on Rivers and Canals , 
they are no more than artificial ones. 
If I miftake not, 
S5$ 
all the navigable canals in England are of the firfl fort; 
that is, have their waters kept up, and let off by Unices. 
This neceffary diftindtion will take away all ambiguity 
from what I have to fay on canals throughout the fol- 
lowing difcourfe. 
2. But that I may fulfil the talk I have undertaken, it 
is neceffary firft of all to lay down fuch principles on the 
nature of rivers and canals in general as have been de- 
monftrated true both by calculation and experience; to 
the end, that we may deduce from thence the true laws of 
motion of their waters, and the quantity of declivity of 
their beds : for this purpofe, and becaufe a large volume 
would hardly fuffice to comprife all the demonft rations 
of thefe principles, which, confequently, I am obliged 
to omit in this treatife, it will not be amifs to mention the 
principal authors who have treated this fubjedt in dif- 
ferent ages and countries, in whofe works the demon- 
fixations of all the principles I fhall lay down may be 
found, if any one doubts the truth of them. Thefe are 
the following. 
sextus julius frontinus, de Aquae-dudiibus Urbis 
Piomae, cum Notis poleni, impreff. 1722. 
John baptist ALEOTTi, Hydrometrician to the Duke 
of Ferrara, and to Pope clement the VUIth. 
4 D 2 
* Don 
