201 
the Trigonometrical Operation * 
the bottom of the Pyrenean mountains, correfponding to an 
arc in the heavens of nearly 8 °| of latitude, differed but little 
from what .Should be its true length, fuppofmg the earth to 
have the figure and dimenfions affigned to it by M. Bouguer 
in his fecond fpheroid. Here, however, it is become neceffafv 
to take notice of fome miftakes * that, through inadvertency, 
were fallen into in the computed lengths of the arcs, which,, 
although they affe<3 in a certain degree the accuracy of the 
numbers brought into comparifon, do not invalidate the general 
reafoning there advanced, and the only thing meant to be efta- 
blifhed, namely, that JVX. Bouguer. s hypothefis anreed better 
with actual meafurement on different parts of the fu.r face of 
* The miftakes adverted to in the text were of three kinds. Firft, an erro- 
neous mode of fumming up the lengths of the arcs from the lengths of the 
degrees, although thefe taken feparately were very accurately computed : for 
enhance, the 43d was taken as that extending from 42 to 43, whereas it fliould 
have been taken for the middle point, that is, from 42 j to 43J, and fo on in 
regard to others. Hence the arcs are all made fomewhat too long. The fecond 
was the omiffion of the value of 93I toifes in eftimating the length of the 
celeflial arc between Greenwich and Perpignan, the fe£tor with which the ft ars 
were obferved having ftood fo much to the- northward of the church of St. Jaumes, 
the point to which the triangular meafurement correfponded. The third was 
fallen into from not knowing that the French obfervations of the ftars had 
been corrected for the nutation of the earth’s axis, in a Paper of M. de la 
Caille s, inferted in the Memoirs of the Academy of Sciences' for the year 
1 758 j whereby all the lengths of the celeftial arcs were thereby in fome degree 
changed from what had been affigned to them refpedlively in the Book, La 
Merldienne verifiee, publiftxed in 1744. From the fame Paper it further appears, 
that they rejedfed altogether their obfervations at Perpignan, as being probably 
affedled by the attradtionof the Pyrenees. With regard to that part of the Table of 
Comparifon in the Paper of 1787, which is affeaed by thefe errors, the only 
dung that now can be done is to annex to this paper a correaed flip, which may 
be leferred to occafionally, or cut off and pafted over the former. 
Vol. LXXX. d a 
the 
