da. Gregory's srictures on don Rodriguez. 263 



139,822 feet, furnished by the survey, gives for the correspond- Dunnose, lin- 

 ing celestial arch, 22' 5Q'"33, while the observations of y Dra- less ther ? be 

 . t,, , . , . , , , . . errors of the 



coins at Blenheim, compared with the observations upon the same, or great- 

 same star at Arbury Hill, give 22' 5Q"6. So that there cannot f r magnitude, 

 possibly be an error of half a second at Arbury Hill, unless vatories at 

 the observations, for five successive years ai Bteyaheimi were all Blenheim and 

 wrong : and Blenheim observatory, be it recollected, has been 

 long celebrated for the excellency of its instruments ; and is select- 

 ed even by Suanlerg for the accuracy of the observations there 

 made. — So, again, with regard to the Dunnose station, the 

 latitude of Portsmouth observatory, as inferred from the said 

 station, and the data in the Trigonometrical Survey, is 50° 48' 

 2'"65 ; while the Requisite Tables, the edition of 178I, give it 

 50° 48 '3*''. So that the observations at Dunnose cannot possibly 

 err half a second, unless there was an error made by Witchell 

 and Bayley, in determining the latitude of Portsmouth obser- 

 vatory, with an admirable mural quadrant, by Bird. These 

 two deductions, then, completely exclude sensible error at 

 punnose and Arbury Hill : and these inferences, it is evident, 

 might as easily have been made by Don Rodriguez as by me. 



This gentleman may find still farther confirmation of the 

 truth of the whole survey, if he will examine the operations 

 by which the meridian of Dunnose is extended to Burleigh 

 Moor, and those for carrying on a new meridian from Black . 

 Down to Delamere Forest. These, it is true, are not to be 

 found (for what reason I cannot say) in the Philosophical Tran- 

 sactions. But they may be seeir in the third volume of the 

 Trigonometrical Survey, published in 1811; by order of the 

 Board of Ordnance ; a volume with which some of Don R.'s 

 friends in England are doubtless acquainted. 



As a last corroboration of the whole portion from Dunnose Confirma- 



to Clifton, amounting to 2° 50' 23"3S ; let me add, that when tion of the 



compared with the meridional arch of 3° f l" at Peru, by furnished by 



means of the valuable theorem, investigated by Professor Play- Professor 



fair, (Edinburgh Transac. vol. v. pp. 8, 9.) for the comparison mu ^ ai f ™ °£ 



of large arcs j it produces ^^.^ for the resulting compres- comparison of 



sion. While Svanberg (pa. 192, " Exposition') gives — _L — the f only ^ 



for the compression, as deducible from a comparison of his mean . of infer " 



. T , _ urn? the ratio 



measure with that at Peru. of the earth's 



Thus, we have confirmation upon confirmation, of the cor- axes > twm ter " 

 * restrial mea- 



rectness sn res. ' 



