[ -7 ] 



climates it would by freezing or growing vifcixl be rendered utterly- 

 unfit for this prarpofe : it has been ^ifapproved of by a perfcm af 

 better judgment than mine, and is only propofed for experiment, 

 Ihould the inftrument be ufed in vifiting hot climates, where the 

 needle would ruft, and other liquors be fcarce. 



The want of adjuftment of the level in this conHbruaion is 

 difcovered by the motion of the image refleded from it a« before- 

 mentioned ; and it is -correfled by noting that diameter of ihe 

 mirror, in which, during its revolution, the image appears mofl: . 

 and leaft elevated ; then fuppofing a vertical plane in that 

 diameter to divide the whole inftrument, marking a line aerofs 

 its bafe, i.e. aerofs the bottom of the weight; if a due portion 

 of the metal of the weight near that line b6 filed away on the 

 fide oppofite to that in which the image is feen higheft or loweft, 

 the mirror will fettle to the juft horizontal pofition. For inftance, 

 fuppofe the moft elevated place of the image feen in the mirror 

 to be at the point a in the diameter a b ; and that a perpendicular 

 plane pafEng through the fame interfeds the bafe or bottom of 

 the weight in the line cd; if fome of the metal be rafped away 

 near d on the fide oppofite to a, then the fide c of the weight will 

 be heavier than the fide d, and will defcend and approach nearer to 

 the rod E E ; and the fide a of the mirror which was too much 

 elevated, will at the fame time fubfide to a level pofition : and 

 when this is once effeded ofadly, feeing that the inftrument is 

 one. folid inflexible mafs, whofe centre of gravity cannot alter, 



O 2 it 



