During the intervals between the infantry attacks the French troops were subjected to an unprecedented artillery fire. Suffering under a strain such as armies had never hitherto known, the French patriots yet held true to their watchword,--"They shall not pass." General Pétain (pā-tăn´), in a stirring address, said to his entrenched heroes, "Courage, we'll get them!" ("_Courage, on les aura!_"), and this phrase became the Verdun battle-cry. Try as the Germans would, from every possible point, they could not break through the living wall of Frenchmen. A little ground was won here and there, but before the end of the year nearly all had been retaken by the French. At a frightful cost the German crown prince and his military advisers had put their fighting machine to the test, and it had failed. A half million men, killed, wounded, or prisoners, were lost to the Germans before they ceased their attacks at this point. THE BATTLE OF THE SOMME.--In July, 1916, while the Verdun struggle was still undetermined, the French and British troops began an advance on the German line along the river Somme (som). Exceedingly heavy artillery attacks first battered down the enemy defenses, and then the infantry went "over the top." During the long course of the Battle of the Somme (July 1 to November 17) the Allies advanced on a front of twenty miles to a maximum depth of about nine miles. Slowly, and at great expense of ammunition and men on both sides, the Allied progress had been won. They had failed to break through the German line, but they had shown how it might gradually be pushed back. And they had relieved the important position of Verdun from further severe attacks, because German forces were needed to the westward. In the course of this battle, on September 15, the British first used their most original military machines--the "tanks." Thereafter these armored cruisers of the land were to play an increasingly important part along the western front. INCREASED USE OF AIRCRAFT.--Aircraft, too, were every day becoming more valuable. In the first year of the war airplanes were used mainly for observation purposes: to find the location of enemy forts, trenches, troops, and batteries; and to direct the fire of the aviator's own batteries. Hundreds of photographs were taken by the airmen, rapidly developed, and within thirty minutes the staff officers could be seen studying them with microscopes to determine what changes had taken place within the enemy's lines. Anchored balloons, too, were used for similar purposes.