1 42 GEOGRAPHY. 



with the river as far up as Kew and Oxford, Mr. 

 Stevenson is of opinion, that the waters of the 

 Thames seldom change, being probably carried up 

 and down with the turn of the tides for an indefinite 

 period of time, to which circumstance he is inclined 

 to attribute the extreme softness of the Thames 

 water. 



In the river Dee, the salt water, when the tide is 

 flowing, insinuates itself under the fresh water of the 

 descending current, so that the river is fairly lifted 

 upwards. 



Length of Rivers. 



Taking the river Thames from its source to the 

 estuary as the unit, the proportional lengths of the 

 other rivers are estimated by Major Rennel as fol- 

 lows : 



Thames ... 1 Jenesia . . . . 10 



Rhine . . . 5J Oby 10J 



Danube ... 7 Amour . . . . 11 



Volga . . . 9J Lena 11 1 



Indus . . . . 5|* Hoanho . . . . 13J 



Euphrates . . 8| Kian Kew . . . 15 



Ganges . . . 0J Nile IgJ 



Brahmaputra . 9% Mississippi . . 8 



Ava River . . 9J Amazons . . . 15 J 



If the length of tbe Thames, including its wind- 

 ings, be reckoned equal to 300 miles, which is not 

 far from the truth, the length of the others may be 

 calculated. 



* Since the above computation was made, the course of the 

 Indus has been found fully equal in length to that of the Ganges ; 

 the Nile is also under-rated, and the Missouri was then unex- 

 plored. Probably the course of the hitter river, from its source to 

 where it joins the ocean under the name of the " Mississippi," is 

 the longest in the world. 



