622, Mr. mann’s freatife 
the two currents , and the over-fwimming of the frefh 
water upon the fait, will be always without the fluice and 
towards the fea, fo long as the furface of the tide is below 
the level of the water in the canal. Many fluice mailers, 
for want of knowing or conlidering this, are accuftomed 
to Amt their gates next the fea a little after half flood , un- 
der the pretence of preventing by this means the fait wa- 
ter from getting into the canal, and communicating 
thereby with the waters that overflow the low grounds 
in many places during winter, which would be of great 
detriment to the foil. Through this falfe perfuafion, 
they lofe no inconfiderable part of that time every day, 
which they might fafely employ in drawing off the wa- 
ters which overflow and incommode low and flat coun- 
tries almoll every winter and rainy feafon, as is the cafe 
in the Dutch and Auftrian Netherlands. 
SECTION IV. 
Experiments to determine the different velocities , in different 
depths of water , of the fame floating body moved uni- 
formly by an equal force . 
58. It is well known already, that for facilitating or 
retarding the motion of boats, &c. in canals, the different 
I depths 
