644 Mr. mann’s T Veatife 
lay down under this laft head of it muft appear very 
plain and eafy. I am very far, however, from giving the 
methods I am going to propofe for taking the levels 
through whole countries and continents as far as rivers 
extend. , as ftrictlv exact ; I know very well that it is 
next to impoffible they fhould be fo, confidering the 
continual variations in the declivities of rivers, and 
in the velocities of their currents in different parts, 
as alfo the impoflibility of knowing the exadt length 
of their courfe through all their turns and winding's 
O O 
1 only give them therefore as a general and eafy me- 
thod of computing the relative heights of countries 
without deviating much from the truth, which, per- 
haps, is all that may be neceffary for the conlide- 
ration of the natural pliilofopher. At all events, they 
may be of fome ufe, for this end, in fo many parts 
of the earth through which rivers pafs, and where no 
barometrical obfervations, or any others whatever, for 
taking heights above the fea, have been, or perhaps ever 
will be made. They may alfo be found ufeful in taking 
the levels through a large extent of flat countries where 
regular canals and rivers pafs, and where the difference 
of elevation is too fmall to be obferved by the barometer, 
and where alfo the taking them through fo great an ex- 
tent by the common methods of levelling would be 
much too, expenfive for the purpofes required. Now in 
1 this 
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