But these divisions may be simplified with respect to the military aspect of the country; for Lisbon taken as the centre, and the distance from thence to Oporto as the radius, a sweep of the compass to Rosaminhal will trace the frontier of Beira; and the space lying between this arc, the Tagus, and the sea-coast, furnished the main body of the defence. The southern and northern provinces being considered as the wings, were rendered subservient to the defence of the whole, but had each a separate system for itself, based on the one general principle, that the country should be wasted, and the best troops opposed to the enemy without risking a decisive action, while the irregular forces closed round the flanks and rear of the invaders. The northern and southern provinces have been already described, Beira remains to be noticed. Separated by the Douro from the Entre Minho and Tras os Montes, it cannot well be invaded on that line, except one or both of those provinces be first subdued; but from Castel Rodrigo to Rosaminhal, that is from the Douro to the Tagus, the frontier touches upon Spain, and perhaps the clearest method to describe the conformation of the country will be to enter the camp of the enemy. An invading army then, would assemble at Ciudad Rodrigo, or at Coria, or at both those places. In the latter case, the communications could be maintained, directly over the Gata mountains by the pass of Perales, or circuitously, by Placentia and the pass of Baños, and the distance being by Perales not more than two marches, the corps could either advance simultaneously, or unite and force their way at one point only. In this situation, the frontier of Beira between the Douro and the Tagus, would offer them an opening of ninety miles against which to operate. But in the centre, the Sierra de Estrella, lifting its snowy peaks to the clouds and stretching out its gigantic arms, would seem to grasp and claim the whole space; the summit is impassable, and streaming down on either hand, numerous rivers cleaving deeply, amidst ravines and bristled ridges, continually oppose the progress of an army. Nevertheless, the invaders could penetrate to the right and left of this mountain in the following directions:-- _From Ciudad Rodrigo._--1º. By the valley of the Douro.--2º. By the valley of the Mondego.--3º. By the valley of the Zezere. _From Coria._--1º. By Castello Branco and the valley of the Tagus; and, 2º. By the mountains of Sobreira Formosa. To advance by the valley of the Douro, would be a flank movement through an extremely difficult country, and would belong rather to an invasion of the northern provinces than of Beira, because a fresh base must be established at Lamego or Oporto, before the movement could be prosecuted against Lisbon.