646 Mr. mann’s Treat if e 
communications are kept conftantlv open, fo as to effec- 
tuate a compleat natural current throughout the whole 
extent thereof ; for in this cafe they are no other than 
rivers, and their waters follow the fame laws of motion. 
4thly, But in canals which are fhut, and their waters 
kept up by fluices fo as to render them nearly flagnant, 
the practice of this method will be different from what 
it is in rivers and open canals : for in this cafe there can- 
not be allowed for the declivity of the furface of the wa- 
ter from fluice to fluice above one inch, or two at moft, 
in each mile of length, according as there may be fewer 
or more accidental drainings of the water in it (N° 74.)* 
Again, as it may happen, in taking the levels of coun- 
tries by the means of artificial canals, that the water in 
different parts may have different directions, attention 
muft be had to add or fubtradi refpedtively the total de- 
clivity of each. 
Moreover, it almoft always happens, in canals where 
the fluices are fhut, that the water on the two fides of 
each fluice is of a very different height, the back waters 
being kept up, while the lower are run off to a certain 
point ; but in fluices next the fea, the tide againft the 
outer gates is fometimes lower and fometimes higher 
than the water in the canal above. In all thefe cafes, the 
difference of height muft be exactly meafured, and the 
quantity 
