;^^ ' -MEASUREMENT -OF ^N ARC . 



-fc for-applying tliC' paints of^lliebeamxompiiffes, thet4vo micr 

 beloagiog to the circular inilruraent were each placed upon . 

 tripod with ihort adjuftbg ,(crev/s for feer, fo as, to raife or 

 microfcope for obtaini-og diftind: vifioR. The brafs Oandar 

 its mahogany bed was, then placed on.the table refii^gon tvv 

 very thin board, each having two flat pieces of wood k , 

 it at fiich a diftance as to receive eafily the mahogany bed ,; j - ^ 

 four pieces were of ftich a ihicXncfs, tiiat their furfaces coincided 

 With the furface of the brafs fcaie. they were tl^en moved to a 

 converii.ent diaance for meafunng oii 2} ket, and ihe rnicrofcopes 

 ^placed upon them and brought over ihe required djvifions on the 

 fcale, md adju0.ed by the feet of the tripods to diftma vifion.. The 

 beam cornpailcs were then laid on .the fcale, and the points 'brought 

 by the hand to .be nearly 2| feet afuoder, and afterwards fixed v^^ith 

 care and accuracy by the adjulling fcrev/ at one end of the beanu 

 ^ I'his being done, the pro'cefs was precifeiy the fame as in the expert- 

 ■ments at Bellary having the wall, brafs butions, &c, in all .refpects 

 the fame when one hundred iee.t was meafiired off. The .chains u^ere 

 .compared as in the former experiments, but to rf.ad off the difFerencs 

 between the chain and the brafs meafarCj one of the micro fcopes (B) 

 ".with ii& niicrometer was made .ufe of, and the fcale with ifs bed was 

 :placed in the lame manner as wben the 2|- feet were "meafured off". 

 The microfcope was then, placed on the wood and the fcale tnoved 

 until the frnall divifions.at its comme.ncement v/ere brought under the 

 inicrofcope, the adjufiing feet of the tripod being moved if neceffary, 

 and difl;in<5t mCion obtained. Thefe divifions are each -^th of an inch ; 

 that is,, half an, inch i^ divided into ten parts. The njicrofcope was 

 then brought ^ve,r;the£rft of thefe parts, and the wires of the micro- 

 iRCter being placed at rigbt angles to the longitudinal line on tb 

 they were feparatcd and made to embrace one of thefe d 



