M ^3 ] 



the wire oppofite the dot ; and its pofxtion is fecured by a fpring 

 concealed in the tube A- The fcrew C, as mentioned before, 

 moves the frame abed north or fouth, and its place is fe- 

 cured by a fpring fhewn in the figure. W is a pinion which 

 works in a rack that fupports the glafs veffel filled with water ; 

 by means of this the veffel can be raifed fo as to relieve the 

 wire from the weight of the plummet when the machine is 

 moved. 



Plate IV. Fig. I. fliews a {keleton of the adjufting tool on its 

 fupports, and parallel to the telefcope B B, which during, the 

 adjuftment is always in a vertical pofition. 



Fig. II. is the fame in profile. Ci, Q. are the dots ; o, o the 

 lenfes, by means of which the images of the dots are formed in 

 the conjugate foci at S, S ; thefe images are diftinflly fhewn by 

 the microfcopes x, z ; the plumb-line is made to fvving through 

 one dot by means of the fcrews at the top of the adjufting tool 

 already fhewn in Plate III. Fig. II. ; whilft the other dot, which 

 is moveable from right to left, has its image brought to the wire 

 by means of a fcrew S in Fig. III. Plate III. which reprefents the 

 piece which carries the moveable dot. We now fuppofe that 

 the plumb-line fwings through the centre of each image, and 

 the true pofition of the tool is fo far obtained, provided the 

 plummet hangs freely in the middle of the glafs veffel, which 

 will be the cafe when the arm, which refts upon the prifmatic 

 bar, that is, when the bar itfelf is horizontal ; this is obtained 

 by 'the capftan headed fcrews t, T, Fig. III. Plate II. Provided 

 alfo that the other arm C, L, is truly horizontal, which pofition 



