C 48 ] 
above the former, and confequently above the Sea^s 
Level, making proper Allowances in the Calculations 
for the Difference of the apparent Level from the true 
one. In this Method nothing is to be apprehended 
but the Variation of Refractions ; but for this a Re- 
medy may be found, for the moft part, by returning 
upon one's Steps, that is, by reciprocally obferving 
the firft Stations feen from the fecond : For if it be 
found, that as much as the fecond Station appears ele- 
vated above the firft, fo much the firft is deprefs'd be- 
low the fecond, except the fmall Difference which 
muft arife according to the given Diftance, it will be 
a Proof, that the Refradtion has. been of no Pre- 
judice. 
The Obfervations and Determinations of the true 
Heights of ail the Places which ar£ to be vifitcd, will 
not be the leaft laborious of thofe that are to be made 
in' theft journies 5 but then their Ufefulnefs will be a 
fufficientRecompenfe for the Trouble; feeing they will 
afford us the Means of knowing all the chief Unevcn- 
neffes of the Ground traverfed by thefe great Tri- 
angles, which being compared with the Length of the 
Courfeof the Rivers, may give us room to judge of 
their Rapidity, of the Eafe or Difficulty of their Com- 
munications, <&c. 
The other confiderable Obfervations and Experi- 
ments to be made in the Journies undertaken for fuch 
Enquiries, are, the Obfervations of the Magnetic 
Needle, both as to its Dip and Variation : But chiefly 
the Obfervations of the Length of the Ample Pendu- 
lum, which, at prefent, is become requifite to be ob- 
ferved with as much Exadtnefs, and in as many Places 
as is poflible 5 but alfo for which there are new Me- 
thods 
