104 GERARDI REGNERI POCKEN. S 



Deiudc 03lendit, nihil esse magis r.ecessariiiin , quam instriimenhj.m exaQlurn. et seriein 

 ohscrvalionum regulärem: cogiiilis enim limililnis erronim , quaccunque stellae dcvialio 

 lios e.vccdens auimiidvertalur , illam causac heterogeneae adscriljeiuiam c'ise. 



Porro laiidat comitem de Macclesfield qiii obscrvationes institucral, Grahamium, 

 qui sectorem confcccrat , et Academicos ad circulum arcticiim missos, qui scclorem 

 ejusdem generis accurate descripserant. Ergo supersedet minuta sectoris sui dcicriplione: 

 scd usum tamcn indicat. Praccipue micrometrum in censum venil , in quo revolulionum 

 valorem magna cum cum delerniinavit , et iibivis eundera invenit. Caelerum siimsit sibi 

 invariabilitalem lineae tclescopü colliueatoriae respectu divisionum arcus , per omnc lem- 

 pus, quod observationes amplectuntur. Sed hoc poslulatum et concedunt observatioi.cs 

 Laponicae et comprobat evciitus , cum, elapsa novi phaenomeni pcriodo, stellae ad eua- 

 dem in instrumento locum redierint, quem in hac constanlis liiieae opticac hyj)uiljesi 

 obtiuere debuerunt. 



« I have already taken notice, in what mamier this phaenomcnon discover'd ilself 

 « to me at tlie end of my first year's observalions, viz, by a gi-ealer apparent change 

 « of declinalion in the stars near the equinoclial colui-e, than could arise from a pre- 

 « cession of 50' in a year; Ihe mean quantily now usually allowed by astronomers. But 

 « Ihsre sppearing at the same lime , an effect of a quile contrary nalure, in some stars 

 « near llie solstilial colure , which seem'd to aller ihcir declinalion less tlian a preces- 

 « sion of 50" required ; I was thereby convinced , that all tlie phaenomena, in Ihe 

 « diSiTfint stars , could not be accounted for, merely by supposing, ibat I had assumed 

 « a wrong quanlily for the precession of the equinoclial points. " 



<f Atfirst, I had a suspicion, that some of these small apparent alterations in the 

 <( places of the slars, might possiWy be occasioned by a chaugc, in the materials, or 

 « in the posilion of llie parls of nj\ sector. But, upon considering how firmly the arc, 

 « on which the divisions or points are niade, is fjstcned to the plate , wherein the wire 

 « is fixcd thal lies m the focus of llie objects-glass ; I saw no reason to appvehend , 

 « that any change could have happened in the posilion of that wire and those points. 



« The Suspension therefore of the plumraet being the most likely cause , from 'whence 

 « I conceivcd any uncertainty could arise ; and the wire of which had been brokea 

 « three or four timcs in the first year of my observalions: I attempted to examine, 

 « whether pari of the 'foremenlion'd apparent motions might not have been owing to 

 « the different plumb-Iines that had been made use of. In order to determine Ihis, 

 « I adjiisted a particular poiut of the arc to the plumb-line, wilh all the exaetness I 

 « could and thea taking off the old wire , I immediately hung on anolher , wilh which 

 « the same spot was again compared. I repeated the experiment three or four limes , 

 << and thereby fully satisCed myself , that no sensible error could arise from the use of 



« difr 



