ON DIVIDING INSTRUMENTS. H 



of the circle, whereby the former, having its diameter at 

 the upper edge about -001 of an inch greater than at the 

 lower edge (being, as before described, a iittle conical), it 

 may easily be brought to the position where it wiil measure 

 the proper portion of the circle. 



Much experience and thought upon the subjer-t have The roller, 

 taught me, that the roller should be equal to one si teenth 

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

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

 into sixteen parts; no matter whether with absolute truth, 

 for accuracy is uot at all essential here. Each of such^.di- 

 visions of the roller will corresi^ond with an angle upon the 

 circle of 1° 24' 22'5", or -^i-^th part of the circle. This 

 number of principal divisio -.s was chosen, on account of 

 its being capable of continual bisection ; but they do not fall 

 in with the ultimate divisions of the circle, which are intended 

 to be equal to 5' each. 



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

 measure the circle. As two microscopes are here necessary, 

 and those which I use are very simple, I will in this niace 

 give a description of them. Fig. 6 is a section of the full 

 size, and sufficiently explains their construction, and the 

 position of the glasses; but the micrometer part and man- 

 ner of mounting it are better shown at H, in Fig. 1 and 2. 

 The micrometer part consists of an oblong square frame. Micrometer. 

 which is soldered into a slit, cut at right angles in the main 

 tube; another similar piece nicely titted into the former, 

 and having a small motion at right angles to the axis of the 

 microscope, has at one end a cylindrical guide pin, and at 

 the other a micrometer screv/ ; a spring of steel wire is also 

 applied, as seen in the section, to prevent play, by keeping 

 the head of the micrometer in close contact with the fixed 

 frame. This head is divided into one hundred parts, which 

 are numbeied each way to 50; the use of which will be 

 shown hereafter. A fine wire is stretched across the movable 

 frame, for the purpose of bisecting fine dots. Two of these 

 microscopes are necessary; also a third, which need not 

 have the divided head, and must have in the movable frame 

 two wires crossing each other at an angle of about 30° ; this 

 microscope is shown at I, Fig. 1. In the two first micro- 

 meters. 



