on Rivers and Canals . 647 
quantity refpedtively added or fubtra&ed in the account 
of the levelling. 
5thly, After this it is neceffary to determine, as nearly 
as poflible, the length of the canals and rivers through 
all their turns and windings, and throughout the whole 
extent of country in which we want the difference of 
elevations. This may be done by an adtual menfuration, 
or by the general opinion of the inhabitants of each part 
of the country, which, being founded upon the long and 
continually repeated experience of an infinity of people, 
will be found to differ very little from the truth, atten- 
tion being had to the quantity of their nominal meafures ; 
even the errors in more or left will nearly compenfate 
each other; or, finally, in great extents it may fuffice to 
compute them from good geographical maps. 
6thly, This being done by one or the other of thefe 
methods, it will be eafy, from the quantity of declivity 
before determined for each part in particular, to find the 
whole quantity of declivity throughout the whole extent 
of country meafured, or from any one part thereof to any 
other along the rivers or canals in queftion, which are 
fuppofed to be continued without interruption from one 
place to the other. If to this be added the relative height 
of the country in each place compared with the level of 
the water in the part of the river or canal next to each, 
4P2 we 
