bCCtor 

 scribed 



«100 ON DIVIDING ASTRONOMICAL INSTRUMENTS. 



VII. 



An Account of a Method of dividing Astronomical and other 

 Instruments, Sj'C. By Mr. Edward Trouguton. 



^Concluded from page 19,.) 



True divHions Jt^^AVING now coini)leted the two first sections of my 

 to be nV'Aj. . ' ... 



from tlie erio- metl'.od of dividing; namely, the first, whicli consists ot 

 iieousdots. making; '25Q small round dots; and the second, in finding 

 the errours of these dots, and forming them into a table; I 

 come now to the third and last part, which consists in using 

 the erroneous dots in comparison with the tabulated errours, 

 so as ultimately to make from them the true divisions. 

 Subdividing It will here be necessary to complete the description of 



the remaining part of the apparatus. And first, a litllte 

 instrument which I denominate a subdividing sector pre- 

 sents itself to notice. From all that has hitherto been said, 

 it must have been supposed, that the roller itself will point 

 out, upon the limb of the instrument to be divided, spaces 

 corresponding to others previously divided upon itself, as 

 was done in setting off the 2.§G points: but, to obviate the 

 difficulty of dividing the roller with sufficient exactness, re- 

 course was had to this sector; which also serves the equallv 

 important purpose of reducing the bisectional points to the 

 usual division of the circle. This sector is represented of 

 half its dimensions by Fig. 5, PI. I. It is formed of thin 

 brass, and centered upon the axis at A, in contact with the 

 •upper surface of the roller: it is capable of beitig moved 

 round by hand ; but, by Its friction upon the axis and its 

 pressure upon the roller, it Is sufficiently prevented from 

 being disturbed by accident. An internal frame B B, to 

 which the arc C C is attached, moves freely in the outer 

 one, and by a spring D is pusiied outwards, while the screw 

 E, the point of which touches the frame B, confines the arc 

 to its proper radius. The arc of this sector is of about four 

 times greater radius than the roller, and upon it are di- 

 vided the spaces which must be transferred to the instru- 

 ment, as represented on a magnified scale by Fig. 4. Now, 

 the angle of one of the spaces of the circle will be mea- 

 sured 



