A little while before the guns were ready and Ney had reported to that effect, Napoleon had received Grouchy's letter, in which it was announced that the mass of the Prussian army had retreated on Wavre. He had replied to it with instructions to Grouchy so to act that no Prussian corps at Wavre could come and join Wellington. Hardly had the Emperor dictated this reply when, looking northward and then eastward over the great view, he saw, somewhat over four miles away, a shadow, or a movement, or a stain upon the bare uplands towards Wavre; he thought that appearance to be companies of men. A few moments later a sergeant of Silesian Hussars, taken prisoner by certain cavalry detachments far out to the east, was brought in. He had upon him a letter sent from Bulow to Wellington announcing that the Prussians were at hand, and the prisoner further told the Emperor that the troops just perceived were the vanguard of the Prussian reinforcement. Thus informed, the Emperor caused a postscript to be added to his dictated letter, and bade Grouchy march at once towards this Prussian column, fall upon it while it was still upon the march and defenceless and destroy it. Such an order presupposed Grouchy's ability to act upon it; Napoleon took that ability for granted. But Grouchy, as a fact, could not act upon it in time. Hard riding could not get Napoleon's note to Grouchy's quarters within much less than an hour and a half. When it got there Grouchy himself must be found, and that done his 33,000 must be got together in order to take the new direction. Further, the Emperor could not know in what state Grouchy's forces might be, nor what direction they might already have taken. It should be mentioned, however, to explain Napoleon's evident hope at the moment of things going well, that _the prisoner had told the Emperor it was commonly believed in the Prussian lines that Grouchy was actually marching to join him, Napoleon, at that moment_. Napoleon sent some cavalry off eastward to watch the advent of the Prussians; he ordered his remnant of one army corps, the Sixth, which he had kept in reserve behind his line,[19] to march down the hill to the village of Plancenoit and stand ready to meet the Prussian attack; and having done all this, he made ready for the assault upon the ridge which Wellington's troops held. That assault was to be preceded, as I have said, by artillery preparation from the great battery of eighty guns which lay along the spur to the north and in front of the French line. For half an hour those guns filled the shallow valley with their smoke; at half-past one they ceased, and Erlon's First Corps d'Armée, fresh to the combat, because it had so unfortunately missed both Ligny and Quatre Bras, began to descend from its position, to cross the bottom, and to climb the opposite slope, while over the heads of the assaulting columns the French and English cannon answered each other from height to height.