104 <>N DIVIDING ASTRONOMICAL INSTRUMENTS. 



circle will not be exa6^1y measured by the corresponding 

 line of the sector, which has been adjusted to the mean in- 

 terval, for the situation of the dot 1°'4 is too far back, as 

 appears by the table of real errours, by — 4*8 divisions of 

 the micrometer head. The ranoe 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 0, where it began before ; for, as it left 

 off in the first interval at | of a division, it has to go for- 

 wards l^ more before it will arrive at the spot where the 17th 

 division of the instrument 1° 25' is to be made, so that in 

 this second course it must begin at {- short of 0. Go tlnough 

 this interval as before, making a division upon the circle at 

 every one of the \6 great divisions of the sector ; and H 

 should now reach the third dot, allowing for a tabular errour 

 of — lO'ii when the division Iths of the sector reaches the 

 cross wires of I. It would be tedious to lead the reader 

 through all the variety of the sector, which consists of eight 

 courses; and it may be sufficient to observe, that at the 

 commencement of every course, it must be put back to the 

 same fraction of a division which terminated its former one ; 

 and that the wire of the micrometer H must always be set 

 to the tabular errour belongiug to every dot, when we end 

 one interval and begin another. The eight courses of the 

 sector will have carried us through ^V part of the circle, 

 11° 15', and during this time, the roller will have proceeded 

 through half a revolution; for its close contact with the 

 limb of the circle does not allow it to return with the sector, 

 when the latter is set back at every course. Having in this 

 manner proceeded, from one interval to another, through 

 the whole circle, the micrometer at last will be found with 

 its wire, at zero, on the dot from which it se' out ; and the 

 sector, with its l6th division, coinciding with the wires of 

 its microsco;>e. 

 Advantages of Having now given a faithful detail of every part of the 

 this method, process of dividing this circle, I wish to remird the reader 

 that, by veritication and correction at every interval, any- 

 erroneous action of the roller is prevented from extending 

 its influence to any distant interval. It will be farther ob- 

 served, that the subdividing sector magnifies the work; 



that 



