V. THE CURTAIN OF FIRE 50 VI. A SHELLED BATTLEFIELD 60 VII. A LIVELY CAMP BEHIND THE LINES 70 VIII. THE SPIES IN THE CAMP 82 IX. THE LAWS AGAINST SPIES 93 X. A DIFFICULT TRIP TO THE MAIN TRENCHES 104 XI. DISCOVERING A GERMAN RANGE-FINDER 116 XII. FINDING THE ENEMY'S BATTERY 128 XIII. THE MYSTERIOUS FIGURES ON THE RANGE-FINDER 140 XIV. CAUGHT IN A TERRIFIC DRIVE 152 LIST OF ILLUSTRATIONS _"At them, boys!" shrieked the Corporal_ _Frontispiece_ PAGE _Method of Signaling from Airplanes_ 53 _Peculiarities of Trajectories_ 56 _Peculiarities of Trajectories_ 57 _The Deadly Shrapnel Shell_ 68 _The Spy's Account Book_ 91 _Pontooning Heavy Guns Across a Stream_ 101 _A German Range-Finder_ 118 _Arrangement of Guns on Hill 203_ 138 THE BOY VOLUNTEERS WITH THE BRITISH ARTILLERY CHAPTER I THE DECISION "It seemed to me as though I should never have the courage to go back to the airplane service since Lieutenant Guyon was killed," remarked Ralph, as he and Alfred were convalescing in the American Hospital, in Paris. "That is the way I feel about it, too," replied Alfred. "To think that he should have escaped the terrific shower of bullets, while we were coming down, to be killed by having the machine hit the ground, the way it did, makes me feel so sad that I sometimes wonder whether it is really so." "I suppose the only thing we can do now is to go home; and, still, that doesn't seem to be the right thing, just now," replied Ralph. "No; I am not in favor of that; suppose we go to England,—anywhere, or anything except that which will remind us of poor Guyon," answered Alfred, as he sat in the huge chair and slowly nodded his head. * * * * * At the outbreak of the war Alfred and Ralph were on the way from southern Germany to Antwerp in an auto, accompanied by a Belgian chauffeur, where they were pursued by the Germans near the frontier. They escaped for a time, but were afterwards arrested by the Germans and finally liberated. On their way to Antwerp they took part with the Belgians in resisting the advances of the foe. Reaching Antwerp, they escaped with the Belgian army, at the time the city was besieged, and after some adventures, crossed the northern part of Belgium and reached Dunkirk on the Channel. From that point, in the endeavor to reach Paris, they had some stirring exploits, which tested their metal on many occasions.