Lacustrine basins of this sort, first emitted, and then again absorbed by a subterranean water-course, are rather rare; there are, however, some other remarkable instances of them in Europe. Thus, in the Oriental Hartz, in the midst of a beautiful spot surrounded by fir-trees, the charming lake called Bauerngraben (Peasants’ Ditch), or sometimes Hungersee (Lake of Famine), sometimes makes its appearance; but when this mass of blue water has filled but for a few days its basin of gypsum rock, it is suddenly swallowed up, and flows away by subterranean channels into the stream of the Helme. The celebrated lake of Copaïs, in Bœotia, may likewise be compared to the Zirknitz lake, at least as regards certain portions of its basin. RIVERS --A. KEITH JOHNSTON Rivers are the result of the natural tendency of water, as of all other bodies, to obey the law of gravitation by moving downward to the lowest position it can reach. The supply of water for the formation of rivers, though apparently derived from various sources, as from rain-clouds, springs, lakes, or from the melting of snow, is really due only to atmospheric precipitation; for springs are merely collections of rain-water; lakes are collections of rain or spring water in natural hollows, and snow is merely rain in a state of congelation. The rills issuing from springs and from surface-drainage unite during their downward course with other streams, forming _rivulets_; these, after a further course, unite to form _rivers_, which, receiving fresh accessions in their course from _tributaries_ (subordinate rivers or rivulets) and their _feeders_ (the tributaries of tributaries), sweep onward through ravines, and over precipices, or crawl with almost imperceptible motion across wide, flat plains, till they reach their lowest level in ocean, sea, or lake. The path of a river is called its _course_; the hollow channel along which it flows, its _bed_; and the tract of country from which it and its subordinates draw their supplies of water, its _basin_, or _drainage-area_. The basin of a river is bounded by an elevated ridge, part of which is generally mountainous, the crest forming the watershed; and the size of the basin, and the altitude of its watershed, determine, _cæteris paribus_, the volume of the river. The greater or less degree of uniformity in the volume of a river in the course of a year is one of its chief physical features, and depends very much on the mode in which its supply of water is obtained.