on Rivers and Canals. 
59 * 
the work muft be begun and directed throughout all 
the reft of the channel, is from the loweft water-mark 
of fpring tides on the fhore at the mouth of the river ; 
or even fomething below it, if it can be done ; though 
this part will foon fill up again by the fand, mud, 8 cc. 
which the tides ceafe not to roll in. 
If it is a canal whofe bed is be to dug anew, or one al- 
ready made, which is to be cleaned and deepened from 
the fea fhore or fome large river back into the country, 
and where no declivity is to be loft ; as is the cafe in all 
flat countries : the work muft be begun, and the depth of 
the whole channel directed, from the low water-mark of 
fpring tides, if the mouth is to the fea, or from fuch a 
depth in the channel of the river, if the canal falls into 
one, that there may be fuch a communication of water 
from the canal to the river, in all fituations of the cur- 
rent, as may let boats freely pafs from one to the other. 
This, of courfe, muft alfo direct the depth of the floor of 
the laft fluice towards the mouth of the canal, be it to the 
fea or into a river. If the bottom or floor of a fluice al- 
ready conftrudted be too low, it will foon fill up with fand 
or mud, and thereby hinder the gates from opening, un- 
lefs it be continually cleaned out; if, on the contrary, 
this floor be too high, and in a canal whofe natural de- 
clivity is too little for the free current of the water, as is 
4 H 2 generally 
