12 MEASUREMENT OF AN ARC 



tion, during the fame temperature, which feldom varies early in the 

 morning for an hour and a half, and this morning happene i re 

 ticularly favorable* 



- Two feet and a half were then taken off from the brafs fl.....uc ,u 

 the moft fcrupulous exaftnefs, after examining with magnif^"^--; 

 the points of the compafies, one perfon keeping oae of. j^ |iCi:.; . 

 carefully £xed to a Hne on the fcale, while the other adjufted tha 

 oppofite point by the fcrew at the end of the beam.— After being fatisfied 

 as to the accuracy of diis diltance of 2i^feet, one point of the beam,, 

 compaffes was fixed on the poiot of interfedion which marked the. 

 commencement, while the Oth^r point was drawn acrofs the line oa 

 the next brafs button, making a point of interfeftioiic The bearu 

 comnafTes were then removed to the next button, and fo on till tea- . 

 feet were meafured off. A long beam was then ufed, and the points 

 with apparatus fixed on it, and adjuiled to that ten feet ; and m a^ 

 manner fimilar to what has already been defcribed, the remaining nine- 

 ty feet were meafured off arid a fine perpendicular hne drawn through. , 

 the lafl point of interfedion. As there was'fuU time to repeat the ope- 

 ration, the meafurement was carried back from, point to painty when aa a 

 exa6l coincidence was. obferyedo: , 



The chaiDj, which, with five thermometers, had beenlying clofe to the - 

 wall all night, was then extended at full length; the weight applied, and : 

 the arrow at the oppofite end brougjit to coincide with the commenceT 

 ment of the meafured line while the whole chain .was adjuiled by the 

 pencil line drawn along the furface of the wall ; and after allowing 

 minutes for the weight to aft freely, the length of the chain wa.' 

 cxamii^ed, and found..to cxcecd.the brafs meafuie by o,©3,^i inclic . 



r 



