C ] 
Unccrtahity or Difference of 2' 18'^ between the 
greateft and lead of their Obfervations : That the late 
Mr. CaJJini makes the Difference 57^' lefs than Mr. 
Caijinty who accounts for this Difference from the 
Obfervations being taken by an ordinary Inftrument j 
but the Inftrument is the fame which was ufed to 
take the Altitude of the Pole of AmienSj which was 
very near that found by Mr. "Picard. 
As to the Trigonometrical Operations for finding 
the Diftance of Places, Mr. Celjius thinks they labour 
under confiderable Uncertainties 5 not only on the 
Account of the many Difficulties they met withal, 
'viz. mountainous Countries, want of proper Signals, 
fo that convenient Triangles could not be 
formed j but add to all thefe, feveral of the Tri- 
angles had but Two Angles obferved, and fome of 
thefe Angles too acute j whence, as Mr. CaJJini him- 
felf very juftly obferves, in his Examination of Snel- 
lius and Riccioli’s Obfervations, great Errors may 
arife. Mr. Picard thinks all Angles lefs than 20 
Degrees ought to be avoided; as alfo that the Tri- 
angles fhould be contrived fo as to have Sides of a 
due Length, neither too great nor too fmall : Then 
follow 16 Triangles, wherein one or more of thefe 
Inconveniences are to be found. 
It may be faid, the Whole of thefe Obfervations 
and Meafurcs of Monfieur CaJJini feem to be fuffici- 
cntly confirmed, if not afeertained; fince the prin- 
cipal Bafe in RouJJllon was found, when computed, 
to differ but Three Toifes from the fame as it was 
adlually meafured j and that, after fome due Cor- 
redions, it was made to agree with the greateft Ex- 
adnefs. Mr. Celfius replies, Why are we not told 
what 
