The one central idea of Napoleon and Soult was clearly to leave a wide discretion for Grouchy, provided always that he kept his communications with the main army open, and that his general direction was one which would insure easy connection, in order either to cut off or check the Prussians. But, however this may be, the hours of Napoleon's inactivity were precious to his enemies; by twelve Buelow was at St. Lambert, and at the same hour two other Prussian corps were leaving Wavre. These movements were apparently tardy, but Gneisenau, feeling that Wellington had been a poor reliance at Ligny, and very much doubting whether he really intended to stand at Waterloo, was unwilling that Bluecher should despatch his troops until it was certain that the Prussian army would not again be left in the lurch. Should the Anglo-Dutch retreat to Brussels, the Prussians must either retreat by Louvain, or be again defeated. Anxiety was not dispelled until the roar of cannon was heard between eleven and twelve. Then the Prussians first exerted themselves to the utmost; it was about four when they were within striking distance, ready to take Napoleon's army on its flank. When Grouchy reached Wavre, at the same hour, he found there but one of Bluecher's corps, the rear under Thielemann. [Illustration: Campaign of 1815. June 15th to 19th.] From Belle Alliance Napoleon returned, and took his station on the height of Rossomme. In front was a vale something less than a mile in width. The highway stretched before him in a straight line until it skirted the large farmstead of La Haye Sainte on the opposite side; then, ascending by a slant to the first crest, it passed the hamlet of Mont St. Jean, only to ascend still higher to the top of the ridge before falling again into a second depression. At Mont St. Jean was Wellington's center. The road from Nivelles to Brussels crosses the valley about a quarter of a mile westward, and on it, midway between the two slopes, lay another farm-house, with its barns, that of Hougomont. More than half a mile eastward, in the direction from which the Prussians were expected, lay scattered the farm buildings of Papelotte, La Haye, Smohain, and Frischermont. The valley was covered with rich crops. Unobstructed by ditches or hedges, it was cut longitudinally about the middle by a cruciform ridge, with spurs reaching toward Belle Alliance on one side, and past Hougomont on the other; the road passed by a cut through the longitudinal arm. Hougomont was almost a fortress, having strong brick walls and a moat; it stood in a large orchard, which was surrounded by a thick hedge. The house at La Haye Sainte was brick also, and formed one side of a quadrangle, inclosed further by two brick barns and a strong wall of the same material; though not as large or solid as Hougomont, it was a strong advance redoubt for Mont St. Jean.