On dlvM'ing Inslmmenis. 89 



and 2 repiesent the ()rincipal parts of the apparatus; fir. I 

 showing the plan, and fi^. 2 the elevation ; m both of winch 

 ihe same letters of reference are affixed lo corres(K)nding 

 parts, and both are drawn to a scale of half dimensions. 

 A A is a part of the circle, the t;iirface of which is seen in 

 (he plan, and the edge is seen in the elevation. B B B is the 

 main plate of the apparatus, restina; with its four feet aaaa 

 upon the surface of the arc; these feet, being screws, mav 

 be adjusted so as to take equal shares of the weight, and thea 

 are fastened by nuts below the plate, as shown in tig. 2. 

 CC and DD are two similar plates, each attached to ^he 

 main plate, one above and the other below, by four pillarsj 

 >and in them are centred the ends of the axis of the roller E. 

 F and G are two friction u heels, the laiier firmlv fastened 

 to B, but the former is fixed in an adjustable frame, bv 

 means of which a'Jjustmtnl these wheels and the roller E 

 may be made to press, the former on the interior, and the 

 latter on the exterior edge of the circle, with an equal and 

 convenient force*. At the extremities of tlie axis of the 

 Toller, and attached to the middle of the plates C and D, are 

 two bridges, havmo a screw in each ; bv means of which an 

 adjustnient is prt)cured io» raising or lowering the roller re- 

 spectinc the edge of the circle, whereby the former, having 

 its diameter at the upper edge about -OOI of an inch greater 

 tlian at the lower edge fleeing, as before described, a little 

 conical), it may easily be brought to the position where it 

 will measure the pioper portion of the circle. 



Much experience and il^iueht ujion the subject have 

 taught n)e, that tiie roller should be equal to one sixteenth 

 part of the circle to be divided, or that it should revolve 

 once in i'2^ 30' ; and that the roller itself should be divided 

 into sixteen pans; no matter whether with absolute truth, 

 for accuracy is not at all essential here. Each of such divi- 

 sions of the roller will correspond with an angle upon the 

 circle of 1" 24' 22'-5, o.r-j4_-th part of the circle. This num- 

 ber of principal divisions wascliosen, on account of ils beinf 

 capable of continual bisection ; but they do not fall in with 

 ilie ultimale divisions of the circle, which arc intended lo 

 be equal to b' each. 



The next thing lo be considered is, how to make the roller 

 measure the circle. As two niicroscopes are here necessary, 

 and those which I use are very simple, 1 will in ihis place 



• .Sufficient siirinij for keeping the roller in close v.m\ uniform cont.nct with 

 rbc tiljji* of llie circle, in found in the app.iniiiis, without any particular con- 

 trivance lor that purpose ; the bcndin;;of the piMars ot the secondary fratnei 

 and ci the uxii) of iliu roller, chicdy suj^ilits liiis ^lO^.eny, 



give 



