[ 84 ] 



r 



eafiily may, and mufi at firfl: be nearly efteded ; which will be 

 known in fome meafure by the plate ^ refting level when the 

 inftrument is fufpended on the needle A^ without any weight 

 iuing from the end of the fiem ; and if it could be efFecfled ac- 

 curately there would be no ufe for the pipe p p, and a folid 

 flem E F might be fixed in the plate D ; but when the weight 

 is hung from the ftem and the inPirument receives a circular 

 motion round the plumb-line (which certainly may be confidered 

 the fame as AG^) it will be found that the image of a fixed 

 objedl feen by refledlion in the mirror, by, an eye kept alfo fixed 

 or viewing it through a fixed fight or vane, will not during the 

 motion of the inftrument appear every where at the fame dif- 

 tance from the centre or edges of the circular plate or mirror, 

 but the image will move or fliift its place in it ; a certain proof 

 that this was not horizontal, and that though the line A G is, 

 infenfibly different from the line of gravity, yet that it was 

 fcarce poffible to fix that line at right angles to the plate, but that 

 there was a neceffity for adjuflment : for this purpofe therefore 

 the four adjufling fcrews s s were added, whofe mode of adtion 

 and eflfe(5l have been explained. 



If during almoft half a revolution of the mirror round the 

 axis NG, (which motion the arm K being made purpofely thin 

 and the pillars a a fraall will allow) the refleded image of a 



fixed 



