•^^ ■ MEASUREMENT OF AN ARC 



was 39,04 divifloDs of the mfcrOmeter bead, an exccx. w.^.^u x- .hoagui 

 Tathef great, but as there had been a fmall bafe meafared on the furface' 

 ofthegroondnear Jmjare, and the recent experiments made, with 

 great care, Irefted fatisfied, though the mcreafe for the rneafiireinent 

 was much greater than oAial, being ^,38 divifioni. I was^oWever' 

 much furprifed on comparing them previous to the meafur^ment near 

 G'^o/y, to find that the- excefs was only 3004 divifions, but being 36.3 

 divifions at the condufion, I apprehended that there might have been 

 fome overfight at Palamcottah, or that the ftandard chain had increafed 

 m Its length 1 in order to determine which, it became abfoiutely necef- 

 fary to compare it with the brafsjtandard, which was done in the fol- 

 lowing manner. 



•^s I had not the means of procuring a caft iron bar,- and executing 

 the meafurement after the manner adopted by the late Mr. Ramsden, 

 it occurred to me that if upon a fine furface the chain could be extend- 

 ed its whole length, one hundred feet might be meafured off from the 

 ftandard fcale at a given temperature, and by accounting for the differ^ 

 ence between the expanfion of brafs and fteel, it would be eafy to de- 

 termine whether the ftandard chain had fuffered any alteration in its 

 length.— For this purpofe, I built a brick wall upwards of two, feet in 

 height, 'Snd fomething more than 100 feet in length, fo .that a weight 

 poft at one end, and a drawing poft at the oth^i^, might be fixed in the 

 brick work, and the neceflary apparatus applied fpr; drawing out the 

 bliahi.— The upper furface of this wall was made pe'ifeetly horizontal by. 

 k fpirit level fixed on a ftraight ruler about four feet in length, and when 

 covered with fine chunam mortar, (a celebrated cement in this coua- 

 try) it was pohlhed, fo as to referable a flieet of glafs, an operation at 

 which the v/orkraen here are remarkably ejepert. After this was <=oni- 



