6 3 .* mann’s 'freatife 
and this, if the direction is in the parallels of latitude 
due Eaft and Weft, is fpherical ; but in all other direc- 
tions it is a portion of an oblate ellipjis , on account of the 
earth’s being a fpheroid comprefled by its axis. Now the 
horizontal plane which continued pafles through the 
fprings of rivers, is always a tangent to the curves of 
their beds at the point of inflexion, infomuch as thefe 
are regular. 
67. The quantity of abfolute declivity from the fpring 
in any determinate part of a river, is that perpendicular 
line drawn from the point of greateft current in that 
place till it meets tbe curve concentrical to the earth's- 
furface which pafles through the river’s fource. The de- 
clivity of the bed below the fpring is had by taking the 
fame perpendicular from the bottom of the bed;, as that 
of the river’s furface is by taking the perpendicular from 
thence. 
68. If a plane be extended horizontally every way 
from the point of tangency to the earth’s furface, or from 
the point where it is perpendicular to any radius of the 
earth, water will run from every other part of the plane 
towards that faid point which is nearer to the center of 
the earth than any other point in the whole plane. 
69. The depreflion of the curve of a river's bed, below 
the concentrical-terrejlrial-curve which interfe&s its 
fource % 
