132 Dr. Gregory* 6 Strictures on Don Rodriguez. 



competent judges, on reading the description of it in the 

 Phil. Trans, tor 1803. To those who have seen neither 

 the instriniienl nor the descnj)tion, it niav suffice it I re- 

 xnark, that the cquahty ot the divisions on the arch is 

 evinced ironi this consideration, that on runninti the mi- 

 crometer screw tn-m division to division over the whole arch, 

 there was nowhere an indication ot an error amounting to 

 half a second; and that the instrument still continues tree 

 from important " derangement," is tolerai)]y well proved 

 by this, that the line ot collimation has been comtant du- 

 ring all the observations and all the journeyings of the 

 sector, and that it still continues the same. 



In the next place, it may be remarked that no error in 

 observation can be imputed to a deviation fiom " vertical 

 position" in tlie sector. Important inaccuracv in this re- 

 spect is precluded by the great length ot the axis, by which 

 the instrument is rectiiitd ; and by the ready and certain 

 means of placing the pkimbline directly over the illumi- 

 iiattd dot which marks the middle ot the axis, or true cen- 

 tre of the dividtd arch. For want of this admirable mode 

 of correction, all previous instruments are necessarily im- 

 jierfect. It appears from Phil. Trans, for 1803, pp. 405, 

 406, that whin the instrument is adjusted in one position 

 by means ot the plumb-line and dot, it is turned to a posi- 

 tion at right angles to the former, and the adjustment con- 

 firmed ; and this being the casein these two situations, the 

 insirtiment must necessarily be vertical in all others. 



Various reas':ns may be assigned to show that the sector 

 could not, at any ot the stations, be out of the plane of the 

 meridia?}.. 1 shall select only two or three. As 1st : If the 

 sector were inclined to that plane, just so much would tlie 

 path of any star in its ajipaient motion be inclined to the 

 liorizontai wire of the telescope ; instead of which, both 

 Col Muilge and Capt. Colbs assure me, that when a star 

 tame into contact \\iih the wire, the light of the star would 

 appear on both siJes of the wire tor about three-quarters of 

 a minute of time, the light on each side being equal at tlie 

 centra! wire: which ol itseif is a positive proof. But, 

 'Jdly : Had the sector been out of the plane of the meridian, 

 the times of the transits of the extreme stars cm]'loved. as 

 compared with two excellent tinu -keepers, njust have 

 shown it. Fur'her, the errors arism-g from a wrong p.lane 

 ot the meridian beinsr comparaiivelv very great in the ex- 

 treme stars and small in those near the zemth, it would 

 lollo-.v that the error in Capv !la, w hich is almost at the ex- 

 tremity of the arch, would be great, coniparcd witli those in 



/3 Draconis, 



