648 'Mr. mann’s freatife 
we (hall have very nearly the difference of elevation of thofe 
two parts of the country. And thus the levels may be 
taken from the fea through any extent of country, nay 
even through whole continents, as far as rivers or canals 
extend without interruption. Cataradfs themfelves, fuch 
as thofe in the Nile and in the river of St. Lawrence, 
need not hinder the operation, lince we have only to take 
the refpedlive heights from which they fall into the ac- 
count as we do in common fluices, and allow for the in- 
creafe of velocity produced by them in the current of the 
river above and below the places where they exift. 
82. Although I do not pretend to equal this method 
(of finding the difference of heights in countries) for 
exaSlnefs to the levels taken by adtual menfuration, or to 
thofe found by a long feries of nice barometrical obfer- 
vations; yet it muff be allowed, that it is free from many 
inconveniencies, and accompanied with many conveni- 
ences, which the others are not. It may be eafily carried 
through great extents of country, where the other me- 
thods cannot be put in practice, on account of the ex- 
pence or time required ; and this may be done with 
very little trouble, and perhaps with fufficient exadt- 
nefs to anfwer all the purpofes of the natural philo- 
fopher in his confiderations on the globe we inhabit. 
Although the method of taking heights by barometrical 
obfervations 
