The rivers of Bosnia, like those of the Jura, flow between parallel mountain ranges towards the north-east, along the furrows traced out for them by nature. But these calcareous mountain ramparts of Bosnia, like those of the Jura, are broken up by narrow gorges, or _cluses_, through which the pent up waters find a way from furrow to furrow. Instead of taking a serpentine course, as do most rivers flowing through a plain, these rivers of Bosnia change from valley to valley by abrupt bends. Gentle and furious in turns, they gradually reach the lower regions, and are finally swallowed up by the Save. Only one river, the Narenta, finds its way into the Adriatic; all others, in accordance with the general slope of the country, flow in the direction of the Danube. These river valleys, with their sudden turnings, would be available as natural roads for reaching the plateau, if most of the gorges were not exceedingly difficult of access; and until regular roads have been constructed, as in the cluses of the Jura, travellers are obliged to scale steep heights in order to pass from valley to valley. It is this want of practicable roads which renders military operations in Bosnia so difficult and perilous. Great armies have at all times remained to the east of the mountain masses referred to, passing from the valley of the Vardar into that of the Morava, whose springs almost intermingle their waters. In that locality we meet with the bed of an ancient lake, through which flows the Sitnitza, one of the upper tributaries of the Servian Morava: this is the plain of Kosovo, the “field of black birds,” which reminds all southern Slavs of painful events. It was there the power of the Servians succumbed in 1389, and, if we may credit ancient heroic songs, more than 100,000 men perished in a single day. Five hundred years have passed away since this great disaster, but the Slavs have never ceased to hope for a day of vengeance, and they look forward to the time when on this very field they may reconquer the independence they have lost.