During the day French and British aviators were engaged in many successful combats in the air. The British downed thirty German aeroplanes, twelve of which were known to have been destroyed. Twelve British machines were missing, two having collided during the fighting and fallen within the enemy lines. On this day, and during the night of August 17-18, 1917, French aviators shot down seven German airplanes and a captive balloon and forced down eight other machines, which, badly damaged, fell in the German lines. A bombing raid in which 111 French machines took part in various sorties dropped 13,000 kilograms (28,600 pounds) of projectiles on aviation grounds at Colmar, Friedrichshafen, and Habsheim. Aviation camps and railway stations at other points were showered with projectiles, producing fires and explosions. British troops on the front northeast of Ypres made a further advance in an attack made in the morning of August 18, 1917, in the vicinity of the Ypres-Poelcappelle road east of Langemarck. By this operation the British advanced their lines 500 yards on a like front, winning all their objectives, including a series of strongly fortified farms. Southeast of Epihy a successful local operation was carried out by the British troops in the early morning of the following day, when German trenches in the neighborhood of Guillemont Farm were captured and a considerable number of prisoners. It was in this the fourth year of the war that the Germans made some radical changes in their methods of defense, owing principally to the preponderance of British artillery, which reduced their front-line trenches to mere furrows of earth and made mantraps of the carefully constructed dugouts. The Germans now scattered their advanced forces over a greater depth. There was no longer an unbroken line of defenses for the British guns to shatter, but strongholds were constructed in isolated shell holes along the front, cunningly concealed from aviators. These stretched back from the first lines to a considerable depth. Along the front strong outposts were established at some distance apart, backed by fortified craters and connected by tunnels and often with dugouts. Back of these shell-hole nests were gun emplacements commanding the openings between the shell holes. Thus when enemy attackers had forced their way through the fortified shell craters they were met with torrents of machine-gun fire. Further back from these defenses there would be found a line of more or less connected trenches, or a series of connected fortified shell holes. The Germans also constructed strong concrete redoubts in every farm house for their machine guns. They built small forts of steel and concrete that were impervious to artillery fire. Many of these strongholds were constructed underground with a steel trapdoor as the only exit, by which the Germans came out to set up their machine guns. CHAPTER II THE FRENCH BREAK THE GERMAN LINES AT VERDUN--CANADIANS GAIN AT LENS