"What in the world is the meaning of this?" Frank asked of Jack. "Don't know," returned the lad, "but I guess I'd better join in." He drew his revolver and fired several shots in the air. "Seems to be expected of us," he said. "We don't want to disappoint them." The German aircraft now headed straight for the city of Calais. Frank sent his machine speeding in the same direction. Then, just as it appeared they would fly directly above the city, the first German craft began to descend. The others did likewise and a moment or so later they all came to earth in the center of what Frank and Jack could see was a small army camp; and as they alighted from their machines, the lads saw that it was an Allied camp and not a German. "Must be Calais," said Frank to Jack in a whisper. "Have we been mistaken? Are these French and British machines?" "Well, it looks like it," returned Jack. "We'll keep quiet and let the other fellows do the talking." A French officer now approached the pilot of the first aircraft. "We heard the firing aloft a moment ago," he said. "Did you encounter the enemy?" "We were pursued all the way from the German lines," was the reply. "Anyone hit?" "I think not, though I believe we accounted for one or two of the enemy." "Good. Will you fly again tonight?" "Yes; but not before midnight." The French officer withdrew. At this one of the aviators raised a hand and the others gathered about him, Frank and Jack with them. All wore khaki clothing and their features were concealed by heavy goggles. "Careful," whispered the aviator. "A false move and we are discovered. Spread out now and see what you can learn. Gather here at midnight." He waved a hand and the Germans, for such Jack and Frank now knew them to be, separated. When the two lads were alone a moment later, Jack said: "Well, this is what I call a piece of nervy business. What shall we do? Inform the French commander immediately?" "No. I have a better plan that that. They can hardly work any mischief tonight. What information they learn will avail them naught for we can warn the French commander later. We must find out what they are up to. We'll stick close and follow them back to the German lines, if necessary." "Good, then! Guess we had better do a little skirmishing about. It will keep suspicion from us should we be watched." "All right," said Frank. "Come on." CHAPTER VIII A STARTLING DISCOVERY With the coming of midnight Frank and Jack returned to the spot where the aeroplanes had been parked. Several of the German aviators already had returned. The man who appeared to be the leader announced that they would await the arrival of the others before taking to the air. The others arrived one at a time until all were present but two. The machines were in readiness to ascend the moment the missing men arrived. The aviators were at their posts.