The march of the French troops through Bry, in the direction of Quatre Bras, became known to the Prussians through Lieutenant Colonel SOHR, who still held his Cavalry Brigade, even at this time, posted in rear of Tilly. Shortly afterwards, some of the French Cavalry having approached, he began to retire slowly towards Mont St Guibert, and, as he frequently formed up, in wait for the Enemy, he did not reach that point until the evening of the 17th. Here he found Lieutenant Colonel LEDEBUR, who had arrived with his Detachment, and had received Orders to maintain the Defile. * * * * * Upon the departure of NAPOLEON, GROUCHY ordered VANDAMME and GÉRARD to get their Corps under arms, and to move them, in the first instance, to the junction of the Gembloux road with that to Namur; and having subsequently received intelligence that a considerable body of Prussians had passed through the former town, he desired that those two Corps should continue their movement upon that point. In the mean time, he repaired to the Advanced Posts of EXCELMANS' Dragoons, which were by this time beyond Gembloux. It was part of this Cavalry which followed Lieutenant Colonel SOHR, on the left. They merely threw out Skirmishers against him; and, as night set in, they abandoned the pursuit in this direction. The Corps of VANDAMME and GÉRARD did not reach Gembloux until very late in the evening. The former was posted in advance, the latter, in rear, of the town; near which also, and on the right bank of the Ormeau, was stationed the Sixth Light Cavalry Division, under General VALLIN, who succeeded to the command, upon Lieutenant General MAURIN being wounded at the Battle of Ligny. The First Brigade of Lieutenant General CHASTEL's Tenth Cavalry Division, consisting of the 4th and 12th Dragoons, under General BONNEMAIN, was pushed on to Sart à Wallain, and the 15th Dragoons (from General VINCENT's Brigade of the Ninth Cavalry Division, under Lieutenant General Baron SOULT), were detached to Perwès. From both these points, reports were sent into Gembloux that the Prussians had retired upon Wavre. PAJOL, with his Light Cavalry and TESTE's Infantry Division, had returned from St Denis, between Namur and Gembloux, to the original position occupied by the latter in the morning, at Mazy, in the immediate vicinity of the Field of Ligny; a movement for which no satisfactory cause has ever been assigned. The extent of information obtained by GROUCHY concerning the Prussian retreat, and the nature of the dispositions which he adopted in consequence, will be best explained by the following despatch which he addressed to the Emperor:-- "Gembloux, le 17 Juin, "Gembloux, 17th June, à dix heures du soir. at ten o'clock in the evening.