tor. Gregory's Strictures on Don Rodriguez. 10 



^ Draconis, x Cygni, &c. which were within a small di- 

 stance of the zenith. But the amplitude of the arch be- 

 tween Dunnose and Arbiiry Hill, as derived from Capella, 

 is 1° 36' 20-"02, while those derived from the other two 

 stars are r 36' 19-"42 and l" 36' iy"g4: a coincidence 

 which proves that the instrument could not possibly have 

 any perceptible deviation from the plane of the meridian at 

 either station. Other reasons for coming to the same con- 

 clusion will appear, on attending to the precautions in ad- 

 justing by double azimuths, &c. as described in the Phi- 

 losophical Transactions. 



The correct position of the sector in all respects is further 

 proved from this: that the observations, however distant 

 in point of time when the proper corrections for aberration, 

 nutation, &c. are applied to them, reduce always very nearly 

 lo the same mean place. 



flence it must be obvious that no error could arise, as 

 Don Rodriguez suspects, from the instrument, whether ia 

 "vertical position, construction, or some accidental de- 

 rangement." { shall now advance still further, and prove 

 thut there is no error, in fact. For, if there were any error 

 in the zenith distances at Arbury Eiill, it would at once be 

 detected on comparison with the observations at Blenheim. 

 Now the distance between the parallel^ of latitude of Blen- 

 heim and that hill, 139,8-22 feet, furnished by the survey, 

 gives for the corrcspondins: celestial arch 22' 59""33, while 

 the observations of y Draconis at Blenheins, compared 

 with the observations upon the same star at Arbury f-Jill, 

 give 22' 59"0. So that there cannot, possibly be an error 

 of half a second at Arbury Hill, unless the observations fof 

 five successive years at Blenheim were all wrong : and 

 Blenheim observatory, be it recollected, has been long 

 celebrated for the excellency of its instrument, and se- 

 lected even by Svanlerg for the accuracy of the observa- 

 tions there made. So again, with regard to the Dunnose 

 station, the latitude of Portsmouth observatory, as in- 

 ferred from the said station and the data in the Trigono- 

 metrical Survey, is 50" 46' 2'"65 ; while the Requisite 

 Tables, the edition of 1781, give it 50" 48' 3". So that 

 the ol)servalions at Dunnose cannot possiblt/ err half a 

 second, unless there was an error made by Witchell and 

 Bayly in determining the latitude of Portsmouth observa- 

 loiy, with an admirable mural quadrant by Bird, These 

 two deductions, then, completely exclude sensible error at 

 Dunnose and Arl)ury Hill: and these inferences, it is evi- 



V(jl .41. No. 1 7 y. March 1813. N dent. 



