The din soon became general, one after another of the American planes joining in the battle. The German aircraft held off a little, fighting from afar, evidently thinking to accomplish their ends without taking too much risk. Had they boldly assaulted, doubtless the result would have been much more disastrous to both sides.

The big bombers had but one object in view, which was to bomb the important target below. To drop an explosive on a certain spot had been the most important training of those aboard these craft. They had been carefully selected from the ranks of the many observers taking service in the aviation branch of the service; and great things were expected of them now.

The Huns had concentrated the glare of numerous searchlights on the hub of the squadron's activities, so that the speeding planes could be seen darting hither and thither like bats during an August evening, darting around some arc-light in the street.

The flash of the distant guns aboard the planes looked like faint fire-flies in action. No longer was the earth wrapped in darkness, for flares dropped by the bombers kept continually on fire. The bridge stood plainly out, and a keen eye, even without the aid of glasses, could distinguish the rush of terrorized German troopers trying to get clear of the danger zone before a well directed bomb struck home.

Jack, leaning from his seat, took all this in. He was keyed to the top-notch by what he saw and heard. Tame indeed did most other incidents of the past appear when compared with this most stupendous event.

"Wow!" burst from his lips, as a sudden brilliant flash below told that the first huge bomb had struck; but with all that racket going on around of course no ordinary human voice could have been heard.

He could see that it had not been a successful attempt, for the bomb struck the ground at some little distance away from the terminus of the structure spanning the river. However, it did considerable damage where it fell, and created no end of alarm among those who were near by.

As yet the Air Service Boys had not been engaged with any of the hostile planes, though most of the other Yankee pilots seemed to be having their hands full in meeting and repelling fierce attacks.

Both kept in readiness for work should their turn come, Tom manipulating the plane, and Jack working the rapid-fire gun which he had learned to handle so cleverly.

Strangely enough, Jack, as he looked, was reminded of a vast circus which he had once attended, and where tumblers, athletes, and trained animals were all performing in three rings at the same time. He had found it utterly impossible to watch everything that went on, and remembered complaining lustily afterwards in consequence.

Now there were some eleven rings in all, besides what was taking place thousands of feet below, where the bombs had started to burst, tearing great gaps in the ground close to the bridge, and causing the water itself to gush upward like spouting geysers.

Lower still dropped the venturesome pilots guiding the destinies of the four huge bombers. What chances they were taking, bent only on succeeding in the important task to which they had been assigned!

Jack knew he would never forget that dreadful crisis, no matter if he were allowed to live to the age of Methuselah; such an impression did it make upon his mind.

But their turn came at length, for in the dim light two big Gothas were discovered swinging in toward them as though bent on bringing about the destruction of the Yankee battleplane.

Jack forgot about what was taking place below, since all of his energies must now be directed toward beating off this double attack. It had come to the point of self-preservation. The Hun airmen were playing a prearranged game of hunting in couples. While one made a feint at attacking, the other expected to take advantage of an exposure and inflict a fatal blow that would send the American aeroplane whirling to death.

Jack saw when the nearest plane opened fire. The spitting flame told him this, for it darted out like the fiery tongue of a serpent. He also realized that the bullets were cutting through space all around them; and a splinter striking his arm announced the fuselage of the plane had already been struck, showing the gunner had their range.

Then Jack began work on his own account, not meaning to let the fight become one-sided. His duty was to pepper any of the enemy craft that came within range, regardless of consequences. To Tom must be left the entire running of the plane motor, as well as the maneuvering that would form a part of the affray.

Heedless of what was taking place around them, the two chums devoted their attention to the task of baffling the designs of their two foes. Wonderfully well did Tom manage his aerial steed. They swung this way and that, dipped, rose, and cut corners in a dizzying fashion in the endeavor to confuse the aim of the Hun marksmen.

Once Jack experienced a sudden sinking in the region of his heart. There was a strange movement to the plane that made him fear the motor had been struck. He also missed the cheery hum at the same time, and felt a sickening sensation of falling.

But immediately he realized that Tom was only executing his pet drop, the nose-dive. One of the Huns followed them down, just as a hawk-might pursue its prey. When the American plane came out of the dive at the new level Jack saw that the Hun was closer than ever, and once again starting to bombard them.

At least they now had only a single adversary to deal with, which could be reckoned a point gained. Most of the fighting was going on above them, but Jack believed the bombers must be somewhere near by, possibly at a still lower level.

Again the maneuvering, or jockeying, for position commenced. In this air duel the pilot who knew his business best was going to come out ahead. It might be they were opposed by some celebrated German ace with a long list of victories to his credit, which would render their chances smaller.

Tom, however, seemed to be keeping up his end wonderfully well. The hissing missiles cut through the canvas of their wings, beat upon the side of the fuselage, and even nipped the Air Service Boys more than once as they stormed past. Neither of the boys knew whether they were seriously wounded or not; all they could do was to fight on and on, until something definite had been achieved on one side or the other.

Once Jack felt something blinding him, and putting up a hand discovered that it was wet; yet he was not conscious of having been struck in the head by a passing bullet. Dashing his sleeve across his eyes he shut his jaws still tighter together, and continued to play his gun as the opportunity arose.

They were coming to closer quarters, and the issue of the battle, however dreadful the result, could not be much longer delayed, Jack knew.

Then it happened, coming like a flash of lightning from the storm cloud!