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Notice irf an erroneous Method of using the Theodolite^ with a 

 strict Analysis of the edicts (if variouH arrangements ivf 

 Readers, By Mr Edward Sang, F.R.S. E., M.S. A., 

 Civil-Engineer and Machine-maker, Edinburgh. Commu- 

 nicated by the Society of Arts.* 



Some months ago, while conversing on the subject of theodo- 

 lites with a gentleman who had been engaged in the Ordnance 

 Survey, I learned that a peculiar arrangement of the readers 

 exists in some of the instruments used therein ; and that a still 

 more peculiar method of determining their average indication is 

 employed. 



On immediately expressing my opinion that the method is 

 erroneous, a discussion ensued, in the course of which my in- 

 formant reiterated his statements in terms sufficiently explicit 

 to remove all doubt as to his exact meaning. I have since been 

 told that this description applies to the Great Theodolite ; in 

 which c^se the minute accuracy of the survey must rest on very ' 

 insufficient data, since the impropriety of this method must af- 

 fect every observation of horizontal angles. 



This accidental circumstance has led me to develope at 

 greater length an analysis which had been sketched out, on the 

 occasion of my report concerning Mr Galbraith"'s pocket re- 

 flecting circle. This analysis I now beg to offer to the atten- 

 tion of the Society of Arts, and also to that of the Ordnance 

 surveyors, in the hope that it may lead to a definite knowledge 

 of the powers of various methods of reading, and that it may 

 help to remove that blind reliance on the authority of names 

 which is too prevalent. Should my information prove to have 

 been incorrect, my labour will not therefore have been in vain, 

 since my object is to exhibit what is accurate in principle ; and 

 should my information prove to have been correct, a strict de- 

 monstration of the impropriety of the process may lead to its 

 rejection, and may thus free the rest of the survey from its in- 

 jurious effects. It is proper to mention, that the errors arising 

 from this source are not of that class which would visibly affect 

 the maps of counties, or the acres of parishes ; they are, how- 



Read before the Society of Arts for Scotland, 28th November 1838. 



