The gallant Leader of the "Union Brigade," when endeavouring to return to the Allied position, after using the most strenuous but fruitless exertions to restrain his men in their wild pursuit, and to withdraw them from a contest in which they had already gained undying fame, became a sacrifice to his chivalrous and patriotic zeal. Intercepted by a party of the Lancers in the soft ground of a newly ploughed field, out of which his exhausted steed had not the power to extricate itself, he fell beneath their deadly thrusts. Sir WILLIAM PONSONBY had highly distinguished himself as a Cavalry Officer in Spain; and, independently of his merits as a soldier, which were justly appreciated by the whole Service, his amiable disposition and private virtues endeared him to all his brother Officers. His equally gallant namesake, Colonel the Hon. FREDERICK PONSONBY, immediately after his brilliant charge with the 12th Light Dragoons, first through a Column of Infantry, and then upon the Right Flank of the Lancers, was endeavouring to withdraw his Regiment from further pursuit, when he was disabled in both arms, and carried by his horse up to the crest of the French position; where, receiving a sabre cut, he was struck senseless to the ground; and it was very generally supposed at the time that he had been left dead on the field. Lieutenant Colonel HAY, who commanded the 16th Light Dragoons, was desperately and dangerously wounded. Colonel HAMILTON, the Commanding Officer of the Scots Greys, after gallantly leading his Regiment through the Enemy's Columns, across the valley, and up the opposite Heights, was last seen far in advance; where it is presumed, from his never having again appeared, he fell in the midst of the French lines, a sacrifice to his distinguished but indiscreet valour. Colonel FULLER, who commanded the 1st, or King's, Dragoon Guards, was killed when pursuing the _Cuirassiers_: he boldly led his Regiment up the French Height immediately upon the Allied Left of the Charleroi road. In addition to the above mentioned, the British Cavalry engaged in this affair sustained a very heavy loss in both Officers and men. With the exception of the bodies of the slain; of such of the wounded as were too far from their respective Lines to be removed; of loose horses, some wildly careering about, others quietly grazing, and many staggering, plunging, or convulsively pawing the ground around them, from the agony of their wounds; the arena of this terrible conflict, which had ceased but a few minutes before, was now perfectly clear. The retiring crowds of French Infantry had disappeared behind the foremost ridge of their position, to collect and reform their scattered remnants.