Upon a first consideration, we are strongly impressed by the striking contrast between this march, in pursuit of a defeated Army, which had commenced its retreat on the previous night, and which presented no check to the advance, and the march of NAPOLEON from the same Field, by Quatre Bras and Genappe, as far as La Belle Alliance, in front of the Waterloo position, a distance of about sixteen or seventeen miles; and this, too, in rear of a victorious Army, with a Cavalry Rear Guard boldly and successfully impeding the advance of its pursuers. It must, however, be taken into account, that in one most important respect NAPOLEON possessed a decided advantage over GROUCHY--an advantage, the magnitude of which increased with every moment that elapsed after the wet weather had set in; for while the former moved the whole distance along a paved high road, the latter had to proceed entirely by cross roads, which may more properly be designated common field roads. It is to this particular point that both GROUCHY and GÉRARD refer in justification of the late arrival of the Infantry at Gembloux. Nevertheless, GROUCHY detached considerably to his Right, with his Cavalry, misled by the same idea which had prevailed with NAPOLEON, that BLÜCHER had retreated upon the Meuse; and the very circumstance of his Dragoons having reached Perwès on the night of the 17th, proves that had he organised a more extended, more combined, and more energetic reconnaissance, with the sixty five Squadrons of Cavalry which had been placed under his orders, he might have connected his operations on the right of the Dyle with those of NAPOLEON on the left of that river, by occupying the line of Nil St Vincent, Corbaix, Mont St Guibert, and the Bridge of Moustier. The only check he would have experienced would have been at the Prussian Post of Mont St Guibert; which, however, in the case of an active reconnaissance, as above, might have been attacked by a strong Detachment in front, and turned by Corbaix on its left. To show the connection which this disposition would have secured with NAPOLEON's movements, it is only necessary to state, that the Third Cavalry Division, under Lieutenant General DOMON, had been detached from the Emperor's Column to reconnoitre the country between the Dyle and the high road to Brussels; and that the 4th Regiment of _Chasseurs à Cheval_ pushed as far as the Bridge of Moustier, on which line its Skirmishers exchanged a few carabine shots with some Prussian Dragoons, who did not, however, appear willing to engage further with them. It was by means of this reconnaissance that NAPOLEON ascertained the retreat, through Tilly and Gentinnes, of the principal Prussian Column, consisting of ZIETEN's and PIRCH's Corps, although the line by which they retired was undiscovered by GROUCHY, in whose immediate sphere of operations it was situated.