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On Trigonometrical Surveying and Levelling, and on the Effects 

 of a Supposed Local Attraction at the Calton Hill* By Wil- 

 liam Galbiiaith, M.A., M.S.A., Edinburgh ; F.R.A.S., 

 London. 



In the course of some years past I have laid before the So- 

 ciety of Arts for Scotland a few remarks relative to the im- 

 perfect state of those usually reckoned our best maps of the 

 country, and pointed out some instances of the amount of 

 error. J 



I also proposed formulae and rules to be observed in the 

 practice of accurate trigonometrical surveying and levelling, 

 which, in my opinion, might be easily understood and readily 

 applied by any one possessing a moderate share of scientific 

 knowledge. As a continuation of these, I shall, with permis- 

 sion of the Society, read the following few remarks on what 

 has already been written, as well as on some connected ob- 

 servations which I have been enabled to make during the 

 months of August and September last in the immediate vi- 

 cinity of Edinburgh. 



I am gratified to find that the data on which the geodetical 

 tables previously given were formed have been confirmed by 

 a recent memoir of M. Bessel, the learned Astronomer of 

 Koenigsberg, Avho had the benefit of the perusal of the results 

 of several arcs of the meridian measured in Germany and 

 Russia, communicated to him in manuscript, which were, con- 

 sequently, unknown to me. It is fortunate, therefore, that 

 M. Bessel's value of the radius of the equator deduced from 

 these, combined with other's formerly known, exceeds mine by 

 166 feet, and his value of the polar semi-axis exceeds mine by 

 334 feet only — small quantities in about twenty millions of 

 feet. 



He also determines the most probable value of the French 

 metre to be 443.321 French lines instead of 443.296 lines, 



* Bead before the Society of Arts for Scotland, 14tli December 1840. 



t Through the errors in our charts, or the ignorance of our pilots, or both 

 combined, the Lords of the Admiralty themselves have lately been put in 

 peril. Will this contribute in any degree to hasten the survey of the Bri- 

 tish Isles 1 



