ASTRONOMY. 



4*9 



The quantity is called the Compression, and de- 

 termines the species of the ellipsis. 



d. We shall now take for the determination of the fi- 

 gure and magnitude of the earth, the five arches 

 subjoined, as those that have been measured with 

 the greatest care, and the best instruments ; as 

 being the largest also, and the most distant from 

 one another. 



As five quantities may be combined, two and two, in 

 ten different ways, ten results may be deduced 

 from the comparison of these degrees ; and if the 

 meridian were truly elliptical, and if there were no 

 error in the observations, all these results would 

 coincide. As the latter supposition cannot be ex- 

 pected to hold, we must look for some difference 

 in the results, and must choose only those combi- 

 nations, in which the degrees are considerably dis- 

 tant from one another, because in that way the er- 

 rors of observation will least affect the conclusions. 

 Such are the 1st and 3d, 1st and 4th, 1st and 5th j 

 2d and 3d, 2d and 4th, 2d and 5th ; 3d and 5th. 



VOL. II. D e. The 



