were working their anti-aircraft guns to their capacity, but by screens of smoke clouds, which were emitted by a special apparatus to hide the big guns. At the same time other cannons were being fired to disguise the sound from the immense long-range weapon, but this was of little effect, now that the location had been discovered.
Meanwhile a score or more of the Hun planes appeared in the air. They had taken flight as soon as their pilots saw the squadron of enemy machines approaching, and were eager, this time, to give battle.
"Our work's being cut out for us," murmured Tom, as he steered his machine to engage a German who seemed eager for the fray. Tom sent a spray of bullets at his enemy, and was fired at in turn. He knew his craft had been hit several times, but he did not think it was seriously damaged.
Jack, too, as he could tell by a quick glance, was also engaged with a German, but Tom had no time then to bestow on mere observation. His antagonist was a desperate Hun, bent on the utter destruction of Tom's machine. They came to closer quarters.
Down below the fighting was growing more furious. It was in the form of an artillery duel. For now the French observation machines were
