"Understand," said the captain, "you will be asked to land not only in the dark but behind the enemy lines, not knowing who or what is below."
"We understand," said Hal quietly.
"I have come to offer you this opportunity," said Captain O'Neill quietly. "Tonight—the exact time is 10 o'clock—we attack in force. In comparison, the assaults before this have been as nothing. I say we, but I mean chiefly, of course, the French. There will be some American troops in the advance, however. The mission I am now offering you was turned over to us by the French general staff."
"We shall be glad of the opportunity to aid, sir," said Hal.
"Good!" said Captain O'Neill, and continued: "One element alone is uncertain; one only is to be ascertained. The force and disposition of the defending troops in shell holes, in their concrete 'pill-boxes,' in their flanking trenches all have been ascertained. They will be blasted out by our artillery. But they have additional forces below the ground, in great caverns too far down to be reached by our shells; they are tremendous underground works concealing whole battalions, many thousands of men, whose presence is known; but the entrances and the means of egress from those great caverns have so far eluded us.
"We have discovered some of these entrances," he continued, "but immediately they have changed. At present we do not know them. But at 10 o'clock tonight the points from which the German reserves will emerge must be instantly and accurately marked. When our infantry goes over the top and the Germans order their shock troops out from the safe underground refuges to meet our men, we must know the points where the enemy battalions are coming up. Some of these points will be cared for by French already in position to inform us. I offer to you the opportunity of marking others of those points."
"We shall be glad," said Hal simply.
"Very well. You understand, of course, that you will be killed if discovered. Both of you come with me."
He arose, and Hal and Chester followed the captain to his motor-car, which they entered and drove to the main road, over which German prisoners captured early in the day were still streaming to the rear. Overhead a few aeroplanes still buzzed—combat and fire control and staff "observation" machines seeking out their aerodromes in the dark. It grew dark so quickly now that Hal, looking up, saw the colored flash of the signal lights from a pilot's pistol; they burned an instant red and blue and red again as they dropped through the air; and, in response to the signal, greenish white flares gleamed from the ground to the right, outlining the aviation field; then the flying machine, which had signaled, began to come down.
From far beyond the drum fire of artillery rumbled and rattled.