The bivouac on the Field of Quatre Bras, during the night of the 16th, continued undisturbed until about an hour before daylight, when a Cavalry Patrol having accidentally got between the adverse Picquets near Piermont, caused an alarm in that quarter that was quickly communicated to both Armies by a rattling fire of musketry, which, rapidly augmenting, extended itself along the line of the Advanced Posts. Among the first who hastened to ascertain the origin and nature of the engagement was PICTON, who, together with other Staff Officers, as they arrived in succession, on discovering that no advance had been attempted or intended on either side, soon succeeded in restoring confidence. Similar exertions were successfully made on the part of the French Officers, and as day began to break upon the scene, both parties resumed their previous tranquillity. In this untoward affair, the Picquets furnished by KIELMANSEGGE's Hanoverian Brigade, and by the 3rd Brunswick Light Battalion were sharply engaged, and a Picquet of the Field Battalion Bremen suffered considerably. It was not long before WELLINGTON, who had slept at Genappe, arrived at Quatre Bras, where he found Major General Sir HUSSEY VIVIAN, whose Brigade of Light Cavalry, consisting of the 10th British Hussars (under Colonel QUENTIN), of the 18th British Hussars (under Lieutenant Colonel the Hon. HENRY MURRAY), and of the 1st Hussars of the King's German Legion (under Lieutenant Colonel VON WISSELL), was posted on the left of that point with two strong Picquets thrown out; one, of the 18th Hussars, under Captain CROKER, on the Namur road, and the other, of the 10th Hussars, under Major the Hon. FREDERICK HOWARD, in front--with a Picquet from the latter, under Lieutenant ARNOLD, on the right of the Namur road. VIVIAN, on being asked what account he could give of the Enemy, communicated to the Duke the result of his observations, which were necessarily very limited, as, with the exception of the firing that had taken place, as before mentioned, along the line of Picquets, the French had continued perfectly quiet, and had as yet given no indication of any offensive movement.