But lord Wellington keeping the head of his army one march behind Celerico, in observation of the routes over the Estrella, and his rear close to the Alva, was master of his retreat; and as the Mondego was fordable in summer and bridged at several points, he could pass it by a flank movement in a few hours. Now the right bank was also one great defile, lying between the river and the Sierra de Alcoba or Caramula. This mountain stretching with some breaks from the Douro to Coimbra, separates the valley of the Mondego from the coast line, and in approaching Coimbra sends out a lofty transverse shoot, called the Sierra de Busaco, exactly in a line with the Sierra de Murcella, and barring the way on the right bank of the Mondego in the same manner that the latter Sierra bars it on the left bank. Moreover this route to Coimbra was the worst in Portugal, and crossed by several deep tributaries of the Mondego, the most considerable of which were the Criz and Dao. The Vouga, however, opened a passage through the Alcoba near Viseu, and that way the French could gain the great road from Oporto, and so continue their movement upon Coimbra. Such being the ground on both sides of the Mondego, the weakest point was obviously towards the Estrella, and lord Wellington kept the mass of his forces there. But Massena was ill-acquainted with the military features, and absolutely ignorant of the lines of Torres Vedras. Indeed, so circumspectly had those works been carried on, that only vague rumours of their existence reached the bulk of the English army; and many British officers imagined that the campaign was only to cloak the general’s intention of embarking when he reached Lisbon. In England the opposition asserted that he would do so: the Portuguese dreaded it; the French army universally believed it; and the British minsters seem to have entertained the same opinion; for at this time an officer of engineers arrived at Lisbon, whose instructions, received personally from lord Liverpool, were unknown to lord Wellington, and commenced thus:--“_As it is probable that the army will embark in September._” CHAPTER VI. THIRD INVASION OF PORTUGAL.