Hal drew a sharp breath and tightened his hold upon the steering wheel.

There was no question that Stubbs' voice had carried to the occupants of the second craft, and as Stubbs had exclaimed aloud in English there was little doubt in the minds of our three friends that the Austrians would seek an explanation. Nor were they wrong.

Came a hail from the Austrian:

"Who've you got aboard, there?"

"Prisoner," replied Hal, thinking quickly.

"What are you doing with him?"

"We--" Hal began, but the Austrian interrupted.

"Spies, that's what you are! Down to the ground now, or I'll put a hole through you."

"Guess it's no use fooling any longer," muttered Hal.

He threw over the elevating lever and the large craft soared rapidly. At the same moment a shot rang out from aboard the Austrian, followed by a cry of surprise, and then the Austrian gave chase.

"Get your guns and see if you can pick 'em off," Hal instructed Chester and Colonel Anderson. "I'll run this thing, but you fellows will have to do the fighting."

"Suits me," responded Chester, examining his revolver carefully.

Colonel Anderson also nodded his agreement to this plan.

Hal now changed his course and the airship headed toward the south, bearing off a trifle to the east, in a direction that he believed, eventually, would land them in Serbia.

It became apparent now that the Austrian had not boasted of the speed of his craft without reason, for he gained perceptibly.

"We can't out-run him, Hal," shouted Chester.

"Then we shall have to try something else," was the reply.

Abruptly he reduced the speed of the craft and the Austrians dashed in range of the revolvers of the fugitives almost before they could have realized it.

"Crack! Crack!"

Chester and Colonel Anderson had fired. There came a scream of pain from behind and the Austrian craft wobbled crazily. A moment later a man sprang to his feet, sought to retain his footing, threw up his arms and went hurtling into s.p.a.ce.

"Got one, Hal!" said Chester, quietly.

"Good!"

Came a volley of small arm fire from behind and bullets whined about the four friends. Again Chester and Colonel Anderson fired almost simultaneously and again their efforts were rewarded. A second man was put out of the fight, as they could see.

At this moment Stubbs came into action.

He arose from his seat and, grasping the side of the speeding craft with his left hand for support, stood to his full height. His right arm drew back, then flashed sharply forward again and a small object went spinning through the air toward the Austrian airship.

It struck home and there was a terrible explosion, followed by several sharp cries of pain, as the Austrian airship seemed to split into a thousand pieces. A moment later these pieces disappeared.

The three friends turned upon Stubbs.

"What is this, magic?" asked Chester in surprise.

"No," replied Stubbs, quietly. "Melenite. I just happened to see a stick of it here, so I threw it."

"Well, you did a pretty good job, Stubbs," said Colonel Anderson.

"I didn't pitch for my college team two years for nothing," returned Stubbs modestly. "But now let's go down. I want to get my feet on the ground again."

"It won't be much longer, Stubbs," said Hal. "Another two hours at this speed should put us across the Serbian frontier. Just be patient."

"I'll wait," replied Stubbs, "but I won't promise to be patient."

He sank back to his place and refused to talk further.

While the big army craft is speeding across Austria it will be a good time to explain the presence of the four friends in their present predicament and introduce them briefly to those who have not met them before.

Hal Paine and Chester Crawford were both American lads. With the former's mother, they had been in Berlin at the outbreak of the great war, and, after a series of interesting and exciting adventures, they made their way to Liege just in time to take part in the defense of that stronghold with the Belgian army.

There they won distinction and lieutenancies in the Belgian service, the latter bestowed upon them by King Albert himself. They had been in France with the British troops that had stopped the German drive on Paris and had gone with the Allied army on its advance. They had seen service on all fronts and now considered themselves veteran campaigners.

Colonel Anderson they had met in Berlin just after the Kaiser had declared war upon France. The colonel, lieutenant then, and Major Derevaux, a Frenchman, had taken the boys with them on their flight and the four had later encountered each other in many strange and unexpected places.

Stubbs they also had met while on one of their many missions and had earned the little man's undying grat.i.tude; but he had repaid whatever they had done for him, with interest, more than once.

The boys, in their latest exploit, had been with the Italian army in the Alps. Two of the four friends having fallen into the hands of the enemy, the others had entered the enemy's lines in an effort to effect their escape.

It was a daring adventure, but after a fight and chase, the four had managed to seize the airship in which we now find them and had at last fought their way clear. They had then held a council of war and decided that it was best to head for the Balkans, rather than to run the gauntlet of the Austrian flying craft which kept constant vigil in the direction of the Italian lines.

Hal and Chester, typical American lads, were large and strong for their ages, which were within a year of each other, seventeen and eighteen now.

In the rough lumber camps of the north, the two had had considerable experience in the use of firearms and the art of self-defense--fists.

Also, during the school term each had practiced the use of the sword until, though by no means experts, they could give a fair account of themselves with this weapon--as each had done more than once.

Fortunately, both lads had made a study of languages and spoke French and German fluently. They never had trouble on that score.

The great war up to this point had not gone as successfully as the Entente Allies had hoped in the early days. The German lines on all fronts were seemingly stronger than ever before. Even the entrance of Italy into the war on the side of the Allies had failed to turn the balance, as it had been confidently expected it would. East and west, the German lines held, while in the Balkans the enemy was even now advancing against the heroic little Serbian army, which, before many days, was to be forced to relinquish its country to the iron heel of the invader.

Montenegro, the smallest factor in the war, still was fighting hard--the rugged and gigantic mountaineers giving a good account of themselves upon all sides.

This was the situation, then, as the airship containing Colonel Anderson, British officer, Anthony Stubbs, American war correspondent, and Hal Paine and Chester Crawford sped southward over Austria.

Several hours after the sinking of the Austrian aeroplane Chester spoke.

"Where do you suppose we are now, Hal?" he asked.

"I believe we must have crossed the frontier," replied Hal. "However, we'll wait another half hour before descending to have a look."

The half hour up, Hal sent the airship lower and lower. Soon, a faint gray speck below became visible, a.s.suming larger and larger proportions, until all aboard made out the ground beneath.