Before the Germans had time to recover from the surprise of Marshal Foch's attack on the Marne, and while they were still retreating to the Vesle, the Allies delivered another heavy blow, this time on the Albert-Montdidier front in Picardy. Here the British and French suddenly attacked in force on the morning of August 8, stormed the enemy positions along a thirty-mile front and on the first day of the attack penetrated to a depth of seven miles. For several days the enemy retreated, closely pursued by allied cavalry and tanks, which for the first time fought in a combination that proved irresistible. The tanks used were of a new small variety, known as "whippets," which rapidly wiped out the machine-gun nests with which the enemy sought to stem the tide of the victorious onrush. Some American troops fought with the British in their advance and gained high praise from the Allied commanders. By August 15 the total number of prisoners captured by the British Fourth Army, under General Rawlinson, was 21,844. In the same period of one week the prisoners taken by the French First Army amounted to 8,500, making a total of 30,344 Germans captured in the operations of the Allied armies on the Montdidier-Albert front, besides 700 heavy guns, quantities of machine guns, and other important spoils of war. North of the Somme, between Albert and Arras, the Germans continued to fall back to the old Hindenburg line, where there were strong defensive positions, with the British and French keeping in close touch with their retreat. On August 15 they had definitely given up the towns of Beaumont-Hamel, Serre, Bucquoy, and Puisieux-au-Mont, and at several points had crossed the Ancre river. Field Marshal Haig announced that the proportion of German losses to those of the Allies in the Picardy offensive were greater than at any other period of the war. The total Allied casualties were not as large as the number of Germans taken prisoner. JOY IN AMIENS AND PARIS One important result of the British drive was that Amiens, the "dead city of Picardy," began to come to life again. Its population of 150,000, including 40,000 refugees, had fled before the German offensive in March, 1918, but the former inhabitants began to return when the menace of the invader disappeared, as the invader himself was chased back toward the Somme. A service of thanks to the Allied arms was held in the Great Cathedral of Notre Dame in Amiens, August 15. Despite the damage from German guns and bombs, the cathedral retained the title of the most beautiful in all France.