Up hill and down dale they raced, through thriving cities, and quaint villages, past peasant cottages and princely chateaux, lying beautiful and serene in the bright sunshine. They were in the garden spot of France, a place that had yet been spared the horror and devastation of war, and the only thing that seemed unnatural was the striking absence of young men. Women everywhere were doing the work, in the fields, in the stores, at the railroad stations, on the streets and country roads. Scarcely any males were seen except old men and boys. There was no need to ask where the young men were. At Verdun, on the Somme, on the Aisne; everywhere on that long line of trenches that stretched from the Vosges to the sea, they were fighting like heroes to keep the Hun at bay. And on the heart of each were written those immortal words spoken at Verdun: "_They shall not pass!_" Hour after hour went by. Suddenly Frank asked: "What was that, fellows? Did you hear it?" "Sounded to me like thunder," said Bart. "With a sky like this?" replied Frank. "Never. Listen!" Borne on the wind came a long, booming sound, growing longer and louder as they sped toward it, falling fitfully at times, only to swell into a mightier rumble like the roar of waves dashing against the coast. They looked at each other with comprehension dawning in their eyes. "It's thunder all right, Bart," said Frank, quietly. "It's the thunder of the guns! We are getting near the fighting front at last!" CHAPTER XVI THE AIRSHIP RAID The signs multiplied now that they were approaching the battle lines. Apart from the ominous roar which had now become unceasing, war showed its grim face on every hand. They dashed through "rest billets"--the towns behind the lines where the exhausted soldiers, who had served their term in the trenches, were sent back for a few days or weeks of rest while fresh troops took their places. The roads became more congested with trucks carrying supplies and ammunition to the front. Ambulances came past in an endless stream, bearing their quota of wounded men. Hospitals were everywhere, marked with a Red Cross that bespoke their mission of mercy and healing. And there were cemeteries too, with their endless rows of simple wooden crosses on which were inscribed the names and regimental numbers of those who slept beneath. Cripples, too, there were, with missing legs or arms, and blinded ones, who had looked for the last time on scenes of warfare. It was the seamy side of war that thrust itself upon their sight. But though it sobered, it did not daunt these eager young Americans who had come to do their part and "see it through." It only deepened their indignation at the merciless military power that had brought such woe and misery upon the world, and each breathed a vow to himself that he would not rest until that power was curbed and punished as it deserved to be. "This looks like the real thing," remarked Frank.