The Post at Abbaye d'Alnes, occupied by the 3rd Battalion of the 1st Westphalian Landwehr, under the temporary command of Captain GROLLMANN, also fell into the hands of the French, between eight and nine o'clock. As soon as the Commander of the First Prussian Brigade--General STEINMETZ--was made acquainted with the attack upon his most Advanced Posts along the Sambre, he despatched an Officer of his Staff--Major ARNAULD--to the Dutch-Belgian General VAN MERLEN at St Symphorien, situated on the road between Binche and Mons, to make him fully acquainted with what had taken place, and with the fact that his Brigade was falling back into position. On his way, Major ARNAULD directed Major ENGELHARDT, who commanded the Outposts on the right, to lose not a moment in withdrawing the chain of Picquets; and on arriving at Binche, he spread the alarm that the French had attacked, and that the Left of the Brigade was warmly engaged, which rendered it necessary that the Right should retire with the utmost expedition. Until this Officer's arrival, the Prussian troops in this quarter were wholly ignorant of the attack; the state of the atmosphere, to which allusion has already been made, having prevented their hearing the slightest sound of any firing. They had a much greater extent of ground to pass over in retreat than the rest of the Brigade, and yet, by the above unfortunate circumstance, they were the last to retire. ZIETEN, having ascertained, about eight o'clock, that the whole French Army appeared to be in motion, and that the direction of the advance of its Columns seemed to indicate the probability of Charleroi and its vicinity being the main object of the attack, sent out the necessary Orders to his Brigades. The First was to retire by Courcelles to the position in rear of Gosselies; the Second was to defend the three Bridges over the Sambre, at Marchienne au Pont, Charleroi, and Châtelet, for a time sufficient to enable the First Brigade to effect its retreat towards Gosselies, and thus to prevent its being cut off by the Enemy, after which it was to retire behind Gilly; the Third and Fourth Brigades, as also the Reserve Cavalry and Artillery, were to concentrate as rapidly as possible, and to take up a position in rear of Fleurus.