[ 4<S ] 
flill fome other fmaller Inftruments, that may be of 
great Ufe in the Operations I propofe, or may fcrve 
to make other curious and ufeful Obfervations at the 
fame time that thofe for the Meafurement of the Earth 
are making. 
When the Sides of the Triangles, taken for mea- 
suring the Earth, terminate at very elevated Places, as 
on the Tops of the higheft Mountains, it is neceffary 
to reduce thefe Triangles to what they would be, had 
they been obferved in horizontal Planes frtuate upon 
a Level with the Sea. For this Purpofe, we muft 
know the Height of the Mountains above the Sea J s 
Level, which cannot always be determined geome- 
trically, or would at leaft be too tedious to perform : 
Wherefore, in the Meridian of "Paris , which crofs'd 
very high Mountains, M. CaJJini was of Opinion, that 
he ought to fix: their Height by a fhorter Method, 
which is that of the Height of the fimple Barometer, 
obferved on the Top of each Mountain, and com- 
pared with that obferved at the fame" time in another 
Place, whofe Elevation above the Sea's Level was 
known. But as that Method fuppofes the Knowledge 
of the Proportion which the different Fallings of the 
Mercury keep with the different Heights to which 
the Barometer is carried j and as natural Philofophers 
are not as yet entirely agreed on this Head, for .want 
of Obfervations of fufficient Accuracy : Thence it 
happen'd, that Dr. "Defaguliers , making appear that 
Mr. Cafjini has not employ'd the mod exadt Propor- 
tion, found Reafons for corre&ing, or at leaft for 
doubting, of fome of M. CaJJini s Calculations. Thus 
it muft be by the Affiftance of new Experiments, bet- 
ter xircumftanced than thofe hitherto made,- and pur- 
, 2 fuant 
