen. "That's the challenge!" he shouted gleefully. "Now for it!" Making a wide sweep, he steered full-tilt at the airship's tremendous gas vessel in such manner as to rip it up with the hooked knife attached to the spar. That was his sole weapon of offence, and he hoped, by repeated attacks, to destroy completely the gas-bag and thus cause the vessel to sink. The task was harder, even, than it looked, for the Zeppelin, like all dirigibles, was provided with a number of separate gas compartments, so that a leakage in one would not materially affect the stability of the whole structure, which would only collapse after several gas chambers had been ripped open. A strange thrill passed through both men as their little machine rushed against the tremendous bulk of the airship, much as a swordfish might attack a whale. Suddenly there was a tearing, ripping sound; the monoplane quivered from nose to tail with the shock of the impact and then passed right across the top of the gas vessel, the knife rending the fabric as it went.