[ 21 ] 



in the other fide of the tube, oppofite to each of thefe, he in- 

 ferts a convex lens ; now it is obvious that an image of each 

 dot will be formed in the conjugate focus of its correfponding 

 lens. The tops of the pillars fupport a brafs frame, carrying a 

 plumb-line and two microfcopes, placed direcflly before the 

 images of the dots, to which their foci are adjufted, fo that 

 the images are diftin£t and magnified ; and the plumb-line 

 being moveable by fcrews at the fufpenfion frame above, is 

 made to fwing through thefe images. Nothing can be better 

 imagined than this. 



In the firft place, the adjuftment is applied totally independent 

 of the inftrumeht ; and fecondly, the plumb-line hanging in the 

 images themfelves, there cannot pofilbly be any parallax on the 

 \%'ire, nor any corpufcular attradion exerted on it, as is probably 

 the cafe when a plumb-line is brought very near to a metallic 

 plate ; inconveniences to which all plumb-lines have hitherto been 

 in fome degree fubjed. 



The three plates are defigned to fhew this valuable adjuftment 

 more fully. 



Plate II. Fig. I. is a bird's-eye view of the top of the eaftern 

 pillar. Fig. II. of the weftern. 



On the top of each pillar, and in the diredion of the me- 

 ridian, is fhewn a prifmatic bar of brafs, A, B, feen in profile, 

 and of a larger fize in Fig. III. The horizontal pofitipn of this 

 bar is obtained by the capftan headed fcrew t, and is fixed by 



the 



