[ n8 ] 



and the altitude of the Moon might be taken with 

 it in the night. But the altitudes of ftars could not 

 be obferved with it, nor the Moon's altitude in the 

 day time, which would however be no great in- 

 convenience, as thefe obfervations might be well 

 enough fupplied by common quadrants. 



The following rules for the fize of the glafTes and 

 the filvering them, and the height of the telefcope 

 may be of ufe. The index glafs and two horizon- 

 glafles mould be all of equal height, and even with 

 one another in height both at top and bottom. The 

 telefcope mould be moveable parallel to itfelf nearer 

 to or farther from the plane of the quadrant, and 

 the range of its motion fhould be fuch that its axis 

 when at the ioweft ftation mould point about ^L.th 

 of an inch lower than the top of the filvering of the 

 horizon-glaiTes, and when at the higheft ftation 

 mould point to the height of the middle of the un- 

 filvered part of the index-glafs. The height of the 

 gjafles, and the quantity of parts filvered and parts 

 unfilvered, mould vary according to the aperture of 

 the objecl-glafs, as in the following table ; where the 

 tirft column of figures mews the ' dimen lions in parts 

 or" an inch anfwering to an aperture of the objecl:- 

 ejais of * ths of an inch in diameter; the fecond 

 column what anfwer to an aperture, of the object- 

 glafs of -j-V^ s °f an * nc h m diameter ; and the third, 

 what are iuitable to an aperture of the object -glafs 

 of -,%-ths of an inch in diameter. 



Diameter 



