GEOGRAPHY. 141 



Straits and Castle of Dover. 



According to Buache, the Straits of Dover are on 

 an average less than 30 fathoms deep, and from 

 thence westward to the chops of the Channel the 

 water gradually deepens to 420 feet. 



The hill on which Dover Castle stands is, at the 

 highest part near the turnpike, 390 feet high. The 

 cliff, where the castle-wall terminates, is about 340 

 feet high. About a mile north of Folkestone is the 

 highest point in the range of Dover hills, being 575 

 feet above the sea. 



Lakes and Rivers. 



All large lakes without an outlet, such as the Cas- 

 pian Sea, have salt water. The deepest part of the 

 Lake of Geneva is between Vevay and Rolle, where 

 it is 164 fathoms; its height above the level of the 

 Mediterranean, 1126 feet (Trench). In August, 

 1806, the water of the Hawkesbury river, in New 

 Holland, rose 86 feet beyond the usual level; in 

 1816, the same river rose 84 feet. 



The entire length of the river Mississippi is 3038 

 miles. 



The Ganges, reckoning its sinuosities, has only a 

 fall of four inches per mile. 



The Thames Water. 



The Thames opposite to London Dock gate was 

 found by Mr. Stevenson to be perfectly fresh. At 

 Blackwall, even in spring-tides, the water was found 

 to be only slightly saline ; at Woolwich the propor- 

 tion of salt water increases, and so on to Gravesend. 

 The strata of salt and fresh water were found to be 

 very imperfectly marked, probably on account of the 

 slowness of the current. From a series of observa- 

 tions made at and below London-bridge, compared 



