568 Mr. mann’s Treafife 
the upper waters, while that in the upper parts proceeds 
from the declivity of the bed. 
From whence it follows, that in rivers which have 
little declivity, it is the depth of the waters which con- 
tributes moft to accelerate their current; and in thofe 
whofe beds have mod; declivity, it is the defcent of gra- 
vity upon an inclined plane which has the greateft fhare 
in producing this acceleration. 
To find w'hether the water in a part of a river where 
the bed is nearly horizontal flows by the velocity ac- 
quired in the preceding declivities, or by the compref- 
fion of the upper waters upon the lower in that place ^ a 
pole muft be thruft down to the bottom, and held per- 
pendicular to the current of the water, wfith its upper 
end above the furface : if the water fwells and rifes im- 
mediately againft the pole, it fhews that its flowing is by 
virtue of a preceding declivity : if, on the contrary, the 
water flops for fome moments before it begins to rife 
againft the pole, it is a proof that it flows by means of 
the compreflion of the upper waters upon the lower. 
16. The abfolute height or elevation of the furface of 
a river which perfeveres in the fame ftate (N° 5.) conti- 
nually decreafes, as the diftance in the river from its 
fource increafes; by reafon that its bed muft continually 
incline and tend towards the center of the earth. 
17. The 
