on Rivers and Canals. 
577 
negative quantity, where the effect vaniflies, or is re- 
duced to nothing. 
26. The beft and moft limple method of meafuring 
the velocity of the current of a river or open canal, that- 
I know of, is the following : 
Take a cylindrical piece of dry, , light wood, and of a 
length fomething lefs than the depth of the water in the 
river : round one end of it let there be fufpended as many 
fmall weights as may be neceffary to keep up the cylin- 
der in a perpendicular fituation in the water, and in fuch 
a manner that the other end of it may juft appear above 
the furface of the water. Fix to the center of that end 
which appears above water a fmall and ftraight rod, pre- 
cifely in the direction of the cylinder’s axis;. to the end, 
that when the inftrument is fufpended in the water, the 
deviations of the rod from a perpendicularity to the fur- 
face of it may indicate which end of the cylinder ad- 
vances the fafteft, whereby may be difcovered the dif- 
ferent velocities of the water at different depths; for if 
the rod inclines forwards according to the direction of 
the current, it is a proof that the furface of the water 
has the greateft velocity; but if it inclines back, itfhews 
that the fwifteft current is at the bottom ; if it remains 
perpendicular, it is a fign that the velocities at the fur- 
face and bottom are equal. 
