[ n8 ] 
and the altitude of the Moon might be taken with 
it in the night. But the altitudes of ftars could pot 
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 glaffes and 
the filvering them, and the height of the telefcope 
may be of ufe. The index glal's and two horizon- 
glades fhould be all of equal height, and even with 
one another in height both at top and bottom. The 
telefcope fhould 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 lowed: dation fhould point about .J-th 
of an inch lower than the top of the filvering of the 
horizon-glades, and when at the higheft dation 
fhould point to the height of the middle of the un- 
filvered part of the index-glafs. The height of the 
glaffes, and the quantity of parts dlvered and parts 
undlvered, fhould vary according to the aperture of 
the objedt-glafs, as in the following table ; where the 
fir-d: column of figures fhews the dimendons in parts 
of an inch anfwering to an aperture of the obje. 61 - 
glafs of -L-ths of an inch in diameter ; the fecond 
column what anfwer to an aperture of the objedt- 
giafs of *ths of an inch in diameter ; and the third, 
O 10 
what are fuitable to an aperture of the objedt-glafs 
of -P-g-ths of an inch in diameter. 
Diameter 
