t <°<s ] 
VII. Considerations on the Convenience of meafuring an Arch of 
the Meridian , and of the Parallel of Longitude , having the 
Qbfervatory of Geneva for their common Interfeldion. By 
Mark Auguftus Pi£let, Profeffor of Philofophy in the Aca- 
demy of Geneva ; in a Letter to Sir Jofeph Banks, Bart . 
P. R. S. 
Read February 24, 1791. 
$ 1 R, 
T HE accurate knowledge of the dimenfions and true figure 
of the earth is not a matter of mere curiofity. Aftro- 
nomy and navigation are fo clofely connected with it, that the 
philofophers of the prefent century have purfued this enquiry 
through the moft difcouraging difficulties; and governments 
themfelves have contributed confiderable fums towards its 
fuccefs. 
We know that, notwithftanding thefe efforts, the end is not 
yet obtained. There are five different conclufions on this fub- 
je£t; one of which is given by Sir Isaac Newton’s theory; 
the others are the refult of four different meafurements, which 
appear the moft creditable among thofe that have been per- 
formed. The extremes give _-I- and f° r difference 
between the polar and equatorial diameters of the earth, that 
is, two fractions, one of which is more than double the other. 
The caufe of thefe difagreements is yet unknown ; perhaps 
the figure of the earth is really irregular ; perhaps the feverai 
meafure- 
