( 263 ) 



On the English Arc of the Meridian. By William Galbraith, 

 Esq., M.A., Vice-President of the Royal Scottish Society of 

 Arts, F.R.A.S., &c.* Communicated by the Royal Scot- 

 tish Society of Arts. 



The paper which I now lay before this Society is one relative to a very 

 important branch of science, whatever may be its own merits. The Eng- 

 lish arc of the meridian, bctweeen Dunnose in the Isle of Wight, and 

 Clifton on the southern borders of Yorkshire, extends tlirough a tract of 

 country of about 200 miles, and has created considerable discussion in fo- 

 reign journals, as well as in those of this country. The notion of errors 

 having been committed in the original zenith-sector observations to de- 

 termine the length of the celestial arc corresponding to that measured on 

 the earth's surface, is now generally exploded ; since instances of much 

 greater differences between the observed and geodetic latitudes have oc- 

 curred in many extensive similar operations on the Continent. 



M. Bessel has indeed discovered that there has been a very considerr 

 able error committed in the original reduction of the observations made 

 on the star Capella, amounting to about 18" ; but, as this error was simi- 

 larly applied to the observations at both the north and south ends of the 

 arc, it produced little effect on the intercepted arc, — the only result then 

 deduced. 



M. Bessel has, however, determined the latitude of Dunnose carefully 

 from the zenith-sector observations, and, in this case, he could not avoid 

 detecting this grave error, since it would have produced an equal effect 

 on the observed latitude. 



The original bases from which the sides of the triangles are determined, 

 were not measured in the imperial standard, and therefore all the dis- 

 tances given in the Survey require to be reduced to it. M. Bessel has 

 made this reduction correctly according to Kater's experiments ; but he 

 has not recomputed the triangulation. He took merely the final result 

 as it stood in the survey. This part of the operation forms the subject of the 

 present paper, and it is hoped that the results (not differing greatly from 

 Bessel's) of my enquiries are stated in such moderate and candid language 

 that no offence can, by any possibility, be reasonably given to any gen- 

 tleman either formerly or now connected \rlth this great national work, 

 so highly beneficial to the interests, and, on the whole, creditable to the 

 science of thecountry. 



These operations have been, and still continue to be, of great service 

 to practical science, as well as to the useful arts. In many instances, the 

 Ordnance Survey in this country has been of great importance in the se- 

 lection of lines of railways, canals, and roads. On the Continent, the 



* Read before the Koyal Scottish Society of Arts, 13th February 1843. 



