3 70 
Dr. Ti arks' account of 
compression considerably differing from can be admitted. 
In order to produce an ellipticity = -j—, it would be neces- 
sary to diminish both & and cc by 5" 5. Although it would 
be difficult to assign the limits of the errors that may have 
been committed in determining ct and a, still it is not very 
likely that so great an error should have been made in both 
places. It is therefore likely that the meridians have, in that 
part of the country, a greater ellipticity than the whole earth, 
which would not be surprising, as some of the arcs measured 
in France appear to indicate even a compression = 
From the foregoing observations we may now conclude, 
that the longitudes laid down in the account of the Survey 
will deviate from the truth, in the same proportion in which 
the parallels of latitude on a spheroid, having the degree of 
the meridian in latitude 5o°4i' equal to that of the earth, and 
the ellipticity differ from those of the terrestrial spheroid, 
the compression of which is nearly The following com- 
parisons will further illustrate the subject. If the radius of 
the Equator be = 3486908 fathoms, and the semi-axis of 
the earth = 3475560 fathoms, which is nearly the result 
of the measurements in France, and Bouguer’s in Peru, and 
corresponds to the compression the length of the degree 
perpendicular to the meridian in latitude 50° 41' will be 
60975.7 fathoms. For the spheroid adopted in the Survey, 
that degree is found 61,182 fathoms. The ratio of these 
numbers is 296 : 297, and the correction of the longitudes 
would be — l -r ; the same correction is, by the chronometrical 
