those of the great emporiums and factories, were either unlit or else shielded from observation above. And out at sea only merchant vessels showed lights, those carried by warships, large and small, being masked.

"Even the passenger flights at Hendon were more exciting than this!" growled the Lieutenant to himself as, just before dawn, he steered a homeward course in the direction of the Forth.

He little thought as he uttered his complaint that within twenty-four hours he would have had enough excitement to last him a lifetime.

The machine had reached the Forth and was crossing it just above the bridge, when Mike, who sat immediately behind the Lieutenant, uttered an exclamation:

"Are me eyes desaving me, or have the hivins turned topsy-turvy?"

"What's the matter?" asked Lawless, for the patent wind-screen with which the machine was fitted made conversation possible even when travelling at a high speed.

"Thim stars below us, sorr. Sure now, is it upside down we're flying?"

Lawless looked below, and saw what at first seemed to justify Mike's anxiety as to the position of the heavens. Immediately beneath the machine was a cluster of luminous specks for all the world like distant stars, shining up instead of down upon them. They were certainly not lamps of any sort, and the dull glow they gave out seemed as if it might be the reflection of some distant lights. Yet there was no moon, nor, so far as Lawless could discover, anything which could account for this extraordinary effect of reflected light.

He circled over the bridge two or three times, hoping to discover the cause of this mystery, but without success. While he was doing this a train rumbled across the bridge, and during its passage Lawless noted that the luminous specks disappeared. They were again observable as soon as the train had passed, however, and the Lieutenant concluded from this that they were situated on the railway track itself.

"Deuced queer business!" he muttered.

But there was no time to make further investigations that night, so he continued his flight to Montrose, still pondering over the mystery.

"By the way, Mike," he said, when they had alighted opposite the hangar, "don't say anything about those lights we saw on the bridge. I'm going to investigate the matter myself."

Mike readily promised, and Lawless was strolling away when a thought suddenly struck him.

"By Jove!" he ejaculated aloud. "I believe I've got it!"

His memory had flashed back to the notice he had read, offering a reward in connection with certain paint marks on the Forth Bridge. No doubt the misguided "person or persons" who had daubed the bridge had used luminous paint. This, of course, would explain the phosphorescent glow which had so puzzled him.

The question now was, had it been done merely as a joke or for some specific object? If for a joke, it was difficult to see where the humour came in; if for an object, it was equally difficult to see what purpose it could serve.

"I'll have a closer look at the bridge when I've rested a bit," he told himself.

That afternoon the Lieutenant took a train to Queensferry. He then set out to walk back over the bridge along the railway track. The paint daubs were clearly visible, though not, of course, phosphorescent in the daylight. It struck him as peculiar that these marks had a definite shape, or rather two definite shapes. One took the form of a St. Andrew's Cross, the other the form of a star—that is, instead of two intersecting lines of the same length, there were four.

But that was not all. A careful examination showed that these stars were placed at the weakest part of the bridge; in other words the parts which would be easiest to destroy by means of dynamite. Finally, the marks could only be seen from above the bridge, not from below it.

Lawless whistled softly when he had made these discoveries.

He returned to the depot, and went at once to his commanding officer and told him of the discoveries he had made. He also explained at some length his own theory concerning them. The commanding officer listened patiently at first because he was a good-natured fellow and liked to see his officers exercise initiative and resource. But his interest in the story soon flagged, and when Lawless began to state his deductions it wilted entirely. An expression of resigned boredom appeared on the officer's face and remained there till the interview came to an end.

"So you see," concluded the Lieutenant eagerly, "it requires no great effort of imagination to realise what the German plans are."

But it required more imagination than the commanding officer was capable of exercising. Indeed, he was not an imaginative man and prided himself upon the fact. He didn't regard imagination as consistent with the duties of a soldier; consequently, he made no effort to encourage it in those under his command.

"It's a very ingenious theory, very!" he said; "but unfortunately the grounds upon which you base it appear to me to be entirely unsatisfactory."

And for the time being that settled the matter so far as the commanding officer was concerned.

That evening, when the Lieutenant arrived at the hangar preparatory to setting out on the nightly scouting expedition, he brought with him a curious looking implement. It consisted of a stout spar about ten feet long, at the end of which was attached a long, curved knife, not unlike a billhook in appearance.

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.

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