17 



As the assumed quantities (from whence the distances, as 

 givea ill this table, are calculated) are supposed to extend to 

 any possible degree of uncertainty in cither element, it may 

 not be improper to mention on what grounds these limits are 

 severally conceived to be so ascertained. The mean semi- 

 diameters of the earth, are deduced from a mean of several 

 very excellent and careful mensurations of degrees of the ter- 

 restrial meridians, such as those of Picard and Norwood, in 

 France and England ; of Snellius, in Holland; of De Ulloa, 

 De la Condamine, &c. in S. America; Maupertuis and his 

 associates, in Lapland; and the more recent ones of Cassini, 

 General Roy, Lalande, and Lambton. To give a satisfac- 

 * tory view of the actual progress already made, towards ascei- 

 taining this element of our calculation, I have subjoined a 

 table of the mean semidiameters of the earth, obtained either 

 by the actual mensuration of degrees near the probable situ- 

 ation of the mean radius of the earth, or else taken from ex- 

 treme measures reduced to means on the most approved hy- 

 pothesis of the figure of the earth, and the ratios of its radii, 

 Avith the names of the mensurators and calculators; from the 

 result of which it will appear, that the accuracy of the deter- 

 mination of the quantity of the earth's semidiameter, lies 

 within the probable limits assigned to it in my table of the 

 possible distances of the earth from the sun; 



The deductions in the following table are compared with a 



mean semidiameter of the earth, deduced from the table of 



degrees of latitude in French toises, in Vince's Astronomy, 



VOL. XI. D Vol. 



