115. =Rivers.=--If a trough be exactly level, the water will be of the same depth at one end as at the other, for the surface of the water at both ends will be at the same distance from the centre of the earth. But raise up one end, and it is now deepest at the other end. If it were not so, the surface at the two ends would not be at the same distance from the centre of the earth. Now if, with the trough thus placed, water run in at the upper end and out at the lower, you have exemplified what is taking place in all rivers--the water is in constant motion from the influence of gravitation, causing it to seek to be on a level. A very slight slope will give the running motion to water, for the particles are so movable among each other that in obedience to gravity they flow down the inclined plane to seek a level. Three inches declivity in every mile in a smooth straight channel will make a river run at the rate of about three miles an hour. The Ganges, which receives its waters from the Himalaya Mountains, in running 1800 miles falls 800 feet. The Magdalena, in South America, falls only 500 feet in running 1000 miles between two ridges of the Andes. 116. =How some Rivers have been Made.=--Changes are constantly produced in the earth by the disposition of water to seek a level. In doing this the water carries solid substances of various kinds from elevated places into depressed ones, tending to fill up the latter. New channels are also sometimes made by the water. The boy who makes a little pond with his mud-dam, and lets the water overflow from it into another pond on a lower level, as he sees a channel worked by the water between the two ponds becoming larger and larger, witnesses a fair representation on a small scale of some extensive changes which have in ages past taken place in some parts of the earth. It is supposed, and with good reason, that many rivers had their origin in the way above indicated. For example, where the Danube runs its long course there was once a chain of lakes. These becoming connected together by their overflow, the channels cut between them by the water continually became larger, until at length there was one long, deep, and broad channel, the river, while the lakes became dry, and constituted the fertile valley through which that noble river runs to empty into the Black Sea. It is said that a similar process is manifestly going on in the Lake of Geneva, the outlet of it becoming continually broader, while the washing from the neighboring hills and mountains is filling up the lake. Towns that a century ago lay directly upon the borders of the lake have gardens and fields now between them and the shore; and Dr. Arnot says, "If the town of Geneva last long enough, its inhabitants will have to speak of the river in the neighboring valley, instead of the picturesque lake which now fills it." [Illustration: Fig. 68.] [Illustration: Fig. 69.]