1O2 PHYSICAL GEOGRAPHY. 



Mississippi and Missouri, from the Rocky or Stony 

 mountains. 



A peculiar phenomenon attending tropical rivers, is 

 that they are subject to a periodical overflowing of their 

 banks. It used to be supposed that the Nile was the 

 only river that was affected in this way; but it is now 

 well known that all the great rivers within the Tropics 

 experience this overflow, although not with the same re- 

 gularity and to the same extent as the Nile. 



These inundations are caused by the rains which 

 fall at stated periods, and which are of a particularly 

 violent nature. The overflowing of the Nile is caused 

 by the rains that flow from the mountains of Abyssinia. 



It sometimes happens that a river is impeded in its 

 progress ; the result is the formation of a lake, the extent 

 of which will depend on the nature of the surface of the 

 land. Sometimes the water surmounts the impediment, 

 and flows on towards the sea ; and sometimes there is 

 no visible outlet, as in the Caspian Sea, and the Sea of 

 Aral, which, although they receive the waters of several 

 large rivers, have no apparent communication with any 

 open sea. In the latter case, the excess of water is most 

 probably evaporated. 



OF THE OCEAN, TIDES, CURRENTS, &C. 



The OCEAN forms the largest mass of waters on our 

 globe. Although the extent of surface of the Ocean may 

 be pretty correctly ascertained, its depth can be only a 

 matter of conjecture. Some have supposed that a great 

 part of the body of the globe consists of water ; but from 

 the Earth's average density being well known to be 5 

 times that of water, such cannot be the fact. 



The most striking quality of sea water is its saltness ; 

 and that taken from the surface has, in addition to its 

 saltness, a bitter taste.* A considerable variation ap- 

 pears to take place in regard to the saltness of the Ocean 

 from the action of currents, tides, &c., as well as from 



* This is attributed to the decomposition of animal and vegetable 

 matter ; for water taken from a certain distance below the surface, has 

 the taste of a solution of salt in water. 



