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On dividing Instruments, l6f 



Things being in this position ; after having given the 

 parts lime to settle, and having also sufficiently proved the 

 permanence ot the micrometer H and the cutting frame 

 with respect to each other, the first division may be made; 

 then, by means of the screv*' for slow motion, carry the 

 apparatus forward, until the next hne upon the sector comes 

 to ^he cross wires of I ; you then cut another division, and 

 thus proceed until the 16th division is cut, = 1° 20': Now 

 the apparatus wants to be carried further, to the amount of 

 fihs of a division, before an interval is complete; but at 

 this last point no division is to be made ; we are here only 

 to compare the division on the sector with the correspond- 

 ing dot upon the insinunent. This interval, however, 

 upon the circle will not be exactly measurt^d by the corre- 

 sponding line of the sector, which has been adjusted to the 

 mean interval, for the situation of the dot I''4 is too far 

 back, as appears by the table of real errors, by — 4-8 divi- 

 sions of the micrometer head. The range of the screw for 

 slow motion must now be restored, the cross wires of H 

 set back to — 4-8 divisions, and the sector moved back by 

 hand, but not to the division where it began before ; for,, 

 as it left off in the first mterval at fths of a division, it has 

 to go forwards J-th more before it will arrive at the spot 

 where the 17th division of ihe instrument 1° -25' is to be 

 made, so that in this second course it must begin at -i-th 

 short of 0. Go through this interval as before, making a 

 division upon the circle at everv one of the 1 6 great divi- 

 sions of the sector; and H should now reach the third dot, 

 allowing for a tabular errorof — 10'2 when the division -fths 

 of the sector reaches the cross wires of I. It would be te- 

 dious to lead the reader through all the variety of the sector, 

 which consists of fig.it courses ; and it may be sufficient to 

 observe, that at iheconmienccment of every course, it mnsl 

 be put back to the same Iraclion of a division which ter- 

 miualed its former one ; and that the wire of the micro- 

 meter H must always be set to the tabular error belonging 

 to every dot, when we end one interval and begin another. 

 'i'he tight courses of the sector will have carried us through 

 -^'j-d part of the circle, 11° 15', and during this time, the 

 roller will have proci;eiIed through half a revolution ; for 

 its close contact wilh the limb of the circle docs not allow 



on some intervals amount to double tliat quantity. It therefore liecomes 

 matter of prudent precaution to examine every interval previous to niakinjj 

 tlie di .isiiins; and, wliere necessary, to adjust the sector, so that iu arc 

 may exactly measure the corre'^potidii)); interval as corrected by the tabu- 

 Liicrd crrurt. 



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