On dlvUivg Instnimev.is. 171 



3^ parts, each of which when transferred to the har will 

 give intervals of 0'4 of an inch each : the angle of tise .sub- 

 dividing sector should of cour.-e be 11° \5\ and subdivided 

 into four parts, w^iich will divide the inch into tenths; 

 the surface may also receive other lines, with subdivisions 

 fuited to the dititrent purposes for which it mav he wanted. 

 1'he revolutions of the roller and its .^'^ parts must be 

 dotted upon the bar; taking care, by sizing the roller, to 

 <."onie as near the true standard measure as pos--ib!e: when 

 this is done, compare the extent of the greater bisectional 

 lunriber that is contained in the lenglh ;- i. e. 128 interval 

 of 51-2 inches, with the standard nicasure; notins the dif- 

 ference as indicated by the micrometer heads :" the ex- 

 an)ination and construction of (he table of errors inav then 

 be Conducted just as was done for the circle. 



Being now ready for the pertorniance of its work the 

 scale to be divided must be laid alongside of the bar, and 

 the true divisions must be cut upon it by an appeal, as be- 

 foie, to the erroneous dots on the bar, corrected bv a cor- 

 responding table of errors. The apparatus, reiTiaininti en- 

 tire in the p(>ssessif)n ot the woikman, with its priiiiiiivc 

 dots, the tal)le of errors, &c., is ready for dividinsi another 

 fctandard, which will be precisely similar to others'thai !• ue 

 been, or may be, divided from it. li may be considered 

 indeed, as a kind of engine ; and, as it is not vitiated by 

 the coarse operation of racking wiih a screw, but perform- 

 ed by only looking at the work, the method will coin- 

 mand about three times the accuracy that can be derived 

 from the ususl straight-line dividing encine. Should it lie 

 asked, if an engine thus appointed would succeed for di- 

 viding circles ? J answer, Yes; but 1 would not rcconi- 

 nicnd it ; because, beyond a certain extent of radius, it is 

 nf)l necessary ; lor the errors, which woukl be introduced 

 into the work by the violence of racking a large wheel, are 

 sufficiently reduced by the comparative t^^liortness of tf.e 

 radius of -uch instruincnis as we divide bv that niLthod ; 

 and, what is still more to the purpose, the dividin"- enoine 

 is lour limes more expeditious, and bears routh usage hit- 

 ter. I cannot cput the subject of dividinix slrai-'hl lines 

 without observing, that I never had niv ajii^aratus compli ic. 

 The siaudaid winch I made for sir George Shuckburi>- Lvc- 

 Jyn in 1706, was done by a mere make-shift coninvance 

 upon the principle of dividing by the eve ; how I succtcd- 

 cd may be seen in Sir George's papers on VVeiebts and 

 Measures (Philosophical Transactions for 17f)S). " I made 

 a second, si'UiC years aftcr^ for j)rijlcjsor Picltt of Geneva, 



v\hich 



