57 8 Mr. mann’s freatife 
This inftrument being placed in the current of a river 
or canal receives all the percuffions of the water through- 
out the whole depth, and will have an equal velocity with 
'that of the whole current from the furface to the bottom 
at the place where it is put in, and by that means may 
4 
be found, both with eafe and exactnefs, the mean velo- 
city of that part of the river for any determinate diftance 
and time. 
But to obtain the mean velocity of the whole fedtion 
of the river, the inftrument muft be put fucceflively 
both in the middle and towards the fides, becaufe the ve- 
locities at thofe places are often very different from each 
other. Having by this means found the difference of 
time required for the currents to run over an equal fpace ; 
or, the different diftances run over in equal times , the mean 
proportional of all thefe trials, which is found by di viding 
the common fum of them all by the number of trials, 
will be the mean velocity of the river or canal. 
If it be required to find the velocity of the current 
only at the furface, or at the middle, or at the bottom, a 
fphere of wood, of fuch a weight as will remain fuf- 
pended in equilibrium with the water at the furface or 
depth which we want to meafure, will be better for the 
purpofe than a cylinder, becaufe it is only affedted by the 
water 
