Upon hearing the words uttered by the mechanic who handled the men working at their battleplane, Tom and his chum exchanged meaning looks.

"Can you make it perfectly safe again before half an hour passes?" asked the former anxiously.

"Surely," came the confident reply. "I know what's in the wind, and you'll be fit for any sort of flight when another fifteen minutes has gone by. We're on the last stay now, and I've carefully examined the motor and every other thing about the plane. Don't fear to risk your lives on my report. I'd go up myself willingly if I had the chance."

"All right, Sessions, we're willing to take your word for it," Tom assured him, and then drew his comrade aside.

Jack on his part was eager for a little talk between themselves. That staggering fact had appalled, as well as angered, him. Why should their particular plane have been selected for such treacherous work, among all the scores connected with the air service in that sector of the fighting front?

"What do you make of this thing, Tom?" he immediately demanded.

"It's an ugly bit of business, I should say," came the guarded reply.

"You mean calculated to make every one feel timid about taking any extraordinary risk—is that it?" continued Jack.

"Yes, if the fact were generally circulated. But according to my mind they'll keep it quiet until after the armada gets off. No use alarming the others, though orders have gone out I presume to have every plane carefully examined. Still, that would only be ordinary caution; we never go up without doing such a thing."

"Tom, do you think there could be any possible connection between this work of a German spy, as it appears on the surface, and my news from Mr. Smedley, the lawyer?"

"It's possible—even probable, Jack. A whole lot depends on whether we learn of any other plane having been meddled with. One thing sure, it'll spur them to greater vigilance about watching things here. This isn't the first time there's been a suspicion of rank treachery. Planes have been known to be meddled with before now."

"I wouldn't put it past him!" muttered Jack sullenly.

"Meaning your cousin Randolph, I suppose," Tom added. "Nice opinion to have of a near relative, I must say. But then I'm inclined to agree with you. It may be only a queer coincidence, your getting such important news this afternoon, and some unknown party trying to bring about our downfall and death in this brazen way only a few hours afterwards."

"And using corrosive acid, too," spluttered the indignant Jack. "I've heard of ropes being partly cut, even wire stays or struts filed to weaken them; but this is the limit. Don't I wish they'd caught the skunk in the act!"

"He'd never have left this aviation camp alive," said Tom sternly. "Why, the boys would be so furious they'd be tempted to lynch him offhand."

"And I'd be glad to help pull the rope!" snapped Jack. "A more cowardly act couldn't be imagined than this. Air pilots take great enough chances, without being betrayed by spies or traitors."

"We'd better say nothing about it," Tom concluded. "I'm going to run over the entire machine on my own account."

"And I'll do the same, Tom; for a pilot can't be too sure of his mount, especially when there's such meanness afoot."

They accordingly busied themselves after their individual fashion. Every brace and stay was looked over carefully and tested as only pilots know how. Long experience, and many accidents have taught them where the weak spots lie, and they understand how to guard against the giving way at these points.

So the minutes passed. Other pilots had already ascended to await the assembling of the picked squadron at some given altitude. Every minute or two could be heard the rush of some unit starting forth. There were few of the accompaniments of an ordinary ascent, for all loud cries had been banned.

"All ready!" came the welcome words at last.

The last strut had been carefully gone over, and now everything was pronounced in perfect condition. At the same time, after such a discovery had been made, it was only natural for the boys to feel a queer tug in the region of their hearts as they climbed to their seats, and with hands that quivered a little proceeded to make fast the safety belts.

"There goes another bomber, which makes four—the full number you spoke of, Tom," remarked Jack. "I suppose we're holding up the procession more or less, worse luck, when usually we can be found in the lead."

"The commander must know about our mishap," replied Tom, "and isn't apt to blame us for any little delay. The night's still young, and we can reach our destination in half an hour, with time to spare. So cheer up, old comrade; everything's lovely and the goose hangs high. Now we're off!"

With that he gave the word, and paid attention to his motor, which started a merry hum. The propellers commenced to spin, and down the slight slope they ran with constantly increasing speed. All around them could be heard the refrain of planes in action; from above came similar sounds, and Jack, looking up, discovered dim scurrying forms of mysterious shape that flitted across the star-decked sky like giant bats.

Now they, too, were rising swiftly in spirals. Both kept a keen watch, for it was at this time they stood the greatest chance of taking part in an unfortunate collision that might result in a fatal disaster.

But every pilot was on edge, and careful to avoid any such blunder. They had been well drilled in all the maneuvers connected with just such a hurried ascent in numbers. Each plane had its regular orbit of action, and must not overstep the bounds on penalty of the commander's displeasure.

After mounting to the arranged height, the Air Service Boys found that it was a very animated region, though fully a thousand feet from the earth's surface. Almost a dozen planes in all were moving in a great circle, their motors lazily droning, and the pilots ready to enter into squadron formation on signal.

In fact, Tom and his chum were the last to arrive, which under the circumstances was not to be wondered at.

"All on deck, I reckon," called out Jack, after he had taken a survey about him. "There's the signal from the flagship, Tom. We've got to keep the red lantern ahead of us and fall into line. There go the bombers to the center, and our place you said was on the left, tailing the whole bunch."

Like a well disciplined aerial navy they fell into place, each taking its position as previously arranged. When the formation was made complete another signal was given. This meant the advance was now to begin, and the crossing of the German lines undertaken.

Unless there chanced to be some mistake made concerning the proper altitude required, so as to clear all possible bombardment when over the Hun lines, this might be accomplished without danger. So far as was known, they had gauged the utmost capacity for reaching them possessed by the German anti-aircraft guns, and Jack promised himself to jeer at the futile efforts of these gunners to explode their shrapnel shells close to the speeding armada.

Something must have been underrated, however; and, in fact, few plans can be regarded as absolutely perfect. The advancing raiders were passing over the enemy front when a furious bombardment suddenly burst forth below.

Jack could see the spiteful flashes of the numerous guns, and while the sound of the discharges came but faintly to his ears, to his consternation, all around them, as well as above and below, came sharp crackling noises, accompanied by bursts of dazzling light.

They were actually in the midst of a storm of bursting projectiles and in immediate peril of having some damage done to their swift-flying planes such as would spell ruin to the enterprise, perhaps bring instant death to some of the fliers!