Mr Sang on Optimum Surveying. 329 



that the earth is right, — the surveyors wrong. Having thus 

 run his first parallel, he continues his operations, and, by a 

 series of papers inserted in the same Magazine, he threatens to 

 luidermine and blow up the defences. 



At length in October 1828, a shot is heard from the place ; 

 but it is like that which was once used in the defence of Stam- 

 boul ; it shakes the wall more than the enemy''s battery does. 

 Dr Tiarks has undertaken the defence of the survey, and has 

 shewn that even if the theorem of Dalby were true, it is entire- 

 ly unfit for the purposes to which it was applied by the Ord- 

 nance Surveyors. He shews, in fact, that the theorem used 

 by them, gave from certain data an oblateness of yjg, but that 

 an error, either way, of 1" in the data would augment the ob- 

 lateness to g^g, or reduce it to ^l-g. One single second can 

 do all this ! A keener satire on the skill of the surveyors than 

 this of Dr Tiarks, it is impossible for me to pen : it can only 

 be matched by a late appeal to the evidence of the heliostate 

 in support of the exactitude of the survey. 



I shall be told that these are old errors, and that they are in 

 course of being corrected. But in reply I urge, that the notices 

 of them are later than the latest volume of the English survey ; 

 and that their exposure preceded the attempt to rectify them. 



Without pretending to class my own labours along with 

 those of Tiarks, Bevan, or Ivory, I may remark, that the error 

 pointed out by me a month ago is one of as much importance 

 as any of the others ; nay, of more importance, for those are 

 errors which affect merely the deductions, and which can be re- 

 medied by a suitable discussion : but this is one which shakes 

 our confidence in the data from which these deductions flow, 

 and which can only be removed by an appeal to the scroll field 

 books, or by an entire remeasurement of the angles. The 

 obliteration of many of the stations prevents the latter method. 



There are two distinct purposes for which an extensive sur- 

 vey may be undertaken ; the one, purely geographical, which 

 has in view the delineation of the district surveyed upon a 

 model of the earth, and which, consequently, involves the de- 

 termination of the earth's figure ; the other, local merely, being 

 directed to the comparison of the dimensions of the district, and 



VOL. XXVI. NO. Lll. — APRIL 1839. Y 



