423 Observations on the Use 



them, as their resemblance to the adjusting screw of the 

 common refracting telescope, renders them very enticing to 

 Visitor*s fingers. It will also be equally prudent to remove the 

 similar contrivance, for giving to the instrument, its azimuth al 

 motion. 



As to the horizontal position of the axis, any deviation from 

 it, may generally be detected and corrected by its own level; 

 but when the situation of the instrument renders it practicable, 

 comparisons made of transits taken by direct Vision, with those 

 obtained by Reflection, will afford a more satisfactory Result. 

 Olive-oil may be frequently used as the reflecting surface, when 

 Quicksilver cannot. 



We have now arrived at the most important, and I fear, the 

 most frequent Source of error, namely, the Deviation of the Instru- 

 ment from the Meridian ; and, upon the most mature considera- 

 tion, I hesitate not in attributing it to the improper Confidence 

 usually placed, in what is denominated a Meridian Mark. The 

 Instrument, if large, and capable of bearing a power of two hun- 

 dred or two hundred and fifty, is most correctly placed in the Meri- 

 dian, hj observations of the superior and inferior Transits of the 

 Pole-star; if small, and having a power equal to 50 or 60, of 

 Circumpolar Stars, at a greater distance from the Pole, whose 

 motion over the wires is therefore better cognizable to the In- 

 strument : such are « Ursae Major, /5 Ursee Minor, a and ^ Ce- 

 phei ; and uhen these cannot be observed, which is sometimes 

 the case, recourse is had to the Transits of high and low stars. 



These observations shewing no discrepancy in the error of 

 the clock, a mark is set up, so as to be bisected by the meri- 

 dian wire ; and if the Astronomer, having assigned to it, its 

 station, could deprive it, as well as its support, of the usual At- 

 tributes of Matter, it would indeed deserve the name it bears ; 

 but till that be within his reach, it should be onli/ considered as 

 what it is, namely, an Approximate Adjustment to the Instru- 

 ment. 



But it may be urged, that any Error in the position of this 

 Mark, (supposing it to be towards the South) may be corrected 



