20 



The mean semidiameter of the earth from a mean of all 

 is 3958,69 British miles, and the mean semidiameter, from 

 a measurement executed under the direction of the French 

 National Institute, which extended from Dunkirk to Barce- 

 lona, and was made between the years 1791 and 1798, as re- 

 duced by Lalande, is equal to 3958 British miles.* 



With regard to the quantity of the other element in this 

 calculation, namely, the Sun's horizontal parallax, it is to be 

 observed, that the determination of this quantity is most cor- 

 rectly deduced, from a comparison of the observed and calcu- 

 lated effects of the solar parallax, upon the several phaeno- 

 mena of the transits of the planet Venus over the Sun, espe- 

 cially on the apparent times of the internal and external con- 

 tacts of the limbs of the planet with those of the Sun. The 

 observations of the external contacts of the Sun's and Venus's 

 limbs, were very carefully and judiciously made at the ob- 

 servatories 



• Mr.Dalby, in an elaborate paper published in the 8Ist Vol. Philosopliical 

 Transactions, read May 19, 1791, gives his determination of the longitude of 

 Dunkirk and Paris, from the triangular measurements made in the years 

 1787 and 88, by the ]jte General Ro}-, on the supposition of the earth's 

 being an elipsoid, whose magnitude is determined by adhering nearly to the 

 measured arc of the meridian between Greenwich and Paris, obtained by 

 the aforesaid operations. On which hypothesis it will appear, that the mea- 

 sured degrees of the meridian in middle latitudes, agree, very nearly indeed, 

 to the assumed elepsoid, whose axes are to each other in the ratio assigned 

 by Sir Isaac Newton, y\z\ 229 to 230, and gives a mean semidiameter of 

 the earth, of 3956,55 English miles ; a quantity, most probably, very near 

 to precision. 



