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On a Method of obtaining the greatest possible degree of Exacti- 

 tude from the Data of a Survey. By Mr Edwaud Sang, 

 F. R. S. E., M. S. A., Civil-Engineer and Machine-maker, 

 Edinburgh. Communicated by the Society of Arts.* 



A FEW weeks ago I laid before the Society of Arts, some re- 

 marks on an erroneous method of using the theodolite, which 

 is practised in the Ordnance Survey of the British Isles. In 

 the course of these remarks it was shewn, demonstratively, that 

 by leaving out certain readings and computations, a superior 

 degree of precision would have been obtained. 



In the present paper I mean to continue the subject, and to 

 inquire whether the like degree of scientific skill which charac- 

 terizes this fundamental operation, has characterized the other 

 departments of this national work. And to do this more ef- 

 fectually, I shall contrast the processes actually used, with that 

 one which gives the greatest possible chance of accuracy. I 

 cannot agree with Mr Bevan, who, after having detected fla- 

 grant errors in the altitudes of certain stations, yet expresses 

 " great confidence in the general result of the terrestrial depart- 

 ments ;"" for, having exposed an error affecting the operations 

 of the great theodolite, I need not profess to feel any confidence 

 until I have examined the grounds on which that confidence is 

 to be founded. 



To some it may appear an invidious task, that of scrutiniz- 

 ing the Ordnance Survey ; but these persons can hardly be 

 aware of the interests which are at stake. The progressive im- 

 provement of Astronomy ; the determination of the form of the 

 earth ; and one element towards the knowledge of the law ac- 

 cording to which its density increases downwards — these are 

 the expected fruits of such an operation : and as the expense is 

 a heavy one even for a nation, we must either obtain these 

 fruits now, or have our expectations deferred for an indefinite 

 term. The matter is not individual but national — not even 

 national — for it is of interest to the whole human race. These 

 fruits are not to be expected from the general and statistical 

 details of the survey, but from the fathoms and the seconds of 



♦ Read before the Society of Arts for Scotland, 26th Dec 1838, and 

 J 6th January 1839. 



