and Levelling, fyc. 339 



past, especially to those taking an interest in trigonometrical 

 surveying, that the latitude of Edinburgh Observatory deter- 

 mined from actual observation there, and that deduced geo- 

 detically from observations made with the Ordnance zenith 

 sector on Kellie Law in Fife, differ from each other by a 

 considerable number of seconds. It occurred to me that this 

 might be owing to local causes, and was of the same nature 

 as that at Arburyhill in England, which at one time caused 

 considerable speculation. Don Joseph Rodriguez, a Spanish 

 mathematician, associated, I believe, with Messrs Biot and 

 Arago in the prolongation of the French arc of the meridian 

 along the eastern coast of Spain, attempted to shew geodeti- 

 cally that this was owing probably to errors made with the 

 Ordnance zenith sector. It was then contended by the 

 defenders of our survey, and admitted by Delambre, that 

 geodetical deductions could never be employed to test effec- 

 tually astronomical observations. 



The Ordnance zenith sector has been lately tested by a 

 comparison with the Greenwich mural circles, and I under- 

 stand the comparison has proved satisfactory. Besides, much 

 greater discrepancies have been recently detected on the Con- 

 tinent, as might naturally have been expected from the action 

 of greater mountain-masses, such as the Alps, Pyrenees, &c. 



It might have been supposed that one of the points in our 

 survey most to be depended upon would have been the ob- 

 served latitude of Greenwich, where astronomical observations 

 have been made for a hundred years with the best instruments 

 which art could contribute to science. 



The result given to the late General Mudge by Dr Mas- 

 kelyne was 51° 28' 40" N., while that lately given by Mr Airy 

 in the Greenwich Observations is 51° 28' 38" N., less than 

 the preceding by 2". How are our surveyors to dispose of 

 these discrepancies, proceeding from the highest authorities \ 

 Again, Professor Henderson gives the latitude of the Calton 

 Hill Observatory at 55° 57' 23" N., Avhile the zenith sector 

 observations on Kellie Law, reduced to the Calton, give 55° 

 57' 16" N., less than that by direct observation by 7". 



Now, I entertain no doubt that both these results are fair 

 deductions from very accurate observations reduced with 



