"Climb, Tom! Climb in a hurry!"

Jack Parmly shrilled these words close to the ear of his chum. Really, there was no need of his saying a single word, since the pilot had sensed their immediate danger just as quickly as had Jack himself. Already Tom was pulling the lever that would point the nose of their aerial craft upward toward the stars, and take them to a much loftier elevation.

The experience was very exciting while it lasted, Jack thought. He saw the numerous planes, forming the raiding squadron break formation in great haste, each pilot being eager to dodge the bursting shells and seek an elevation where they could not reach his flimsy craft.

It would take only one accidental shrapnel shell to cause the destruction of the best machine among them, and thus reduce the number of available airmen serving the cause of liberty.

For a brief interval the explosions continued to sound all around them. But presently Jack was enabled to breathe easily again. They had climbed beyond the range of the German guns, no matter how heavily charged; and, besides this, they sped along rapidly, so that the Hun lines were soon left behind.

"Trouble's past. Admiral signaling keep on this level, Tom!" called out the observer.

"Got you, Jack!" came the answer, heard above the rushing noises that "made the welkin ring," as Jack told himself.

The firing ceased as the German gunners realized, to their chagrin doubtless, that again their intended prey had eluded them. They must have set those anti-aircraft quick-firers of theirs in fresh elevated emplacements after the Yankees had taken the measure of their power to do harm; but the trap, if such it was intended to be, had failed to catch a single victim.

"Did they get any of our crowd?" Tom called out, feeling considerable uneasiness as to the result of the bombardment.

"Never touched us," he was immediately assured by the observant Jack. "All the same it was a smart trick, and somebody's bound to be hauled over the coals on account of the blunder."

"Yes," admitted Tom, speaking loud so as to be heard above the roar of the numerous planes around them, "because it might have played hob with the squadron, and even ruined the success of the whole expedition."

After that they relapsed into silence. It was exceedingly difficult to try to keep up any sort of conversation while going at such a furious pace through the upper air currents. Besides, the night was cold at such an elevation, and consequently both boys had their heads well muffled up, making use of hoods with goggles for the purpose. They also wore gloves on their hands, as well as heavy sweaters under their leather-lined coats.

The formation, in a way, reminded Jack of many a flock of wild geese that he had seen flying north or south over Virginia in their spring and autumn migrations. In the lead went the battleplane containing the squadron commander, forming the apex of the triangle, and showing a fiery red eye in the shape of an automobile rear light as a rallying point for all the other machines.

Then the seven other battleplanes sank away from the apex, three on one side and four on the other, that of the Air Service Boys being the one to the rear of all the rest.

Flying two and two abreast, and guarded on both sides by those sturdy fighting craft came the four huge bombers, each heavily laden with the most destructive of explosives. They, too, could show teeth if cornered and compelled to depend on their own defensive powers; for each of them carried a machine gun, of which the observer had been trained to make good use, just as he must know how to drop his bombs successfully when the proper instant arrived.

All seemed quiet just at present, but none of those guiding the aerial racing craft deceived themselves with the belief that this could last long. It went without saying that the Huns must realize the necessity for guarding the important bridge across which their beaten armies were flocking day and night in constantly increasing numbers. Unless the guns could be taken across in safety, they stood to lose many of their best batteries.

Consequently they would be apt to assemble a flotilla of fighting planes in that vicinity, ready to soar aloft and give furious battle to any Allied squadron venturesome enough to make the attempt at destruction.

If the blowing up of the bridge could only be accomplished, the sacrifice of a few planes with their crews might be counted a cheap price to pay for the great benefits reaped.

The minutes passed, and all the while the raiders were drawing nearer and nearer their intended goal. Every pilot and observer in that squadron had been carefully selected with a view to his fitness for the gigantic task that had been laid out for accomplishment.

There would be no hesitation when the eventful moment came, since none was present save those who had been tried in the furnace of battle and found to be fine gold, eighteen carat pure. Such a thing as flinching when the test came was not to be considered; they would carry through their appointed tasks or fall while in the endeavor, paying the price the airman has ever had dangled before his eyes.

Jack was using his night-glass, and he now broke out with a cry.

"We must be getting close to the bridge, Tom! I can see flickering lights darting about, and I believe they must be planes rushing up into the air!"

"Like as not they've been warned of our coming by the row we're making," replied the pilot, in a shout. "Then again those Huns along the line would send word back, for they must know what we're aiming at. It's all the same to us. We came out after action, and we'd be terribly disappointed if we didn't get a lot of it."

Then came signals from the leading plane. Closer formation was the rule from that time forward, since the bombers must be amply protected in order to allow their gunners an opportunity to get to work with those frightful explosives and hurl them at the place where the bridge was supposed to lie.

Both boys began to feel their pulses thrill with eagerness, as well as excitement. Looking down, Jack could detect moving lights, the source of which he could only speculate upon. Then came a flash which must mark the discharge of the first anti-aircraft gun. The enemy was showing exceeding nervousness, for as yet the leading American plane could not be anywhere within range.

With the burst of shrapnel there came a realization that the gunners below were only trying to get their range. The whole pack would break loose in another minute or less; but Jack had reason to believe their altitude was such as to render the fusillade harmless.

Then down below he saw a sudden brilliant flash. That must mark the falling of a flaming bomb, dropped from one of the big planes in order to get a lead on their location. Jack believed he had even glimpsed the bridge itself in that brief interval. How the prospect thrilled him!

Tom, on his part, had little opportunity to observe anything that was taking place earthward. His duty lay closer at hand, for he knew that a swarm of fighting Gothas had started up to engage the attacking squadron, and realized that one or more of these hostile aircraft might suddenly appear close at hand, bent on bringing about their destruction.

Besides, constant vigilance was the price of safety in other particulars. With almost a dozen of their own planes speeding through space, a false move on the part of a careless pilot was apt to bring about a collision that could have only one result.

Jack made a discovery just then that caused him to cry out.

"The signal, Tom! We are to drop down and give the bombers a better chance to get there. No matter what the cost, we've got to reach that bridge to-night!"

Already Tom was changing the course. They had begun to swing lower, each unit of the attacking squadron in its appointed place. A brief interval followed, and then came the bursting shrapnel again around them, while from several quarters close by hovering German planes commenced using their machine guns, to be answered by the challengers in like manner.