that hammering hour after hour. It might be the German High Command suspected that the Americans were ready to make a more stupendous push than had as yet been undertaken, with the idea of capturing a whole division, or possibly two, before they could get away; and this bombardment was continued in hopes of discouraging them.

The two Air Service Boys did not bother themselves about this, being content to leave all such matters to those in command. They had their orders and expected to obey them to the letter, which was quite enough for them.

Once more in their dugout, Tom and his comrade crawled into their limited sleeping quarters simply to rest, neither of them meaning to try to forget themselves in slumber.

When the time came for action they were soon crawling out of the hole in the ground. As pilots came and went unnoticed, each intent on his individual work, their departure caused not the faintest ripple. In fact, there were two other airmen who also came out and joined them when making for the place of the temporary canvas hangars, they, too, having had secret orders concerning this same night raid.

Arriving on the open field, they found a busy scene awaiting them. Here were mechanics by the score getting planes ready for ascension. The hum of motors and the buzz of propellers being tuned up could be heard in many quarters.

Those sounds always thrilled the hearts of the two boys; it seemed to challenge them to renewed efforts to accomplish great things in their chosen profession. When, however, they reached their own hangar and found a knot of mechanics working furiously, Tom's suspicions instantly arose.

"What's wrong here?" he asked the man who was in charge of the gang.

"There's been some sort of ugly business going on, I'm afraid," came the reply; "for we're replacing several wire stays that look as if they'd been partly eaten by a corrosive acid. Smacks of rank treachery, Sergeant."

CHAPTER V

THE AIR RAIDERS

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