ipS ESSAYS AND OBSERVATIONS 



be perfe(5lly jufl; ; whereas there is ground 

 to doubt of their precifenefs. 



Mathematical inftruments ia ge*- 

 fteral, and fuch particularly as "ferve for 

 aftronomical purpofes, have been, of late, 

 brought to an exa<5lnefs never known in 

 former times. Hence fome minute mo- 

 tions, or inequalities of motions, in the 

 heavens, may be difcovered, that have 

 hitherto efcaped the moll diligent obfer- 

 vers. They had difcovered indeed fome 

 motions that are very minute or flow, 

 fuch as the preceffion of the equinoxes ; 

 but this was owing to the continual iu' 

 creafe of the efFecT:s of thofe very minute 

 motions, which, though infenfible in one 

 revolution, become very manifeft in ma- 

 ny revolutions. Motions of this kinct 

 are ealily found, and are accurately de- 

 termined, by comparing diftant obferva- 

 tions. But there are other minute mo- 

 tions which are hard to be difcovered : 

 Thefe which do not grow up to be fenfi- 

 ble in a number of revolutions ; their 

 efFecft in one part deftroying their efFeifl 

 in another part of a revolution. Of this" 



kind 



