Fig. 713.—The Staubbach (Lauterbrunnen).
Rivers and lakes add not only to the wealth of nations by their usefulness, but, by the additional picturesqueness of their appearance, to the beauty of the landscape. The velocity of rivers would be very much increased if it were not for the strong resistance offered by the banks and the stones to the current, and by friction. The Rhine and the Rhone, if thus unimpeded, would flow at a rate considerably over one hundred miles an hour; and our own little stream (the Thames), instead of eddying peacefully and twirling gracefully by Medmenham or Cookham, would rush along at the speed of the train which so often crosses it on its way to the sea.
The slopes of river-beds, like the slopes of mountains, vary very considerably, and the inclination of a river varies at different places; in a distance of seven hundred miles the Amazon only falls twelve feet, and the current flows chiefly by impetus already acquired. A slope of one foot in two hundred precludes all navigation, and at still greater inclines rapids and cataracts are formed—the great falls wearing away the river-bed by degrees; so it is calculated that hundreds of years ago Niagara Fall was much farther down the river, and the cataract is slowly moving up stream. In time, as the rock wears away, the height will disappear as the celebrated “Falls,” and will become a rapid within a few miles of the lake.
Lakes are derived from river-drainage and springs. Some are very salt, owing to evaporation carrying away so much water, and leaving the accumulated mineral salts. These very salt lakes are likely to dry up, as the supply of water is not equal to the demands of evaporation. Floating islands appear and disappear on many lakes. Derwentwater is one instance. On the uses of lakes and rivers it would be superfluous to dwell. We are more concerned to examine their influence on climate, and in this sense we must also consider mountains. But we will now group all the phenomena of the air and water, and their effect upon climate, under “Meteorology” in the chapters next following.
CHAPTER XLVIII.
PHYSICAL GEOGRAPHY. METEOROLOGY.
THE ATMOSPHERE—WINDS AND AIR CURRENTS—WIND PRESSURE—STORMS—RAIN-CLOUDS—WATER-SPOUTS—ATMOSPHERICAL PHENOMENA.