OB, PLAIN 



tween internal parts, and that 

 great ocean which is the high- 

 way of the world, and the bond 

 between nations. The whole ex- 

 tent of land drained by a river, 

 including all its branches and tri- 

 butaries, is its basin. Adjoining 

 basins are divided from each other 

 by tracts of land more or less 

 elevated, which form the water- 

 shed, or water-parting. Hivers 

 and their tributaries are of the 

 utmost importance in draining 

 and purifying the lands. The vo- 

 lume of water discharged mainly 

 depends upon the extent of 

 country drained. Rivers flowing 

 from mountainous countries are 

 liable to periodical floods, from 

 the melting of snow on the moun- 

 tains ; and the same occurs where 

 the waters are supplied by pe- 

 riodical rainy seasons The long- 

 est European river is the Volga, 

 which measures 2200 miles ; the 

 longest American river, the 

 Amazon, 3900 miles ; the longest 

 Asiatic river, the Yenisei, 2900 



17 



164. 



miles ; the longest African river, 

 the Nile, 2800 miles. Inunda- 



2 



teaching. 25 



Hons, 17, are the result of the 

 quantity of water, either from 

 rain, or the melting of ice or 

 snow, exceeding the capacity of 

 the natural and artificial drain- 

 age of a country. — A long, 

 deep, and narrow hollow, worn 

 in the earth by a stream of wa- 

 ter through a long period of 

 time is a ravine, Via. The rugged 



17a 



165. 



crags and deep ravines which 

 prevail in the Crimea presented 

 great difficulties to the move- 

 ments of the armies engaged in 

 the late war. A shallow stream, 

 which may be crossed by travel- 

 lers upon horseback or on foot, 

 constitutes a ford, 17b. These are 



17 h 



166. 



generally characteristic of coun- 

 tries where roads have <not been 

 constructed, and where, from 

 the smallness of the population, 

 bridges are too costly. The long- 

 continued action of water upon 

 rocks, and especially upon blocks 

 of limestone, leads to their 



