[ 8o ] 



knot of a looped thread /, which is inferted through a very fmall 

 perforation made in the axis of the wire, and only fo wide as 

 juft to admit the thread tight through it, or inftead thereof a 

 Yery fine and flexible ftring of cat-gut or fiddle-ftring, in order 

 that when a weight is appended to the ftring it may pull the 

 wire only by a point in its axis. A ring or hoop rr (of iron) is 

 brazed on the lower end of the pipe />/>, juft fo broad as to admit 

 four equidiftant and oppofite adjufting fcrews s s, (made of tem- 

 pered fteel, and having fquare heads, in order to be turned by a 

 key as w"ell as by a turn-fcrew) to be inferted through it j and 

 lac hoop is to be fo thick as to fuflain, without change of its 

 circular fhape, the prefl'ure of the fcrews againft the wire G E. 

 Tne pipe p p is to be fixed perpendicular in the plate Z>, that it 

 and alfo the wire E G, vvhen in the middle of it, may be at right 

 ar.gles to the mirror ; and then any little deviation of the wire 

 from this will be correded by moving the wire (which muft be fo 

 fixed as to yield eafily) towards that fide where it makes too great 

 an angle with the mirror, which is done by firft withdrawing the 

 adjufting fcrew on the fame fide and then advancing the oppofite 

 fcrew : and when the wire is thus firmly preiled by the four oppo- 

 fite fcrews it cannot change its pofition after being made ver- 

 tical to the mirror, nor confequently the inftrument alter its ad- 

 juflment.* 



* The plate D might be omitted, and the pipe with the wire EG might be faf- 

 tened in the plate C, were it not that there fliould be a hole left in the middle of 

 this plate to allow the needle-cap (hereafter mentioned) to be fixed in the plate B, 



and 



