The upper course of rivers is characterized rather by plunges than by equable flowing, and determines its way by a series of leaps through zigzag cuts and various ravines. It traverses bowl-shaped hollows and narrow defiles, and makes its way even through mountain lakes, depositing in them its residuum of sand and gravel which it has caught up and swept along. In its wild plunges it draws into its body considerable air, which appears as bubbles, and makes it a white mass of foam. By-and-by it reaches more level ground, becomes clear as crystal, and assumes a rich emerald green, or a deep blue. It is unnavigable, wild, romantic, and is always found in mountain districts. The brawling brooks of Salzburg, of the Pyrenees, and of Sweden and Norway, all partake of this character. Those of the Pyrenees have a fall of an inch in every foot, and in some places cataracts of two or three feet. The same is observable in the Alps, where the continual stir of the water mixes in air enough to turn all into a mass of silvery-white foam. The Carpathian waters are the same before they reach the high plateaus lying at their feet. The Alpine lakes, too, which lie within the upper course of the rivers which feed them, have a considerable fall; Lago Maggiore, for instance, has a descent of 52 feet between Magadino and Arona. In all the most marked mountain systems of Europe, the upper course of the rivers is especially prominent. Northern Europe is characterized by the fact that its streams have, throughout the most of their length, the peculiarities of the upper course—whether observed in northern Russia, in all Sweden, Norway, and Scotland. With the exit of the river from the mountain district, all these relations are changed, and a new character begins. The Middle Course. Far more moderate is the descent after the river emerges from the mountain region, or where it has never experienced the wild turbulence of the upper course, as is the case in most of the rivers of eastern Europe. In the middle course the angle of inclination is much modified. The upper Main has a fall of 342 feet within three miles after leaving Fichtel Mountains. The descent of most of the rivers of central Germany is much less than this. The Neckar, whose sources lie 2084 feet above the sea, in passing to Heilbronn, which is 450 feet above the sea, falls at the rate of about an inch to every 32 feet. The fall of the Saale, after leaving the Fichtel range, is about 20 feet to the mile; that of Naab, about 14; that of the Eger, less; and that of the upper Oder, in Silesia, still less. More gradual yet is the slope of the Volga bed, which falls but 1400 in about 2050 miles, considerably less than a foot to a mile; and in its lower course its inclination must be still less. The effects of the current must necessarily be very different from those observable under the influence of the dashing and wayward upper course.