It was during the fighting at this time that the Germans introduced a curious device which they employed when withdrawing their batteries, and which would cause the opponents to believe that their guns were still in action. This was a mechanism with capsules filled with explosives which they placed on the site of the battery that had been, or was about to be, withdrawn. These capsules exploded at intervals of about half a minute, and heard at some distance would be mistaken for the reports of a field gun. Even an old campaigner would have been deceived by this device and led to believe that he was really facing artillery. By employing the mechanism the Germans were able to get their guns away unknown by their opponents, and it also prevented untimely attacks. At an early hour in the morning of August 10, 1917, General Haig's troops by a dashing drive penetrated the German lines to a depth of several hundred yards, carrying completely the village of Westhoek and the remaining positions held by the enemy on Westhoek Ridge. Haig's successful stroke was delivered on a front of nearly two miles south of the Ypres-Roulers railway. Every forward position held by the Germans east of the town of Hooge on the fighting front between Frezenberg and the Ypres-Menin road was won. This section had been the scene of some of the fiercest fighting on the first day of the great Flanders battle. The terrain offered many obstacles in the way of attack. The wooded sections had been strongly fortified by the Germans, and south of Westhoek the ground was broken by marshes that made military operations difficult. Such was the spirit of the British troops on the morning of the 10th that every objective was won in a short time despite the frenzied efforts of the Germans to defend their positions. There was very heavy fighting in Glencorse Wood, where the British established themselves after inflicting heavy casualties on the Germans and taking 240 prisoners. The British airmen during the day hovered over the scene of battle and engaged the enemy machines whenever they appeared. In every aerial engagement the British were victorious. Five German machines were destroyed and five others driven out of control. In addition two German observation balloons were brought down in flames and four others sent to earth badly damaged. Late in the day of August 10, 1917, the German troops made no less than six desperate attacks on the British position on Westhoek Ridge, but in each offensive were driven off, their ranks shattered, and with heavy losses. The last futile attack was made at 10 o'clock at night, and after it failed the Germans began a furious bombardment of the sector above the village of Westhoek, which was continued throughout the night.