OP THE UNITED STATES. 353 



Instead of the pillars d, d, which cany the needle, there are 

 three screws, liy wjiich the iippeiiiiost circular piece n, n is 

 screwed last to (he middle of the plane tal)le, which has for 

 that purpose a circular part, giving an additional thickness to 

 the table in this place. 



The piece;?,;/ is a strong circular rim, eiglit inches in dia- 

 nieter, witli six strong radii. The outer part has a l)ell me- 

 tal ring, upon whicli it revolves upon tiie lower plate/,/, to 

 give a smoother motion than brass on brass. In the centre 

 m is a boll metal axis, about three inches and a hall" long, 

 passing through the centre of the piece /, /. Tl»e piece /, / is 

 similar to tlie above in shape, only stronger, and [irojecting 

 somewhat over it, so as to admit the clamping part />, which 

 goes in a small rim cut in it all roimd. and i)y wliicli the 

 plane table is placed in the pro[)cr dii-eclion by a tangent 

 screw, after beins; approximately placed by hand. 



Fn the centre piece /. / is a strong piece o. n. through which 

 the axis of the upper plate passes, and in which its revolu- 

 tion may be fully stopped by the milled h; ad mother screw 

 /. This centre piece has a small neck at o, o, below which it 

 is formed in a part of a sphere of aI)out an inch and a half in 

 diameter, which is held down to the lower |)iccc ;•, ;- by the 

 sections of a hollow spherical piece //, q, covering the above 

 part of a sphere. 



In the plate i\i\ which is again formed like the others, but 

 niade the strongest of the tliree. (the rim being about an inch 

 and a half bioad and one-third of an incii thick.) tliere are 

 three perpendicular screws e, at one hundred and twenty de- 

 grees distance (Vom eacli other, supporting; the piece /. / upon 

 round nobs, and being turned by their milled heads below the 

 plate, which lidl exactly in the middle between the three 

 sockets s. These sockets receive the Joints of the brass fer- 

 rules a. whicii move round tlic pin passina; throucjii them, 

 and have a stron<; Wf)od screw inside, in which woodi'U legs 

 five feet long are screwed fast, and extend far enough out in 

 all situations of (lie trronnd, in ordei- to ajivc suffieient solidity 

 to the table. These legs have iron ferrules and points below. 



