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Apparatus for reverjlng the Injirument. 



As the line of collimation requires conftant verification, the 

 reverfing of the axis fhould be rendered as commodious as poffible ; 

 particularly as this is always done to the greateft nicety by celef- 

 tial obfervations, which will not admit of delay. I always prove 

 it by the polar ftar*, and this is the method: I obferve the 

 palTages over the three firft wires ; I then reverfe the inftrumcnt, 

 and take the paflages over the two laft wires, that is, in fad 

 over the two firft again. Now it is manifeft, that if the colli- 

 mation be perfed, the intervals from the firft wire to the middle 

 one, and from the middle one to the fifth, will be precifely 

 equal, as alfo the intervals from the fecond to the middle, and 

 from the middle to the fourth. The apparatus for this purpofe, 

 fhewn in Plate V. Fig. I. is fo manageable, that I can reverfe this 

 weighty inftrument without help in the fhort half paflage of (5 

 Urfae minoris, in time to obferve the pafTage over the laft wire, 

 having before reverfing obferved the pafTages over the firft, fecond 

 and middle wires. The horns E, E, being placed under the axis : 

 the winch A is turned, which works with a double pinion in the 

 rack B, which is double, having the teeth fet at alternate inter- 

 vals, as fhewn in Fig. II. by which means the fmootheft 

 motion poflible is obtained : this rack elevates the block C, hol- 



* The reafon why I prefer the polar ftar is, becaufe the fmalleft error in the 

 collimation will, on account of the flow motion of this ftar, fhew a difference 

 which cannot be attributed to the unavoidable error of obfervation, or cafual 

 irregularity of the clock ; at leaft we know the limit by praftice. 



E lowed 



