650 iWh mann’s Treatife 
fphere, the Bate of which feldom remains long the fame. 
Now it is no eafy matter either to determine the quantity 
of thefe exactly, or to calculate the effedts and errors in 
the vifual rays proceeding therefrom, which yet mull be 
done to come at the truth by the common method of 
levelling; whereas, in the method I propofe, no fuch con- 
liderations are necelfary, as is evident from the nature of 
it. 
But this is more than enough on a method lb obvious 
and eafy; I Brail now give a few examples of it, and 
thereby conclude this eBay, already perhaps much 
too long. 
83. Suppofing the length of the Scheld, between 
Antwerp and Ghent, following all its meanders, to be 
forty meafured Englifh miles, as it is reckoned nearly to 
be ; and fuppoling the length of the faid Scheld between 
Ghent and Tournay to be fifty of the fame miles; and 
that of the Lys from Ghent to Commines, where it ap- 
proaches neared: to the city of Ipres, forty-fix miles; it is 
required to know the refpective differences of elevation 
between all thefe places. 
It may be found above (N° 78. 79.) that the river 
:Scheld, between Ghent and Antwerp, has not above one 
foot declivity in each mile of its courfe; and that the 
1 Scheld 
