Leaving Cracow the river runs north-east, and for about one hundred miles forms the boundary between Austria and Russian Poland. At the town of Sandomir the Vistula is joined by the San, which rises on the northern slopes of the Carpathians and flows past the fortress and busy manufacturing town of Przemysl.[27] About fifty miles to the east of Przemysl, on the railway which runs from Odessa on the Black Sea into Silesia, we find Lemberg, an old city which is now a busy place of trade, because it stands in the broadest part of the Galician plain, with excellent communications north, south, east, and west. [Illustration: This picture gives you an idea of a typical landscape in Poland. Notice the difficulties which the Russians have had to overcome in bringing up food and ammunition to their armies. _Photo, Daily Mirror._] From Sandomir the Vistula runs north and north-west across the high plateau of Southern Poland, in a broad valley hemmed in by wooded bluffs. Passing the Russian fortress of Ivangorod on its right bank, it afterwards receives the river Pilica on its left bank, and crosses the plain of Central and Northern Poland. About thirty miles north of the confluence with the Pilica is Warsaw,[28] the most important town in the whole of Russian Poland. It is a beautifully situated city, and before the war was one of the brightest and gayest places in Western Europe. A glance at the map will show you why it is so important both to the Russians and to the Germans. It is the meeting-place of three great Russian railways, by which alone men and ammunition can be carried swiftly into Russian Poland. Warsaw also commands the main stream of the Vistula, which the Russians largely use for transport. Were it to be captured, the communications of the Russians would be cut, and they would be powerless to meet the foe in Poland. Were the Germans to seize it and hold it, they could keep the Russians so far back from their frontier that all fear of invasion through Poland would vanish. Warsaw is the chief stronghold of Poland, and is one of the strongest citadels in Europe. North of Warsaw the Vistula swings round to the west, and at the fortress of Novo Georgievsk receives the river Bug, which rises not far from Lemberg, and sweeps across the plain to the east of, and almost parallel with, the Vistula for more than two hundred miles. A right bank tributary of the Bug, the river Narew,[29] is worth notice, because along it we find a chain of Russian forts. About thirty miles west of Novo Georgievsk the Vistula receives on its left bank the sluggish river Bzura, which rises within a short distance of the Warta[30] or Warthe, a northward and westward flowing tributary of the German river Oder. The Vistula now sweeps north-west past Plock, and enters Prussia some ten miles above Thorn.