APPENDIX. 345 
the incerfperfed Wands prove that it is 
not fo. 
25. If we fuppofe the Sea to deepen from 
the Shores in nearly the fame Proportion that 
Land rifes from the Shores, then from the 
following Eftimate the greateft Depth of the 
Sea will not exceed five or fix Miles. For 
fince flow Rivers are found to have a Fall of 
about a Foot in a Mile, if we fuppofe the 
River Niger in Africa (which is one of the 
longeft Rivers in the World, and runs about 
2400 Miles in length) to fall at the rate of 
four Feet each Mile, then its whole Fall from 
its Rife, to its difeharging itfelf into the Sea 
will be I. 81 Miles ; if it falls at the rate of 
fix Feet each Mile, then its whole Fall will 
be 2. 72 Miles. If eight Feet each Mile, its 
Fall will be 5. 72 Miles. But if the Fall be 
fee at ten Feet each Mile, then the Fall of 
the River will be 4. 54 Miles, which is a 
large Allowance, and may therefore well in- 
clude the height of the Tops of the Moun- 
tains, from whofe fides thofe Springs break 
forth : The higheft Mountains being eftimat- 
cd to be fcarce of the Earths fcmi« 
diameter. 
