3 66 Dr. Tiarks* account of 
Having now fully proved the errors of the longitudes of 
the Survey, and likewise shown in what manner they are to be 
corrected, I conceive it to be of some interest to investigate the 
cause of the mistake into which those distinguished men have 
fallen, who conducted this great national undertaking with so 
much ability and perseverance. The safest and most obvious 
method of reducing the results of a survey of a country with 
respect to longitude and latitude, would be (if practicable) to 
determine astronomically, in the country itself, arcs both of 
the meridian and of a parallel. The spheroid nearest ap- 
proaching to the figure of the earth for that country, would 
be easily deduced from these measurements, and all the re- 
ductions would be perfectly correct ; but as the determina- 
tion of an arc of longitude is exceedingly difficult, this method 
has hardly ever been practised. An arc of the meridian is 
more easily determined, and the reductions with respect to 
latitude are readily obtained. In order to find, however, in 
the most correct manner the arcs of parallels of latitude, it 
is necessary to combine an arc of a meridian measured in the 
country, with another measured in the same hemisphere as 
different from it as possible, in order to determine the eccen- 
tricity of the meridians, and the dimensions of the correspond- 
ing spheroid. The measurements near the Equator and 
near the Pole may thus be combined with the arcs measured 
in several parts of Europe, and the errors in the longitude, 
arising from the adoption of a spheroid thus determined, will 
be very small. In this manner Bouguer's degree near the 
Equator and the degree of the meridian in the middle between 
Greenwich and Paris, as they are given in the account of the 
Survey, would have led to a spheroid of the compression jjp 
